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Sex Linked Traits.

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Presentation on theme: "Sex Linked Traits."— Presentation transcript:

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.


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

6 Female Male Homozygous Heterozygous

7 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

8 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

9 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.

10 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.

11 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.

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

13 In males, there is no matching allele on the X and Y chromosome
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.

14 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.

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

16 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

17 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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