X Linked Traits Definitions Autosomes Chromosomes 1 22

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

First Cross XR Xr Y XR XR Xr X RY ALL RED-EYED OFFSPRING

Second Generation (F 1 Cross) XR XR Y Xr XR XR XR Xr X RY Xr Y - 3 RED OFFSPRING - 1 WHITE MALE What is the probability of these flies having a redeyed daughter? 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.

Cross 2 XR Xr Xr Xr Y X RY What is the probability of these flies having a redeyed son? 0. 25 or 25 % or ¼ Xr Y 1 Red female, 1 Red male, 1 White female, 1 White male

Morgan’s Conclusions • 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 Y XR Xr X RY Xr Xr 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 • Male Genotypes • C c • X X • X c C C X X • c • X Y C X Y • only 2 possible phenotypes • 3 possible genotypes • 2 phenotypes

• Link to Ishihara Test for Color Blindness • http: //www. toledobend. 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