Exam 2: Sex Determination and Sex Chromosomes

Question 1

What is the heterogametic sex in humans?

Answer 1

The male (provides both an X and a Y)

Question 2

Dioecious

Answer 2

Have only male or female reproductive organs

Question 3

Monoecious

Answer 3

Have both male and female reproductive organs

– Common in plants and animals
– Can produce egg and sperm

Question 4

What are the two sexes of C. elegans and the percentage of each after a hermaphroditic self-fertilization?

Answer 4

Hermaphroditic (XX)- 99%

Male (XO)- 1%

Question 5

C. elegans, grasshopper, butterfly, etc. XO mode of sex determination

Answer 5

Males are XO. Only about half of the sperm produced by a male have an X chromosome. The other half of the sperm produced by a male lack a sex chromosome. Therefore, in a mating cross one would expect half of the progeny to be males and the other half hermaphrodites.

Question 6

ZW Mode of sex determination

Answer 6

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

Ex: chickens, some amphibians and snakes, and fish

Question 7

Homogametous

Answer 7

Producing gametes with like chromosomes

– Females with two X chromosomes can only produce gametes that contain an X
– Males with two Z chromosomes can only produce gametes that contain a Z

Question 8

Heterogametous

Answer 8

Producing gametes with different chromosomes

– XY Males can produce produce gametes that contain a X or gametes that contain a Y

Question 9

What is genic sex determination?

Answer 9

No obvious differences in the chromosomes of males and females.

– Genes that determine sex are present on
autosomes.
– Example: some plants, fungi, and protozoan

Question 10

Explain sex determination in Drosophila

Answer 10

-Achieved by a balance of female determinants on the X and male determinants on the autosomes

– Ratio of X chromosomes to number of haploid sets of autosomes is predictive of sex

-Metafemale X>A
-Normal females (2X:2A)
–Triploid females (3X:3A)
-Intersex (3X: 4A)
-Male (X/XY:2A)
-Metamale (XY:3A)

Question 11

Environmental sex determination examples

Answer 11

-Position on the stack of slipper limpets, the marine mollusk Crepidula fornicata (ex: females on bottom, males on top)

-Temperature in alligators, turtles, some lizards

Question 12

True or False: X and Y are homologous chromosomes

Answer 12

False- X and Y are pseudo homologues/heteromorphic chromosomes because they pair during meiosis despite not being the same and will synapse and travel as a tetrad during meiosis 1 but have no crossing over.

Question 13

Sex is determined by genes on chromosomes

Answer 13

PAR – Pseudoautosomal Region

Critical to segregation of X and Y chromosomes during male gametogenesis.

SRY: Sex-determining Region Y

Controls male development. (If found on X chromosome, would present as male.)

MSY: Male-Specific Y,

Non-recombining region of the Y

Question 14

Non-disjunction in meiosis 1 vs meiosis 2 causes what kind of aneuploidy?

Answer 14

2 trisomic 2 monosomic

1 trisomic 1 monosomic 2 normal diploid

Question 15

What is dosage compensation?

Answer 15

Dosage compensation is a mechanism to balance dose of X chromosome gene expression in males and females. This prevents excessive expression of X-linked genes in humans and other mammals.

Question 16

Why is dosage compensation important?

Answer 16

A delicate balance of gene products is necessary to maintain normal development of an organism.

Question 17

XO in humans presents as

Answer 17

Turner Syndrome

Question 18

XXY, XXYY, XXXY, XXXXY

Answer 18

Klinefelter Syndrome

Question 19

XXX

Answer 19

Triple X

Question 20

XXXX+

Answer 20

Poly-X Female

Question 21

If the ratio of X chromosomes to autosomal chromosomes on a a Drosophila melanogaster exceeds unity, what does it tell you?

Answer 21

A inviable female is produced (metafemale).

Question 22

What is difference between heterochromatin and euchromatin?

Answer 22

Heterochromatin is defined as the area of the chromosome which is darkly stained with a DNA-specific stain and is in comparatively condensed form. Euchromatin is defined as the area of the chromosome which is lightly stained and rich in genes that actively participate in the transcription process.

Question 23

Lyon Hypothesis

Answer 23

In 1961, Mary Lyon described the concept of X-chromosome inactivation, which is still referred to as ‘Lyonization,’ and made possible an understanding of inheritance of X-linked disorders such as hemophilia, Duchenne muscular dystrophy, and fragile X syndrome.

Question 24

Why are all calico cats female?

Answer 24

The gene for coat color is carried on the X chromosome. To be calico, a cat must have two copies that are both activated for different parts of the coat.

Question 25

True or False: A normal human female heterozygous for two X-linked alleles will express only one of those alleles in any given cell in her body.

Answer 25

True

Question 26

Barr Bodies

Answer 26

Barr Bodies are condensed, randomly inactivated X chromosomes that are typically found exclusively in female mammals.

-They are a genetic mechanism that compensates for X dosage disparities.

– Darkly stained body toward the periphery of the nucleus

-Barr bodies occur early in embryonic development

Question 27

Explain mammalian X-inactivation

Answer 27

– Explains dosage compensation
– Follows N - 1 rule (N = total number of X chromosomes)

Question 28

Dosage Compensation in Drosophila

Answer 28

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

Question 29

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

Answer 29

Chromosome inactivation not in early stages of development for cells destined for gonadal tissue

– Not all X chromosomes forming Barr bodies are inactivated, around 15% escape inactivation

Question 30

How many Barr bodies would you anticipate in a female with Turner syndrome?

Answer 30

0

Question 31

How many Barr bodies would you anticipate in a male with Klinefelter syndrome?

Answer 31

1

Question 32

SRY

Answer 32

SRY (which stands for sex-determining region Y gene) is found on the Y chromosome. In the cell, it binds to other DNA and in doing so distorts it dramatically out of shape. This alters the properties of the DNA and likely alters the expression of a number of genes, leading to testis formation. Most XX men who lack a Y chromosome do still have a copy of the SRY region on one of their X chromosomes. This copy accounts for their maleness. However, because the remainder of the Y chromosome is missing they frequently do not develop secondary sexual characteristics in the usual way.

Question 33

Hemizygous

Answer 33

Hemizygous describes the condition of having only one copy of a gene or chromosome segment, often used to describe genes on the X chromosome in males due to their possession of only one X chromosome.