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

Chromosome theory of inheritance

• Genes located on chromosomes = gene locus• Thomas Morgan

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?

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

XCXc

• Patches of colorblind cells in retina

• Normal phenotype

Blue colorblindness is rare

• Genotype of orange female? • X-orange X-orange• Genotype of black female?• X-black X blackGenotype of calicoX-orange X-black• Genotype of orange male?X-orange Y• Genotype of black male?• X-black Y

LinkageUnlinked 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

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 statureLymphadema of the hands and feetBroad chest Low hairlineLow-set earsReproductive sterilityRudimentary ovaries gonadal streak absence of a menstrual periodIncreased weight, obesityShield shaped thorax of heartShortened hand bonesSmall fingernailsCharacteristic facial featuresWebbed neckAorta abnormalitiesPoor breast developmentHorseshoe kidneyVisual impairmentsEar infections and hearing lossHigh 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)

Possible symptomsCommon physical signs include:Excess skin at the nape of the neckFlattened noseSingle crease in the palm of the handSmall earsSmall mouthUpward slanting eyesWide, short hands, short fingersWhite spots on eye Physical development is often slower than normal. Delayed mental and social development.

Chromosome structure abnormalities

1. Deletion – fragment lost2. Duplication – repeated fragment3. Translocation – fragment attaches to other

chromosome4. Inversion – fragment inverts

Inheritance of mitochondrial DNAand chloroplast DNA (plants)

• Extranuclear