20. Sex determination

Question 1

What are the 5 ways in which sex can be determined?

Answer 1

1. environment
2. genes
3. haplo/diploidy
4. balance of autosomes to sex chromosomes
5. a sex chromosome

Question 2

What are 2 organisms which show an example of sex determination? Environmental

Answer 2

1. Bonella viridis, a marine worm. Sex is determined by where the larvae lands, male if lands near probiscus
2. reptiles - incubuation temperature of eggs

Question 3

How can temperature influence sex determination?

Answer 3

Due to temperature sensitivity in the synthesis of sex hormones

Question 4

How does sex determination work in turtles?

Answer 4

Acromatase production is temp sensitive, increases in high temps
More aromatase means more oestrogen

Question 5

How do drosophila demonstrate how a certain genotype can change the sexual phenotype?

Answer 5

If a female has the tra tra mutation then becomes male

Question 6

What is an example of genes determining sex in humans?

Answer 6

Disorder of Sexual Development (DSD) where there is an androgen insensitivity locus on X chromosome. AR* is the mutation, males with X^AR* appear as females

Question 7

What are the phenotypic affects of androgen insensitvity?

Answer 7

• XY individuals produce testosterone but the cells are insensitive to it due to defect in testosterone (androgen) receptor
• Outwardly female but no female reproductive organs

Question 8

How does the haplo/diploidy sex determining mechanism work?

Answer 8

• In bees females are diploid and males are haploid

Question 9

How do drosophila use the ratio of sex chromosomes to autosome as a mechanism for determining sex?

Answer 9

• Sex determined by the ratio of X chromosomes to sets for autosomes
• Females have ratio of 1
• Males have ratio of 1:2 of X;A, but Y still needed for fertility

Question 10

What are meta females in drosophila?

Answer 10

Ratios greater than one are meta female

Question 11

What are the female genotypes for Turner syndrome?

Answer 11

X for females

Question 12

What causes Turner syndrome?

Answer 12

Mostly result from sperm that has lost a sex chromosome

Question 13

What is the genotype for females with triple X?

Answer 13

XXX

Question 14

What are some phenotypic characteristics of females with triple X?

Answer 14

• Usually taller
• Normal sexual development and fertility, no other physical characteristics
• Increased risk of learning disabilities and delayed development

Question 15

What phenotypic characteristics do females with XXXX have?

Answer 15

• Mean IQ 55

- Speech and language delays and behavioral problems

Question 16

What is the male genotype for KIinefelter syndrome?

Answer 16

XXY

Question 17

What are some characteristics of males with XYY?

Answer 17

Most neither mentally retarded or criminal, but can be emotionalyl immature and have impulsive behaviour

Question 18

What important DNA sequence does the Y chromosome contain?

Answer 18

TDF - testis determining factor

Question 19

What does the testis determining factor protein actually do?

Answer 19

• Regulatory factor which binds to the other genes to promote their expression

Question 20

What sorts of genes does the TDF regulate?

Answer 20

• DAX1 is an anti-testis factor so SRY has to shut this down

- SOX activated to differentiate cells, leading to the seminiferous tubules of the testis

Question 21

What happens if TDF is not present?

Answer 21

Oogonia begin meisosi in female, undifferentiated gonad becomes functional at the start of 12 weeks

Question 22

How can TDF also be sex determining in mutations?

Answer 22

• XY females have a deletion of TDF

- XX males have an insertion of part of Y chromosome with TDF locus

Question 23

What is pseudohermaphroditism?

Answer 23

Where children appear female until puberty, after which they change to male with a surge of testosterone and dihydrotestosterone
- Autosomal recessove wotj sex limited expression

Question 24

How can TDF mutations come about?

Answer 24

• X and Y have some homologous regions
• Known as pseudoautosomal region which pairs in meisos
• If the crossover is not exact then TDF locus may be transferred to X giving XX males and XY females

Question 25

What is heterogametic?

Answer 25

The sex that produces 2 types of gametes

Question 26

What is the ZW sex determining system?

Answer 26

Females are heterogametic ZW and males are homogametic with ZZ