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

Bithorax Wildtype White eye

Wingless eyeless

Wildtype = normalMutant = abnormal

Drosophila genetics

White eye allele = wWildtype allele = w+

gene locus onX chromosome

Sex-linked!

Genotype? ww, w+w w+w+

Sex-linked genes

Cross a red female with a white male (pg. 288)

Xw+ Xw+ X Xw Y

Punnett square results

Cross f1 females with f1 males

f1 Xw+ Xw X Xw+ 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 genotypes Male genotypes

XCXC XCXc XcXc XC Y XcY

Terms: homozygous, heterozygous, hemizygousConcept 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

XCXc

• 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?

LinkageUnlinked 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 genesCrossing 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, XOnly viable monosomy in humans

2. Trisomy (2n+1)

Ex. Trisomy 21 (Down syndrome)

Higher risk in women > 35Abnormal meiosis

• Nondisjunction anaphase I• Nondisjunction anaphaseII

• Fertilization animation at Learn Genetics

Chromosome structure abnormalities

1. Deletion 2. Duplication 3. Translocation 4. Inversion

Univ. Wisc. Genetics center tumors

Inheritance of mitochondrial DNAand chloroplast DNA (plants)

• Extranuclear genes

• Maternal inheritance (cytoplasm)