Lecture 5 - Sex Determination and Sex Linked Characteristics

Question 1

What genetic factor is responsible for determining the sex of an organism?

L5 S24

Answer 1

The presence or absence of the Y chromosome is responsible for determining if the sex will be male or female respectively.

It is specifically the Sry gene that is located on the Y chromosome which produces a protein called testis determining factor that is responsible for this.

Question 2

How does heterogametic sex differ from the homogametic sex?

L5 S37/38

A. The heterogametic sex is male; the homogametic sex is female.
B. Gametes of the heterogametic sex have different sex chromosomes; gametes of homogametic sex have the same sex chromosome.
C. Gametes of the heterogametic sex all contain a Y chromosome.
D. Gametes of the homogametic sex all contain an X chromosome.

Answer 2

B. Gametes of the heterogametic sex have different sex chromosomes; gametes of homogametic sex have the same sex chromosome.

Question 3

What genetic conditions are there related to abnormal sex chromosome number?

L5 S44

Answer 3

Turner syndrome:

• XO genotype (only one X chromosome with no additional Y or X)
• sex is females

Klienfelter syndrome:

• XXY, XXXY, XXYY, etc.
• sex is males

Poly-X:

• more than two X chromosomes
• sex is female

Question 4

What does the term hemizygous mean and how does it relate to disease?

L5 S56

Answer 4

Males only have one X chromosome and the Y chromosome does not carry paired genes. This means males only have one allele for each gene on the X chromosome.

Because of this, if that allele is recessive, the male will display the recessive phenotype despite having only one recessive allele.

Question 5

What is dosage compensation and how does it relate to mosaicism?

L5 S57

Answer 5

Only one copy of each gene is needed from the X chromosome. In women, one X chromosome per cell is randomly condensed into a Barr body.

Because of this neighboring cells can have different genotypes and phenotypes based of which X chromosome is deactivated.

Question 6

What are the most common X-linked recessive disorders?

L5 S59

Answer 6

• Red-green colorblindness
• hemophilia
• Duchene muscular dystrophy
• Becker’s muscular dystrophy

Question 7

A woman who is red/green color blind has children with a man who has normal vision. What is the probability that their first child will be color blind?

L5 S62

Answer 7

50%

If the child is a male it will be color blind because the mother can only pass down the recessive gene.
If the child is a female it will be a carrie.