16.1: Sex determination and sex-linkage

Environmental Sex Determination

In reptiles, sex is determined by the incubation temperature of eggs.

• lizards and alligators = higher temps = more males

• turtles = higher temps = more females

Genetic Mechanisms of Sex Determination

In bees, sex determination depends on ploidy. Females are diploid (resulting from fertilized eggs), and males are haploid (resulting from unfertilized eggs).

Presence of Sex Chromosomes

In humans, the XY system is used. Females have XX chromosomes, and males have XY chromosomes.

Sex determination in Drosophila

ratio of X chromosomes to autosomes.

• 1:1 = females

• 1:2 = males (2 sets of autosomes)

Sex Determination in Grasshoppers

Grasshoppers have X chromosomes but no Y.

• females = XX

• males = X

ZW Sex-Determination System

In birds

• males = ZZ

• females = ZW

Sex-Linked Traits

Traits with distinct inheritance patterns due to differences in how the sex chromosomes are partitioned in males and females.

Autosomal Recessive Inheritance

If either mum or dad has it = neither female nor male offspring will show the trait

Sex-Linked Recessive Inheritance in Mother

carried on X chromosome

• all male offspring inherit affected allele and display the trait as receives X from mum and Y from D

• female offspring inherits the allele but doesn’t display as also receives non-affected allele from dad (presence of other X masks affected X)

Autosomal Dominant Inheritance

All offspring will inherit the trait if either the mother or father has it.

Sex-Linked Dominant Inheritance on X Chromosome of Mother

Both male and female offspring will inherit and display the trait.

Sex-Linked Dominant Inheritance on X Chromosome of Father

Only female offspring will display the trait because the male offspring inherits the Y chromosome from the father.

amount of chromosomes in humans

Each cell's nucleus contains 23 chromosome pairs - 22 autosomes and 1 set sex chromosomes = 46 total

Sex Chromosome Composition in Females and Males

• females = XX

• Males = XY

Chromosome Number in Sperm and Egg Cells

Sperm and egg cells carry half the number of chromosomes (23), including one sex chromosome. All eggs carry X, sperm carries either X or Y.

Sex Determination During Fertilization

Sex is determined during fertilisation.

• Sperm with X chromosome + egg = female

• Sperm with Y chromosome + egg = male

Structural Differences Between X and Y Chromosomes

The X chromosome is larger than the Y and contains more genes (800-1000 vs. 55).

Genes on Y Chromosome

Genes on the Y chromosome are called Y-linked genes and are only expressed in males.

• e.g. SRY gene is responsible for male sexual traits.

X-Linked Traits

Traits encoded by genes located on X chromosomes. These traits show different patterns of expression between the sexes due to the presence of two X chromosomes in females and one in males.

Expression of Recessive X-Linked Genes

affect males more than females because females have two X chromosomes with one being a normal copy of the gene

Sex-linked Disorders

Inherited disorders caused by mutations to single gene located on sex chromosome. Can be

• Y- linked

• X-linked dominant

• X-linked recessive

Y-linked Disorders

Caused by mutations on genes located on the Y chromosome and can only be passed from father to son

• e.g., Y chromosome infertility

X-linked Dominant Disorders

Requires only one copy of the mutated gene and affects both females and males. Cannot be passed from fathers to sons.

X-linked Recessive Disorders

Affects males much more often because females need two copies of the mutated genes. Unaffected heterozygous women can be carriers.

Duchenne Muscular Dystrophy (DMD)

An X-linked recessive disorder almost exclusively affecting boys, caused by mutations in the gene encoding dystrophin. Results in progressive muscle degeneration and weakness.

ploidy

the number of sets of chromosomes in a cell.

factors that influence determination of sex

environmental influences, genetic mechanisms or presence of sex chromosomes

Why are X chromosome mutations more harmful in male offspring

Because males have only one X chromosome, any mutation on it has no corresponding allele to compensate, leading to the manifestation of X-linked recessive disorders.