Lecture 13

Question 1

phenotypic dimorphism

Answer 1

differentiation of sexses

Question 2

Heteromorphic chromosomes

Answer 2

• dissimilar
• sex chromosomes X and Y

Question 3

Sex chromosomes

Answer 3

characterize one sex or the other in a wide range of species

Question 4

Sex determination

Answer 4

by specific genes not entire chromosomes

Question 5

Mode of sex determination: protenor

Answer 5

• XX/XO mode of dex determination
• depends on random distribution of X chromosome in half of male gametes
• presence of 2 X = female
• presence of 1 X = male

Question 6

Lygaeus mode of sex determination

Answer 6

• XX/XY mode of sex determination
• female gametes have 1 X chromosomes
• male gametes have either an X or Y chromosome

Question 7

Lygaeus ratio is

Answer 7

1:1 between male and female offspring

Question 8

Homogametic sex

Answer 8

• producing “like” chromosomes
• zygotes with 2 X chromosomes
• results in female offspring

Question 9

Heterogametic sex

Answer 9

• Producing unlike chromosomes
• zygotes with one X and one Y chromosomes
• male offspring

Question 10

Females as heterogametic sex

Answer 10

• ZZ/ZW sex determination
• females are the heterogametic (ZW) sex
• males are the homogametic (ZZ) sex
  Chickens

Question 11

Klinefelter and Turner Syndrome

Answer 11

• 2 human abnormalities
• characterized by aberrant sexual development
• both occur from nondisjunction

Question 12

nondisjunction of sex chromosomes

Answer 12

failure of X chromosomes to segregate during meiosis

Question 13

Klinefelter syndrome

Answer 13

47 chromosomes - XXY
- tall, and long limbs
- large hands and feet
- internal ducts are male, rudimentary testes don’t make sperm (sterile)
- feminine development not suppressed (large breasts, and round hips)

Question 14

Turner syndrome

Answer 14

45 chromosomes - X
- phenotypically female
- rudimentary ovaries, underdeveloped breasts
- short
- cognitive impairment

Question 15

Triplo-X

Answer 15

47 chromosomes - XXX
- normal set of autosomes
- female differentiation
- sometimes women are completely normal
- sometimes women have underdeveloped secondary sex characteristics
- sterility and mental retardation

Question 16

47 - XYY condition

Answer 16

• only consistency shared characteristic- males are over 6 feet tall
• subnormal intelligence
• personality disorders

Question 17

Gonadal primordia

Answer 17

• the tissues that will form the gonad
• by 5th week of gestation, a pair of gonadal ridges associated with each embryonic kidney
• gonadal phenotype is sexually indifferent (primordial germ cells migrate to ridges

Question 18

Biopotential gonads

Answer 18

gonadal ridges can form either ovaries or testes

Question 19

Y chromosome and male development

Answer 19

• Y chromosome has at least 50 genes
• fewer than X (100 genes)

Question 20

PARS: PseudoAutosomal regions

Answer 20

• present on both ends of Y chromosomes
• share homology with regions on X
• synapse and recombine with X during meiosis
• located at the ends

Question 21

Pairing regions…

Answer 21

critical to segregation of X and Y chromosomes during male gametogenesis

Question 22

MSY

Answer 22

male-specific region of the Y
- non-recombining region of Y chromosome
- 23 million base pairs
3 regions
- X-transposed region (15% of MSY)
- X-degenerative region (20%)
- Ampliconic region (30% abd encodes proteins specific to development and function of testis)

Question 23

SRY

Answer 23

sex-determining region Y
- located adjacent to PAR of the short arm of Y chromosome
- controls male development
- located right under PAR region

Question 24

Testis-determining factor (TDF)

Answer 24

protein encoded by SRY
- at 6-8 weeks SRY becomes active in XY embryos
- encodes protein that triggers testes formation

Question 25

Sex ratio

Answer 25

actual proportion of male to female offspring

Question 26

primary sex ratio

Answer 26

reflects proportion of males to females conceived in population

Question 27

secondary sex ratio

Answer 27

reflection proportion of each sex born

Question 28

theoretical basis

Answer 28

1:1 ratio - each type of sperm has equivalent viability and motility in female reproductive tract - egg surface equally receptive to both X and Y bearing sperm

Question 29

Dosage compensation

Answer 29

- genetic mechanism - balances does of X chromosome gene expression in males and females - prevents excessive expression of X-linked genes in humans and other mammals

Question 30

Barr bodies (sex chromatin bodies)

Answer 30

- genetic mechanism compensates of X dosage disparities - inactive X chromosome, highly condensed - darkly stained body in interphase nerve cells observed: Barr bodies - random activation - occurs early in embryonic development

Question 31

X-inactivation

Answer 31

- explains dosage compensation - follows N-1 rule (N = total number of X chromosomes)

Question 32

Why does X-inactivation not affect syndromes such as Turner or Klinefelter?

Answer 32

- chromosome inactivation not in early stages of development for cells destined for gonadal tissue - not all X chromosomes forming Barr bodies are inactivated (15% escape inactivation)

Question 33

Lyon hypothesis

Answer 33

- inactivation of X chromosome is random - occurs in somatic cells at early stage of embryonic development - all descendant cells have same X-inactivation ex) Calico cats and fur color/patterns

Question 34

G6PD

Answer 34

- glucose-6-phosphate dehydrogenase - synthesis of G6PD enzyme controlled by X-linked gene - experiment with G6PD mutants provides strong support of Lyon hypothesis (random permanent inactivation of X chromosome)

Question 35

Xic: X inactivation center

Answer 35

- active only on inactive X - Has X-inactive specific transcript (XIST) gene critical for X-inactivation - two noncoding genes in Xic locus (Tsix Xite play important roles in X chromosomes inactivation)