Question

4) Cystic fibrosis is a human disease caused by an autosomal recessive mutation. About 1 in 22 people in the human population are heterozygous carriers and have no symptoms of the disease. A woman who has a brother with CF marries a man who has no history of CF in his family a) What are the possible genotypes of the woman? b) What is the most likely genotype of the man? c) What is the percentage of offspring that would be affected with CF, depending upon the genotype of the mother?

5) Lesch-Nyhan syndrome is a metabolic defect caused by the lack of the enzyme hypoxanthine-guanine phosphoribosyl transferase (HGPRT). This disease is the result of an X-linked, recessive mutation and results in mental retardation, self- mutilation and early death. A normal man marries a woman who had a brother who died of LNS. Your are the genetic counselor who will advise them on the likelihood of their having a LNS baby. a) What are the possible genotypes for the woman? What is the man's genotype? b) For each of the woman's possible genotypes, what are the possible genotypes of her offspring and the implications for survival for each of them?

Answer 1

4. let the alleles be C and c (dominant and recessive)

(a) The woman can be Cc (carrier) or CC

(b) The man is CC (no history of CF)

(c) If the mother is CC then no offspring are affected, if the mother is Cc then 50% offspring are carriers.

	C	c
C	CC	Cc
C	CC	Cc

5. For an X-linked disease, let the alleles be X^H and X^h (dominant and recssive)

(a) The woman had a brother who died of LNS so she can have X^HX^h or X^HX^H genotype.

(b) The man being unaffected, has X^HY genotype.

(c) If the mother is X^HX^H then none of the offspring are affecyed since the recessive allele is not expressed in any of them.

For X^HX^h mother,

	XH	Xh
XH	XHXH	XHXh
Y	XHY	XhY

50% progeny are affected.

25% are carrier females whichc an suvive.

25% affected males which won't survive.

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