Sex Determination and Sex Chromosomes Flashcards
Genic sex determination
Single allele at gene locus determines sex of that individual
Ex- yeast (a or alpha mating types)
Genotypic sex determination
Combination of sex chromosomes determines the sex of that individual
Ex- fruit flies (ZW-female; ZZ-male), mammals (XX-female; XY-male)
Y determination mechanism
Presence of Y chromosome dictates male
Absence of Y chromosome dictates female (default)
Pseudoautosomal regions (PARs)
Portion of Y chromosome that aligns with X chromosome during synapsis
Male-specific region of the Y chromosome (MSY)
Euchromatin has 50-60 genes involved in sex determination
Sex-determining region of Y (SRY)
Becomes active at 6-8 weeks during development
Encodes Testes Determining Factor (TDF)
Testes Determining Factor (TDF)
Encoded by SRY
Controls testes development
Absence of this causes gonads to develop into ovaries
Barr bodies and gene dosage compensation
Silencing of 1 X chromosome in females allows for the same level of expression for genes on the X chromosome in males and females
Entire chromosome isn’t completely inactivated- some parts are a little bit active
of Barr bodies in somatic cells
Total # of X chromosomes - 1
Lyonization
1 X chromosome is changed from euchromatin into heterochromatin about 16 days after fertilization
Occurs randomly between 2 X chromosomes
Doesn’t have to be the same chromosome in each cell
All mitotic descendants of the cell inherit that same activation pattern
Ex- calico cats (some cells express black X and some express orange X)
X-linked recessive inheritance
Trait occurs more frequently in males (only have 1 X chromosome)
Females express the trait when they are homozygous for the allele
Female heterozygotes are carriers
X-linked dominant inheritance
Very few traits follow this pattern of inheritance
Affected mother: all sons will have trait and 50% of daughters will have trait
Affected father: all daughters will have trait and no sons will have trait
