Chapter 7: Sex Determination and Sex Chromosomes

Question 1

Phenotype dimorphism

Answer 1

the differences in appearance between males and females of the same species

-In animals, including humans, differentiation of sexes is evident via phenotype dimorphism

Question 2

Heteromorphic chromosomes

Answer 2

Dissimilar

Example: 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 (Butterfly)

Answer 5

XX/XO mode of sex determination

-Depends on random distribution of X chromsome into half of male gametes

-Presence of two X chromosomes in zygote results in female offspring

-Presence of one X chromosome results in male offspring

PRESENCE OF A CHROMSOME DETERMINES MALE OR FEMALE

Question 6

Lygaeus (milkweed bug) mode of sex determination

Answer 6

XX/XY mode of sex determination

Female gamets have two X chromosome

Male gametes have either an X or Y chromosome

Example: XX for females
XY for males

THE Y CHROMSOME DETERMINES THE SEX

Question 7

Homogametic sex

Answer 7

-Producing LIKE chromsomes
-Zygotes with two X chromosomes
-Results in female offspring

Question 8

Heterogametic Sex

Answer 8

-Producing unlike chromosomes
-Zygotes with one X and one Y chromosome
-Results in male offspring

Question 9

This might sound very interesting:

Some females in different animals are actually expressed as a heterogametic sex

Answer 9

-ZZ/ZW sex determination
-Females are the heterogametic (ZW) sex
-Males are the homogametic (ZZ) sex

Example: Chikens

Question 10

Y chromosome determines maleness in…

Answer 10

HUMANS YO!

Question 11

What is the human karyotype?

Answer 11

22 pairs of autosomal chromosomes
2 sex chromosomes
Reveals one pair of chromosomes differs in males and females
-Failure of X chromosomes to segregate during meiosis
Females: XX
Males: XY

Question 12

Klinefelter and Turner Syndrome

Answer 12

-Two human abnormalities
-Characterized by abberant sexual development
-Both syndromes result from nondisjuction
-failure of X chromosome to segregate during meiosis
The word abberant means: diverging from the normal type.

Question 13

What is nondisjunction?

Answer 13

the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei.

Question 14

Klinefelter syndrome (47, XXY)

Answer 14

-A male inherits an extra X chromosome

Symptoms:
-Tall, long arms and legs
-Large hands and feet
-Internal ducts are male, rudimentary testes fail to produce sperm
-Feminine development NOT supressed
-Enlarged breasts are common, rounded hips

Question 15

Turner Syndrome (45, X)

Answer 15

-Only one sex chromosome present (X)

-Phenotypically female
-Female external genitalia and internal ducts
-Ovaries are rudimentary (undeveloped)
-Underdeveloped breasts
-Short stature
-Cognitive impairment

Question 16

Trisomy X, Triplo-X syndrome:

47,XXX syndrome

Answer 16

-Three X chromsomes
-Normal set of autosomes
-Results in female differentiation
-Sometimes women are perfectly normal
-Sometimes, underdeveloped secondary sex characteristics occur
-Sterility and mental retardation

Question 17

47, XYY Condition, Known as Jacob’s Syndrome

Answer 17

-Only consistently shared characteristic–males are over six feet tall
-Subnormal intelligence
-Personality disorders

Question 18

What is gonadal primordia?

Answer 18

The tissues that will form the gonad

-By the fifth week of gestation, a pair of gonadal (genital) ridges associated with each embryonic kidney

-Gonadal phenotype is sexually indifferent
-Primordial germ cells migrate to ridges

Question 19

Bipotential gonads

Answer 19

Gonadal ridges can form either ovaries or testes

Question 20

Y chromsome has at least ____ genes

Answer 20

50

-Fewer genes than the X chromosome, which has 100 genes

Question 21

What are PARs: Pseudoautosomal Region

Answer 21

-Present on both ends of Y chromosome
-Share homology with regions on X chromosome
-Synapse and recombine with X during meiosis

Question 22

What are MSY and SRY Regions?

Answer 22

Pairing region critical to segregration of X and Y chromosomes during male gametogenesis

Question 23

What is the MSY region?

Answer 23

Male-specific region of the y chromosome
-Nonerecombining region of the Y chromosome

Question 24

What is the SRY region?

Answer 24

Sex-determining region of the Y
-Located adjacent to PAR of the short arm of Y chromosome
-Controls male development
-Encodes protein: Testis-determining factor (TDF)

Question 25

What is the TDF?

Answer 25

Testis -determining factor
-At 6-8 weeks of development, SRY gene becomes active in XY embryos
-Encodes protein that triggers testes formation

Question 26

What is dosage compensation?

Answer 26

-It is a genetic mechanism
-It balances the dose of X chromosome gene expression in males and females
-Prevents excessive expression of X-linked genes in humans and mammals

Question 27

Barr bodies (Sex chromatin bodies)

Answer 27

-Genetic mechanism compensates for X dosage disparities
-Inactive X chromosome, highly condensed
-Darkly stained bodies in interphase nerve cell observed: Barr bodies
-Random inactivation
-Occurs early in embryonic development

Question 28

X inactivation

Answer 28

explains dosage compensation
-Follows the N - 1 rule (N is equal to the total number of X chromosomes

Question 29

The Lyon Hypothesis

Answer 29

The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated early in embryonic development in mammals.

-Inactivation of X chromsome is random
-Occurs in somatic cells at early stages of embryonic development
-All descendant cells have same X-inactivation

Question 30

Why does X-inactivation not affect syndromes like Turner and Klinefelter?

Answer 30

-Chromosome inactivation not in early stages of development fir cells destined for gonadal tissues

-Not all X chromosomes forming Barr bodies are inactivated
-15% escape inactivation

Question 31

G6PD

Answer 31

Glucose-6-phosphate dehydrogenase

-Synthesis of this enzyme is controlled by X-Linked gene
-Experiment with G6PD mutants provides strong support of Lyon hypothesis

Remember, random permanent inactivation of X chromsome

Question 32

Xic: X inactivation center

Answer 32

-Active only on inactive X
-Has X-inactive specific transcript (XIST) gene critical for X-inactivation
-Two noncoding genes in Xic locus
-Tsix and Xite play important roles in X chromsome inactivation

Question 33

Sex-determination in drosophila

Answer 33

Ratio of X chromosomes to number of haploid set of autosomes

-Y doesnt determine sex
Normal females have 2X;2A
Triploid females (3X:3A) –> metafemale

Question 34

Metafemale

Answer 34

low viability Drosophila fruit fly with a female phenotype in which the ratio of X chromosomes to sets of autosomes (A) exceeds 1.0.

Question 35

Dosage compensation in Drosophila

Answer 35

Drosophila females have two copies of X-linked genes
X inactivation not observed in Drosophila
Male X-linked genes transcribed at twice the rate of females

Question 36

C. elegans (Major model organism

Answer 36

Nematode worm Caenorhabditis elegans has two sexual phenotypes

Males have only testes

Hermaphrodites have both testes and ovaries

Question 37

Self-fertilization

Answer 37

Eggs are fertilized by stored sperm

Vast majority prodiced are hermaphrodites

Less than 1% of offspring are males

As adults, makes can mate with hermaphrodites
This will produce half male and half hermaphrodite offspring

Question 38

TSD: Temperature-Dependent Sex determination

Answer 38

-Controls sex determination in reptiles
-Three different patterns of temperature sex determination in reptiles
-Crocs, most turtles, and some lizards