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Chapter 6 - Matters of Sex

Section 4 - X Inactivation

Section 5 - Parent-of-Origin Effect

X-Inactivation

  • Early in female development, the maternal or paternal X chromosome is turned off in each cell. So females exhibit traits from mom in some cells and traits from dad in other cells.
  • X inactivation compensates for differences between males in females in the number of gene copies on the X chromosome.
  • The XIST gene encodes RNA that inactivates these genes.
  • The inactivated X takes up stain and appears as a Barr body. The hypothesis proposed that the Barr body was the inactive X chromosome
  • A female may express a sex-linked trait if the mutant allele is on the active X in affected tissues.

Barr Body Picture

Mosaicism

X-Inactivation

X-Inactivation

Examples

X-Inactivation Examples

  • Anhidrotic ectodermal dysplasia - skin lack sweat glands in a mosaic pattern
  • Women who are carriers of hemophilia have a less than normal blood clot time but this may reduce risk of cardiovascular disease
  • Calico cats - black, orange, and white patches because of epistasis that shuts off pigment synthesis
  • tortoiseshell cats - black and orange
  • X-inactivation can be used to detect Lesch-Nyhan syndrome -Mutation defective HGPRT an enzyme
  • Hair can be taken from a suspected carrier at different parts of the head to see of hairs contain HGPRT

Genomic Imprinting

  • The disease may be more severe or different depending upon which parent transmitted the gene

Genomic Imprinting

Angelman Syndrome & Prader-Willi Syndrome

Angelman & Prader-Willi

  • Small deletion in the same region of chromosome 15
  • Prader-Willi Syndrome-is obese, small hands and feet, eats uncontrollably and does not mature sexually, fathers chromosome has a deletion
  • Angelman Syndrome – is mental impairment, extended tongue, large jaw and poor muscle coordination and convulsions that make the arms flap. Mothers chromosome has a deletion

Additional Resources

Science in Pajamas

Other Videos

Additional Resources

Science in Pajamas

Sex-Linked Traits

Birds Sex-Linked Dihybrid Cross

Science in Pajamas

Sex-Linked Traits

Birds Sex-Linked Dihybrid Cross

Other Videos

AK Lectures - Sex Chromosomes and Sex-Linked Traits

FuseSchool - Sex Determination

Amoeba Sisters - Punnett Squares and Sex-Linked Traits

UCD Medicine - X-Linked Recessive

Khan Academy - Example Punnett Square for Sex-Linked Recessive Trait

Catalyst University - X-Chromosome Inactivation, Barr Bodies, and Calico Cats

Bozeman Science - X Inactivation

Dirty Medicine - Prader-Willi vs Angelman Syndrome (Imprinting)

Other Videos

AKL - Sex Chromosomes and Sex-Linked Traits

FS - Sex Determination

AS - Punnett Squares and Sex-Linked Traits

UCDM - X-Linked Recessive

KA - Example Punnett Square for Sex-Linked Recessive Trait

KA - Example Punnett Square

CU - X Chromosome Inactivation, Barr Bodies, and Calico Cats

CU - X-Chromosome Inactivation

BS - X Inactivation

DM - Prader-Willi vs Angelman Syndrome (Imprinting)

DM - Prader-Willi vs Angelman

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