Patterns Of Inheritance
Genetics (II) -- Patterns Of Inheritance
A
s recently as the 1850s, biologists had still to unravel the patterns of heredity. Darwin in the ‘The Origin of Species’ wrote: . .The results of the various but dimly understood laws of variation are infinitely complex and diversified... The laws governing inheritance are for the most part unknown. . .
|[pic] |It was a common brief in Darwin’s and Mendel’s time, that inheritance is a bl_____ process (paint-pot). It was |
| |Gregor Mendel in the 1860s who laid the foundations of modern-day genetics proposing the p_________ nature of |
| |heredity and unravelled the principles of seg________ and independent ass_________. |
| | |
| |It was not until 1900 that the scientific community started to appreciate Mendel’s insights. An understanding of |
| |the behaviour of ch_________ during g_____ formation was a basic step for the clarification of the role of |
| |chromosomes as the physical agents of Mendelian genetics. |
I) Mendelian Inheritance
A) Monohybrid Inheritance
Gregor Mendel was an Austrian monk. He began his experiment with garden pea in 1856. He first
|investigated the inheritance of only o__ pair of characters: what |[pic] |
|is now called M________ inheritance. He took p___-breeding tall | |
|plants and prevented them from carrying out their normal | |
|self-pollination*a. The tall plant’s stigmas were then carefully | |
|dusted with Pollen collected from short plants*b. These tall and | |
|short plants are said to belong to the Par_____ generation. The | |
|seeds produced are the First f____ (F1) generation, these were | |
|planted: they were h_____ with a tall mother and a short father. | |
|The F1 generation grew, without inter_____ height, into tall plants. Mendel tried the same |[pic] |
|experiments using tall plants to provide the pollen for the stigmas of short plants (that is, he | |
|perform a reciprocal cross) and again all members of the F1 generation were t___. | |
| | |
|The tall F1 plants were allowed to s___-pollinate (or selfed) and their seeds were then sown to | |
|produce the second filial or F2 generation. Mendel counted the individuals and he recorded | |
|approximately _____ tall and ____ short. | |
| | |
|Similar crosses were carried out with the other seven pairs of characters, always with the same | |
|result: | |
a) How could Mendel prevent self-fertilization of the tall plants ?
b) How could Mendel ensure that no other pollens will reach the tall plants’ stigmas besides those of the short plants?
a) Particulate Inheritance and Mendel’s First Law
In Mendel's experiments, the kind of blending of characters that troubled Darwin did not occur. Mendel knew nothing of meiosis, genes or chromosomes. He spoke of a pair of factors (now called alleles) determining a character and he drew the following conclusions :
|Mendel’s conclusions from Monohybrid cross : |In terms of modern genetics, Mendel’s monohybrid experiments can be represented |
| |as follows: |
|Since the original parental stocks were pure breeding, the tall | |
|variety must have possessed two tall factors and the short variety |[pic] |
|two short factors. | |
| | |
|The F1 generation possessed one factors from each parent which were | |
|carried by the gametes. | |
| | |
|These factors do not blend in the F1, but retain their | |
|individuality. | |
| | |
|The tall factor is dominant to the short factor which is recessive. | |
| | |
| | |
|The separation of the pair of parental factors, so that one factor | |
|is present in each gamete, became known as the Mendel’s First Law | |
|Mendel’s First Law -- Law of Segregation : |[pic] |
| | |
|:Particles or factors (now called genes) which determine characters existed in each | |
|parent in pairs. | |
| | |
|Each gamete carries only one of the two parental factors. | |
*The ratio of dominant phenotypes to recessive phenotypes of 3 : 1 is called the monohybrid ratio. (This ratio will only be apparent if samples are large enough).
b) Monohybrid Test-cross
|An organism with a dominant phenotype may be homozygous or | |
|heterozygous. In order to find the genotype of the organism it is |[pic] |
|crossed with a homozygous recessive individual. | |
| | |
| | |
|If All the offspring show the dominant phenotype the organism is | |
|__________________ _________________, | |
| | |
|but if a 1:1 ratio of dominant to recessive phenotypes results then | |
|the organism is ______________________. | |
B) Dihybrid Inheritance
a) The ‘Dihybrid Ratio’ : 9331
Mendel went on to ascertain the pattern of inheritance when t__ characters were considered at the same time.
