Lecture for Thursday September 25, 2003 What’s due? CH3 problem set Assignments: Study for Exam! Today’s lecture: Review material from 9/23 Genes on the X Chromsome Sex-linked, -limited, -influenced Inheritance

Review: *Pedigree- a family tree that shows the phenotype of a particular trait for each family member *Modification of Mendelian Ratios: Incomplete dominance- expression of a heterozygous phenotype which is distinct from, and often intermediate to, that of either parent P:Red Flower C R C R White Flower C W C W X F1:F1:Pink Flower C R C W F2:F2: ¼ Red C R C R ½ Pink C R C W ¼ White C W C W

Review con’t: *Codominance- The effects of two alleles are equally and simultaneously expressed in the heterozygote *Multiple Alleles- three or more alleles of the same gene *ABO Blood group in humans Genotype I A I A I A I O I B I B I B I O I A I B I O I O Antigen A A B B A,B Neither Phenotype A A B B AB O simple dominance codominance A=B>O *Lethal Allele- an allele whose expression results in the death of the individual expressing it *Can be either dominant or recessive

Review con’t: Gene Interactions: *Epistasis: situation in which one gene masks the expression of another *Novel phenotypes: interaction between genes yields new phenotype Example: shape of summer squash In the F 2 genotypes: A_B_ disc phenotype A_bb or aaB_ sphere phenotype aabblong phenotype

Genes on the X Chromosome: Sex chromosomes- chromosomes involved in sex determination Autosomes- non sex determining chromosomes Example: In Drosophila and all mammals sex chromosomes designated as X and Y XX=female XY=male *1909 Thomas Hunt Morgan determined that there were differences in chromosomes of male and female Drosophila II III IV XX XY or

X-linkage- transmission and expression of genes located on the X chromosome X-linkage in Drosophila: white mutation (eyes) Genes on the X Chromosome con’t: ½ red ½ white

Genes on the X Chromosome con’t: *Morgan proposed that the white eyed mutation was X-linked *Reciprocal cross did not yield identical results *Males were XY and females were XX *Since females could be white eyed, the trait was not determined by a locus on Y ½ red Symbols: red eye = w + white eye = w w + w + X wY w+ww+w w+Yw+Y Gametes from mother: ½ w + ½ w Gametes from father: ½ w + ½ Y ¼ w + w + ¼ w + Y ¼ ww + ¼ wY ½ red female ¼ red male ¼ white male

Sex-limited Inheritance: Sex-limited trait: A trait that is expressed in only one sex even though the trait may not be X-linked *holandric genes: genes on the Y chromsome Example: hypertrichosis (ear hair) P: XX x XY h note: h is only on Y F 1 : ½ XX + ½ XY h *autosomal genes Example: milk production in mammals; L=lots, l=little Genotype Phenotype femalemale LLlotsnone Lllotsnone lllittlenone

Sex-influenced trait: Phenotypic expression that is conditioned by the sex of the individual Sex-influenced Inheritance: Examples: *cleft palate in humans *horns in sheep *pattern baldness in humans Genotype Phenotype female male BBbald bald Bbnot bald bald bbnot bald not bald B=bald b=not bald Summary: Sex-linkedon X or Y sex-chromosome Sex-limitedall or none expression by sex Sex-influencedgenotype + sex determines phenotype

Phenotypic Expression: Penetrance: (quantitative term) the frequency with which individuals with a given genotype, exhibit some degree of a phenotype associated with that trait Expressivity: (qualitative term) the degree or range in which a phenotype for a given trait is expressed *Gene expression often governed by genotype and environment Example: if 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant *Onset of genetic expression (when is a gene expressed?) Tay-Sachsautosomal rec.lipid metabdeath by 3 years Lesch-NyhanX-linked rec.nucleic acid metabonset >6mo. DMDX-linked rec.muscular degenerationdiagnosed at 3-5yrs Huntingtonsautosomal dom.brain deteriorationonset >30yrs