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Pattern baldness is a sex-linked recessive trait. If a woman whose father is bald and a man
who is bald has a son and the woman’s mother does not carry the allele for pattern baldness. The
The probability that the son will inherit the allele for pattern baldness is
(A) $0\%$
(B) $25\%$
(C) $50\%$
(D) $75\%$
(E) $100\%$

Answer
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Hint: Pattern baldness is a sex-linked recessive trait. X-linked inheritance means that the gene which is causing the disorder or the trait is found on the X chromosome. The males just have one X and one Y chromosome and the Females have two X chromosomes.

Complete answer:
A female will always inherit only one X chromosome from her father. Because the woman’s mother only carries one copy of the allele and doesn’t carry the allele for pattern baldness. If the couple has a son, there’s a $50\%$ chance that the son will inherit the X-linked allele for pattern baldness from his mother.

Additional Information: In humans, the inheritance of X-linked recessive traits follows a novel pattern made from three points.

The primary is that affected fathers cannot pass x-linked recessive traits to
their sons , this is because of the reason that fathers give Y chromosomes to their sons. This suggests that males suffering from an x-linked recessive disorder inherited the responsible X chromosome from their mothers. The second is that the x-linked recessive traits are more commonly expressed in males than females. This is often because of the fact that males possess only a single X chromosome, and thus require just one mutated X to be affected. Women possess two X chromosomes and thus must receive two of the mutated recessive X chromosomes only, which is possible when one X chromosomes are received from each parent. So, baldness is hair loss that affects the top and front of the scalp.
So the correct answer is ‘$50\%$’.

Note: In male-pattern hair loss (MPHL), it was seen that the hair loss often presents itself as either a receding hairline, loss of hair on the crown of the scalp, or a mixture of both, on the other hand in female-pattern hair loss , which typically presents as a thinning of the hair.