Sex Is Determined by the X and Y ChromosomesHumans have 23 pairs of chromosomes, and the 23rd pair determines sex.Females have two X chromosomes (XX), carrying about 900 genes.Males have one X and one Y chromosome (XY).The Y chromosome is much smaller, carrying only about 55 genes, most tied to male development.The Sperm Decides the Sex of the OffspringThe egg always carries an X chromosome, so the sperm decides the sex of the zygote.An X-carrying sperm produces an XX (female) zygote.A Y-carrying sperm produces an XY (male) zygote.The SRY gene on the Y chromosome acts as a switch that triggers the testes to form.Without the SRY gene, the embryo follows the default pathway and develops as female.Sex determination in punnet gridsCommon MistakeIt is a common mistake to think two X chromosomes cause female development.It is really the absence of the SRY gene that leads to female development.One X Chromosome Leaves Males Exposed to X-Linked DisordersThe X chromosome carries many genes that have nothing to do with sex determination but are essential for both sexes.Males are more vulnerable to X-linked recessive disorders because they have only one X chromosome and no second copy to mask a faulty allele.A female needs the faulty allele on both X chromosomes to be affected, so she is usually a carrier instead.ExampleRed-green colour blindness is a common X-linked recessive disorder.Haemophilia is another X-linked example, covered in detail in D3.2.12.Colour Blindness Shows the X-Linked PatternWe write X-linked alleles as superscripts on the X, such as $X^N$ for normal vision and $X^c$ for colour blindness.A son inherits his single X from his mother, so a carrier mother can pass colour blindness to her sons.ExampleCross a carrier mother ($X^NX^c$) with a father who has normal vision ($X^NY$).There is a 25% chance of a carrier daughter.There is a 25% chance of a colour-blind son.Colour Blindness Cross and X-linked TraitsFruit Flies Revealed That Genes Sit on ChromosomesGeneticist Thomas Morgan studied eye colour in fruit flies (Drosophila) and found it was inherited differently in males and females.His reciprocal crosses showed eye colour is an X-linked trait, linking a specific gene to a specific chromosome for the first time.Click to interactFruit flies in XY sex determination FullscreenOpen in new tabTipUse a Punnett grid with alleles written as superscripts on the X to predict sex-linked inheritance.Always include the Y chromosome for males so the male genotypes are clear.Check Your UnderstandingActive recallWhat does the SRY gene do, and what happens in its absence?Why are males more likely than females to be affected by X-linked recessive disorders?From which parent does a son inherit his single X chromosome?What are the possible children of a carrier mother ($X^NX^c$) and a father with normal vision?
