II. Inheritance patterns on Sex Chromosomes

A. Sex Chromosomes B. X- linked Genes C. X Inactivation

Sex Chromosomes

• 1 pair

• Act homologous during meiosis 

• Contain sex determining genes AND genes unrelated to sex determination

• Sex linked genes (X-linked or Y-linked ): genes on either sex chromosome

• Males typically XY -> heterogametic

  • Half of sperm get an X. half get a Y

  • Y chromosome has genes for male development 

• Females typically XX -> heterogametic

  • All eggs have an X

  • Female phenotype due to absence of Y

• Sex determination varies 

X- linked Genes

X chromosomes- many genes, many required

X linked traits- controlled by genes on x chromosome

Unique inheritance pattern

Inheritance of sex chromosome

XX x XY

Gametes: X or X x X or Y

50% XX, 50% XY

• Males

  • Just one x chromosome- every allele expressed

  • Hemizygous: having just one allele for a gene

  • Recessive alleles never masked

X Inactivation

• Genes on x chromosome expressed in females and males

• Females- 2 copies

• Males- 1 copy

• Females makes 2x proteins from X-linked genes?

• Barr bodies 

  • 1, X chromosome inactivated

  • Dense structure dna +  proteins along inside of nuclear envelope

  • Involves modification of DNA and histone proteins (often methylation)

  • Inactivation is random- different x in different cells

• Tortoise Shell Cat

  • Some fur genes are x- linked

  • Heterozygous females. -> mosaic expression pattern 

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