|One of his crosses was between true-breeding plants giving round, yellow seeds with those giving |[pic] |
|wrinkled, green seeds. Such a cross, in which two Pairs of alleles are considered, is called a di_____ | |
|cross. It is carried out in the same way as the monohybrid cross. | |
| | |
|Mendel’s F1 generation each developed round, yellow seeds, indicating that the round character is | |
|dominant to wrinkled and yellow is dominant to green. The F1 plants were selfed, Mendel counted 556 | |
|seeds of the F2 generation - of these there were: | |
| | |
|315 round and yellow 101 wrinkled and yellow | |
|108 round and green 32 Wrinkled and green. | |
| | |
| | |
| | |
|The proportions of each phenotype approximated to a ratio of | |
|9 : 3 : 3 : 1. This is known as the dihybrid ratio. Mendel made two deductions from these | |
|observations. | |
| | |
|Two new com________ of characteristics had appeared in the F2 generation : wrinkled and yellow, round | |
|and green. These new combinations are called re__________ types to distinguish them from the original | |
|P_______ types. | |
| | |
|The ratios of each pair of characteristics appeared in the monohybrid ratio of ___ : ___. i.e. 423 | |
|round to 133 wrinkled, and 416 yelow to 140 green. | |
It was obvious that the roundness of the original parent seeds was in no way tied to their yellowness, since both round green and wrinkled yellow F, seeds appeared. Taking each character separately, it is clear that approximately 3/4 of the F2 seeds were round and 1/4 wrinkled; 3/4 were yellow and 1/4 green. Considering the characters together, they approximate to a ratio of 9 round yellow: 3 wrinkled yellow: 3 round green: 1 wrinkled green.
From these results, Mendel stated that the two pairs of characteristics, se_______ and behave in___________, while combining in the F1 generations. This forms the basis of the Mendel’s Second Law -- the Law of Independent Assortment which stated that :
Any one of a pair of factors may pass with any one of another pairs into a gamete during gametogenesis.
Q. Flower colour in sweet pea plants is determined by two pairs of genes (R,r and S,s). If at least one dominant gene from each pair of genes are present the flower are purple. All other genotypes are white. If two purple plants, each having the genotype RrSs, are crossed, what will be the phenotypic ratio of the offspring ?
b) Dihybrid Test-cross
|The plants grown from the round, yellow F1 seeds can be used as parents | |
|in a testcross / backcross-- with double re_______ plants (that is, |[pic] |
|those which are recessive in both characters under consideration). | |
| | |
|The four different phenotypes are produced in equal proportions, which | |
|is what the Mendelian theory predicts. | |
C) A Summary of Mendel's hypotheses
The following summary includes terms taken from our present knowledge of the nature of genetics.
1. Each characteristic of an organism is controlled by a pair of a______.
2. If an organism has two unlike alleles for a given characteristic, one may be expressed (the d________ allele) to the total exclusion of the other (the recessive allele)
3. During g_______ formation, each pair of alleles separates (segregates) and each gamete receives o___ of each pair of alleles (the principle of segregation).
4. During gamete formation in each sex, either one of a pair of alleles may enter the same gamete cell (combine randomly) with ei_____ one of an______ pair (the principle of independent assortment).
5. Each allele is transmitted from generation to generation as a dis_____ unchanging unit.
6. Each organism inherits one al____ (for each characteristic) from each parent.
Mendel published his research data and hypotheses in 1866 in a journal, The Proceedings of the Brunn Natural History Society, which was sent to most of the learned scientific societies throughout the world. In all cases they failed to appreciate the importance of his findings, The importance of Mendel’s finding was rediscovered in 1900, around 40 years after he published his findings, -- 16 years after his death!
During the following ten years, Mendel’s work was successfully repeated and his laws found to apply to other plants and animal species.
NB The mechanism of dihybrid inheritance, the examples quoted and the typical dihybrid ratio of 9:3:3:1 only apply to characteristics controlled by genes on different chromosomes. Genes situated on the same chromosome may not show this pattern of independent assortment as described in ‘linkage’ later
D) The Chromosomal Basis of Inheritance -- Mitosis and Meiosis
Chromosomes are composed of DNA, which takes the form of a long double h____ which in places forms aggregates with nucleoprotein histones. These structures are called nucleosomes and have the appearance of beads on a string. The DNA-protein complex is called chr______.
a) Mitosis
When cell division takes place, cells pass through a regular sequence of stages known as the cell cycle. This includes two overlapping events, n______ division followed by cell division (cyto_______ cleavage).
Mitosis results in an increase in cell numbers and is the method by which gr_____, replacement and re____ of cells occurs in all higher organisms. It provides an equal distribution of duplicate chromosomes between daughter cells and the production of genetically i_______ offspring.
