Chapter 15~ The Chromosomal Basis of Inheritance ________

The Chromosomal Theory of Inheritance

Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

Chromosomal Linkage Thomas Hunt Morgan Drosophilia melanogaster

– Observed white eye mutation in males only

– Red eyes are more common phenotype, known as “wild type”

– Concluded that eye color gene was located on X chromosome

Sex-linkage: genes located on a sex chromosome

More common in men, due to XY chromosomes, inability to masked faulty X conditions

Sex Linkage inheritance If mother is a carrier of X linked

condition– Her son(s) will have a 50/50

chance of having the condition– Her daughters will have 50/50

chance of being carriers If Father has an X-linked condition

– His sons can only inherited Y-chromosome

– His daughters will inherited the faulty X chromosome, 100% chance of being at least carrier

How to get a female with X-linked condition?

– Need mother who is a carrierXcX, and father with condition XcY

– 50/50 chance of female with condition XcXc

Human sex-linkage

Sex-Linked Disorders– Color-blindness

– Duchenne muscular dystropy (MD)

– hemophilia

X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene in calico cats)

Genetic recombination Linked genes

– Genes that are located very close on a chromosome and will be inherited together.

– Only way to “unlink” them is due to random chance of crossing over

Genetic maps– The further apart 2 genes are, the

higher the probability that a crossover will occur between them and therefore the higher the recombination frequency

Linkage maps– Genetic map based on

recombination frequencies

Chromosomal errors, I Nondisjunction:

– Homologous chromosomes or sister chromatids do not separate properly during meiosis I or meiosis I

Aneuploidy: chromosome number is abnormal

– Monosomy~ missing chromosome

– Trisomy~ extra chromosome

– Polyploidy~ extra sets of chromosomes

Chromosomal errors, IIAlterations of chromosomal structure: Deletion: removal of a chromosomal segment Duplication: repeats a chromosomal segment Inversion: segment reversal in a chromosome Translocation: movement of a chromosomal segment to another