1. Give complete genetic explanations for each of these situations:
a) Phenotypically normal parents who have red-green colorblind XO daughters, and phenotypically normal parents who have red-green colorblind XXY sons. [Remember that this form of colorblindness is X-linked.]
b) A pair of otherwise identical twins, one of whom is normal and the other has Down syndrome.
2. On your first day as a volunteer in the neonatal unit of a hospital, you make the unfortunate mistake of getting 3 babies mixed up so you don't know which is which. All 3 babies look much the same to you, so you can't tell from the babies features which baby belongs to which couple. Not wanting to confess your blunder, and having blood typing kits at your disposal, you decide to try to figure out whose baby is whose on your own. You determine that baby #1 has blood type A, #2 has blood type O, and #3 has blood type AB. From medical records, you find out that the blood types of the 3 sets of parents are as follows:
Jensens: mother is type AB, father is type O
McDonalds: mother is type A, father is type O
Andersons: mother is type A, father is type B
a. For each baby whose parents can be determined with certainty from this information, indicate above whose baby is whose.
b. Based on your answers in part a, what blood types could future children of the McDonalds possibly have, and what is the probability of each type?
3. Three recessive mutations in Drosophilia are all on the same autosomal chromosome: scute bristles (sc), facet eyes (fa) and cut wings (cu). In the P generation, true-breeding males with cut wings were crossed to true-breeding flies with scute bristles and facet eyes. F1 females heterozygous for all 3 mutations were then crossed to triple homozygous recessive males to generate the following F2 population:
Scute, cut: 9 facet: 6
Scute, facet, cut: 90 wild type: 95
Cut: 403 scute, facet: 394
Scute: 1 facet, cut: 2
Draw a map showing the relative positions of these genes on the chromosome and the distances between them.
4. Deafness in humans can be caused by mutations in a variety of different genes encoding proteins involved in development and function of the inner ear. The pattern of inheritance of deafness in two human families (related by the marriage of III-2 and III-3) is shown here.
a. Assuming this trait is fully penetrant, circle each of the following modes of inheritance that can be excluded by the data shown. For each mode you name, give a reason why it can be excluded, referring to specific individuals in the pedigree:
i. X-linked recessive:
ii. autosomal recessive:
iii. X-linked dominant:
iv. autosomal dominant:
v. maternal inheritance:
b. III-1 and III-4 met at the wedding of III-2 and III-3 and eventually also got married. Still assuming the trait is fully penetrant, what is the probability that their first child will be deaf? If the probability is different for a boy vs. girl, clearly state the difference. Show the fractions you are multiplying together to get your final answer.
c. Years later, IV-5 marries a deaf man from an unrelated family, and they have six children, all of whom have normal hearing. Which of the following phenomenon the most likely explanation of this outcome? Circle only one.
ii. incomplete penetrance (assuming, unlike you did in parts a and b, that this is possible)
v. variable expressivity
5. You are doing a genetics experiment with the fruit fly. In the "P" generation, you cross two true-breeding flies. The female parent is brown and wingless and the male parent is black with normal wings. All of the flies in the F1 generation are brown and have normal wings.
* Indicate the alleles associated with dominant phenotypes by uppercase letters and alleles associated with recessive phenotypes by lowercase letters. Assume the genes are not found on a sex chromosome. Indicate the color alleles as "B" and "b" the wing alleles by the letters "N" and "n".
a) The genotypes of the flies in the P generation are: female and male.
b) The genotype of the flies in the F1 generation is:
c) You now take an F1 female and cross her to a true-breeding black, wingless male. This male's genotype is:
d) You count 1600 offspring in the F2 generation. If the wing and the color traits were linked and no recombination occurred, you would expect to count:
e) When you count the F2 generation, you really get:
85 brown winged flies
728 black winged flies
712 brown wingless flies
75 black wingless flies
What is the genetic distance between the color and wing genes?
f) A series of fruit fly matings shows that the recombination frequency between the gene for wing size and the gene for antenna length is 5% (i.e. the genetic distance between them is 5 centimorgans). List all possible recombination frequencies between the gene for color and the gene for antenna length.