Chapter 15 Chromosomes

The chromosomal basis of sex

In humans, Y chromosome determines sex

XX X XY

Gametes?

p(son)

p (daughter)

Humans 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

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

Wingless eyeless

Wildtype = normal Mutant = abnormal

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 Xw+ Xw X Xw+ Y

f2

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?

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

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

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

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

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 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

Chromosome 12 deletion along with an inversion (in the long arm) of chromosome 12

Inheritance of mitochondrial DNA and chloroplast DNA (plants)

• Extranuclear

• Maternal inheritance (cytoplasm)