Lecture 20-21 - Sex Determination and Sex-related Inheritance Flashcards
Define heterogametic and homogametic sex.
heterogametic sex: the sex which has two types of sex chromosome
homogametic sex: the sex which has one type of sex chromosome
Mention all 5 mechanisms of sex determination.
Environment, genes, haploidy/diploidy, ratio of autosomes to sex chromosomes, a sex chromosome
In turtle, the mechanism of sex determination is dependent on temperature. In higher temperatures, production of the enzyme _______ increases, leading to an ________ in oestrogen levels and _________ in testosterone levels. Hence, it is more likely to have a _______ offspring.
aromatase, increase, decrease, female
A particular genotype may change the expected sexual phenotype. Describe two examples of this type of sex determination mechanism.
In Drosophila, the tra normal gene is required for normal female development. The presence of the tra mutant gene in the locus can reverse the sex of a female if homozygous.
In humans, the androgen insensitivity locus in the X chromosome can also affect sex. If males have a copy of the AR mutant gene, the subject would appear as female.
Explain how does males (XY) individual experiencing Androgen Insensitivity possess a female phenotypic appearance.
XY individuals still produce testosterone, which is necessary for development of male sexual secondary characteristics. However, the cells are insensitive to its due to the defect in the androgen (testosterone) receptor, hence leading to the failure of the development of those characteristics.
Define hemizygous.
possessing only one allele of a gene, rather than the normal two
Autosome No. = A
X Chromosome No. = X
A/X<1/2 –> __________
A/X>1 —–> __________
male, female
In Drosophila, the _______ is needed for fertility.
Y chromosome
Explain how the ratio between the X and autosomal chromosome can affect the sex in Drosophila
Gene products of the X and autosomal chromosome have the tendency to dimerize together. When the ratio between X and A is equal is equal or larger than 1, the leftover products of X forms its own dimer, which is required for the transcription of the Sxl gene. Sxl protein is required for the normal function of tra gene which is expressed only in females.
How does the presence of a Y chromosome in humans lead to the male phenotype?
Up to 6 weeks of gestation, there is a undifferentiated tissue mass. If the Y chromosome is present, the TDF (testis determining factor) will specialize the tissue to form the testis. he testes then lead the production of anti-Mullerian hormone (which prevents development of the Mullerian duct to the female reproductive system). It also produces testosterone, which is responsible for the development of primary and secondary male sexual characteristics
TDF (Testis Determining Factor) is a form of transcription factor which affects other genes to promote/inhibit their expression. Mention two examples of gene that is affected by the TDF.
DAX1 - anti-testis factor (deactivated by TDF)
SOX9 - activation leads tot he differentiation of cells, leading to the formation of the seminiferous tubules of the testis
Pseudo-hermaphroditism is a condition in which children appear female-like until puberty, at which the subject changes to male. This is caused by a deficiency in _________, which leads to reduced levels of ________ which is involved in the development of the external penis and scrotum.
5 α reductase, dihydrotestosterone
Explain how then genetic sequence of the X and Y chromosomes can lead to the translocation of the TDF gene between the non-homologous chromosomes.
Even though the X and Y chromosomes are totally non-homologous (X is a metacentric, Y is acrocentric, differing lengths), they have the same pseudoautosomal region at the tips. Since the TDF is located just below the pseudoautosomal region, it increases the risk of translocation of the TDF locus between the chromosomes.
Give examples of organisms which follows the ZW sex-determination system.
Birds, some reptiles and fish, butterflies and moths
What is a reciprocal cross? What is its function?
Reciprocal crosses involve two crosses done, where in one cross, a male expressing the trait of interest will be crossed with a female not expressing the trait. In the other, a female expressing the trait of interest will be crossed with a male not expressing the trait.
It is designed to test the role of parental sex on a given inheritance pattern (possible sex-linked genes).
