Lecture 11: Sex chromosomes and dosage competition

Question 1

Who first identified the X chromosome in wasp sperm?

Who later discovered the XY pairing in beetle chromosomes?

Answer 1

Hermann Henking (1891)

Nettie Stevens (1905)

Question 2

sex chromosomes have three other names, what are they?

Answer 2

1. Heterosomes
2. Allosomes
3. Gonosomes

Question 3

What is a hermaphrodite?

Answer 3

Species that have both male and female reproductive organs.

• some hermaphroditic species can self-fertilize while others require partners
• Some hermaphrodites are males or females at different times in their life cycle (sex reversal in clownfish, bluehead wrasse)

Question 4

Often in animals, how is sex determined?

Answer 4

Sex determination is associated with a pair of “sex” chromosomes that genetically determine sex

Question 5

How are three ways in which sex is environmentally determined in some species?

Answer 5

1. Temperature during embryonic development (alligators, tuatara, sea turtles)
2. Social organization (sex-ration) - most reef fish can change sex during their lifetime
3. Some species are parthenogenic: females lay fertile eggs - no male required (walking stick insects, some fish/lizards - sharks in captivity)

Question 6

What is monoecious?

Are monoecious plants bisexual or hermaphroditic?

Provide an example.

Answer 6

Houses both sexes (male and female reproductive organs) on the same plant but are separated

-two houses

• bisexual (NOT hermaphrodite)
• eg Banana

Question 7

What is a hermaphroditic plant vs a bisexual plant?

Answer 7

Hermaphrodite flowers have male and female reproductive organs on the same flower

Bisexual (monoecious) plants have both male and female reproductive organs, but they are on DIFFERENT flowers

Question 8

What is a dioecious plant?

Answer 8

Has only male or female reproductive parts.

Eg holly tree, cannabis

Question 9

What is interesting about sex determination in clown fish (aka anemone fish)

Answer 9

Able to change sex when exposed to changing conditions

• If dominant female in a group dies, the dominant male becomes the female and the next male in line becomes the dominant male

Question 10

Define simultaneous hermaphroditism

Answer 10

Individuals possess fully functional male and female gonads

Question 11

Define sequential hermaphroditism

(give an example of an organism that displays this)

Answer 11

Clownfish:

• fish change sex sometime during its life (either from male to female protandry or from female to male protogyny)

Question 12

What is protogyny?

Protandy?

Answer 12

• Protogyny: Female to male
• Protandry: Male to female

Question 13

In some, particularly protandrous (M->F) species, sex change is _______ dependent and eventually every fish in the population will change sex

Answer 13

In some, particularly protandrous (M->F) species, sex change is size-dependent and eventually, every fish in the population will change sex

Question 14

How is sex determined in alligators?

Answer 14

• Sex is fully determined by the time of hatching
• temperatures of 30C produce all females
• Temp of 34 produce all males
• Natural ration is 5F:1M

Question 15

What is parthenogenesis?

Answer 15

Where a female can lay fertile eggs - don’t require a male

Question 16

X-Y system is based on _________ distinct chromosomes

Answer 16

X-Y system is based on morphologically distinct chromosomes

Question 17

In the XX-XY system, what is homogametic and heterogametic?

Answer 17

Homogametic - XX = female

Heterogametic - XY = male

Question 18

Mammals use heteromorphic sex chromosomes. What does this mean?

Answer 18

The chromosomes look different (ie morphilogically distinct)

Question 19

What are 3 notable mammal species that do not conform to the XY system?

Answer 19

Spiny rat and mole vole have lost the Y chromosome and the Sry gene

• Both males and females are X0

Akodan south American grass mice - females can be either XX or XY

Question 20

How is sex determined in the X0 system?

Answer 20

Females will have 22 chromosomes + two X chromosomes (22+XX)

Males will have 22 + X (only one X)

Question 21

How is sex determined in the ZW system?

Answer 21

ZZ = male

ZW=Female

Males are homogametic while females are heterogametic (opposite of XY system)

Question 22

How is sex determined in a ploidy system (haplo-diploid system)?

Answer 22

• Females are diploid
• Males are haploid (monoploid)
  
  • produced from unfertilized haploid eggs

Question 23

What is the default sex in humans?

Answer 23

Female (XX)

X chromosomes have most of the genetic information

Question 24

How are male humans produces?

Answer 24

Presence of Y chromosome

• Y chromosome has genes that encode ‘maleness’
• SRY/TDF - testis determining factor
• Gonad tissue is transformed from ovary into testes
• testosterone is produced
  
  • turns on other male-specific genes

Question 25

Follow human sex determination in the absence of a Y chromosome (5)

Answer 25

1. No Y chromosome
2. Gonads develop into ovaries
3. Ovaries produce estrogen (steroid hormone)
4. estrogen binds to Estrogen receptor - results in:
5. Female development

Question 26

Follow human sex determination in the presence of a Y chromosome (what is on the Y chromosome that results in Male)? (6)

Answer 26

1. Y chromosome
2. Has SRY gene (testis determining factor)
3. embryonic gonads develop into testis
4. testis produce steroid hormone testosterone
5. testosterone binds to testosterone receptor (AR = androgenous receptor)
6. Active AR triggers male development

Question 27

What are bipotential gonads?

Answer 27

Produced by embryo - have the potential to go in either direction (hence bipotential)

• ie can become male or female

Question 28

In the presence of SRY gene (on Y chromosome) the bipotential gonads develop into ________ cells and then into _________cells. This occurs at ~___ weeks after conception

Answer 28

In the presence of SRY gene (on Y chromosome) the bipotential gonads develop into sertoli cells and then into leydig cells

This occurs at ~5 weeks after conception

Question 29

The sertoli cells produce _____ (hormone) which results in a degeneration of the _____ duct (which normally develops into the uterus and fallopian tubes)

Answer 29

The sertoli cells produce AMH (anti-mullerian hormone) (hormone) which results in a degeneration of the Mullerian duct (which normally develops into the uterus and fallopian tubes)

Question 30

The _____ cells and _____ cells make up the testis which produce ______ (steroid hormone)

Answer 30

The sertoli cells and leydic cells make up the testis which produce testosterone (steroid hormone)

Question 31

Testosterone acts on the ______ duct to stimulate the development of the epididymis and vas deferens

• when does this occur?

Answer 31

Testosterone acts on the wolffian duct to stimulate the development of the epididymis and vas deferens

• ~8weeks post conception

Question 32

DHT (derived from testosterone) has what effect during development?

Answer 32

DHT = dihydrotestosterone

• stimulates the urogen and sinus to develop into the external genetilia in males

Question 33

How are sex-reversals caused?

Answer 33

Aberrant SRY loci

• SRY+ in XX = male genitilia
• SRY- in XY = female genitilia

Question 34

In which region do human x and y chromosomes pair and undergo crossover?

Answer 34

Psudoautosomal region