When does crossing over occur in meiosis?
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Answer:
Explanation:
From the question: Duchenne muscular dystrophy is an X-linked recessive disease.
Now for an X-linked recessive disorder to be affected by a male individual; it only requires the presence of only one copy of the recessive allele of the disease to be present BUT in a female individual. both copies of the recessive allele must be present.
Let the X-linked recessive disease(i.e Duchenne muscular dystrophy) be = (ⁿ)
However, we are told that the couples are normal and are unaffected. Therefore;
the male partner (XY) will be:
the female partner (XY) will be:
Similarly, the question proceeds by telling us that: the woman's brother has the disease.
Definitely, it's possible that this unaffected woman is a carrier of the disease.
So, if she is a carrier; we have her traits to be:
NOW, if a cross exist between these couples; we have
×
So, we have offspring as follows:
= normal unaffected female
= normal unaffected male
= female carrier for the Duchenne muscular dystrophy disease
= affected male with the Duchenne muscular dystrophy disease
So, the probability of the child to be affected with the disease =
Also, the probability that the first child will be a male or a female =
∴
the probability that the couple’s first child will be affected =
=