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Flashcards in Sex Determination Systems Deck (32):
1

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

Genic sex determination

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

Genotypic sex determination

3

Presence of Y chromosome dictates Male or Female

Y determination mechanism

4

XX females

Homogametic

5

XY males

Heterogametic

6

Aligns with X chromosome during meiotic synapsis.

Pseudoautosomal regions (PARs)

7

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

Male-specific region of the Y (MSY)

8

-Becomes active at about 6-8 weeks during development.
-Region encodes protein called Testes Determining Factor (TDF) which directly controls testes development

Sex-determining Region of Y (SRY)

9

Absence of TDF results in:

Development of gonads into ovaries.

10

How was SRY first identified?

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

11

Cytogenic; has deletion in the Y chromosome.

XY females

12

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

XX males

13

SRY first identified, cont...
experiment in mice: SRY gene ortholog cloned and introduced into XX mouse embryos

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

14

Evidence for Y chromosome mechanism

Turner Syndrome
Klinefelter syndrome
XYY male

15

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

Turner syndrome

16

XXY (male)
Breast development
Above average height

Klinefelter syndrome

17

XYY (male)
Taller than average
antisocial?

XYY male

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.

Barr Bodies

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.

Gene dosage compensation

20

total # if X chromosomes - 1 =

# of Barr bodies in somatic cells

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.

Lyonization

22

All mitotic descendents of the cell inherit that inactivation pattern.

Epigenetic phenomena

23

Femalse are mosaics

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

24

Genes on X or Y chromosome

Sex linked genes

25

Many linked genes are "innocent passengers", meaning;

they don't have anything to do with sex determination

26

Sex linked genes in humans:

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

27

Sex-linked traits in humans:

1. X-linked recessive inheritance
2. X-linked dominant inheritance

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.

X-linked Recessive Inheritance

29

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

100%

30

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

50%

31

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

0%

32

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

0%