i) Nuclear division
|1) Interphase | |
|The cell increases in size and synthesizes many new or_______. Just prior to the next phase, DNA and | |
|histones replicate. At this stage each chromosome exists as a pair of chromatids joined at the centromere. |[pic] |
| | |
|2) Prophase | |
|This is the longest phase. The chromatids sh____ and th_____ by spiralization and condensation. In animal | |
|cells and lower-plant cells the centriole pairs move towards opposite p___ and a sp____ forms. Spindle | |
|fibres are formed, made up of micro______ of tubulin. | |
| | |
|Towards the end of prophase the spindle consists of long continuous fibres stretching from one pole to the | |
|other and shorter ones attached to the chromesome centromeres. The nuclear m_______ finally breaks down, | |
|marking the end of prophase. | |
| | |
|3) Metaphase | |
|Pairs of chromatids become attached to spindle fibres at their centromeres and are manoeuvred into a | |
|position across the spindle equator. | |
| | |
|4) Anaphase | |
|The centromeres divide and the chr______ (now chromosomes) separate. They are slowly pulled apart, | |
|centromere first, towards op_____ poles. | |
| | |
|5) Telophase | |
|Therefore identical sets of chromosomes have been moved to each pole. At the poles the chromosomes uncoil | |
|and elongate. The spindle fibres disintegrate and each mature centriole replicates. The nuclear membrane | |
|reforms and cyto________ cleavage begins. | |
ii) Cytoplasmic cleavage
|In animal cells microfilaments at the |[pic] |
|equator draw in the cytoplasm producing a| |
|furrow around the cell's circumference. | |
|The cell membranes meet and cell | |
|separation occurs. | |
| | |
|A cell plate is formed in plants by | |
|vesicles produced by Golgi bodies. A | |
|primary c___ w___ is laid down on either | |
|side of the cell plate, followed | |
|frequently by a secondary cell wall. | |
| | |
|Perforations in the wall house | |
|plasmodesmata which connect adjacent | |
|cells together. | |
b) Meiosis
This occurs during ga____ and sp___ production. Alternatively called reduction division, it reduces the diploid number of chromosomes in a cell to a h_____ number. This ensures that the chromosome number in offspring of sexually reproducing organisms is retained at a constant level from generation to generation.
Essentially, the process consists of a dup______ of genetic material (as in mitosis) in the parental cell followed by t__ successive meiotic nuclear divisions and cell divisions to form f___ h_____ cells.
|a) Interphase |[pic] |
|The cell increases in size and many organelles and the chromosomes rep_____. | |
|b) Prophase I |[pic] |
|Chromosomes condense. H_______ maternal and paternal chromosomes p___ up (synapsis) and form structures called biv_____. | |
| | |
|Chiasmata may occur. These are the sites of ex_____ of genetic material during the process of c_______ over and account for the | |
|major source of genetic variation. | |
| |[pic] |
|The centrioles, if present, move to opposite poles, a spindle apparatus forms and the nucleoli and nuclear | |
|membrane break down. | |
|c) Metaphase I |[pic] |
|Bivalents attached to the spindle by its centromere and aligned across the spindle eq_____. | |
|d) Anaphase I |[pic] |
|Spindle fibres pull the homologous chromosomes apart by their centromeres . Thus two h_____ sets of chromosomes are created. | |
|e) Telophase I |[pic] |
|Nuclear membranes ref___ around the chromosomes, forming two new haploid nuclei. | |
|f) Prophase II | |
|This stage is absent in cells without interphase II.. New spindle fibres appear at right angles to the first |[pic] |
|spindle axis. The chromatids condense and centrioles, if present, migrate to opposite poles. | |
|g) Metaphase II |[pic] |
|The chromosomes are aligned on the eq_______ plate and attached to the spindle fibres by their centromeres. | |
|h) Anaphase II |[pic] |
|The centromeres divide and the spindle fibres pull the separating chromatids (now called chromosomes) towards | |
|opposite poles | |
|i) Telophase Il |[pic] |
|Spindle fibres disappear, centrioles replicate and nuclear membranes reform. Chromosomes lengthen and uncoil and | |
|cell division proceeds as in mitosis. | |
| | |
|Four haploid daughter cells (gametes) are formed. They contain mixed p and m genetic | |
|material. This has occurred as a result of c______ over during meiosis and independent a_________ of bivalents. | |
|[pic] |
|Q. The picture shows metaphase of two types of cell division. |[pic] |
|a. Which picture represents metaphase in mitosis and meiosis respectively ? | |
| | |
| | |
|b. What structures separate during | |
|i) anaphase I in meiosis | |
| | |
|ii) anaphase in mitosis | |
| | |
c. What is the significance of this difference for the two daughter cells produced in
i) meiosis
ii) mitosis
d. At what stage do chromatids separate in meiosis ?
e. Give three reasons why the product of anaphase II of meiosis is different from that of anaphase of mitosis ?
|[pic] |[pic] |
|f. Write the sequence of numbers that correctly describe the order of | |
|stages of meiosis shown on the right. | |
| | |
| | |
|g. Identify the stages of meiosis in the picture above. | |
Significance of meiosis
1) Sexual reproduction
|If meiosis did not occur, fusion of gametes would result in a d_____ of the chromosomes for each |[pic] |
|successive sexually reproduced generation. This situation is prevented in the life cycle of all | |
|sexually reproducing organisms by the occurrence of a cell division involving a r_______ in the | |
|diploid number of chromosomes (2n) to the haploid number (n). | |
| | |
|Q. Which meiotic division is the reduction division? | |
| | |
| | |
|2) Genetic variation | |
Meiosis also provides opportunities for new com______ of genes to occur in the gametes. This leads to genetic v______ in the genotype and phenotype of offspring produced by the fusion of gametes:
The orientations on the equatorial plane of bi______ during metaphase I and of chromosomes in metaphase II are r______. This is called independent assortment and results in the assortment of m______ and p_______ chromosomes between daughter nuclei produces new allelic com_______ in gametes.. This is the basis of Mendel' _______ law.
