2.3c Sex determination

Question 1

How is the sex determined in birds, insects and mammals?

Answer 1

The sex of birds, mammals and some insects is determined by the presence of sex chromosomes.

Question 2

How is the development of male characteristic’s determined

Answer 2

In most mammals the SRY gene on the Y chromosome determines the development of male characteristics.

Question 3

Sex linked patterns

Answer 3

Heterogametic (XY) males lack most of the corresponding homologous alleles on the shorter (Y) chromosome. This can result in sex-linked patterns of inheritance as seen with carrier females (XBXb) and affected males (XbY)

Question 4

Inactivation of X chromosome

Answer 4

In homogametic females (XX) one of the two X chromosomes present in each cell is randomly inactivated at an early stage of development. X chromosome inactivation is a process by which most of one X chromosome is inactivated.

X chromosome inactivation prevents a double dose of gene products, which could be harmful to cells.

Question 5

Carriers

Answer 5

Carriers are less likely to be affected by any deleterious mutations on these X chromosomes.

Question 6

X chromosome inactivated

Answer 6

As the X chromosome inactivated in each cell is random, half of the cells in any tissue will have a working copy of the gene in question.

Question 7

Hermaphrodites

Answer 7

Hermaphrodites are species that have functioning male and female reproductive organs in each individual.

Plants: They both produce both male and female gametes and usually have a partner with which to exchange gametes.

Question 8

Benefit of Unisexual plant

Answer 8

The benefit to the individual organism is that if the chance of encountering a partner is an uncommon event, there is no requirement for that partner to be of the opposite sex.

Question 9

Environmental sex determination

Answer 9

For other species, environmental rather than genetic factors determine sex and sex ratio.

For example, environmental sex determination in reptiles is controlled by environmental temperatures of egg incubation.

Question 10

Sex change

Answer 10

Sex can change within individuals of some species as a result of size, competition, parasitic infection or temperature.

In some species the sex ratio of offspring can be adjusted in response to resource availability.