Achondroplastic dwarfism in humans is caused by an incompletely dominant allele.

Question

Achondroplastic dwarfism in humans is caused by an incompletely dominant allele. The dwarfism allele is denoted D, and the normal allele is denoted d. The phenotypes of the genotypes are given in the table below. Two dwarfs mate and produce a zygote. Use the rules of genetics, the characteristics of the three Phenotypes, and rules of probability to answer the questions below. a. What is the probability the zygote is D/d? b. The zygote eventually becomes a 10 year old boy. What is the probability the boy is a Dwarf? c. What is the probability the 10 year old boy has the D/D genotype? d. If the 10 year old boy is a dwarf, what is the probability he has the D/d genotype?

Explanation / Answer

Possible outcomes are
D/D
D/d
D/d
d/d

(a) There are two outcomes with genotype D/d
hence probability that the zygote is D/d is 2/4 = 0.5

(b) For a 10 year boy to be a dwarf, probability = 3/4 = 0.75

(c) As D/D genotype does not survive, hence probabiltiy that the 10 year old boy has the D/D genotype is 0

(d) Dwarfism is observed with the genotype D/d, hence if the 10 year old boy is a dwarf, probability that he has the D/d genotype is 1.

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