As a result of ch_______, crossing over of segments of chromatids occurs between homologous chromosomes during prophase I, leading to the formation of new combinations of alleles on chromosomes of the cells. This produces new l groups.
|Q1.When does chiasmata formation and cross-over occur ? |[pic] |
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|Q2.Between which structures does exchange of genes occur ? | |
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|Q3.The figure shows chiasmata formation in meiosis, complete the four separated | |
|chromosomes to show the result of crossing-over. | |
c) Chromosomal Theory of Heredity
By 1900, as a result of improvements in the optical properties of microscopes, the behaviour of c_________ in gametes and zygotes had been observed. William Sutton, noticed the striking similarities between the behaviour of c_________ during gamete formation and fertilisation, and the transmission of Mendel's hereditary f______.
|A summary of the similarities % events during meiosis & Mendel’s hypotheses |
|Mendel's hypotheses | |Match | |Meiosis and fertilisation |
|Characteristics are controlled by pairs of factors | | | |Homologous chromosomes separate during gamete formation |
|Pairs of factors separate during gamete formation | | | |Diploid cells contain pairs of chromosomes (homologous |
| | | | |chromosomes) |
|Each gamete receives one factor | | | |Homologous pairs of chromosomes are restored at fertilisation, |
| | | | |each gamete contributing one homologous chromosome |
|Each organism inherits one factor from each parent | | | |One homologous chromosome passes into each gamete cell |
On the basis of the evidence, Sutton proposed that:
Chromosomes were the car_____ of Mendel's factors, the so-called chromosome theory of heredity. According to this theory, each pair of factors is carried by a pair of homologous chromosomes, with each chromosome carrying o___ of the factors.
i) Mendel's Law of segregation of factors
This could now be explained in terms of the s_________ (segregation) of h________ chromosomes which occurs during anaphase I of meiosis.
|[pic] |
ii) Mendel's principle of independent assortment
This may also be explained in terms of the movement of chromosomes during meiosis.
During gamete formation the distribution of each allele from a pair of homologous chromosomes is entirely independent of the distribution of alleles of other pairs.
It is the r______ alignment or assortment of homologous chromosomes on the equatorial spindle during metaphase I of meiosis, and their subsequent separation during metaphase I and anaphase I, that leads to the variety of allele recombinations in the gamete.
|[pic] |
Q. Man has 23 pairs of chromosomes, calculate the possible no. of different chromosome combinations among his sperms.
Q. What are the possible no. of different combinations in the zygote resulting from fertilization of a human egg by a human sperm (assuming no crossing-over occurred during meiosis) ?
Q. The deposition of starch in pollen grains in maize is controlled by the presence of one allele of a certain gene. The other allele of that gene results in no starch being deposited. Explain why half the pollen grains produced by a given maize plant contain starch.
II) Problems of Explaining Continuous Variation
The kinds of patterns of inheritance which commonly occur appear to bear little relationship to Mendelian ratios. (The sharply defined characters studied by Mendel were discontinuous.)
This is because the result of many natural crosses is a series of offspring which vary continuously, that is, the expression of a character have many intermediates within a certain range. This had, in the Mendel and Darwin’s time, led to the widely accepted belief that inheritance is a blending process -- the ‘paint pot’ theory.
A) Discontinuous and Continuous Variation
The term variation describes the di_______ in characteristics shown by organisms belonging to the same natural pop______ or sp_____.
a) Discontinuous variation
There are certain characteristics within a population which exhibit a limited form of variation. Variation in this case produces individuals showing clear-cut differences with no i_________between them. E.g. blood groups in Man, wing length in Drosophila, melanic and light forms in Biston betularia, style length in Primula and sex in animals and plants.
Characteristics showing discontinuous variation are usually controlled by just o__ of a very f__ number of genes and their phenotypic exp_______ is relatively unaffected by en___________ conditions.
Since the phenotypic variation is restricted to certain c____-c__ characteristics, this form of variation is alternatively known as qual_____ inheritance, as opposed to quantitative inheritance which is characteristic of continuous variation.
b) Continuous variation
Many characteristics in a population show a complete grad_____ from one extreme to the other without any break. E.g. mass, height, shape and colour of organs and organisms. The frequency distribution for a characteristic exhibiting continuous variation is a n______ distribution curve.
Characteristics exhibiting continuous variation are produced by the com_____ effects of m____ genes (polygenes) and en________ factors. Individually each of these genes has little effect on the phenotype but their combined effect is significant.
|[pic] |
i) Environmental Effects
The ultimate factor determining a phenotypic characteristic is the gen_____. At the moment of fert_________ the genotype of the organism is determined, but the subsequent degree of exp_______allowed to this genetic potential is influenced greatly by the action of en___________ factors during the dev_______of the organism.
A genotypically tall child may be dwarfed by starvation and so appear similar to one whose genotype is for shortness. No character of any organism can be said to be solely due to the effects of the environment or solely due to heredity: b___ are always operating. In the case of continuously varying characters, such as human height, the genotype determines the phenotypic r_____ within which an individual will fall and the environment determines the exact point in the range.
