1. Sex Linked Inheritance
September 22, 2018

2. I. Sex Linked Genes
The X and Y chromosomes carry the genetic information that makes us male and female
They also contain genetic information for some other traits
What combination of sex chromosomes do girls have?
What combination of sex chromosomes do boys have?
X X
X Y

3. Sex Linked Genes (cont.)
Who determines the gender of the baby?
Father determines the sex of the baby because the mother can only give one X.
The father can give either the X or Y chromosome to their child.

4. II. Who is Affected More by Sex-Linked Traits?
Men are usually affected more than women
Since men only have one X chromosome they will display the characteristics of these traits even if they are caused by a recessive allele
Male The "a" recessive allele will be expressed in his phenotype   
Female The "a" recessive allele will not be expressed in her phenotype

5. III. Examples of Sex Linked Genes
There are about 1,098 human X-linked genes.
Most of them code for something other than female anatomical traits.
Many of the non-sex determining X-linked genes are responsible for abnormal conditions such as
Hemophilia
Red-green color blindness
Congenital night blindness
Some high blood pressure
Duchenne muscular dystrophy
Fragile-X syndrome
Male pattern baldness

6. IV. Sex-Linked Punnett Square
Exactly like completely a regular monohybrid Punnett Square except you need to indicate if the trait is on the X or Y chromosome
Genotype examples
Heterozygous female = XT Xt
Homozygous dominant female = XT XT
Homozygous recessive female = Xt Xt
Non-affected male = XT Y
Affected male = Xt Y

7. C. Sex Linked Punnett Square Example

8. V. Heterozygous Vs. Homozygous
If a woman is a carrier of an X-linked recessive allele for a disorder and her mate does not have it, their boys will have a 50% chance of inheriting the disorder. 
None of their girls will have it, but half of them are likely to be carriers
If a man has an X-linked recessive disorder and his mate does not carry the allele for it, all of their girls will be carriers.
None of their boys will inherit the allele. 
Only girls receive X chromosomes from their fathers.
Carrier
Affected

9. VI. Sex-Linked Recessive Pedigrees
Completely shaded in individuals possess the trait
Half shaded in individuals are carriers of the trait

10. Queen Victoria’s Pedigree
(Hemophilia)

11. Red-Green Color Blindness Problem
In humans, red-green colorblindness is a recessive sex-linked trait. It is found on the X chromosome, not the Y. Because, males only have one X chromosome, they have a much greater chance of having red-green colorblindness. Females would have to be homozygous recessive in order to have red-green colorblindness.
Let’s run a color-blind test to see if any of you are colorblind!

12. Colorblindness

13. Colorblindness
Normal vision – 8 red green color blind - 3

14. Colorblindness

15. Colorblindness

16. Colorblindness

17. Colorblindness

18. Sex Linked Inheritance Practice
A recessive allele on the X chromosome is responsible for red-green color blindness in humans. A woman with normal vision whose father is color-blind marries a color-blind male. What is the probability that a son of this couple will be color-blind?