20. Sex determination Flashcards
What are the 5 ways in which sex can be determined?
- environment
- genes
- haplo/diploidy
- balance of autosomes to sex chromosomes
- a sex chromosome
What are 2 organisms which show an example of sex determination? Environmental
- Bonella viridis, a marine worm. Sex is determined by where the larvae lands, male if lands near probiscus
- reptiles - incubuation temperature of eggs
How can temperature influence sex determination?
Due to temperature sensitivity in the synthesis of sex hormones
How does sex determination work in turtles?
Acromatase production is temp sensitive, increases in high temps
More aromatase means more oestrogen
How do drosophila demonstrate how a certain genotype can change the sexual phenotype?
If a female has the tra tra mutation then becomes male
What is an example of genes determining sex in humans?
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
What are the phenotypic affects of androgen insensitvity?
- 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
How does the haplo/diploidy sex determining mechanism work?
- In bees females are diploid and males are haploid
How do drosophila use the ratio of sex chromosomes to autosome as a mechanism for determining sex?
- 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
What are meta females in drosophila?
Ratios greater than one are meta female
What are the female genotypes for Turner syndrome?
X for females
What causes Turner syndrome?
Mostly result from sperm that has lost a sex chromosome
What is the genotype for females with triple X?
XXX
What are some phenotypic characteristics of females with triple X?
- Usually taller
- Normal sexual development and fertility, no other physical characteristics
- Increased risk of learning disabilities and delayed development
What phenotypic characteristics do females with XXXX have?
- Mean IQ 55
- Speech and language delays and behavioral problems
What is the male genotype for KIinefelter syndrome?
XXY
What are some characteristics of males with XYY?
Most neither mentally retarded or criminal, but can be emotionalyl immature and have impulsive behaviour
What important DNA sequence does the Y chromosome contain?
TDF - testis determining factor
What does the testis determining factor protein actually do?
- Regulatory factor which binds to the other genes to promote their expression
What sorts of genes does the TDF regulate?
- 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
What happens if TDF is not present?
Oogonia begin meisosi in female, undifferentiated gonad becomes functional at the start of 12 weeks
How can TDF also be sex determining in mutations?
- XY females have a deletion of TDF
- XX males have an insertion of part of Y chromosome with TDF locus
What is pseudohermaphroditism?
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
How can TDF mutations come about?
- 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
What is heterogametic?
The sex that produces 2 types of gametes
What is the ZW sex determining system?
Females are heterogametic ZW and males are homogametic with ZZ
