Bio 1101 Lecture 4 Part A Chapter 9: Inheritance & DNA

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1 Bio 1101 Lecture 9 Chapter 9: Inheritance & DNA

? Recall DNA ? A double-stranded molecule ? Each strand consists of a chain of nucleotides ? Sugar, phosphate group, and nitrogenous base ? 4 different bases: A, T, G, and C ? The chains are hooked together by hydrogen bonds between bases ? Strands are complementary: A-T, G-C

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? DNA must be replicated before a cell can divide ? ? When DNA replicates, the two strands are "unzipped", and a new DNA strand

complementary to the existing one is constructed ? Hydrogen bonds are relatively easy to break apart, compared to the covalent bonds ?

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Process = DNA REPLICATION

DNA Replication is SEMI-CONSERVATIVE

DNA polymerase enzymes attach individual nucleotides to the growing strand

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? Cells divide by Mitosis to make exact copies of themselves (somatic cells) ? ? Animal cells divide by Meiosis to make haploid gametes ? ? Occasionally, mistakes happen in the sorting of the chromosomes ?

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? Genes are carried on the chromosomes that are passed from parent to offspring through the gametes

? ? If we know what traits the parents carry, then we can calculate the probabilities of the

offspring inheriting various combinations of those traits ? ? Today we discuss Inheritance

7 Chapter 9: Inheritance ? Gregor Mendel ? Austria, mid-1800s ? First person to analyze patterns of inheritance in a scientific way ? DNA wasn't known at the time ? "Heritable factors" were passed from parent to offspring ?

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? Today we discuss Inheritance

7 Chapter 9: Inheritance ? Gregor Mendel ? Austria, mid-1800s ? First person to analyze patterns of inheritance in a scientific way ? DNA wasn't known at the time ? "Heritable factors" were passed from parent to offspring ?

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? Mendel studied pea plants ? Could control their reproduction ? Self-fertilization ? Cross-fertilization ? Had several traits that occurred in 2 distinct forms: ? Flower color: white or purple ? Seed color: yellow or green ? Seed shape: round or wrinkled ? Pod color: green or yellow ? Etc.

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? Monohybrid Cross: two plants are crossed that vary in only 1 characteristic ?

? Example: True-breeding purple flowers with true-breeding white flowers ? The resulting plants (F1 hybrids) all had purple flowers. What happened to the trait for

white flowers? Did it disappear? ?

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? Mated the F1 hybrids ("monohybrids"), and found that some in the next generation (F2) had white flowers

? The trait was still there, but obscured by the trait for purple flowers ? Specifically, ? of the resulting plants had white flowers, and ? had purple flowers ?

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Using basic statistics, Mendel developed the following 4 hypotheses:

(1)There are alternative forms of genes (alleles) (2)For each inherited trait, and organism has two genes, one from each parent; they may

be of the same or different alleles (3)A sperm or egg carries only 1 copy of that gene; after fertilization, the paired condition

is restored (4)When 2 genes of a pair are of different alleles, one is fully expressed (the dominant

allele) while the other is masked (recessive allele)

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Some Terminology

? Homozygous = having 2 copies of the same allele

? Heterozygous = having different alleles for a gene

? AA = Homozygous Dominant

? Aa = Heterozygous

? aa = Homozygous Recessive

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(4)When 2 genes of a pair are of different alleles, one is fully expressed (the dominant allele) while the other is masked (recessive allele)

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Some Terminology ? Homozygous = having 2 copies of the same allele ? Heterozygous = having different alleles for a gene ? AA = Homozygous Dominant ? Aa = Heterozygous ? aa = Homozygous Recessive ?

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? Recall that in Meiosis, a diploid cell goes through two divisions ? Each resulting daughter cell therefore carries only one copy of each chromosome, and therefore only one copy of each gene ? Gametes are haploid

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? For heterozygotes, there is a 50% chance of either allele being passed on to a gamete

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? A Punnett Square can be used to calculate probabilities of characteristics of offspring if two individuals of known genotype are mated ? Genotype = genetic composition (expressed as "PP" or "Pp" or "pp", for example, where P = dominant allele for purple flowers, and p = recessive allele for white flowers) ? Phenotype = the trait an organism expresses (such as "purple" or "white" flowers)

? Back to Mendel's experiments with pea plants... ? The F1 generation plants were carrying one copy of each of the flower color alleles (purple

and white); in the F2 generation, some individuals inherited two copies of the white (recessive) alleles and expressed white flowers

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? So in a Punnett Square, we list the gamete(s) that each parent can produce across the top and down the left side of the square

? The interior squares show the potential offspring ? But a Punnett Square doesn't HAVE to be a square...

? What if you crossed a "PP" plant with a "Pp" plant?

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? Test Crosses: If genotype of an individual expressing the dominant trait is unknown, can do a "test cross" to find out ? Cross it with a homozygous recessive

? Example: coat color in Labrador retrievers

24 Activity #1

1. Define homozygous and heterozygous. Explain why it is necessary to add the terms "dominant" or "recessive" after the term "homozygous," but not after the term "heterozygous."

2. Consider pea plants, where the allele for purple flowers is dominant to the allele for white flowers. If a pea plant has purple flowers, can you tell what its genotype is? Explain why or why not. If a pea plant has white flowers, can you tell what its genotype is? Explain why or why not.

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3. A pea plant that is heterozygous for flower color (Pp) is crossed with one that is

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"dominant" or "recessive" after the term "homozygous," but not after the term "heterozygous."

2. Consider pea plants, where the allele for purple flowers is dominant to the allele for white flowers. If a pea plant has purple flowers, can you tell what its genotype is? Explain why or why not. If a pea plant has white flowers, can you tell what its genotype is? Explain why or why not.

