Download Exceptions to Mendel`s Genetics

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11.3—Exploring Mendelian Genetics
I.
II.
III.
Independent Assortment
A. Mendel asked a question…
1. “Does the segregation of one pair of alleles affect the segregation of another pair of alleles?”
B. Completed a test cross (two heterozygous parents) and found the following ratio:
1. 9:3:3:1 ***
C. Law of Independent Assortment
1. States—Genes for different traits segregate independently during the formation of gametes
2. This allows for the great genetic variation within a species
Other genetic patterns…the “exceptions”
A. Incomplete dominance
1. Neither allele is completely dominant
2. The heterozygous phenotype is falls somewhere between the two homozygous phenotypes
3. Example:
a. Red flower (RR) x White flower (rr)
B. Codominance
1. Both alleles are equally dominant
2. Both alleles contribute to the phenotype
a. Examples
i. “Erminette” chickenshave black and white feathers
ii. Humans may have two different forms of a gene that codes for a protein that
controls levels of cholesterol in the blood
iii. Human blood type: IAIB (AB)
C. Multiple Alleles
1. Means that there are two or more alleles that EXIST for a trait in a population
2. Examples:
a. Rabbits (p.273 Figure 11-12)  one of the best studied and understood examples
b. Humans
i. Blood (IA, IB, i)
ii. Eye color (controlled by 3 or more alleles)
D. Polygenic Traits
1. Traits are controlled by two of more genes
2. Polygenic traits show a variety of phenotypes
a. Human skin color believed to be controlled by 4 or more genes
b. Blood pressurecontrolled by a number of genes responsible for weight, cholesterol,
kidney function, smoking, and possibly many more!
Applying Mendel’s Principles
A. Thomas Hunt Morgan (1900’s—American)
1. Chose the “model” organism for studying genetics
a. Drosphila melanogastor (fruit fly)
2. Reasons for choosing the fruit fly:
a. Reproduce quickly
b. Reproduce many offspring
i. 1 pair can produce approximately 100 offspring
B. Affect of the Environment of gene expression
1. Characteristics can be affected by many factors:
a. Examples include: climate, soil, water, light, food, temperature, radiation, pollution etc.
2. Genes provide the plan…but the environment can change the plan.