General Genetics Lecturer Alshehri, Dokhnah Saeed dalshehri@ut.edu.sa (Practical Session) BIO221 Lecturer Alshehri, Dokhnah Saeed dalshehri@ut.edu.sa Alshehri, Dikhnah Saeed

The 6th Lab Sex-Linked Genes                                                                                                                           Alshehri, Dikhnah Saeed

Outlines: Gender Determination. Sex-Linked Traits. X-Linked Inheritance. Sex-Linked Disorders. Some practical Exercise and class activates. Alshehri, Dikhnah Saeed

Sex determination Sex chromosomes – determines the sex of an individual ( Y, X ) * Males have X and Y Two kinds of gametes * Female have two X’s Only one type of gamete Alshehri, Dikhnah Saeed

Which parents determines the sex of the offspring? XY male X Y XX Female X X It is the male that determines the sex of the offspring. Alshehri, Dikhnah Saeed

Sex Linked Traits 1- Traits controlled by genes located on sex chromosomes (X or Y) 2- Most are X-linked: The X chromosome is larger and has more genes. 3- A Y has very few genes. 4- Show inheritance pattern that differs for autosomal traits. Alshehri, Dikhnah Saeed

X-linked Inheritance Alshehri, Dikhnah Saeed

* Males only have one copy of the X chromosome (hemizygous), plus a Y. * Female have two X chromosomes, and may be homozygous or heterozygous for a trait X-linked genes are never passed from father to son. * The Y chromosome is the only sex chromosome that passes from father to son. Alshehri, Dikhnah Saeed

Females with one copy of the normal gene and one copy of the mutated gene are called carriers. They don’t show the trait. * Males are never carriers – if they have a mutated gene on the X chromosome, it will be expressed Alshehri, Dikhnah Saeed

X-linked disorders Recessive Dominant Red-green color blindness Hemophilia Duchenne muscular dystrophy Ichthyosis Dominant Hypertrichosis Alshehri, Dikhnah Saeed

Red-green color blindness Can not distinguish red from green. Normal red-green Red-green colorblind Some 10 million American men—fully 7 percent of the male population—either cannot distinguish red from green, or see red and green differently from most people. This is the commonest form of color blindness, but it affects only .4 percent of women. There are three basic variants of color blindness. Red/green color blindness (deuteranopia) is the most common deficiency, affecting 8% of Caucasian males and 0.5% of Caucasian females. The prevalence varies with culture. Blue color blindness (protanopia) is an inability to distinguish both blue and yellow, which are seen as white or gray. Protanopia is quite rare and has equal prevalence in males and females. Alshehri, Dikhnah Saeed

What a red-green colorblind person would see Normal Red-green colorblind Alshehri, Dikhnah Saeed

Alshehri, Dikhnah Saeed

Hemophilia Alshehri, Dikhnah Saeed

X Chromosome Inactivation While females have two X chromosomes, only one is active. The other is turned off. This is random, in some cells the maternal X is turned off and in others the paternal X is turned off. All descents of that cell will have same X turned off It is condensed and known as a Barr Body. Alshehri, Dikhnah Saeed

Exercise 1 Color blindness A normal man married to a normal woman (her father affected with color blindness). What is the probability of children (male and female) to be affected with the disease? female male Xᴱ Xᴱ Normal Xᴱ Y Normal Xᴱ Xᵉ Carrier Xᵉ Y Affected Xᵉ Xᵉ Affected Alshehri, Dikhnah Saeed

Exercise 2 Hemophilia Hemophilia is an X-linked recessive trait. Write the genotype for the following individuals: A normal man An affected man An affected woman Unaffected and not carrier woman Unaffected and carrier woman Female Male Xᴴ Xᴴ Normal Xᴴ Y Normal Xᴴ Xʰ Carrier Xʰ Y Affected Xʰ Xʰ Affected Alshehri, Dikhnah Saeed

Exercise 3 A woman carried hemophilia disease married to a normal man. Write the genotype of the parents. What is the probability of the male children to be affected with the disease? Alshehri, Dikhnah Saeed

Assignment: How many males of their progeny will show the trait? A woman “affected with red color blindness" which is a recessive, sex- linked trait, marries a normal male. How many males of their progeny will show the trait? A. All B. 1/2 C. 1/4 D. 0 E. 3/4 Alshehri, Dikhnah Saeed

Exercise 4 White eyes in Drosophila are an X- linked recessive trait. Red eyes is the X-linked dominant form. If a Red eyed, heterozygous female fly crossed with a red eyed male fly, How many females of their offspring will have white eyes? How many males of their offspring will have white eyes? Alshehri, Dikhnah Saeed

Alshehri, Dikhnah Saeed