Lecture 7: Modification of Mendelian ratios: Partial dominance, codominance, multiple alleles, lethal alleles
1. Describe the genotypic and phenotypic ratios for each of the following (assume the parental generations are true breeding when not stated otherwise):
a. The F2 of monohybrid cross with full dominance2. How would you distinguish between partial dominance and codominance (Be careful, you can get yourself thoroughly confused if you try to carry this too far).
b. The F2 of monohybrid cross with partial dominance
c. The F2 of a monohybrid cross with codominance.
3. How would you distinguish between partial dominance and full dominance (again be careful).
4. Describe a situation in which full dominance by certain criteria becomes partial dominance when examined by other criteria.
5. Briefly describe three different genetic mechanisms that can cause the phenotype of a heterozygote A1A2 to be different from that of either of the corresponding homozygotes. A1 and A2 are alleles at the same genetic locus, but you may designate any type of relationship between the two of them that you wish, as long as it corresponds to something that happens in real life. Cite an example of each of the mechanisms that you propose. (Yes, there really are at least three possibilities!)
6. When two indivduals exhibiting dominant traits associated with homozygous lethality (for example, Manx cats) are crossed, altered Mendelian ratios are observed among the progeny.
a. What phenotypic ratio is observed among the immediate progeny of such a cross?7. Explain how a short deletion can generate phenotypic behavior that appears to be dominant lethal, but actually involves two separate genes.
b. What mechanism is responsible for the altered phenotypic ratio?
c. Explain why it is not possible to obtain a true-breeding strain of Manx cats.
d. What is the rationale for calling such a trait dominant when it is impossible to obtain a true-breeding line in order to do a classical dominant x recessive cross?
8. Why are dominant lethal alleles rarely observed in nature, as opposed to laboratory or domesticated strains?
9. There are three different alleles at the genetic locus responsible for human ABO blood types.
a. What mechanism prevents all three alleles from being expressed in a single individual?10. How many different genotypes are possible at a locus that has 5 alternative alleles?
b. Would it make any difference if all three alleles were codominant? Explain your answer.
c. A woman with blood type A is married to a man with blood type B. Their first child is type O. Describe two distinctly different mechanisms that might allow this to happen.
d. If only the I locus that controls ABO blood types is involved, what blood types would be expected and in what ratio if the couple in part c have enough additional children to include all possible types. Ignore statistical problems due to small sample size.
11. How many of the genotypes in problem 10 can be observed in any one individual?
12. A man is blood type A and his wife is blood type B. Their first child is a girl with blood type O.
a. What are the genotypes of the parents?
b. What was the probability of that child's birth among all of the other possible children that the couple could have had?
c. What is the probability that the next two children will both be blood type A?
d. What is the probability that those two children will be one boy and one girl?
e. The couple ultimately has six children. Knowing that the first is a girl with blood type O, what is the probability that they will have two more girls with blood type O? (Be careful -- this question is designed to confuse you!)
f. What is the probability that the fourth child will be a boy with blood type AB and the fifth a girl with blood type B?
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