Chapter 4: Sex Determination and Sex Chromosomes

Sex determination

• mechanism by which an individual develops into a female or male

  • Relies on sex chromosomes present in individuals

sex chromosomes

a pair of chromosomes that determines sex in a species

heterogametic sex

• males produce 2 kinds of gametes (sperm), X and Y

  • Sperm determines sex of offspring (zygote)

  • X or Y + 22 autosomes

homogametic sex

• females only produce X gametes (eggs), one kind of gametes

  • X + 22 autosomes

X-0 sex determination

• Ratio of X chromosomes to autosomes that determines sex

• 1X / 2n autosomes = male

• 2 Xs / 2n autosomes = female

Found in many insects and C. elegans

Z-W sex determination

• Found in birds and some fish 

• Male is Z-Z (homogametic sex)

• Female is Z-W (heterogametic sex)

Gyandromorph

half male, half female

Haplodiploid sex determination

• sex is determined by the number of sets of chromosomes 

  • Found in social insects (bees, wasps, ants)

  • Male (drone) is produced from unfertilized haploid eggs - set of 16 chromosomes

  • Females (workers + queen) grow from fertilized diploid eggs - 2 chromosome sets = 32 total 

    • Only females arise through sexual reproduction 

Temperature-dependent sex determination

• Found in some reptiles and some fish 

• Want 50:50 ratio

Behavioral sex determination

behavioral interactions can determine sex

• Ex: clownfish (amphiprion)

Protandrous hermaphrodites

• switch from male to female 

  • Rotate through roles of sex

• when females die, male becomes female and largest juvenile matures into a reproductive male (hormone levels control expression of particular genes as kids have both male + female immature sexual organs)

• female controls other clownfish through aggressive dominance (keeps them male)

Monoecious plants

• produce both male and female gametes from same individual

Dioecious

• each individual plant is either male or female 

  • Ex: ginkgo

Dosage compensation

• mechanism that keeps levels of expression of X chromosome genes similar in both sexes

X-chromosome inactivation

•  female mammals inactivate one of their X chromosomes in each somatic cell → forms Barr body 

variegation (mosaicism)

• X chromosome is randomly inactivated 

X-inactivation center (Xic)

• mechanism of counting X chromosomes 

  • Region of X-chromsome that contains the X-inactive specific transcript gene (Xist) 

  • Only active on Barr body → its activation inactivates X chromosome

Phases of X Inactivation

1. Initiation: X chromosome is randomly selected for inactivation 

2. Spreading

   1. Xist expressed on chromosome to be inactivated

   2. Xist transcripts coat chromosome 

   3. Proteins recruited to chromosome condense it 

3. Maintenance (through mitosis and beyond)

lyon hypothesis

A hypothesis to explain the pattern of X-chromosome inactivation seen in mammals. Initially, both X chromosomes are active. However, at an early stage of embryonic development, one of the two X chromosomes is randomly inactivated in each somatic cell of a female.

Sex-linked genes

some genes are unique to X or Y 

X-linked genes

genes only found on the X chromosome

Y-linked genes

genes only found on the Y chromosome

Pseudoautosomal genes

 found on both X and Y chromosomes

• the inheritance pattern of the Mic2 gene is the same as the inheritance pattern of a gene on an autosome even though it is located on a sex chromosome

Sry gene

• sex-determing gene Y, single gene on the Y chromosome is responsible for male development 

  • Sry encodes a transcription factor that activates male-specific genes

  • Default sex in mammals is female

X-linked inheritance

• breaks Mendel’s rules (differ from autosomal genes) 

  • Male is hemizygous for X-linked genes b/c there’s only one copy 

Testcross

a phenotypically dominant individual (unknown genotype) is mated with a recessive individual 

Reciprocal cross

2 crosses that differ in which sex carries the trait 

mosaic females

female that is heterozygous on the X chromosome

• consists of a mixture of two kinds of cells—each with different functional X chromosomes

• those with the active paternal X and those with the active maternal X

hemizygous

a single copy of the x-linked gene in males