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Genetics questions

1. Duchene muscular dystrophy (DMD) is due to an X-linked recessive gene.
A.Explain why 8 to 10 % of female carriers of the DMD gene have muscle weakness.
B. A Female carrier of DMD marries a phenotypically normal man.
(i) Given the genotypes of the parents using appropriate genetic symbols.
Female____ Male_____
(ii) On average what proportion of their male offspring will be affected, and what proportion of their female offspring will be affected with the disorder?

2. Females have been observed with 5 X chromosomes in each somatic cells. How many Barr bodies would such females have?

3. Ichthyosis hystrix gravior ( a greatly thickened-----condition of the skin) is a rare human condition, but in the single extensive pedigree which has been studied it occurs only in males. All the sons of each affected father have the condition Females are not only unaffected, but never transmit the gene for this defect. Can you suggest a possible explanation for this curious and unusual type of inheritance?

4. A form of color blindness (c) in humans is caused by a sex-linked recessive mutant gene. A woman with normal color vision and whose father was color blind marries a man of normal vision whose father was also color-blind.
A. Use appropriate genetic symbols and give the genotypes of the following individulas:
(i) the woman__________
(ii) the woman's father_____
(iii) the man ______
(iv) the man's mother_______

B. What proportion of their offspring will be color-blind? (Give your answer separately for males and females)

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