Question 1

What is the best explanation of a child having the physical phenotype of Down syndrome but having a totally normal karyotype on chromosomal analysis of blood?

Accepted Answer

Mosaicism of trisomy 21 in various tissues

Question 2

A man and his sister are both affected with a mitochondrial disorder.Because the sister has two children who also have the disorder,the man is concerned about the possibility of also having children with the disorder.What is the estimated risk for each pregnancy of this man producing a child with the same mitochondrial disorder that he has if his wife does not have a mitochondrial disorder?

Accepted Answer

Mitochondrial disorders are exclusively inherited maternally, which means that the risk of the disorder being passed on by the affected man to his offspring is 0%. Even though the man has the disorder, he cannot pass the affected mitochondria to his children as it is not inherited through the sperm. Therefore, the risk of producing a child with a mitochondrial disorder is dependent on the mitochondrial status of the man's partner.

Question 3

Which factor has the greatest influence on the expression of a genetic disorder such as Prader-Willi syndrome or Angelman syndrome through the process of genomic imprinting?

Accepted Answer

Genomic imprinting involves the differential methylation of genes depending on whether they are inherited from the mother or the father. In disorders such as Prader-Willi syndrome and Angelman syndrome, the affected genes are located on the same region of chromosome 15, but they are expressed differently depending on whether they are inherited from the father or the mother. Therefore, the gender of the parent transmitting the deletion (either paternal or maternal) has the greatest influence on the expression of the disorder through genomic imprinting. Age at time of conception and the number of bases deleted may also be factors, but they do not have as strong and direct an impact as parental gender.

Question 4

What factor most strongly influences the development of the paramesonephric tissue into female sex organs?

Accepted Answer

The Y chromosome contains the SRY gene, which triggers the development of male sex organs. In the absence of a Y chromosome, the default pathway is the development of female sex organs. Therefore, the absence of a Y chromosome influences the development of the paramesonephric tissue into female sex organs.

Question 5

Which process can cause a person with an XY genotype to have a female phenotype?

Accepted Answer

Failure of gene expression resulting in a complete absence of androgen receptors can cause a person with an XY genotype to have a female phenotype. This is because androgens, which are male hormones, normally bind to androgen receptors to promote male sexual development. Without functioning androgen receptors, male sexual development cannot occur and the individual will exhibit female phenotypic traits. This condition is known as complete androgen insensitivity syndrome.

Quiz 5: Chromosomal and Mitochondrial Inheritance and Disorders

What is the best explanation of a child having the physical phenotype of Down syndrome but having a totally normal karyotype on chromosomal analysis of blood?

Which factor has the greatest influence on the expression of a genetic disorder such as Prader-Willi syndrome or Angelman syndrome through the process of genomic imprinting?

What factor most strongly influences the development of the paramesonephric tissue into female sex organs?

Which process can cause a person with an XY genotype to have a female phenotype?