Which allele combination represents a female who is a carrier for an x-linked recessive disorder?
xrxr
xrxr
xry
xry

The correct answer would be XRXr.

In humans, X and Y are the sex chromosomes which determine the gender of the person.

The males are heterozygous, that is, they are XY whereas the females are homozygous for the X chromosome, that is, XX.

The x-linked recessive disorder is the one in which the alleles for the disease are carried by the X chromosome. As females are XX, thus, both X chromosome must carry the recessive allele in order to produce the phenotype.

Therefore, the genotype of the carrier of a recessive disease must be heterozygous in nature.  

For example, the carrier of the recessive disease would carry one recessive allele and one dominant allele in its genotype.

Thus, the genotype of the X-linked recessive disease would be XRXr.

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