Mendelian Genetics



Mendelian Genetics

GREGOR MENDEL: 1860, father of genetics. An Austrian monk who developed the laws of inheritance by studying garden peas. Showed that parents can pass heritable factors to their children (genes).

• Genes: stretches of DNA on chromosomes that can determine a trait. Always located on the same spot or locus on a chromosome.

Genes have alternate forms called alleles, and every organism has 2 alleles for every trait that are found on homologous chromosomes (one from mother, one from father).

• Dominant allele: form of the trait that will always be expressed. use capital letter to denote it.

• Recessive allele: form of the trait that will be masked or overtaken by the dominant trait. Use lower case letters to denote it. need two copies of the allele to express it.

• Homozygous: have 2 copies of the same allele. Ex: BB, bb

• Heterozygous: one copy of each alternative allele. Ex: Bb

• Codominant alleles: alleles that are expressed equally. Have the same dominance.

• Incompletely dominant alleles: one allele is not completely dominant over the other. The phenotype is somewhere in between or an intermediate between both traits.

Principles of Segregation: Mendel performed experiments where he followed the inheritance of one trait only. This is called a monohybrid cross because the parent plants differ in one characteristic only.

- Parental generation is called P generation.

- The offspring of the P generation are the F1 generation.

- The offsprings of the F1 generation are the F2 generation.

Used purple flowers versus white flowers on pea plants.

Mendel deduced from his experiments:

1. there are 2 forms of each trait called alleles

2. the organism inherits one allele from the mother and one from the father

3. allele pairs segregate during meiosis, each sperm and egg (gamete) carry one allele for each inherited trait

4. Fertilization gives the offspring 2 alleles. alleles can either be dominant or recessive.

Use PUNNETT square to solve a genetic problem. keep track of the gametes.

• figure out the genotype of the parent

• figure out possible gametes

• use the square to determine all possible genotypes and phenotypes of the offspring.

From the punnett square, Mendel showed that the generation of F1 will all be purple with a Pp genotype, and that F2 will have 3/4 purple and 1/4 white flowers. genotype: PP, 2Pp, pp

F1 generation:

| |P |P |

|p |Pp |Pp |

|p |Pp |Pp |

• All F1children are heterozygous

• this is always the case if you mate a homozygous dominant with a homozygous recessive gene

F2 generation: Pp x Pp

| |P |p |

|P |PP |Pp |

|p |Pp |pp |

• 3/4 have purple phenotype but one is homozygous ( PP) and 2 are heterozygotes (Pp)

• 1/4 are white, homozygous (pp)

• phenotype ratio is 3:1 purple vs. white

• genotype ratio is 1:2:1 ( 1PP, 2 Pp, 1pp)

• this is always the ratios for a heterozygous monohybrid cross

The principle of independent assortment is shown by following 2 traits at the same time.

It states:

- each pair of alleles segregates independently during gamete formation

- all combinations can occur in the gametes

Dihybrid cross: mating of parents differing in 2 traits.

RY: round yellow seeds, dominant

ry: wrinkled green seeds, recessive

crossed RRYY x rryy -------> gametes RY and ry

F1 generation are RrYy ----> round and yellow peas. all offsprings are heterozygous for one trait.

F2 generation: RrYy x RrYy-------> gametes RY, rY, Ry, ry

F2 generation:

| |RY |rY |Ry |ry |

|RY |RRYY |RrYY |RRYy |RrYy |

|rY |RrYY |rrYY |RrYy |rrYy |

|Ry |RRYy |RrYy |RRyy |Rryy |

|ry |RrYy |rrYy |Rryy |rryy |

We have: 9 round yellow peas (dominant, dominant), 3 round green peas ( dominant, recessive), 3 wrinkled yellow peas ( recessive, dominant), 1 wrinkled green pea ( recessive, recessive). The ratio of phenotypes for a dihybrid cross is always 9:3:3:1.

This behavior is attributed to the behavior of genes after meiosis. The alleles segregate as the homologous chromosomes separate in anaphase I. This can lead to many different combinations of gametes depending on how the chromosomes align during metaphase I.

1/4 RY, 1/4 ry, 1/4 rY, 1/4 Ry

Testcross: use it to determine an unknown genotype. If you are not sure of the genotype, homozygous or heterozygous for a trait, then cross it with a homozygous recessive.

Two possibilities:

AA x aa

a a

A Aa Aa

A Aa Aa

All dominant offspring ( heterozygous).

Aa x aa

a a

A Aa Aa

a aa aa

1/2 dominant and 1/2 recessive. 1:1 ratio.

How are human traits inherited? We study the results of matings that have already occurred and we track the inheritance of a trait over many generations in a family.

Pedigree Charts or Trees: analyze inheritance of a disorder or a gene in a family.

• males are squares

• females are circles

• affected individuals will have shaded circle or square, unaffected are not shaded

• an individual who has one copy of the recessive allele is a carrier.

Incomplete Dominance: is a situation where one allele is not completely dominant over another. so the result is somewhere in between the two traits that are being studied. The heterozygote shows traits intermediate in between the two homozygote parents.

Ex: RR -----> red flowers

rr -------> white flowers

F1 generation: RR x rr ------> Rr pink flowers, intermediate color.

