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?