T.H. Morgan: first to associated genes with chromosomes

• worked with Drosphila melanogaster(fruit flies)

Fruit flies produce 100s of offspring, new generation every 2 weeks, only 3 autosomal and 1 sex chromosome

wild type: the normal/most common phenotype in natural populations

mutant: the opposite of wild type

sex-linked/X-linked: genes located on sex/X chromosome

Y chromosomes has few genes, so most mutants are on the X chromosome

genes on the same chromosome passed along as a unit

genes on the same chromosome are linked

linked genes don’t follow the Law of Independent Assortment

Genetic map: an ordered list of loci along a chromosome

genetic map was constructed by: Sturtevant, Morgan’s student

linkage map: created by associating frequencies of recombinants along the chromosomes

the farther apart the two genes, the high the chance that a crossover will occur, and higher the recombination frequency

1 map unit(mu) = 1% chance of recombination

more than 50 mu indicates no linkage

males determine gender of offspring

For a female to express a recessive sex-linked allele, she must be: homozygous recessive

hemizygous: only one locus(one X to be expressed)

X-linked recessive disorders: re-green color blindness, Duchenne muscular dystrophy, hemophilia

X-inactivation: only one of two X chromosomes are active in each cell

Barr body: the inactive, condensed X chromosome

Where are the Barr body chromosomes reactivated? ovaries

Calico cats are only female, only male when: XXY (Klinefelter’s)

Nondisjunction: gametes having one extra and one less chromosome

Nondisjunction is caused by: error in homolog separation in anaphase I and sister chromatid separation in anaphase II

aneuploidy: abnormal chromosome number

trisomy: three chromosomes in a gamete

monosomy: one chromosome

polyploidy: entire extra set of chromosomes; common in plants

trisomy 21: down syndrome

Klinefelter’s: extra X chromosome

Jacob’s syndrome: extra Y chromosome

trisomy X: shows no difference from the normal XX

Turner’s Syndrome: monosomy X

Cri du chat: deletion in chromosome 5

Leukemia: reciprocal translocation