Multiple Choice
Which of the following best describes how possible genotypes for each generation are determined in a pedigree chart for an autosomal recessive trait?
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2. Mendel's Laws of Inheritance
Pedigrees
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Given a pedigree showing that Ryan is a male affected by an X-linked recessive disorder, which of the following is Ryan's genotype?
A
XRY
B
XrY
C
XRXR
D
XRXr
Verified step by step guidance1
Step 1: Understand that the disorder is X-linked recessive, meaning the gene causing the disorder is located on the X chromosome and the recessive allele (denoted as X\r) causes the disorder when present without a dominant allele.
Step 2: Recognize that males have one X and one Y chromosome (XY), so they have only one allele for genes on the X chromosome. Therefore, if a male is affected by an X-linked recessive disorder, he must have the recessive allele on his single X chromosome.
Step 3: Identify that Ryan is male and affected, so his genotype must have the recessive allele on the X chromosome and a Y chromosome, which does not carry the gene for this disorder.
Step 4: Write Ryan's genotype as X\rY, where X\r represents the X chromosome carrying the recessive allele and Y is the male sex chromosome.
Step 5: Confirm that other options like XRY (dominant allele on X), XRXR (female homozygous dominant), or XRXr (female heterozygous carrier) are not possible genotypes for an affected male.
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