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Sex Linked Traits

Sex Linked Traits. Male or Female? The gender of a baby is determined by genes on chromosomes. There are 23 pairs of chromosomes in each of our cells. One of pair of chromosomes are called sex chromosomes. The sex chromosomes determine whether a person is male or female.

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Sex Linked Traits

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  1. Sex Linked Traits

  2. Male or Female? • The gender of a baby is determined by genes on chromosomes. • There are 23 pairs of chromosomes in each of our cells. • One of pair of chromosomes are called sex chromosomes.

  3. The sex chromosomes determine whether a person is male or female. • The sex chromosomes are the only pair of chromosomes that • do not always match.

  4. Females The two sex chromosomes match - XX Males The two sex chromosomes do not match - XY

  5. Male Female Homozygous Heterozygous

  6. Since both of a female’s sex chromosomes are X, all eggs carry one X chromosome • Males have two different sex chromosomes, so sperm cells will either carry an X or a Y chromosome

  7. Sex-Linked Genes Some human traits occur more often in one gender than the other. Sex-Linked Genes: Genes on the X and Y chromosomes, whose alleles are passed from parent to offspring on sex chromosomes

  8. What is a sex-linked trait? • Sex-linked traits are due to genes located on sex chromosomes. • As we know, male sex chromosomes are XY while female sex chromosomes are XX. • The X chromosome contains over 1000 genes while the Y chromosome contains as few as 26. Therefore, many sex-linked traits are discussed in terms of the X-chromosomes.

  9. 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.

  10. 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.

  11. In females, a dominant allele on one X chromosome will mask a recessive allele on the other X chromosome.

  12. In males, there is no matching allele on the X and Y chromosome. As a result, any allele on the X chromosome will produce the trait in a male who inherits it.

  13. Because males have only one X chromosome, males are more likely than females to have a sex-linked trait that is controlled by a recessive allele.

  14. Example of sex-linked trait controlled by a recessive allele: red-green Color Blindness affects more males than females.

  15. Examples of X-linked genes • Other than determining sex, genes on the X chromosome are responsible for traits. Some examples are: • Hemophilia • Red-green color blindness • High blood pressure • Muscular dystrophy • Fragile-X syndrome

  16. Recessive Lethals • In some rare instances, recessive traits are lethal-- meaning the organism is born very weak and sickly or dies not long after birth. • When looking at non-sex linked chromosomes, we can apply standard Punnett Square probability. • However, when looking at sex linked chromosomes we see that males have a much higher tendency for recessive lethals. Why?

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