A boy with Klinefelter syndrome (47,XXY) is born to a mother who is phenotypically normal and a father who has the X-linked skin condition called anhidrotic ectodermal dysplasia. The mother's skin is completely normal with no signs of the skin abnormality. In contrast, her son has patches of normal skin and patches of abnormal skin.
Which parent contributed the abnormal gamete?

Verified step by step guidance

1. Step 1: Understand the genetic condition and inheritance pattern. Klinefelter syndrome (47,XXY) means the boy has two X chromosomes and one Y chromosome. The father has an X-linked skin condition (anhidrotic ectodermal dysplasia), which is inherited through the X chromosome.
2. Step 2: Analyze the mother's genotype and phenotype. The mother is phenotypically normal with no signs of the skin condition, indicating she likely does not carry the mutant allele on either of her X chromosomes.
3. Step 3: Consider the son's phenotype of patches of normal and abnormal skin. This mosaic pattern suggests X-chromosome inactivation (lyonization) in a female or in this case, a 47,XXY male, where one X chromosome is randomly inactivated in different cells, leading to patches expressing either the normal or mutant allele.
4. Step 4: Determine which parent contributed the abnormal X chromosome. Since the mother is normal and the son has the condition, the abnormal X chromosome must have come from the father, who has the X-linked skin condition.
5. Step 5: Conclude that the father contributed the abnormal X chromosome carrying the mutation, while the mother contributed a normal X chromosome, and the Y chromosome came from the father as well, resulting in the 47,XXY karyotype.

Key Concepts

Klinefelter Syndrome (47,XXY)

Klinefelter syndrome is a chromosomal condition where a male has an extra X chromosome, resulting in a 47,XXY karyotype. This affects male development and can cause physical and reproductive abnormalities. Understanding this helps explain the genetic background of the boy in the question.

X-linked Inheritance and Mosaicism

X-linked conditions are caused by mutations on the X chromosome. In males (XY), a single mutated X causes the disease, while females (XX) may be carriers without symptoms due to X-inactivation. Mosaicism, such as patches of normal and abnormal skin, occurs when some cells express the mutated gene and others do not, often due to random X-inactivation.

Parental Origin of Chromosomes and Gametes

Determining which parent contributed the abnormal X chromosome involves understanding gamete formation. The mother provides one X chromosome, and the father provides either an X or a Y. Since the father has the X-linked condition, his X chromosome carries the mutation, but the son’s extra X could come from either parent, affecting the phenotype.