Genes Lecture 4-Sex Determination Flashcards
types of sex determination in animals
genetic and environmental
X specific region
> 1000 genes just on the X chromosome
MSY
Male specific region of the Y
80 genes
pseudo autosomal region
shared between the X and Y ( ̴ 20 genes in humans)
required for X-Y pairing in male meiosis as pairing is dependant on sequence similarity
sex determination in mammals
Females are the homogametic sex
Males are the heterogametic sex
XO individuals
female
XXY individuals
male
where did evidence that the Y chromosomes confer the male sex chromosome come from
aneuploidy observations
Is all of the Y chromosome required to be defined as male?
Only ONE gene is required:
SRY (sex-determining region on the Y
evidence suggesting the Y gene is enough to determine maleness
- Translocation of SRY to the X chromosome is found in rare XX males (sex reversal)
- Mutation of the SRY gene can give XY females
- Transgenic mice expressing SRY will give rise to males
How does SRY work?
transcription factor that binds to promoters to activate gene expression
Outline gonad development
Week 4: begins at the genital ridge with just somatic cells
Week 5 & 6: future germ cells will start migrating from another area and settle into the genital ridge to form the indifferent gonad region
Week 7: SRY is activated enabling the indifferent gonad to become testes
*without the SRY the indifferent gonad will go to become the ovaries
ZW system in birds
males: ZZ (homogametic)
females: ZW
gynandromorphs/ sexual mosaic in birds
e.g chicken that is half male and half female
-cells have a female set of chromosomes on half their body (zw) and a male set of chromosomes on the other half of the body (zz)
cell autonomous sex identity (CASI)
each cell define its sex; Every cell decides whether to express male or female characteristics
why do mammals not show CASI
due to sex hormones that define sex for all tissues
Thomas Hunt Morgan
showed that sex-chromosome can pass on their genes in a way that didn’t fir Mendelian phenotypic ratio
X-linkage
specific genes located on the X chromosome
spermatogenesis
Y-linked genes with many male-specific functions
males are hemizygous for genes on the x chromosome
males only have 1 copy of the x genes
examples of X-linked traits in humans
haemophilia A and B, Duchenne muscular dystrophy, and colour-blindness, which is very common in males.
colour-blindness
-genes regulating photosensitivity on the x chromosome are very similar therefore are more likely to recombine in meiosis
-3 key amino acids in each opsin gene critical for their ability to detect the correct colour
-loss of discrimination between red and green gave rise to a specific type of colour-blindness called deuteranomaly
X chromosome inactivation
One of the X chromosomes in each female cell becomes inactivated - a type of dosage compensation
Barr bodies
cytologically visible inactivated X chromosomes seen in interphase
Random XCI in early development
random and happens early in development and persists for all subsequent miotic cell divisions
example of epigenetic control of gene expression
why are there female tortoise shell cats
-Tortoiseshell cats are heterozygous for large O and small o alleles on the X chromosome. The large O allele produce the orange pigment and the small o allele black
-melanocytes, the cells producing pigmentation, will randomly inactivate one or the other X chromosome
-descendants from this initial melanocyte will later form a patch and all these descendant cells will express the black small o allele. Because this occurs randomly, it is inevitable that other melanocytes will have the other X chromosome inactivated, then a patch of orange fur will form.
how can male cats show tortoise shell
- XXY male ( ̴1/3000 males)
- X chromosome inactivation occurs in males with more than one X chromosome
-In individuals with multiple X chromosomes (XX females, XXY males, XXX females) each cell expresses only one X chromosome, the others are forming Barr bodies
