Lecture 12 Sex Determination

Question 1

Females are the homogametic sex, true or false

Answer 1

True, Females are XX

Question 2

Males are the heterogametic sex, true or false

Answer 2

True Males are XY

Question 3

‘Not all of the Y chromosome is required for sex determination in males’
True or False

Answer 3

True. Maleness is conferred by a single gene, the SRY gene.

Question 4

What does the SRY gene stand for

Answer 4

Sex determining region

Question 5

Which gene confers maleness

Answer 5

SRY gene on the Y chromosome

Question 6

What is sex reversal, how does it occur

Answer 6

This occurs due to translocation of the SRY gene from the Y chromosome to the X chromosome, producing a rare male with the genotype, XX

Question 7

Why are XX males sterile

Answer 7

not sure

Question 8

Is it possible to get females that are XY

Answer 8

Yes, this can occur when the SRY gene is mutated so it is no longer functional,thus maleness is not conferred.

Question 9

How does the SRY gene confer maleness

Answer 9

• SRY gene codes for the SRY protein.
• SRY is a transcription factor
• This transcription factor is thought to regulate expression of genes necessary for testis formation.
• SRY expression causes gonad cells to differentiate into testis cells.

Question 10

State the system present in birds for sex determination

Answer 10

ZW system

Question 11

Male birds gametic genotype is

Answer 11

ZZ

Question 12

Females birds gametic genotype is

Answer 12

ZW

Question 13

What is a Gyndandromorph

Answer 13

• sexual mosaic
• cells on the right side of the body have the female chromosome set ZW
• cells on the left side of the body have the male chromosome set ZZ

Question 14

Define autosomes

Answer 14

these are non sex determining chromosomes

Question 15

What is Sex linked inheritance

Answer 15

Involves genes located on the X chromosome, known as X linked genes.
However, most genes on the X chromosome are unrelated to sex determination or sex function and are expressed in males and females.

Question 16

Why are X linked traits more common than Y linked traits

Answer 16

Because there are more genes located on the X chromosome, as the X chromosome is larger than the Y chromosome.

Question 17

Why do X linked recessive conditions affect males mor commonly than females

Answer 17

Because males are hemizygous for recessive genes on the X chromosome.
Males only require one X chromosome for them to have the recessive trait, while a female can have this recessive trait masked by a dominant allele on the X.

Question 18

Which parents must be affected in order for a X linked trait to be passed on

Answer 18

to inherit an X linked trait, this requires a carrier or affected mother or an affected father.

Question 19

In X linked recessive conditions, what is the probability of a son born to a carrier mother and unaffected father?

Answer 19

50 percent chance of inheriting the mothers recessive allele on the X chromosome, and therefore being hemizygous for the trait.

Question 20

Why do females undergo X chromosome inactivation

Answer 20

• Because females have 2 X chromosomes therefore have double the gene dosage of X linked genes than males.
• Thus one of the X chromsomes becomes inactivated.
• This is a type of dosage compensation

Question 21

Describe the procrss of X chromosome inactivation in females

Answer 21

• The inactivated X chromosomes becomes highly condensed - heterochromatin
• heterochromatin does not permit genes to be expressed
• X chromosome is visible cytologically in interphase as a barr body.

Question 22

What is a barr body

Answer 22

a highly condensed inactive X chromosome.

Question 23

What is the name given to the transcriptionally inactive form of the X chromosome in X chromosome inactivation

Answer 23

Heterochromatin

Question 24

Which phase of the cell cycle is the Barr body visible

Answer 24

Interphase