In the development of human characteristics such as personality, temperament and intelligence, there is evidence to suggest that both nature (hereditary factors) and nurture (environmental factors) interact to varying degrees in different individuals to influence the final appearance of the characteristic. It is these genetic and environmental differences which act to produce phenotypic differences between individuals.
There is no firm evidence, as yet, to suggest that one factor is universally more influential than the other, but the environment can never increase the extent of the phenotype beyond that determined by the genotype.
ii) Polygenic Inheritance
Many of the most obvious characteristics of organisms are produced by the c_______ effect of many different genes. These genes form a special gene complex known as a polygenic system.
Whilst the effect of each gene alone is small on the phenotype, the combined effect of these genes (polygenes) can produce almost infinite variety of the character. Polygenic inheritance forms the genetic basis of c_________ variation.
If the height of a large number of humans of the same age and sex is measured and the data plotted graphically, a curve similar to that shown here is obtained. It is called a n_______ d_________ curve.
Most of the individuals are of m______ height. There are many fewer who are very short or very tall. This continuous variation is quite typical of quantitative characteristics. Thus quantitative characteristics are mainly determined by the a______ effects of several genes. E.g. Human skin colour, weight, height.
The observations can be explained by assuming that height are determined by several genes :
| |Suppose that three genes, Aa, Bb and Cc, control the height of a human and assume that each of the dominant |
| |alleles is responsible for the addition of two units of height, while each recessive allele allows for 1 |
| |unit. The tallest individual will have the genotype AABBCC(12 units) and the shortest aabbce (6 units). |
|[pic] | |
| |Individuals heterozygous for all three genes (AaBbCc) will have a height exactly halfway between the extremes|
| |(9 units). The distribution of height among the progeny of such heterozygotes can be worked out as follows: |
| |[pic] |
III) Pedigree Analysis
The study family trees or pedigree diagram*.
|*Family histories are studied for as many |[pic] |
|previous generations as possible. In the chart, | |
|the males are represented by squares and the | |
|females as circles. Shading indicates the | |
|incidence of the particular phenotype under | |
|study. Each generation is set out along one | |
|line, the birth sequence running from left to | |
|right. Suceeding generations are shown on | |
|following lines. | |
IV) Other Inheritance Patterns
A) Incomplete Dominance
There are several conditions where two or more alleles do not show complete dominance or recessiveness due to the failure of any allele to be dominant in the heterozygous condition.
|This state of incomplete dominance (co-dominance or blending) is an |[pic] |
|exception to the situation described by Mendel in his monohybrid | |
|experiments. | |
| | |
|In most cases the heterozygote has a phenotype which is intermediate | |
|between the homozygous dominant and recessive conditions. | |
|An example is the production of blue F.1 Andalusian fowls by crossing|[pic] |
|pure-breeding black fowl with splash white parental fowls. | |
| | |
|If the F1 fowls are allowed to interbreed, the F2 generation shows a | |
|modification of the normal Mendelian monohybrid ratio of 3:1. | |
| | |
|Q1.Work out the expected phenotypic ratio of F2 of interbreeding the | |
|F1. | |
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|Q2.What is the typical phenotypic ratio in F2 in incomplete dominance | |
|? | |
N.B. As there are no accepted genotypic symbols for alleles showing incomplete dominance, It is important to specify symbols in genetic explanations.
B) Multiple alleles and Inheritance of ABO blood groups
There are several conditions where a single characteristic may appear in several different forms controlled by three or more a______, of which any two may occupy the same gene loci on homologous chromosomes. This is known as the multiple allele condition and it controls such characteristics as coat colour in mice, eye colour in mice and blood group in Man.
A, B and O Blood group is controlled by an autosomal gene. The gene locus is represented by the symbol I and there are three alleles represented by the symbols A, B and O. The alleles A and B are equally dominant (co-dominant) and O is recessive to both.
RBCs may carry on their surface either one or both of the antigens A and B. In addition, antibodies which specifically bind with the antigens, anti-A and anti-B, may also be present in the plasma.
The phenotypic appearance of blood groups as determined by genotypes
|Blood group |Genotype(s) |Antigens on RBC |Antibodies in plasma |Remarks |
|A | | | |\ |
|B | | | |\ |
|AB | | | |universal acceptor |
|O | | | |universal donor |
Q1.Explain, using appropriate genetic symbols, the possible blood groups of children whose parents are both heterozygous, the father being blood group A and the mother being B.
Q2.If these parents have two children, what is the probability that both will have blood group A ?
Reference Reading: Origin of multiple allelic forms; blood group and susceptibility to diseases
The allelic forms of a gene are thought to orginate as a result of gene mutation, If gene mutate more than once there would be more than two alternative forms of a gene. These are called multiple alleles. One interesting fact is the correlation between blood groups and susceptibility to diseases. For instance, individuals of blood group O show a greater tendency to suffer from duodenal and gastric ulcers and group A individuals are prone to develop cancer of the stomach, sugar diabetes and pernicious anaemia. This example shows that a mutation which is not eliminated by natural selection will persist in the genotype of the descendents of the original mutant organism.
