Sex Determination and Sex-Linked Traits

Ch. 4

Genetic Basis of sex

Female: larger gametes (n) (egg in mammals)

Male: smaller gametes (n) (sperm in mammals)

Hermaphrodites

Either individual can make small/large gametes (invertebrates, plants, etc.)

Mechanisms of Sex Determination

• Environmental: temp and population/hormonal cues

• Genic

• Chromosomal: Haplo-diploid and differentiated sex chromosomes (XY, ZW, X0)

Environmental Sex Determination

Temperature (turtles, crocodiles, skinks): sex is based on the temperature the egg is incubated at (cold vs hot).

• Earliest mode of sex determination

• Concerns for climate change (skew sex to only warm temp sexes)

Genic Sex Determination

• Based on a gene

• No differentiated sex chromosomes (plants)

  • Sex is a trait like color, height, etc.

• Genes in the nucleus or the cytoplasm

Chromosomal Sex Determination

• Haplo-diploid sex determination

  • Diploids are female (fertilized)

  • Haploids are male (unfertilized)

  • Wasps, bees, ants

Chromosomal Sex determination cont.

• Differentiated sex chromosomes:

  • Mammals: female XX, male XY

  • Birds: female ZW, male ZZ

  • Grasshoppers: female XX, male XO

• **Always exceptions to every rule (common muntjac)

Environmental Sex determination cont.

Population/hormonal Cues (limpets, fish):

• Sequential hermaphroditism (Limpets produce both types of gametes at different times)

• Controls male/female ratio (In fish there is one male and the rest are female but the male dies, a female can transition to a male from hormonal cues)

Chromosomal Sex Determination Term: Heterogametic Sex

• The sex that can make two different gametes (one chromosome will lose genes over time, Y very small)

  • XY, males in mammals

  • ZW, females in birds

Pseudoautosomal Regions (both ends of chromosome)

This region helps the X and Y chromosomes pair up and separate correctly in meiosis (similar genetics helps to find each other)

Sex Chromosome Evolution: Sex Chromosome Degeneration

• The “unpaired” chromosome loses genes over time

• Humans: Y chromosome has no pair in meiosis and genes move off it and onto the X chromosome

• Started long ago in animals and is starting to happen in plants

SRY (Sex Determining Region Y) Gene on Y Chromosome Function

1. SRY protein (transcription factor) binds to DNA, affecting transcription of other genes (turns on genes)

2. Other gene expression → testes development → testosterone

3. Testosterone → phenotypic effects

About the Y Chromosome

• Y doesn’t carry many genes

• Most genes in males and females are the same

• It just depends on how they are regulated

How does testosterone lead to the male phenotype?

1. Testosterone binds to Androgen receptor in cell

2. Bound compound (testosterone and androgen) go into the nucleus and DNA

3. Affects transcription of other genes

Sex vs. Gender

Sex: Type of gamete produced, some secondary characteristics

Gender: social roles, self-identification, etc.

X Aneuploidy

• Turner Syndrome

• Poly X Females

• Klinefelter Syndrome

Aneuploidy

Not true ploidy, different from number of chromosomes expected to see

X Aneuploidy: Turner Syndrome

• XO

• Female phenotype

• Rare

• Normal, may be sterile

• Shows that one X is sufficient for development

X Aneuploidy: Poly-X Females

• XXX or higher

• Female phenotype

• Somewhat common

• Normal, taller, lower fertility

X Aneuploidy: Klinefelter Syndrome

• XXY (or more Xs)

• Male phenotype

• Somewhat common

• Shows that the Y chromosome (and SRY gene) determines male development

Y Aneuploidy

• XYY Syndrome

  • Somewhat common

  • Very mild effects compared to XXY, XXX, XO

  • Shows the lack of effect of the Y chromosome

• YO - no survival, need at least one X to be alive

Mutations in Sex Related Genes

• Androgen insensitivity syndrome (XY female)

• SRY Translocation

Mutations in Sex Related Genes: Androgen Insensitivity Syndrome

• XY female

• Recessive X-linked allele → nonfunctional androgen receptor

• testes still develop, but are undescended

Mutations in Sex Related Genes: SRY Translocation

• XX males and XY females

• SRY gene or part of the Y chromosome is moved to the autosomes

Inheritance of Sex-linked traits

• Sex chromosome with a superscript X⁺, X^-, X^a

  • X⁺ - For wildtype (usually dominant)

  • X^- or X^a - For mutant (usually recessive)

Hemizygous (1X)

• X⁺Y: have ½ the genetic content for gene determining trait (eye color)

F₂ generation (X-linked)

• Any allele on the X will be expressed in males

How to balance XX dosage with XY?

Dosage Compensation: One X is inactivated early in development (turned into a Barr body)

• The X that is inactivated is random, one starting cell and its descendants

How does dosage compensation explain the effect of X aneuploids?

• It explains effect of X aneuploidy are less severe than trisomy 21 (down syndrome)

• Why are there any effects at all? Because only one X is turned off (not every gene is inactivated) and regulation is complex

Barr Body

One X is inactivated early in development and turned into this

Genetic Mosaics

• Female mammals are this

  • Humans: anyone with XX is this

• Tortoiseshell cats example: some cells black chromosome is expressed and some orange