SEX DETERMINATION & SEX LINKAGE

Sex determination systems

1. genotypic sex termination

2. environmental sex determination

Genotypic sex determination

due to specific DNA sequences

• genic balance

• chromosome determination

• mating-type determination

Genic balance

amount of gene products

Chromosome determination

presence or absence of gene products

Mating-type determination

which alleles are present and are therefore expressed

Environmental sex determination

signals from environment

• ex: day length, temperature, social clues

XX

• wild-type (normal) female

• causes gonads to develop into ovaries and produce eggs

XY

• wild-type (normal) male

• causes gonads to develop into testes and produce sperm

Y chromosome

• pseudoautosomal region (PAR)

  • synapses with X chromosome during meiosis

• male-specific region of the Y (MSY)

  • euchromatin containing SRY locus

Sex-linked genes

• genes present on X & Y chromosomes

  • many linked genes don’t influence sex development

  • most sex-linked genes are X-linked because Y doesn’t contain many genes

X-linked recessive inheritance

• traits more frequently observed in males

  • males are hemizygous for genes on the X chromosome

  • no second allele to compensate for recessive allele

• females express the trait when they are homozygous for the allele

  • female heterozygotes are carriers

Barr bodies

inactive X chromosomes

• blastocyst stage → X is deactivated in XX mammals

• barr body is made of heterochromatin

• form of imprinting

  • monoalleleic expression allows for gene dosage compensation

  • equal amounts of X-linked gene expression between males and females

• number of barr bodies in somatic cells = all X chromosomes except one are deactivated

Lyonization

how barr bodies are formed

• XIST gene on X chromosome

  • weakly expressed

• one XISt gene copy is silened

  • becomes the active X chromosome

  • other X continues to express XIST and becomes the barr body

• XIST encodes a long non-coding functional RNA (lncRNA)

  • coats chromosome and recruits epigenetic machinery (DNA methyltransferases and HDAC) to pack up the chromosome

Mosaicism

• mixed phenotype resulting from different genotypes or gene expression levels in the cells of a multicellular organism

• in this case, due to monoallelic expression resulting from random barr body lyonization

  • diff monoallellic expression in diff cells of the body

  • some cells have paternally derived X chromosome silenced and only express maternally dervied X chromosome genes and vice versa

• all mitotic descendants of a cell inherit the parent cell’s inactivation pattern