2.3 c) Sex Determination Flashcards
The sex of which types of animals are determined by sex chromosomes?
birds, mammals and some insects
How are male characteristics determined in most mammals?
the SRY gene on the Y chromosome
Since males are ___ (XY), they lack most of the corresponding ___ ___ on the shorter Y chromosome.
heterogametic, homologous alleles
The fact that males are heterogametic (XY) is the reason behind ___-linked patterns of ___, as seen with ___ females and ___ males.
sex-linked, inheritance, carrier, affected
Explain carrier females if you please
Carrier females have X^B and X^b chromosomes. The B allele is dominant, and is a normal (non-diseased) allele. It takes precedence over the recessive, diseased b allele.
The possible females are X^B X^B (unaffected), X^B X^b (carrier) and X^b X^b affected
Explain affected males
Affected males have X^b and Y chromosomes. The b allele is diseased/affected, and it affects the male individual since there is no normal allele which is dominant over it. The Y chromosome cannot carry a copy of this gene
The possible males are X^B Y (unaffected) and X^b Y (affected)
Males have a greater chance of being affected by certain recessive conditions (such as… ) because…
(colour blindness, haemophilia), one copy of the recessive allele, whereas females would need two.
*the recessive allele can only appear on X chromosomes
Females are ___ (XX).
One of the two X chromosomes are ___ ___ at an early stage of ___.
homogametic.
randomly deactivated, development
X chromosome deactivation is a process by which…
most of one X chromosome is deactivated
X chromosome inactivation prevents…
a double-dose of gene products, which can be harmful
Carriers of deleterious mutations are likely to be unaffected by the condition because…
Carriers are always (male/female).
the X chromosome that is inactivated in each cell is random, so only half of the cells in a tissue will be affected, and the other half will have a working copy of the gene (half b half B).
Female
Why can’t males be carriers of deleterious X chromosome mutations?
because they only have one X chromosome - so if they have it, they will be affected, and if they don’t they will be unaffected.
Carriers have it but are unaffected due to the presence of a dominant, normal allele on the other X chromosome
Many organisms (usually vertebrates) are ___.
hermaphroditic
What are hermaphrodites?
organisms that have functioning male and female reproductive organs (and produce both types of gamete)
Hermaphrodites produce both ___ and ___ gametes, and usually have a ___ which which to exchange ___.
male, female, partner, gametes
What is the main benefit of being a hermaphrodite?
if the chance of meeting a partner is low, there is no requirement of that individual to be of the opposite sex
Sometimes sex determination and sex ratio is under ___ control, rather than ___ control.
environmental, genetic
Environmental sex determination in reptiles is determined by
the temperature of egg incubation
Sex can change within individuals of some species as a result of s___, c___ or p___ ___.
size, competition, parasitic infection
In some species the sex ratio of offspring can be adjusted in response to ___ ___.
resource availability (males typically need more food)
Explain why carrier females are usually not affected by conditions like colourblindness even though they carry a copy of the gene. (2)
because random X chromosome inactivation occurs, meaning that half of the cells in the tissue will have a working copy of the gene
What are the three factors that can trigger a sex change?
Size, competition and parasitic infection
