Sex Determination & Sex Linkage Flashcards
What are sex chromosomes?
Chromosomes that determine the sex of an organism
In mammals, they are X and Y chromosomes.
What is the genetic sex determination system in birds?
ZW system: males are ZZ, females are ZW
This system differs from the XY system in mammals.
What is dosage compensation by X inactivation?
One of the X chromosomes in each female cell becomes inactivated
This results in a Barr body, which is a condensed, inactive X chromosome.
What is the phenotypic ratio in a dihybrid cross with dominant reciprocal epistasis?
15:1 phenotypic ratio
This occurs when one gene masks the expression of another.
What are the possible genotypes for a scalloped female in Drosophila crossed with a normal male?
X^sd X^sd x X^+ Y
F1 results will include a carrier female and a scalloped male.
What is the expected F2 ratio from a normal female crossed with a scalloped male in Drosophila?
3:1 normal to scalloped, with sex bias
This reflects the sex-linked nature of the scalloped mutation.
What does it mean for males to be hemizygous for genes on the X chromosome?
Males have only one copy of the X chromosome
This makes them more susceptible to X-linked recessive conditions.
What is the role of the SRY gene?
It is the sex-determining region on the Y chromosome that triggers male development
- Translocation of SRY to the X chromosome is found rare in XX males (sex reversal)
- Mutation of the SRY gene can give XY females
- Transgenic mice expressing SRY will give rise to males
What is a gynandromorph?
An organism that has both male and female characteristics
In birds, this occurs when one side of the body has a male chromosome set (ZZ) and the other side a female set (ZW)
This is known as cell-autonomous sex identity = each cell decides whether to express male of female characteristics
Fill in the blank: In humans, nearly all X-linked diseases are ______.
recessive
Most X-linked diseases manifest more frequently in males due to their hemizygous state.
What is the significance of the pseudoautosomal region (PAR)?
It contains genes shared between the X and Y chromosomes, essential for X-Y pairing during male meiosis
PARs allow for proper segregation of sex chromosomes.
True or False: Female mammals are the heterogametic sex.
False
Females are the homogametic sex (XX) while males are the heterogametic sex (XY).
What are some examples of X-linked diseases in humans?
- Haemophilia A (Factor XIII)
- Haemophilia B (Factor IX)
- Duchenne muscular dystrophy
- Red-green colour vision deficiency
These conditions primarily affect males due to their hemizygous nature for X-linked genes.
What is the expected probability that a son will be color blind if his mother is a carrier and his father has normal vision?
0.5
This is due to the X-linked recessive inheritance pattern.
What is a Barr body?
An inactivated X chromosome in female mammals
It is visible during interphase and results from X chromosome inactivation.
What happens during random X chromosome inactivation?
One of the X chromosomes in a female is randomly inactivated during early development
Inactivated X chromosome becomes highly condensed, does not express genes and is visible cytologically in interphase as a Barr body
This leads to a mosaic expression of maternal and paternal X-linked genes.
What is the expected outcome of a cross between a white-eyed male Drosophila and a red-eyed female?
All F1 offspring will have red eyes
Red is dominant over white in this genetic scenario.
What is the male-specific region of the Y chromosome (MSY)?
It contains genes that are primarily involved in male sex determination and spermatogenesis
The MSY is approximately 60 Mb in size.
What is the pseudoautosomal region (PAR)?
- Tip of the chromosome
- Shared between the X and Y chromosomes (~20 genes in humans)
- Required for X-Y pairing in male meiosis
= Homologous region
What are non-sex chromosomes called?
Autosomes
How is sex determined in mammals?
- Females are the homogametic sex (XX)
- Males are the heterogametic sex (XY)
Evidence that the Y chromosome confers the male sex chromosome came from aneuploidy observations
- XO = female
- XXY = male (Y overrides X)
How does SRY work?
- it is a transcription factor that recognises specific DNA sequences found in promoters of genes (enhancers)
- binding of SRY at promoters activates gene expression
- activation of SRY produces testis (ovaries if not activated)
What is sex-linked inheritance?
- Involves genes located on the X chromosome (X-linkage)
- There are also Y-linked genes, many with male-specific functions but this is a small percentage of sex-linkage
- Most genes on X are unrelated to sex determination or sex function and are expressed in both males and females
- Males are hemizygous for genes on the X chromosome = only have 1 copy of the X gene (if 2 copies, twice as much protein in females than males so it is lethal)
What is the importance of X chromosome inactivation?
Involves dosage compensation to ensure there isn’t too much protein
