13 Jun PCB 3063– the pedigree of a rare neurodegenerative disease in humans
a. Based on the pedigree, characterize each of the following modes of inheritance as likely, unlikely, or impossible: autosomal dominant, autosomal recessive, Y linked, X-linked dominant, and X-linked recessive. Justify your reasoning.
Further scrutiny of members of this family reveals the following: 1) The disease’s degree of expression varies among individuals, with some mildly affected, others moderately affected and still others severely affected; and 2) Individual III-14 was previously married. Her ex-husband is phenotypically normal, with no history of the disease in his family going back ten generations. He has custody of the couple’s two children, a girl and a boy, both of whom are affected with the disease. The partial pedigree is shown below (the “slash” represents separation or divorce)
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b. Do these findings eliminate any of the remaining “likely or unlikely” modes of inheritance listed in part (a)? Are any of these modes of inheritance still possible? If so, which one(s) and why?
c. Is mitochondrial inheritance a likely, unlikely or an impossible mode of inheritance for this disease? Does it provide an explanation for the disease’s variable expressivity?
d. How can mitochondrial inheritance explain individual III-14?
Question 6: (2 points)
In a human population, the frequencies of alleles A and a are 0.8 and 0.2, respectively. Assume gene A is autosomal and the population is under Hardy-Weinberg equilibrium. If five people are randomly selected, what is the probability that exactly 3 AA and 2 Aa individuals will be picked?
Question 7: (3 points)
As we discussed in “lecture” earlier in the semester,Rh is the most complex of the blood group types, involving at least 45 different antigens. The most clinically important antigen, D or RhO, is encoded by the gene RhDwhich is found on chromosome 1. Individuals that are Rh-positive have either one or two RhD genes, whereas the Rh-negative phenotype is caused by the absence of the RhD gene. (The antithetical allele d does not exist, however the letter “d” is used to indicate the D-negative phenotype). For the purpose of this homework, we will simplify things. Assume that the Rh blood group has only two alleles: the Rh-positive allele (D) and the Rh-negative allele (d).
Erythroblastosis fetalis (EF)is a condition that causes the mother’s red blood cells to attack those of the baby as if they were any foreign invaders. It is referred to as hemolytic anemia of the newborn. It is caused by anti-Rh antibodies from the mother which pass through the placenta and attack fetal blood cells that happen to be Rh-positive. Babies that are at risk for this condition are those with Rh-positive blood, whose mothers are Rh-negative (dd).
Side note:Here’s a headline from last year that addresses this condition. It’s well worth a read!
http://www.cnn.com/2015/06/09/health/james-harrison-golden-arm-blood-rhesus/index.html
Consider a population under Hardy-Weinberg equilibrium, where the frequency of the Rh-negative allele, d, is 0.3. What is the frequency of crosses that could potentially produce children with erythroblastosis fetalis?
Question 8: (3 points)
ABO blood type is studied in a Lebanese population, and these allele frequencies are determined.
f(IA)
0.30
f(IB)
0.15
f(IO)
0.55
What are the frequencies of the various genotypes and various phenotypes in this population? Assume Hardy-Weinberg equilibrium.
Question 9: (3 points)
In humans, the presence of chin and cheek dimples is dominant to the absence of dimples, and the ability to taste the bitter compound PTC (phenylthiocarbamide) is dominant to the inability to taste this compound. Both traits are under the control of autosomal genes that are unlinked.
A population from Ghana is examined, and the following allele frequencies are calculated.
Dimples
Frequency
Tasting
Frequency
D
0.62
T
0.76
d
0.38
t
0.24
a. Determine the frequency of genotypes for each gene.
b. What are the expected frequencies of the four possible phenotype combinations:
dimpled tasters, undimpled tasters, dimpled nontasters, and undimplednontasters?
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