C) Linkage
For just 23 pairs of chromosomes to carry over tens of thousand of genes, it follows that each chromosome must carry many genes. Genes that are carried on the s____ chromosome is said to linked and they together form a linkage group. How many linkage groups are there for human inheritance?
Usually the linked genes remain tog_____ throughout gamete formation, and so pass on / inherited as one unit to the offspring. Thus they are n__assorted independently according to Mendel’s 2nd Law.
|Q. Consider two genes A and B, what are|[pic] |[pic] |
|the possible types of gametes produced | | |
|if | | |
| | | |
|i) A an B is not linked ? | | |
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|ii) A and B is linked ? | | |
a) Deviations from the 9 : 3 : 3 : 1 ratio (dihybrid ratio)
Since genes belonging to the same linkage group do not show i________ a________. They fail to produce the expected 9:3:3:1 ratio in a breeding situation involving the dihybrid inheritance.
|In Drosophila the genes for body colour and wing length have the following allelomorphs (phenotypic characteristics determined by different |
|alleles): grey and black body, and long and vestigial (short) wings. Grey body and long wing are dominant. |
| |
|If pure-breeding grey-bodied long-winged Drosophila are crossed with black-bodied vestigial-winged Drosophila, the expected F2 phenotypic ratio would|
|be 9:3:3:1. This would indicate a normal case of Mendelian dihybrid inheritance with random assortment resulting from the genes for body colour and |
|wing length being situated on non-homologous chromosomes. However this result is not obtained. |
| |
|Instead the F2 show an approximately 3 : 1 ratio of parental phenotypes. This may be explained by assuming that the genes for body colour and wing |
|length are found on the same chromosome, that is they are linked. |
|[pic] |[pic] |
|However, in practice, this 3:1 ratio is never achieved and four phenotypes are invariably |Pure |
|produced. This is because total linkage is ra__. |breeding F1 F2 |
| |parents |
|Most dihybrid interbreeding involving linkage produce approximately 3 : 1 ratio of the | |
|parental phenotypes and a variable number of phenotypes showing new combinations of |Yellow flower Yellow flower |
|characteristics, in equal numbers. These latter phenotypes are described as recombinants*. |Red fruit Red fruit |
| |(68) |
|*The proportion of recombinants depends on how often crossing-over occurr to produce | |
|gametes with the recombinant chromosome. |Yellow flower |
| |Yellow flower yellow fruit (7) |
| |Red fruit |
| |white flower |
| |Red fruit (7) |
| | |
| |white flower white flower |
| |yellow fruit Yellow fruit (18)|
b) Deviations from the 1 : 1 : 1 : 1 ratio (the dihybrid test-cross ratio )
Consider the F1 generation of the experimental cross shown, predict the results of a backcross between heterozygous grey-bodied long-winged Drosophila (F.1) and homozygous recessive black-bodied vestigial-winged Drosophila (the double recessive parent). The possible outcome depend on whether l______ is involved:
|What are the possible outcome if | |
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|1) If the four alleles for grey and black body, and long | |
|and vestigal wings, were on different pairs of chromosomes | |
|that is not linked) ? | |
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| | |
|2) If the alleles for body colour and wing length were | |
|situated on the same pair of chromosomes (that is linked) ?| |
| | |
| | |
| | |
| | |
Morgan, a geneticist famous for his work involving linkage, carried out this backcross several times and never obtained either of the predicted outcomes. Each time he obtained the following results:
41.5% grey body long wing
41.5% black body vestigial wing
8.5% grey body vestigial wing
8.5% black body long wing
Q. On the basis of these results,
a. Do you think the genes for wing length and body colour are linked or not ?
b. If the genes are linked, how would you explain the appearance of the recombinants ?
Morgan postulated that there must be some form of exchange of alleles between homologous chromosomes during gamete formation. This is later known as crossing-over. From this it is possible to produce the following definition of linkage :
‘Two or more genes are said to be linked when phenotypes with new gene combinations (recombinants) occur less frequently than the parental phenotypes’.
Q. Give two reasons why Mendel was fortunate in his choice of characteristics for studies ?
D) Crossing-over
It was Morgan who proposed that crossing-over of alleles occurred as a result of the breakage and recombination of homologous chromosomes during chiasmata.
The alleles of parental linkage groups separate and new associations of alleles are formed in the gamete cells, exchange of genes take place between non-s_____ chromatids of homologous chromosomes, a process known as genetic re .
Offspring formed from these gametes showing 'new' combinations of characteristics are known as recombinants. Thus crossing-over is a major source of observable g v within populations.
