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 Wildtype = normal Mutant = abnormal Wingless eyeless
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)
X w+ X w+ X X w Y Punnett square results Cross f1 females with f1 males
f1 X 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 genotypes Male 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
X C X c 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
Possible symptoms 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)
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)