Test Your Understanding Describe the inheritance of sickle-cell disease. Include

Question

Test Your Understanding Describe the inheritance of sickle-cell disease. Include the terms genotype, phenotype, homozygous, dominant, and recessive in your description. 6. 7. Describe the inheritance of color blindness, and explain why color blindness is much more common in men than women. Test Your Analytical Ability Ifa man with sickle-cell disease marries a woman with sickle-cell trait, what is the probability that their children will have (a) sickle-cell trait and (b) sickle-cell disease? 8. A man with normal blood clotting marries a woman who is a carrier for hemophilia, a sex-linked trait. What is the probability that their first child will have hemophilia? If this child is hemophilic, what is its sex? What is the probability 9. that this couple's next chidwil e hemohia Explain.

Explanation / Answer

6. Ans: It is a homozygous recessive disorder where Hbs is a recessive gene and HbA is dominant gene. The disease, sickle cell anaemia is caused by a gene (Hbs) which is lethal in the homozygous condition (HbsHbs) but has a slight detectable effect in the heterozygous condition. The genotype of homozygotes is Hbs/Hbs and they die of fatal anaemia. Individuals having genotype Hba/Hbs do not have sickle cell phenotype but are carriers of sickle-cell trait. They show signs of mild anaemia as their RBCs become sickle shaped in oxygen deficiency. A marriage between two carriers, therefore, results in carrier and normal offspring in the ratio 2:1. The variation in 3:1 ratio is due to the death of homozygous recessive (HbsHbs).

7. Ans: People suffering from colour blindness are unable to distinguish red and green colours particularly in dim light. This is due to the fact that the red and green colour pigment containing cells in the retina are lacking. It is a sex-linked disorder produced by a recessive gene and gene for normal eyesight is dominant to colour blindness. In male, X and Y are the sex-chromosomes. Gene for colour blindness lies in X-chromosomes. It produces the disorder because there is no homologous gene in Y-chromosome to check its expression.

In female, XX are the sex-chromosomes. When both the sex-chromosomes have a gene for colour blindness, the female is colour-blind. In case only one X-chromosome bears a gene for colour blindness, its dominant homologous gene on the other X-chromosome checks the expression of the recessive gene. Because of this reason colour blindness occurs more often in males than in the females.

8. Ans: a) If a man with sickle-cell disease marries a woman with sickle cell trait, there is a 50% probability that their children will have sickle cell trait.

b) If a man with sickle-cell disease marries a woman with sickle cell trait, there is a 50% probability that their children will have sickle cell disease.

9. Ans: Genotype of normal Man: XY

Genotype of carrier woman: XhX

Thus, XY cross XhX can be shown below

X

Y

Xh

XXh

XhY

X

XX

XY

Number of normal children = 2 (XX and XY)

Number of carrier female = 1 (XXh)

Number of hemophilic male = 1 (XhY)

It can be seen that 1 out four Children will have hemophilia.

Thus probability that their 1st child will have hemophilia is 25%

The child having hemophilia will be male (XhY)

It can be seen from the Punnett Square test that proability of the next child being hemophilia is 25% or 0.25.

If you are satisfied with my answer please rate it.

X

Y

Xh

XXh

XhY

X

XX

XY

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