Question

a.a mother and father produce a son with hemophilia even though neither have the disease themselves. If this son has children with a woman who is carrier of the disease, what is the chance of having a son who has the disease and what is the chance of having a daughter with the disease? Briefly explain your answer.

b. a father with a sister who has Tay Sachs has a son and daughter with a woman who also has a sister with the disease. What is the chance that the son has the disease as well as the daughter? Is it the same or different and why? Briefly explain

c.what role does a virus play in gene therapy and name two criteria that must be met to choose an appropriate virus

d. is it possible to test for Cruetzfeldt Jacob syndrome using amniocentesis? Explain why or why not.

e. why is that Down’s syndrome produces changes in several places in the body and not just one tissue?

Answer 1

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Haemophilia is a sex-linked disease. If a person is suffering from this disease, even a minor cut will result in continuous bleeding because of the absence of blood clotting factors and is also known as bleeder’s disease. There is no permanent cure for this disease however normal blood transfusion helps in blood clotting because all the vital factors necessary for clotting are available in transfused blood.

It is caused due errors occurred on the gene h located on the X-chromosome and hence is known as a sex-linked genetic disorder and shows crisscrossed inheritance. A female becomes haemophilic is both the X-chromosome carry the gene h i.e., Xh Xh. Haemophlic females generally die before birth as the presence of gene h on both the X chromosome is a lethal combination. If the gene h is present only on one X-chromosome then females become carriers (XhX) while males become haemophilic (XhY). The reason for this is that the Y-chromosome of males is devoid of any corresponding allele for the haemophilia gene.

If a normal man and normal woman produce a son with haemophilia then the possible reason for this would only be the presence of gene h on one of the chromosomes of the mother. It suggests that the father is normal and the er is a carrier for haemophilia.

Parents:           XY(♂)          ×          XhX(♀)

Gametes:           X, Y          ×          Xh, X

F1 generation:

♂/♀ gametes	X	Y
Xh	XhX Carrier daughter	XhY Haemophilic son
X	XX Normal daughter	XY Normal son

Now, the haemophilic son produced by normal mother and father is married to a woman carrier of the disease then the offsprings produced by them would be:

Parents:          XhY(♂)          ×          XhX(♀)

Gametes:          Xh, Y          ×          Xh, X

F1 generation:

♂/♀ gametes	Xh	Y
Xh	XhXh Haemophilic daughter (dies)	XhY Haemophilic son
X	XhX Carrier daughter	XY Normal son

The chances of having a son with disease XhY is 25% i.e., one out of four. It is because males are not the carrier for this disease and can express this disease even in the presence of a hemophilic gene on their only X-chromosome.

There are 25% chances to have a daughter with a disease XhXh in this case but as this is a lethal combination of recessive alleles, mostly such girls die before birth in the womb.