Sex Linkage

X –Linked Traits Definitions: Autosomes - Chromosomes #1 – 22 Sex chromosomes - #23 In males, the 23rd pair of chromosomes is NOT homologous Females have a homologous 23rd pair. Traits located on the 23rd pair are sex-linked or X-linked. Males: XY Females: XX

X –Linked Traits Genes for sex-linked (X-linked) traits are carried on the X chromosome but not on the Y chromosome Therefore in a male, the gene on the X chromosome is expressed whether it is dominant or recessive In a female, she must have two recessive alleles to have the recessive phenotype

Thomas H. Morgan: Gender and Inheritance Breakthrough work done by Thomas Hunt Morgan – American Geneticist 1908 – Work on fruit flies (Drosophila melanogaster) provided deeper understanding of heredity based on gender

Drosophila melanogaster Ideal for genetic experiments for several reasons: Rapid life cycle (10-15 days) Hundreds of eggs at one time Crossing experiments can be repeated many times Offspring mature quickly Males/females different Small Markedly contrasting traits Only 8 chromosomes (4 pairs)

Female Male How are they different??

Male or Female? Male (XY) Non Homologous Chromosomes

Chromosomes in Fruit Flies Morgan found the following: Females  4 homologous pairs of chromosomes  3 autosomal pairs, one sex pair (XX) Males  3 homologous pairs: 3 autosomal pairs Sex chromosomes were not homologous one X and one Y (XY)

Solving X-Linked Problems: Eye Color In Drosophila Let R = red, r = white Assume these alleles are found on the X chromosome only (they are sex-linked) Cross 1. Cross a red eyed homozygous female with a white eyed male. r R R Female: Male: X Y X X Now, put these gametes into a Punnett Square...

ALL RED-EYED OFFSPRING First Cross XR XR XR Xr XR Xr Xr XRY XRY Y ALL RED-EYED OFFSPRING

Second Generation (F1 Cross) XR Xr 3 RED OFFSPRING - 1 WHITE MALE XR XR XR Xr XR What is the probability of these flies having a red-eyed daughter? XRY XrY Y 0.50 or 50 % or ½

Solving X-Linked Problems: Eye Color In Drosophila Let R = red, r = white Assume these alleles are found on the X chromosome only (they are sex-linked) Cross 2. Cross a red eyed heterozygous female with a white eyed male. r R r Female: Male: X Y X X Now, put these gametes into a Punnett Square...

1 White female, 1 White male Cross 2 XR Xr What is the probability of these flies having a red-eyed son? XR Xr Xr Xr Xr 0.25 or 25 % or ¼ XRY XrY Y 1 Red female, 1 Red male, 1 White female, 1 White male

Morgan’s Conclusions Traits controlled by genes carried Genes controlling eye color must be on X Chromosome Traits coded for by genes on sex chromosomes are called SEX-LINKED TRAITS Y does not carry gene for eye color Males cannot be heterozygous for sex linked traits Female Male XR XR XR Xr Xr Xr Xr Y XRY Traits controlled by genes carried on Y chromosome only affect males

Examples of Sex-Linkage in Humans Color Blindness genes for visual pigments responsible for perception of red and green are found on the X chromosome Hemophilia A caused by the lack of a blood protein, called Factor VIII, that is critical for blood clotting Gene for Factor VIII on X Chromosome Both known as X-linked recessive

Color Blindness Caused by gene carried on X only Female Genotypes XCXC XCXc XcXc 3 possible genotypes 2 phenotypes Male Genotypes XCY XcY only 2 possible phenotypes 2 possible genotypes

Link to Ishihara Test for Color Blindness http://www.toledo-bend.com/colorblind/ishihara.html

Other Gender Related Inherited Traits Sex Limited Eg. Milk production in cows is controlled by an autosomal gene Bulls carry the genes...but they do not make milk!!!!!! The gene is not expressed for bulls (because they have low levels of female hormones which switch on this gene) Sex Influenced Eg. Male only needs one allele for baldness – male hormones stimulate the expression of this allele females need two recessive alleles to be bald Gene is dominant in males and recessive in females more common in males