Sex-Linked Traits

Characteristics that are inherited from genes found on the sex chromosomes.

Human Chromosomes

Humans typically have 23 pairs of chromosomes

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The 23 pair is our sex chromosomes these are the only chromosomes that don’t have the same genes on them

Sex-Linked Traits Vocabulary

• Autosomal – traits that are carried on chromosomes other than the sex chromosomes
• Sex linked traits – traits carried on sex chromosomes
• Sex Determination in a binary system
• Females – XX
• Males – XY
• There are other combinations such as XXY, XO,XYY, XXX
• Almost all sex-linked traits are found on the X chromosome
• Y chromosome contains very few genes and is mainly involved in sex determination

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Sex Chromosomes

• The X and Y chromosome carry different genes

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• Some genes code for turning on and off, or expressing, other genes

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• Science now sees that genes on the Y chromosome work with genes on the X chromosome to express other genes

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The Y Chromosome

• The Y chromosome is about 1/3 the size of the X chromosome
• The Y chromosome contains about 55 genes, the X has about 900
• The Y chromosome contains a “male determining gene”, the SRY gene, that causes testes to form in the embryo and results in the external and internal genitalia
• If there is a mutation to the SRY gene the embryo may develop female genitalia despite have XY chromosomes
• Epigenetics may affect the SRY gene and how the gene is expressed

The Y Chromosome

• As they age some men lose the Y chromosome from their cells

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• If there is a mistake or mutation on the Y chromosome it will be expressed because there is only one

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• Epigenetics can play a large roll in sex determination and expression

Sex-Linked Traits

• It is possible for a female to be a carrier of an X-linked trait, but not express it

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• Men will express all X-linked traits they inherit because they only have one X chromosome

Carriers

• Because females have two copies of the X chromosome, it is possible to have certain traits “hidden” by a dominant copy.
• However, because males only have one X chromosome, the observable phenotype is obvious and identifies the genotype.

Carriers

• When a female contains a recessive allele that is hidden by the dominant allele, we call them carriers.
• A carrier maintains the ability to pass on a trait even if they do not express/show it.

Example: Color Blindness

The allele for colorblindness is on the X chromosome

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If a male gets the allel for colorblindness he will be colorblind

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A female must have two alleles for colorblindness to be colorblind