Chapter 15 Chromosomes

Chromosome theory of inheritance Genes located on chromosomes = gene locus Thomas Hunt Morgan, Columbia Univ. “Fly room”

Drosophila – 100s of offspring – 2n = 8 3 prs autosomes X and Y sex chromosomes

BithoraxWildtypeWhite eye Winglesseyeless Wildtype = normal Mutant = abnormal

Drosophila genetics White eye allele = w Wildtype allele = w + gene locus on X chromosome Sex-linked! Genotype? ww, w + w w + w +

Sex-linked genes Cross a red female with a white male (pg. 288)

X w+ X w+ X X w Y Punnett square results Cross f1 females with f1 males

f1X w+ X w X X w+ Y f2  Some genes are located on the X chromosome

The chromosomal basis of sex In humans, Y chromosome determines sex XX X XY Gametes? p(son) p (daughter)

Humans have an X/Y system

Development <2 months gestation – embryo has rudimentary gonads

2 mos. SRY gene on Y chromosome active -> testes develop -> testosterone -> male If no SRY female default pathway  ovaries

X-linked genes in humans Female genotypesMale genotypes X C X C X C X c X c X c X C Y X c Y Terms: homozygous, heterozygous, hemizygous Concept check: From whom do males obtain the Y chromosome? From whom do females obtain the X chromosomes? Why are X-linked disorders more prevalent in males?

Example: colorblindness 1/1O males Colorblindness is an X-linked trait. A man is colorblind. What % of his sons and daughters are expected to be colorblind. His wife does not carry the colorblind allele on either X chromosome.

No green photoreceptors

X-chromosome inactivation in females One X inactivated during embryonic development  Barr body (see nuclear envelope) # in female cells? male cells?

Genes on this X are not expressed Lyon hypothesis – Females mosaics for X-linked traits – Allele key

XCXcXCXc Patches of colorblind cells in retina Normal phenotype Blue colorblindness is rare

Genotype of orange female? Genotype of black female? Genotype of orange male? Genotype of black male?

Linkage Unlinked genes -On different chromosomes -Independent assortment! Example in humans: Blood type (Chromosome 9) Lactose intolerance (Chromosome 2)

Linked Genes -on same chromosome -do not assort independently Example humans: Freckles (Chromosome 16) Red hair (Chromosome 16)

Genetic recombination of linked genes Crossing over Meiosis Non-sister chromatids of homologous chromosomes 25% of each

Crossing over results in recombinants

Chromosomal abnormalities Disjunction anaphase of meiosis

If chromosome do not disjoin Non-disjunction – Meiotic spindle error – Sister chromatids do not separate in Anaphase II – Gametes  ?

 Fertilization (human) Aneuploidy – abnormal number of chromosomes

1. Monosomy (2n-1) Ex. Turner syndrome 45, X Only viable monosomy in humans

2. Trisomy (2n+1) Ex. Trisomy 21 (Down syndrome) Higher risk in women > 35 Abnormal meiosis

Nondisjunction anaphase I Nondisjunction Nondisjunction anaphaseII Nondisjunction Fertilization animation at Learn Genetics Fertilization animation

Chromosome structure abnormalities 1.Deletion 2.Duplication 3.Translocation 4.Inversion Univ. Wisc. Genetics centerUniv. Wisc. Genetics center tumors

Inheritance of mitochondrial DNA and chloroplast DNA (plants) Extranuclear genes

Maternal inheritance (cytoplasm)