F2 generation: 1red(RR), 2 pink(Rr), 1 white(rr)

1:2:1 ratio.

ABO Blood Group: Some genes have multiple alleles, more than two possible alleles exist fro the same gene. For the ABO blood group , there are 3 possible alleles, A, B, and O which can produce 4 phenotypes A, B, AB, O. AB is an example of codominance in genes.

A: RBC have carbohydrate A

B: RBC have carbohydrate B

O: RBC have no carbohydrates

Rh factor: Rh +ve or Rh -ve, is a separate carbohydrate that is inherited separately than the ABO system.

Polygenic Inheritance: some traits are influenced by many genes. Example skin color, height, eye color. This occurs when genes have an additive effect on the phenotypic appearance of a person.

skin color has at least 2 pairs of alleles

very dark dark medium light v.light

AABB AABb AaBb Aabb aabb

AaBB AAbb aaBb

aaBB

Many inherited disorders are controlled by a single gene.

Recessive Autosomal Disorders:

Usually born to parents that are normal but heterozygotes for the trait. Carriers.

1. Albinism: lack of pigment in skin, hair, and eyes. sensitive to light.

2. Tay-Sachs Disease: common in jewish people, lipid accumulation in brain, development slows down , blind, seizures, and paralysis.

3. Cystic Fibrosis: excessive secretion of tick mucus which interferes with breathing, digestion, and liver function. gene on chromosome 7. Untreated life expectancy is about 5 years.

Dominant Autosomal Disorders: the presence of one allele is sufficient to show the phenotype. heterozygote.

1. Achondroplasia: dwarfism, head and torso are normal but arms and legs are short. homozygous genotype causes fetal death.

2. Huntington's Disease: neurological disorder, mental deterioration, depression, no swallowing or speaking. death in 10-20 yrs.

Many genetic disorders can be detected before birth.

1. Amniocentesis: sample of amniotic fluid is examined by Karyotyping to check for genetic disorders. cells need to grow in culture for a while before they are tested. 14 - 20 wks of pregnancy. Ex: Downs Syndrome.

2. Chorionic Villus Sampling(CVS): a tube is inserted in cervix and sucks a little of fetal tissue from the placenta. 10-12 wks. Karyotyping can be carried out immediately.

3. Ultrasound: direct examination of the fetus by sound waves. Can get enhanced and detailed images of the spine, head, feet, hands, toes, and major organs.

Linked Genes: genes that are located close together on the same chromosome tend to be inherited together. These genes do not follow the Mendelian pattern of independent assortment. The genes are inherited together on adjoining portions of the chromosome. This pattern is similar to the 3:1 ratio of monohybrid cross rather than the 9:3:3:1 ratio of a dihybrid cross. Sometimes, crossing over occurs between homologous chromosomes which leads to the production of new alleles not found in the parents.

Sex Chromosomes: most chromosomes in cells are not sex chromosomes ( 1 sex chromosome), they are called autosomes ( 22 pairs autosomes).

X and Y are in humans, fruit flies

X and O in grasshoppers, crickets.

Z and W in birds, fish, butterflies.

• XX individuals are females. All the eggs contain one X chromosome in them.

• XY individuals are males. Sperm can either carry an X or a Y chromosome.

• If an X sperm fertilizes an X egg, a female fetus develops. If a Y sperm fertilizes an X egg, a male fetus develops.

• There are other genes on the sex chromosome that do not determine sex. These genes are called sex-linked genes.

• Mainly these are found on the X chromosome

• Males usually inherit this from their mothers only, since they get their X chromosome from their mothers.

• males usually need only 1 copy of the allele to express it, while females require 2 copies of the recessive sex allele to be affected.

• Women can be carriers for a sex linked allele if they carry it on one of their x chromosomes. They can pass it on to their sons or have carrier daughters.

• Ex: color blindness, hemophilia, Duchenne muscular dystrophy

Red green color blindness: carried on the X chromosome.

XBXB = female normal vision

XBXb = carrier female with normal vision

XBY = normal male

XbY = color blind male

Hemophilia: sex linked recessive gene that prevents normal blood clotting which may cause excessive bleeding after a minor cut. need clotting factors and blood transfusions.

XHXH = female, normal clotting

XHXh = female, carrier for recessive gene

XhXh = hemophiliac female

XHY = normal male

XhY = hemophiliac male

Duchenne Muscular Dystrophy: sex linked recessive disorder characterized by the progressive weakness of muscles. Eventually all the muscles in the body become paralyzed and breathing becomes difficult. Death occurs around age 20. Cases occur in males.

Mistakes can occur during meiosis that can alter the number of chromosomes in the gametes produced in our ovaries and testes.

Nondisjunction: members of a chromosome fail to separate leading to abnormal numbers of chromosomes.

Sometimes gametes can have the wrong number of X and Y chromosomes. Either 2 X's or 2Y's or one is missing.

XXY: Klinefelter syndrome: sterile males with abnormally small testes. long arms and legs, some breast enlargement, slow but not mentally retarded.

XYY, Jacobs syndrome: taller than average, may have some reading and speech problems.

XXX, Metafemales: no physical abnormality, some learning disabilities, limited fertility.

XO, Turner syndrome: short stature, web of skin from the neck, sterile, normal intelligence.

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