Sex Determination Flashcards
How is the sex of birds, mammals and some insects determined?
By the presence of sex chromosomes
What is the definition of a sex chromosome?
A type of chromosome that participates in sex determine e.g. male XY, female XX. They determine whether an individual is male or female
What is the SRY-Gene?
In most mammals the SRY-gene on the Y chromosome determines development of male characteristics. Heterogametic (XY) males lack most of the corresponding homologous alleles on the shorter (Y) chromosome
What does the lack of the corresponding homologous alleles on the shorter chromosome result in?
It results in sex-linked patterns of inheritance as seen with carrier females (X^BX^b) and affected males (X^bY)
B=normal division
b=colour blind
In homogametic females (XX) how are one of the two X chromosomes present in each cell?
It is randomly inactivated at an early stage of development
What is X chromosome inactivation?
A process by which most of one X chromosome is inactivated
What does X chromosome inactivation prevent?
A double dose of gene products which could be harmful to cells
What are X carriers less likely to be affected by?
Any deleterious mutations on these X chromosomes
What happens because 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
How can X-chromosome inactivation explain tortoise shell cats?
In cats one of several genes controlling fur colour is located on the X chromosome. The gene has two versions, or alleles. One form of the gene codes for orange fur (X^B)and the other form codes for black fur (X^b) In any given cell of a heterozygous female could end up as either of the following:
uX^b
X^Bu
What are hermaphrodites?
They are species that have functioning male and female reproductive organs in each individual. They produce both male and female gametes and usually have a partner with which to exchange gametes
What is the benefit to the individual organism who is a hermphrodite?
It 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
What determines sex and sex ratio for species that are not hermaphrodites?
Environmental factors rather than genetic factors
How is environmental sex determination in reptiles controlled?
It is controlled by environmental temperature of egg incubation
Sex can change within individuals of some species as a result of what?
Size, competition or parasitic infection
