Transcript Chapter 15~ The Chromosomal Basis of Inheritance ______

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 Ychromosome
– 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, II
Alterations 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