Lecture for Thursday September 25, 2003

1. 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

2. P: Red Flower CRCR White Flower CWCW X F1: Pink FlowerCRCW ¼ RedCRCR½ PinkCRCW¼ WhiteCWCW F2: 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

3. simple dominance simple dominance codominance GenotypeIAIA IAIO IBIB IBIO IAIB IOIO Antigen A A B B A,B Neither Phenotype A A B B AB 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: *Codominance- The effects of two alleles are equally and simultaneously expressed in the heterozygote *Multiple Alleles- three or more alleles of the same gene A=B>O *ABO Blood group in humans

4. 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 F2 genotypes:A_B_ disc phenotype A_bb or aaB_ sphere phenotype aabb long phenotype

5. II III XY IV or XX Genes on the X Chromosome: *1909 Thomas Hunt Morgan determined that there were differences in chromosomes of male and female Drosophila 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

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

7. *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 Gametes from mother: Gametes from father: w+w ½ w+ ½ w ½ w+ ½ Y w+Y ½ red Genes on the X Chromosome con’t: *Morgan proposed that the white eyed mutation was X-linked Symbols: red eye = w+ white eye = w w+w+ X wY ¼ w+w+ ¼ w+Y ¼ ww+ ¼ wY ½ red female ¼ red male ¼ white male

8. Example: hypertrichosis (ear hair) P: XX x XYhnote: h is only on Y F1: ½ XX + ½ XYh Genotype Phenotype female maleLL lots none Ll lots none ll little none 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 *autosomal genes Example: milk production in mammals; L=lots, l=little

9. Genotype Phenotype female maleBB bald bald Bb not bald bald bb not bald not bald Sex-influenced Inheritance: Sex-influenced trait: Phenotypic expression that is conditioned by the sex of the individual Examples: *cleft palate in humans *horns in sheep *pattern baldness in humans B=bald b=not bald Summary: Sex-linked on X or Y sex-chromosome Sex-limited all or none expression by sex Sex-influenced genotype + sex determines phenotype

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