Question
Asked Sep 16, 2019
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Consider the pedigree of a family with a disorder of unknown inheritance. Note that individual 11 is known to have no affected alleles. Superscript a indicates the affected allele for sex-linked traits.

(image attached)

What are the possible genotypes of individual 1?
XAXA
aa
Aa
AA
XAXa
XaXa
4
8
3
12
14
15
10
11
13
17
16
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4 8 3 12 14 15 10 11 13 17 16

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Expert Answer

Step 1

For X-linked dominant diseases, one allele carrying the disease trait is enough to express the disease in an individual. For X-linked recessive diseases in females, both the alleles must carry the disease. However, for males, the presence of only one allele expressing the trait results in the disease, because males have only an X chromosome.  

Step 2

In the given pedigree, a indicates the affected allele for sex-linked traits. Therefore, the normal allele would be XA and the allele carrying the disease would be Xa. Individual 2 is affected, and hence, he must have the genotype XaY. As no individual from the second generation shows the occurrence of the disease, it can be said that the individuals of the second generation are either normal or carrier of the disease. If the disease was X-linked dominant, then individuals carrying even one allele of the trait would have expressed the disease. But as in the second generation, there are only carriers and normal individuals, it can be said that the disease is X-linked recessive.

Step 3

If individual 1 had the genotype XaXa, then all the individuals in the second generation would have been affected because individual 2 definitely has the genotype XaY. Resulting individuals would then have the genotype XaXa  and XaY, and thus diseased.

If individual 1 had the genotype XAXa, then the offsprings would have the genotypes XaXa, XAXa ,XaY and XAY. In this case also, some of the individuals of the second generation would have been diseased, which is not the case here.

The genotype of individual 1 cannot be aa, AA or Aa as the disease is X-linked and the allele for the diseased trait is Xa.

If individual 1 had the genotype XAXA, then ...

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