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3. A pea plant that is heterozygous for flower color (Pp) is crossed with one that is homozygous recessive for flower color (pp). What percentage of the resulting offspring will have purple flowers? What percentage of the offspring will be heterozygous?

4. You have a purple pea plant and cross it with a white pea plant. 100% of the resulting offspring have purple flowers. What was the genotype of the purple-flowered parent? Draw a Punnett Square to help explain.

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? Break...

27 Dihybrid Cross Extending the Punnett Square for analysis of two characteristics simultaneously ? ? What if the plants differ in 2 characteristics? How do you determine probability of characteristics in offspring? ? Simply expand the Punnett Square. ? When 2 factors are considered and both parents are heterozygous for both, it is called a Dihybrid Cross ? Remember, ONE copy of each gene is passed on to each gamete ?

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? A cross between two individuals that are heterozygous for 2 traits will result in a 9:3:3:1 ratio of phenotypes: ? 9 Purple flower/Green pod ? 3 Purple flower/Yellow pod ? 3 White flower/Green pod ? 1 White flower/Yellow pond (homozygous recessive for both traits)

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? Mendel's observations of pea plants caused him to develop two important principles: ? Principle of Segregation ? pairs of alleles separate during gamete formation; the fusion of gametes at fertilization creates allele pairs again ? Principle of Independent Assortment: Each pair of alleles segregates independently of the other pairs during gamete formation

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32 Non-Mendelian Genetics ? Not all genetics follows the rules of Mendel ? ? In Incomplete Dominance, the heterozygote has a phenotype that is intermediate between the two homozygotes ? Example: Flower color in snapdragons (cross between a homozygous red flower and a homozygous white flower produces pink flowers) ?

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32 Non-Mendelian Genetics

? Not all genetics follows the rules of Mendel ?

? In Incomplete Dominance, the heterozygote has a phenotype that is intermediate between the two homozygotes

? Example: Flower color in snapdragons (cross between a homozygous red flower and a homozygous white flower produces pink flowers)

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? More than two alleles may be present for one trait ? Example: Blood Type ? ABO Blood Group (three different alleles), producing four different phenotypes: A, B,

AB, or O ? type blood ? A and B are both dominant to O (AO = A-type blood; BO = B-type blood) ? But A and B are co-dominant; both are expressed; a person with both A and B alleles

has type AB blood ? In order to have type O blood, you must have two copies of the O allele ?

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? Pleiotropy = when one gene has an effect on more than one trait ? Example: sickle-cell anemia

? Makes red blood cells produce abnormal hemoglobin molecules, and red blood cells become sickle-shaped

? The allele for this disease is recessive; only homozygous recessives express the disease

? Heterozygotes are protected against malaria ? Sickle cell disease (and thus the gene) is most common in Africa, where Malaria is

also common

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? Polygenic Inheritance: when more than one gene affects one trait ? Many traits vary along a continuum (not just two or three different phenotypes) ? It is the combination of several genes that accounts for this variation ? Example: skin color and height in humans ?

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? The Role of Environment ? Not all traits are strictly controlled by genes ? Often, a result of interaction between genes and environment ? Height, risks for diseases such as heart disease or cancer, intelligence, appearance, etc.

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? Sex-linked genes ? Genes located on the sex chromosomes ? The X chromosome carries about 1,100 genes, while Y carries about 78 genes, mostly affecting testes development ? Therefore, most sex-linked genes are on the X chromosome

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? Sex-linked recessive traits include red-green color blindness and hemophilia

? Primarily affect males, because they are only carried on the X chromosome; there is no

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? Sex-linked genes ? Genes located on the sex chromosomes ? The X chromosome carries about 1,100 genes, while Y carries about 78 genes, mostly affecting testes development ? Therefore, most sex-linked genes are on the X chromosome

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? Sex-linked recessive traits include red-green color blindness and hemophilia ? Primarily affect males, because they are only carried on the X chromosome; there is no

corresponding locus on the Y chromosome ? Females have two chances to get a good working copy of the color vision gene (XX) ? Males only have one chance, because they only have one copy of the X chromomes

(XY) ? If females have one copy of the recessive allele, they are said to be "carriers" (they

have the gene for color blindness, but do not express it) ? Alleles for sex-linked traits are written as superscripts on the X chromosome

? For example, the genotype of a female who is a carrier for color blindness could be written as XNXn (where "N" is normal color vision, and "n" is for the color blindness allele)

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44 Activity #2 1. In pea plants, the allele for purple flowers is dominant to white flowers, and the allele for yellow seeds is dominant to the allele for green seeds. A PpYy plant is crossed with a ppYY plant. Complete the Punnett Square below. What are the chances that this cross will result in a purple flower with yellow seeds?

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2. If a Ppyy plant is crossed with a PPYy plant, what percentage of the offspring will have purple flowers AND green seeds? Draw a Punnett Square to help answer the question.

3. Flower color in snapdragons is an example of incomplete dominance. If you cross a red snapdragon with a pink snapdragon, what percentage of the offspring will have red flowers? What percentage of the offspring will have pink flowers? What percentage of the offspring will have white flowers? Draw a Punnett Square to help explain your answers.

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4. Consider the X-linked recessive trait for hemophilia (use H = normal, h = hemophilia; remember to place these allele designations as superscripts on the X chromosome). If Mom has hemophilia, what is/are her possible genotype(s)? If Dad has hemophilia, what is/are his possible genotype(s)?

5. Dad has hemophilia, and Mom does not have hemophilia and is not a carrier. They are going to have a baby. If the baby is a boy, what are the chances he will have hemophilia? If the baby is a girl, what are the chances she will have hemophilia? Draw Punnett Squares to help explain your answers.

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? All for today...

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