Sex Determination (U2, c3/4)

Question 1

What is a hermaphroditic organism?

Answer 1

One that has both male and female reproductive organs (usually an invertebrate).

Question 2

What are some examples of hermaphrodites?

Answer 2

Earthworms, most land molluscs.

Question 3

What three environmental factors affecting sex ratio?

Answer 3

Temperature
Parasites
Competition

Question 4

What are the two ways in which temperature can affect sex determination (and an example for each way)?

Answer 4

1) There is a trend in the effect of temperature on the sex ratio, e.g. red-headed rock lizard
2) Extreme temperatures cause the production of one sex, while intermediate temperature causes the production of the other, e.g. Hermann’s tortoise, Nile crocodile.

Question 5

What are two ways in which infection by the bacteria Wolbachia influence sex ratio?

Answer 5

1) Relying on infection to cause sex determination

2) By already having a chromosomal sex, but the bacteria killing or feminising the males

Question 6

What kind of organism does Wolbachia mostly infect and determine the sex of?

Answer 6

Arthropods, particularly insects.

Question 7

What effect does Toxoplasma gondii have on humans?

Answer 7

Cause a higher proportion of male offspring in mothers (and may also affect behaviour).

Question 8

Some species can actively change the sex ratio of their offspring - why would they do this?

Answer 8

To reduce competition! Often, males disperse away from home faster, so switching to produce more males can have this effect.

Question 9

What two things may cause female organisms to control sex ratio (increase proportion of males) to reduce competition?

Answer 9

1) Detecting urine from other females, which indicates the presence of competition (Lesser Mouse Lemur).
2) Being in a poor quality habitat (Seychelles Warbler).

Question 10

What are three common situations in which an organism might change sex in its life?

Answer 10

Size
Competition
Parasitic infection

Question 11

What does the SRY Gene refer to / stand for?

Answer 11

Sex-determining Region of Y-chromosome.

Question 12

What does SRY produce to switch on typically male genes?

Answer 12

A transcription factor.

Question 13

What has happened to SRY in XX males?

Answer 13

SRY is translocated to X-chromosome.

Question 14

What has happened to SRY in XY females?

Answer 14

SRY is deleted from Y-chromosome.

Question 15

Are X and Y chromosomes homologous?

Answer 15

Yes!

Question 16

Why are X and Y chromosomes considered homologous?

Answer 16

They pair up during meiosis I, like other homologous chromosomes, due to a small area of homology around its centromere.

Question 17

What is sex linkage?

Answer 17

A pattern of inheritance referring to most males being heterogametic - the Y-chromosome lacks many genes found on its homologous X-chromosome.

Question 18

What is a carrier female?

Answer 18

A female who possesses a recessive gene on one the X-chromosome, but it is masked by the dominant gene on their other X-chromosome.

Question 19

One X-chromosome is randomly partially inactivated in females. What regions of the chromosome are inactivated?

Answer 19

Regions that are lacking on the Y-chromosome.

Question 20

Why is X-chromosome inactivation beneficial?

Answer 20

It prevents a double-dose of gene products.

Question 21

Why can an XX female be described as a mosaic of cells?

Answer 21

Each cell has different X-chromosomes active and inactive.