Sex Linked Inheritance September 22, 2018

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

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.

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

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

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

C. Sex Linked Punnett Square Example

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

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

Queen Victoria’s Pedigree (Hemophilia)

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!

Colorblindness

Colorblindness Normal vision – 8 red green color blind - 3

Colorblindness

Colorblindness

Colorblindness

Colorblindness

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?