Question

A hypothetical form of dwarfism inherited in an autosomal dominant fashion. Males and females are affected equally. Individuals carrying the allele have shortened appendages with normal size torso and head. Individuals carrying two copies of the allele die. The disorder is carried by less than 5% of the population.

a. Determine the genotype for the majority of the population.

b. Two normal individuals can have a child expressing this disorder. What does this indicate about the origin of most cases (>80%)?

c. A male with this form of dwarfism has a child with a normal female. What is the probability there child will be normal?

d. If the allele was carried on chromosome 23 making it a X-linked trait, would that change the probability in letter c? If so, how?

This is all the information given.

Answer 1

(a) Majority of the population has the genotype dd.

Reason: Let us the dominant allele (defective trait) be D and the recessive allele (normal) be d.

Since the defect is autosomal dominant,the genotype Dd expresses the disorder.

Also, the genotype DD is lethal (they will be dead). Hence those who are homozygous for the dominant allele (D) will not be present in the population.

And the population has 5% affected individuals which shows us that the majority of the population (95%) has the genotype dd.

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(b) If two normal individuals (genotype dd) can have a child with this disorder, then it indicates that the defect is a result of de novo mutation. That is, one the parent's gametes might have undergone a mutation which then formed into zygote after fertilization and the resulting individual is affected with the disorder. Since the mutation occured in the gametes, the parents will be normal and will have normal alleles.

This could indicate that most cases of this disorder will be present in individuals with same ancestry or in other words, most of the affected individuals could be related by blood.

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(c) Parents: Male with disorder Dd X Normal female dd

Gametes:   D , d d

F1: Dd , dd

Therefore 50% of the next generation (0.5) has the chance of inheriting this defect from the parents.

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(d) If the allele was carried on the X chromosome, the probability would change in the following way:

Let the defective allele carrying X chromosome be X^D

and the normal allele carrying X chromosome be X^d

Now the cross between the two changes to:

Parents===>   X^DY (affected) x X^d X^d (normal)

Gametes===>    X^D ,   Y    X^d

F1====>   X^DX^d (female affected), X^dY (normal male)

Therefore 100% of the females of the next generation will be affected with the disorder and 100% of the males will be normal if the dominant allele is present on the x chromosome.