Genes Lecture 4-Sex Determination

Question 1

types of sex determination in animals

Answer 1

genetic and environmental

Question 2

X specific region

Answer 2

> 1000 genes just on the X chromosome

Question 3

MSY

Answer 3

Male specific region of the Y
80 genes

Question 4

pseudo autosomal region

Answer 4

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

Question 5

sex determination in mammals

Answer 5

Females are the homogametic sex
Males are the heterogametic sex

Question 6

XO individuals

Answer 6

female

Question 7

XXY individuals

Answer 7

male

Question 8

where did evidence that the Y chromosomes confer the male sex chromosome come from

Answer 8

aneuploidy observations

Question 9

Is all of the Y chromosome required to be defined as male?

Answer 9

Only ONE gene is required:
SRY (sex-determining region on the Y

Question 10

evidence suggesting the Y gene is enough to determine maleness

Answer 10

1. Translocation of SRY to the X chromosome is found in rare XX males (sex reversal)
2. Mutation of the SRY gene can give XY females
3. Transgenic mice expressing SRY will give rise to males

Question 11

How does SRY work?

Answer 11

transcription factor that binds to promoters to activate gene expression

Question 12

Outline gonad development

Answer 12

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

Question 13

ZW system in birds

Answer 13

males: ZZ (homogametic)
females: ZW

Question 14

gynandromorphs/ sexual mosaic in birds

Answer 14

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)

Question 15

cell autonomous sex identity (CASI)

Answer 15

each cell define its sex; Every cell decides whether to express male or female characteristics

Question 16

why do mammals not show CASI

Answer 16

due to sex hormones that define sex for all tissues

Question 17

Thomas Hunt Morgan

Answer 17

showed that sex-chromosome can pass on their genes in a way that didn’t fir Mendelian phenotypic ratio

Question 18

X-linkage

Answer 18

specific genes located on the X chromosome

Question 19

spermatogenesis

Answer 19

Y-linked genes with many male-specific functions

Question 20

males are hemizygous for genes on the x chromosome

Answer 20

males only have 1 copy of the x genes

Question 21

examples of X-linked traits in humans

Answer 21

haemophilia A and B, Duchenne muscular dystrophy, and colour-blindness, which is very common in males.

Question 22

colour-blindness

Answer 22

-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

Question 23

X chromosome inactivation

Answer 23

One of the X chromosomes in each female cell becomes inactivated - a type of dosage compensation

Question 24

Barr bodies

Answer 24

cytologically visible inactivated X chromosomes seen in interphase

Question 25

Random XCI in early development

Answer 25

random and happens early in development and persists for all subsequent miotic cell divisions
example of epigenetic control of gene expression

Question 26

why are there female tortoise shell cats

Answer 26

-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.

Question 27

how can male cats show tortoise shell

Answer 27

• 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