Question

5. Identify any one (1) genetic disorder not caused by nondisjunction. Explain why.

6). Given that Phenylketonuria (PKU) is inherited as a recessive trait. What is the chance for two heterozygous parents to have a phenylalanine-sensitive child? Explain.

7). Given a mating between a non-hemophiliac male & a heterozygous female what is the probability for a hemophiliac female child? Explain.

8). Given a mating between an albino male & a carrier female, what is the probability for a child who is a carrier for albinism? Explain.

Answer 1

4. Non-disjunction refers to the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division. Cri du chat syndrome is a genetic condition present from birth that is caused by the deletion of genetic material on the small arm (the p arm) of chromosome 5. It is also known as 5p- syndrome or cat cry syndrome. The clinical features of the disease include

• drooling;
• small head (microcephaly) and jaw (micrognathism);
• widely-spaced eyes (hypertelorism);
• severe cognitive and motor disabilities
• difficulty in swallow, stunted growth

There is no known treatment for this condition. Mortality is very high in the first three years of life. They often have cardiovascular anomalies that require surgical correction.

5. Phenylketonuria (PKU) is inherited as an autosomal recessive trait.

Mother -Aa

Father - Aa

A - dominant allele, a - recessive allele

	A	a
A	AA	Aa
a	Aa	aa

From the above Punnet square, it can be inferred that when both the parents are heterozygous, the chances of having a phenylalanine sensitive child is 25%. The genotypes AA and Aa (75%) are normal as the aa genotype is necessary for the disease to manifest.

7. Hemophilia is an X-linked recessive disorder.

Xh - recessive allele of hemophilia

XH - dominant allele (normal)

The mother is heterozygous (XH Xh) for hemophilia and the father (XHY) is normal. The Punnet square is as follows:

	XH	Y
XH	XHXH	XH Y
Xh	XH Xh	XhY

25% chance of having a child with hemophilia. But among the female children, none of them have the hemophilia genotype. A normal or a carrier genotype is possible. The chances of having a female hemophiliac child, in this case, are zero.

8. Albinism is an autosomal recessive disorder.  

Albino male - aa (recessive)

Carrier female - Aa (heterozygous)

The Punnet square will be as follows:

	a
A	Aa (carrier)
a	aa (albino)

There chances of a child being a carrier are 50%.