Sex Determination Systems

Question 1

Single allele at a gene locus determines the sex of that individual.
ex. yeast exist as 2 different haploid sexes: a or alpha.

Answer 1

Genic sex determination

Question 2

Combination of sex chromosomes determine the sex of that individual
ex. ZW female and ZZ Male in birds or XX female and XO in insects

Answer 2

Genotypic sex determination

Question 3

Presence of Y chromosome dictates Male or Female

Answer 3

Y determination mechanism

Question 4

XX females

Answer 4

Homogametic

Question 5

XY males

Answer 5

Heterogametic

Question 6

Aligns with X chromosome during meiotic synapsis.

Answer 6

Pseudoautosomal regions (PARs)

Question 7

Euchromatin has about 50-60 genes involved in sex determination.

Answer 7

Male-specific region of the Y (MSY)

Question 8

• Becomes active at about 6-8 weeks during development.

- Region encodes protein called Testes Determining Factor (TDF) which directly controls testes development

Answer 8

Sex-determining Region of Y (SRY)

Question 9

Absence of TDF results in:

Answer 9

Development of gonads into ovaries.

Question 10

How was SRY first identified?

Answer 10

Observation: sex reversal in humans
Hypothesis: SRY is essential for male sex determination

Question 11

Cytogenic; has deletion in the Y chromosome.

Answer 11

XY females

Question 12

Cytogenic; has small pieces of Y chromosome attached to X.

Answer 12

XX males

Question 13

SRY first identified, cont…

experiment in mice: SRY gene ortholog cloned and introduced into XX mouse embryos

Answer 13

Result: Introduced trans gene caused normal male development from the XX mice.
Conclusion: SRY causes male testes development

Question 14

Evidence for Y chromosome mechanism

Answer 14

Turner Syndrome
Klinefelter syndrome
XYY male

Question 15

XO female
Usually embryonic lethal
Secondary sexual characteristics poorly developed
short stature

Answer 15

Turner syndrome

Question 16

XXY (male)
Breast development
Above average height

Answer 16

Klinefelter syndrome

Question 17

XYY (male)
Taller than average
antisocial?

Answer 17

XYY male

Question 18

the inactive X chromosome in a female somatic cell, rendered inactive in a process called lyonization, in those species in which sex is determined by the presence of the Y (including humans) or W chromosome rather than the diploidy of the X or Z.

Answer 18

Barr Bodies

Question 19

Silencing of one X chromosome in female allows for the same level of expression for genes on the X chromosome in males and females.

Answer 19

Gene dosage compensation

Question 20

total # if X chromosomes - 1 =

Answer 20

of Barr bodies in somatic cells

Question 21

Only X chromosome is changed from euchromatin to heterochromatin about 16 days after fertilization
-deactivation occurs randomly between 2 X chromosomes in every cell.

Answer 21

Lyonization

Question 22

All mitotic descendents of the cell inherit that inactivation pattern.

Answer 22

Epigenetic phenomena

Question 23

Femalse are mosaics

Answer 23

Lyonization

ex. calico cat: only some genes are coded for orange and some for black. Determined randomly.

Question 24

Genes on X or Y chromosome

Answer 24

Sex linked genes

Question 25

Many linked genes are “innocent passengers”, meaning;

Answer 25

they don’t have anything to do with sex determination

Question 26

Sex linked genes in humans:

Answer 26

more than 100 traits known, examples:
color perception
blood clotting factors
dystrofen: large protein for muscle cell structure

Question 27

Sex-linked traits in humans:

Answer 27

1. X-linked recessive inheritance

2. X-linked dominant inheritance

Question 28

• Trait occurs more frequently in males because males are hemizygous for genes on the chromosome.
• Females express the trait when they are homozygous for the allele. Heterozygotes are carriers.

Answer 28

X-linked Recessive Inheritance

Question 29

What proportion of the sons of an affected mother would show the trait?

Answer 29

100%

Question 30

What proportion of the sons of a carrier mother would show the trait?

Answer 30

50%

Question 31

A homozygous normal mother marries an affected male, what proportion of their daughters would show the trait?

Answer 31

0%

Question 32

An affected mother marries a normal male, what proportion of their daughters would show the trait?

Answer 32

0%