The chromosomal basis of sex In humans, Y chromosome determines sex XX
X
XY
Gametes? p(son) p (daughter)
Humans X/Y system
Development testes develop -> testosterone -> male
If no SRY ovaries female
SRY discovered in 1990 Y chromosome has ~78 genes
Chromosome theory of inheritance • Genes located on chromosomes = gene locus • Thomas Morgan
• Drosophila – 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+ on X chromosome Sex-linked gene!
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
Cross f1 females with f1 males
f1
f2
Xw+ Xw
X
Xw+ Y
X-linked genes in humans Female genotypes
XCXC XCXc
XcXc
Male genotypes
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?
No green photoreceptors
Example: colorblindness is X-linked • 1/1O males • 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.
X-chromosome inactivation in females • One X inactivated during embryonic development Barr body (see nuclear envelope) • # in female cells? male cells?
FYI • Barr body testing was introduced in the 1966 Olympic games, in an effort to detect male athletes trying to "pass" as females, to gain a . competitive advantage
• Genes on this X are not expressed • Lyon hypothesis – Females mosaics for X-linked traits
C c X X • 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
Chromosomal abnormalities • Disjunction anaphase of meiosis
If chromosome do not disjoin • Non-disjunction – Meiotic spindle error – Gametes ?
Fertilization (human)
Aneuploidy – abnormal number of chromosomes
• Nondisjunction anaphase I • Nondisjunction anaphaseII • Fertilization animation at Learn Genetics
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)
Higher risk in women > 35 Abnormal meiosis
1/700 births
Possible symptoms Common physical signs include: Excess skin at the nape of the neck Flattened nose Single crease in the palm of the hand Small ears Small mouth Upward slanting eyes Wide, short hands, short fingers White spots on eye Physical development is often slower than normal. Delayed mental and social development.
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
Chromosome 12 deletion along with an inversion (in the long arm) of chromosome 12
Chromosome 6p25.1 deletion and 7q21.13 duplication This is Hayes. He was born in July 2007 with a rare chromosome disorder Chromosome 13 deletion This is a story of Maya who has a chromosome abnormality. She has a gross motor delay in which she isn't walking or crawling 22q13 deletion syndrome – Insulin treatment. The family of a little child born with a deletion of the 22q13 region of his chromosome 22 Chromosome 16 Deletion Ace's Story Chromosome 15q Duplication Syndrome - Cody's Poem
Inheritance of mitochondrial DNA and chloroplast DNA (plants) • Extranuclear
