Lecture - Chromosomes

Haploid (N)

one copy of genetic material subdivided into chromosomes

Diploid (2N)

two copies of genetic material subdivided into chromosomes

Haploid example

gamete

Diploid example

Somatic cell

Independent assortment

when genes are on different chromosomes

Crossing over (crossover)

when genes are on the same chromosome

• Note: not all organisms are capable of crossing-over (ex. male fruit flies cannot do it but females can) and have to rely exclusively on independent assortment for genetic recombination

Sex chromosomes

different in females (XX) and males (XY)

Autosomes

all other chromosomes

Popular model organism

Drosophila melanogaster (fruit fly)

XY System: XY , XX

• Y determination: mammals (including humans)

• X-to-autosome ratio: fly, worm, some plants

ZW System: ZZ, ZW

birds, butterflies, moth, some fish

Ploidy: Diploid and Haploid

ants, bees, wasps

Genic determination (alleles)

• Yeast mating type genes a and alpha

• Nurospora A and a

Turner Syndrome

XO

Klinefelter Syndrome

XXY

SRY (Sex-determination Region on Y)

encodes a product called testis-determining factor that promotes the formation of testes instead of ovaries

• only one X chromosome is needed in males and females (after a while)