Sex Determination

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Sex Determination in Humans

Autosome- a chromosome other than a sex chromosome

Sex Chromosome- chromosome determining the individuals sex

Gamete- Has just 1 of each chromosome

The Sex Chromosomes

PAR stands for Pseudo- Autosomal Region. these regions are the regions of sequence similarity where X and Y pair in meiosis.

The presence of SRY directs male sexual development

Sex determination starts in early development ā†’ Gonad Dependent

  • Expression of the SRY on males results in development of the testes and production of testosterone. SRY is necessary but not sufficient for male sexual development (aka expressing SRY is not enough alone)

  • No SRY results in the development of the ovary, complex expression patterns of hormones like estrogen and usually female secondary characteristics. Absence of SRY is necessary but not sufficient for sexual development (aka not expressing SRY is not enough lone)

SRY and Male Development

SRY expressedā†’ turns on SOX9ā†’ SOX9 expression induces testes developmentā†’ Testes development allows testosterone to be produced in necessary amounts for male characteristics to developā†’ Testosterone binds to Androgen Receptorā†’ complex enters nucleus to bind to DNA and changes expression

  • There is genetic variation in genes determining sex determination and sexual development, leading to variation on primary and secondary characteristics

    • No functional receptor the body does not respond to androgens (internal intersex +external female characteristics)

    • Accumulation of androgen leads to differences in development (mix of male, female, and intersex)

    • Testosterone canā€™t be converted to DHT leading to differences in development (intersex characteristics)

This form of development is responsible for sexual characteristic development ā¬‡

Dosage Compensation

Genes outside of PARs have different dosage (amounts of product)

  • Dosage Compensation- Dosage compensation is the process by which organisms equalize the expression of genes between members of different biological sexes

  • XX- condensed and decondensed similarly, one x will be turned off by being condensedā†’ ā€œActive Xā€ and ā€œInactive Xā€

Mosaicism

(example in Tortoiseshell cats)

Sex Determination: Many Types

1) Genetic Sex Determination

  • Inherited sex chromosomes that result in different morphology between sexes

    • XX/XY ā† Male

    • ZZ/ZW ā† Female (bird)

    • XX/X_ ā† insect (male) (fertilized egg= female; unfertilized egg= male)

  • 2) Enviornmental Sex Determination

    • factors in the environment result in affects on the genes that code for sex (they do not have sex chromosomes)

3) Hermaphrodites

  • Ex: clown fish

  • organism is able to make both eggs and sperm

Sex Dimorphism

Some species have trait differences between males and females in addition to primary sexual characteristics. This is called sexual dimorphism.

Two Types of dimorphic traits:

  • Sex Influenced- A trait whose expression is conditioned or influenced by the sex of the individual

  • Sex Limited- A trait that is expressed in one sex even though both sexes carry the allelesā†’ not sex Chromosome linked

Sex Linkage

mode if inheritance resulting form genes being located on the sex chromosome

  • sex chromosome= possibility of sex linkage

  • no sex chromosomes= no sex linkage

Sex Chromosomes

  • Y-linked genes (and traits) are called Holandric very few of these little genetic information

  • sex linkage is mostly inherited on the X-chromosome

    • due to hemizygosity, sex linked traits (like colorblindness) is more likely to affect males

    • affected females will have to have an affected father. the mother will either be colorblind or a carrier

    • when a the sex of the carrier organism is swapped and the offspring that carries it percentage goes from 50% to 100% (half carrier to all affected in some way) thats a sign of a sex linked trait in mendelian genetics

Hemizygosity- when there is only one copy of an allele in a diploid cell

  • modes of inheritance can include autosomal recessive and autosomal dominant

  • key clues- (XY children) do no inherit the trait from fathers (XY parent)- the same as for the X-linked recessive

For the Exam

  • SI Sessions: Mondays + Wednesday from 5-6:30

  • predict phenotypic ratio , determine degrees of freedom \, concepts, terminology, and synthesis