Genetics

Question 1

How does an autosomal recessive disorder occur when both parents are carriers?

Answer 1

Both parents have one normal allele (A) and one mutated allele (a).
A child must inherit two mutated alleles (aa) — one from each parent — to be affected.

Question 2

What are the probabilities when both parents are carriers (Aa × Aa)?

Answer 2

25% (aa) → affected
50% (Aa) → carrier
25% (AA) → unaffected

Question 3

How does an autosomal recessive disorder occur when only one parent is a carrier?

Answer 3

One parent is a carrier (Aa), the other is unaffected (AA).
The child cannot inherit two recessive alleles, so they will not be affected.

Question 4

What are the probabilities when only one parent is a carrier (Aa × AA)?

Answer 4

0% affected (aa)
50% carriers (Aa)
50% unaffected (AA)

Question 5

How does an X-linked (allosomal) recessive disorder occur when one parent is a carrier?

Answer 5

Usually refers to a carrier mother (XᴺXᵃ) and unaffected father (XᴺY).
Males (XY) are more affected because they only have one X chromosome
If they inherit the mutated X, they will express the disorder

Question 6

What are the probabilities when the mother is a carrier (XᴺXᵃ) and father is unaffected (XᴺY)?

Answer 6

Sons:
50% affected (XᵃY)
50% unaffected (XᴺY)
Daughters:
50% carriers (XᴺXᵃ)
50% unaffected (XᴺXᴺ)

Question 7

Why are males more affected in X-linked recessive disorders?

Answer 7

Males only have one X chromosome, so one mutated allele causes the disorder, whereas females need two copies.