Red-green colour blindness is an X-linked recessive trait with the alleles
$X^R$: normal color vision
$X^e$: red-green colour blind
Examine the following pedigree - VCE - SSCE Biology - Question 16 - 2011 - Paper 1
Question 16
Red-green colour blindness is an X-linked recessive trait with the alleles
$X^R$: normal color vision
$X^e$: red-green colour blind
Examine the following pedigree.
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Worked Solution & Example Answer:Red-green colour blindness is an X-linked recessive trait with the alleles
$X^R$: normal color vision
$X^e$: red-green colour blind
Examine the following pedigree - VCE - SSCE Biology - Question 16 - 2011 - Paper 1
Step 1
II3 must be $X^R X^R$.
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Answer
To determine if II3 can be XRXR, we analyze the pedigree. Since II2 is normal (filled square), and both parents can contribute an X chromosome, if II2 is XRXe, II3 cannot be XRXR since there’s a chance II3 could inherit Xe. Thus, this statement is false.
Step 2
II4 must be $X^R X^e$.
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Answer
To evaluate II4, we notice II3 (male) cannot carry XR while being normal, implying that II4 presents a genotype that includes Xe for the colorblind trait. Thus, II4 most likely must be XRXe, making this statement true.
Step 3
II4 has a two in three chance of being $X^R X^R$.
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Answer
For II4 to have a two in three chance of being XRXR, considering two males contributing an X chromosome to II4, we must conclude that individual probabilities must align directly with direct pattern inheritance, giving a greater likelihood of being XRXe instead. Hence, this statement is false.
Step 4
IV1 has a one in four chance of being colour blind.
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Answer
Since IV1 inherits X chromosomes from both parents, one being Xe, the likelihood of inheriting the colour blind trait is 1 in 4, as it can be XRXR, XRXe, or XeXe. Therefore, this statement is true.