All four chromatids can be involved in chiasmata formation simultaneously; chiasmata formation can occur at any point along a chromosome, the position usually being determined by ch______.
|Look at the chromatids of homologous chromosomes shown below. The crosses show 5|[pic] |
|points where chiasmata can form. | |
| | |
|a. If one chiasma forms during meiosis, is there a greater chance of genes A and | |
|B being separated or genes A and F ? | |
| | |
| | |
| | |
|b. If two genes lie very close together on the same chromosome, is it likely that| |
|a chiasma will separate them ? Explain your answer. | |
| | |
| | |
| | |
| | |
| | |
The result of deviation from di____ ratio (9331 to 3:1 parental types with small number of recombinants) and dihybrid test cross ratio (1111 to 1:1 parental types and small number of recombinants) may now be explained in terms of formation of chi____ and cr___-over in me____.
| Testcross result |Explanation of deviation from dihybrid testcross ratio in terms of linkage & |
| |crossing-over during meiosis |
|41.5% grey body long wing |[pic] |
|41.5% black body vestigial wing | |
|8.5% grey body vestigial wing | |
|8.5% black body long wing | |
| | |
|Since chiasmata formation is a relatively r___ event, most linked | |
|genes are inherited as one u___ and do not show independent | |
|assortment. | |
| | |
|The few number of recombinant merely represent a s proportion| |
|of gametes with recombinant chromosome resulting from a small | |
|proportion of cells that undergo crossing-over during meiosis and | |
|hence reversed the linkage. | |
| | |
|This explains why there are larger proportions of p______ types | |
|compared to that of the r_______________. | |
Q. In maize the genes for coloured seed and full seed are dominant to the genes for colourless seed and shrunken seed. Pure-breeding strains of the double dominant variety were crossed with the double recessive variety and a backcross of the F1 generation produced the following results. Explain how the result might be obtained with a genetic diagram.
|coloured, full seed |380 |
|colourlessl shrunken seed |396 |
|coloured, shrunken seed |14 |
|colourless, full seed |10 |
E) Sex Determination
Pairs of chromosomes (homologous chromosomes) are found in all cells, but one pair of chromosomes always shows differences between the sexes. These are the sex chromosomes or h_______somes.
All other chromosomes are known as autosomal chromosomes or _______somes. The chromosomes are known as X and Y chromosomes, and the Genotype of the female is XX and that of the male is XY. These characteristic sex genotypes are found in most animals, including Man.
but in the case of birds (including poultry). moths and butterflies the sex genotypes are reversed: the females are XY and the males are XX. In some insects, such as the grasshopper, the Y chromosome may be absent entirely and so the male has the genotype XO.
|In the production of gametes the sex chromosomes segregate in typical Mendelian |[pic] |
|fashion. For example, in mammals each ovum contains an X chromosome; in males one| |
|half of the sperms contains an X ch________ and the other half contains a Y | |
|ch_________ as shown. | |
| | |
|The sex of the offspring depends upon which type of sperm fertilises the ovum. | |
|The sex having the XX genotype is described as h____gametic as it produces gamete| |
|cells containing only X chromosomes. Organisms with the XY genotype are described| |
|as h_____gametic since half their gametes contain the X chromosome and half the Y| |
|chromosome. | |
|The function of the Y chromosome appears to vary according to species. In Man the| |
|presence of a Y chromosome controls the differentiation of the testis which |[pic] |
|subsequently influences the development of the genital organs and male | |
|characteristics. | |
| | |
|In most organisms, however, the Y chromosome does not carry genes concerned with | |
|sex. In fact it is described as genetically inert or genetically em_____ since| |
|it carries so few genes. | |
F) Sex Linkage
The sex chromosomes bear other g___ in addition to those directly concerned with sex determination. Genes carried on the sex chromosomes are said to s__-linked. Most sex-linked traits involve X-linkage, since the Y chromosome is much smaller and carries very few genes.
Q. Study the figure left below which shows a cross between pairs of Drosophila differing with respect to eye colour. The wild-type flies have red eyes, but a mutant strain exists with white eyes
|[pic] |[pic] |
.
a) Which gene for eye colour is dominant?
b) Does this cross appear to fit the pattern of monohybrid inheritance?
c) What is the unusual feature of the ratio in the F2 generation?
Figure on the right shows another similar cross, but this time the sex/eye colour combination of the parents has been reversed. This sort of cross is called a reciprocal cross.
d) Does the result of the reciprocal cross generate the same inheritance pattern as its previous one ?
e) How is reciprocal cross particularly useful in relation to sex-linked inheritance ?
f) Given that the genotypes of the parents in the two previous crosses are as follows, draw genetic diagrams to show the two crosses showing the genotypes and phenotypes at each stage.
a) XRXR XrY (left)
b) XrXr XRY (right)
In Human male, there is a portion of the X chromosome for which there is no ho_________ region in the Y chromosome. Thus human male always express the phenotypes for all sex-linked genes, since they have only one X chromosome, regardless of whether it is recessive or dominant. This condition is known as hemizygous.
Human female have t____ the chance of obtaining an X, thus female express the dominant phenotype whenever there is at least one dominant gene, but need two copies of the recessive gene for expression. This explains why the r________ sex-linked trait such as red-green colour blindness, haemophilia occurs mostly in the _____.
