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 = 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+

Chromosome with white gene

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, 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

XC

Xc

• 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 pea plants: color gene (Chromosome 1) shape gene (Chromosome 7) 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)

Drosophila

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 1/5,000 births

Only viable monosomy in humans

Short stature Lymphadema of the hands and feet Broad chest Low hairline Low-set ears Reproductive sterility Rudimentary ovaries gonadal streak absence of a menstrual period Increased weight, obesity Shield shaped thorax of heart Shortened hand bones Small fingernails Characteristic facial features Webbed neck Aorta abnormalities Poor breast development Horseshoe kidney Visual impairments Ear infections and hearing loss High waist-to-hip ratio (the hips are not much bigger than the waist) Attention Deficit/Hyperactivity Disorder Nonverbal Learning Disability (problems with math, social skills and spatial relations)

Possible symptoms

2. Trisomy (2n+1)

Ex. Trisomy 21 (Down syndrome) 1/700 births

Higher risk in women > 35

Abnormal meiosis

Possible symptoms Common physical signs include: Excess skin at the nape of the neck Flattened nose Separated joints between the bones of the skull (sutures) Single crease in the palm of the hand Small ears Small mouth Upward slanting eyes Wide, short hands with short fingers White spots on the colored part of the eye (Brushfield spots) Physical development is often slower than normal. Most children with Down syndrome never reach their average adult height. Children may also have delayed mental and social development. Common problems may include: Impulsive behavior Poor judgment Short attention span Slow learning

• Nondisjunction anaphase I

• Nondisjunction anaphaseII

• Fertilization animation at Learn Genetics

Chromosome structure abnormalities

1. Deletion – fragment lost

2. Duplication – repeated fragment

3. Translocation – fragment attaches to other chromosome

4. Inversion – fragment inverts

Univ. Wisc. Genetics center tumors

Inheritance of mitochondrial DNA and chloroplast DNA (plants)

• Extranuclear genes

• Maternal inheritance (cytoplasm)