In all recessive sex-linked traits, female who are heterozygous are described as c_______ of the trait. They are phenotypically n_____ but h___ of their gametes carry the recessive gene. Despite the father having a normal gene, there is a ____ chance that s of carrier females will show the trait.
Q. Some sex-linked traits are expressed more often in girls than in boys (e.g. Vitamin D resistant rickets), while others are expressed more often in boys than in girls. How is this possible ?
Q. A study of crosses reveals that the genes for sex-linked diseases (eg. Colourblindness) are exchanged from one sex to the other at each generation. The father passes it to his daughters, who thus become carriers. The daughters in turn may pass it to their sons, who are thus colourblind. Explain the phenomenon.
a) Example 1 : Haemophilia - a human sex-linked disease
H_________ is a disease in which one of the factors (factor 8) required for the normal c______ of blood is deficient. As a result, the blood fails to clot or clots very slowly. Thus, even minor injuries can cause profuse internal and external b______ which can lead to death.
Haemophilia is g________ determined and the gene concerned behaves as a s__-l____ r_______ gene (h), associated with the X chromosome. There is a fairly high spontaneous mutation rate of the normal gene (H) to the form causing haemophilia.
For a male to have haemophilia, he must carry just one gene for the disease on his X chromosome. His Y chromosome bears no genes affecting the character. In a female, there is a site or locus on both X chromosomes where the gene controlling the clotting factor is found. Both these loci must be occupied by the r_______ allele for a female to have the disease. Such a situation is a very rare occurrence. Besides, haemophiliac female are unlikely to children because the onset of puberty is often fatal, why ?
Figure below shows the pedigree of Queen Victoria and her descendants, some of whom were haemophiliacs. Examine this carefully and answer the following questions.
|[pic] |
Q1.If a haemophiliac man marries a normal woman, what will be the possible genotypes of their children?
Q2.Which family in the pedigree illustrates this situation?
Q3.If a carrier woman marries a normal man, what would be the possible genotypes of their children?
Q4.Name two families from the pedigree which illustrate this situation.
b) Example 2: Colourblindness
C_____________ is one of the most common genetic disorders that occurs in the human race. About 8 percent of males in the U.S. are red-green color defective. The trait for colourblindness is caused by a r_________ gene located on the ___ chromosome.
Since the Y chromosome does not contain an allele for this gene, men are hemizygous for either the normal allele or the color-defective one. Because of this, the usual "dominant" and "recessive" terms do not apply in males. Whichever allele present is expressed.
The terms can be used in females since they have two X chromosomes. Because it is a recessive gene, a woman h___________ for the trait will be phenotypically normal.
|Females heterozygous for color |Carrier woman X normal man |Carrier woman X Colourblind man |
|blindness will pass the mutant gene | | |
|on to 1/2 of their children. Males | | |
|who get this allele will be color | | |
|blind because, possessing only one X | | |
|chromosome, this is the only allele | | |
|they have. Female children with the | | |
|mutant allele will be heterozygous | | |
|like their mother as long as the | | |
|father has normal vision. Females | | |
|homozygous for the trait are rare, | | |
|since both parents would have to | | |
|possess the defective allele. | | |
|The daughters of normal females and |Normal woman X Colourblind man |Colourblind woman X normal man |
|color-blind males will all be | | |
|carriers with normal vision. They | | |
|will all inherit the defective gene | | |
|from their father and a normal X gene| | |
|from their mother. Because males do | | |
|not inherit X chromosomes from their | | |
|fathers, sons of color-blind males | | |
|and normal females will have normal | | |
|vision. | | |
| | | |
Q1.A girl of normal vision, whose father was colourblind, marries a man of normal vision. What type of vision can be expected in their offspring?
Q2.A colour-blind woman marries a man of normal vision and their first child is a boy. The boy's maternal grandmother had normal vision, but his maternal grandfather was colour blind.
a) What are the genotypes, as to vision of the boy, the parents and the grandparents?
b) If the boy had a sister, what would her genotype be?
c) If the boy grew up and married a woman genotypically like his sister, what type of vision would be expected among the offspring?
c) Sex-linked inheritance in other organisms
i) In Drosophila, the X chromosome of pair I can carry a recessive allele (m) causing miniature wings. Normal flies have long wings. Chromosome pair II may carry a recessive allele (b) which produces black body colouration. Normal flies have grey bodies.
Describe the Fl and F2 genotypes and phenotypes resulting from a cross between the following purebreeding individuals:
a black female with long wings X a grey-bodied male with miniature wings
ii) In Drosophila the genes for wing length and for eye colour are sex-linked. Normal wing and red eye are dominant to miniature wing and White eye.
a) In a cross between a miniature wing, red-eyed male and a homozygous normal wing, white-eyed female, explain fully the appearance of (1) the F1 and (2) the F2 generations.
b) Crossing a female from the F1 above with a miniature wing, white-eyed male gave the following results:
normal wing, white-eyed males and females 35
normal wing, red-eyed males and females 17
miniature wing, white-eyed males and females 18
miniature wing, red-eyed males and females 36
Account for the appearance and numbers of the phenotypes shown above.
End
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