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Chapter 15
Chromosomal Basis Of
Inheritance
Chromosome Theory of
Inheritance
Mendelian genes have specific
loci on chromosomes and it is the
chromosomes that undergo
segregation and independent
assortment not the alleles.
I. Morgan and Friends
First to associate a specific gene to a
specific chromosome
Used drosophila / looked at wild types and
mutants
He discovered sex-linked traits, linked
genes, and crossing over
I. Morgan and Friends
1. Sex-linked traits – genes located on sex
chromosomes (usually X in humans)
examples: color blindness and
hemophilia
I. Morgan and Friends
1. Linked Genes – genes located on the
same chromosome and inherited
together
a. Started with wild and double mutant
B-gray (wild)
G-normal (wild)
b-black (mutant)
g-vestigial (mutant)
P – BBGG x bbgg
F1 - BbGg
I. Morgan and Friends
b. Crossed F1 with a double mutant
F1 – BbGg x bbgg
c. What did Morgan expect to get from this
cross? Do the Punnett Square
He expected a 1:1:1:1 ratio
d. What did Morgan actually get?
965 – gray/normal
944 – black/vestigial
206 – gray/vestigial
185 – black/normal
I. Morgan and Friends
e. How did Morgan explain this?
Maybe the traits were on the same
chromosome (linked)
f. If they are linked what results should
Morgan have seen?
He should have seen a 1 gray/normal :
1 black/vestigial ratio
g. Why didn’t he get 1:1?
Crossing Over
I. Morgan and Friends
3. Crossing Over – sections of chromosomes
switching places
- genetic recombination – new traits in
offspring
- parent types – offspring like parents
- recombinant – offspring w/ new traits
- a 50% frequency of recombination is
observed for 2 genes on different
chromosomes
- If Morgan’s flies were completely linked it
should be 1:1:0:0 but there were some
recombinant types because of crossing over
II. Using Recombination to Map Loci
 Sturtevant
 Genetic Map – ordered list of loci on a chromosome
 The farther apart 2 genes are the better chance of
crossing over and a higher recombination frequency
 Linkage Map – genetic map based on recombination
frequency
 Map Unit – unit used to distance genes, =1 recomb.
Frequency (centimorgan)
 Some genes are so far apart crossing over is inevitable
resulting in over 50% rf. Can’t tell the difference
between it and 2 chromosomes
 Cytological Map – locate genes w/ respect to
chromosomal structures, like centromere
III. Sex Chromosomes
 Not all organisms are X and Y based
- insects are X – 0 / males XO, females XX
- birds are Z – W / males ZZ, females ZW /
ovum determines the gender not sperm /
opposite of the XY system, that is why ZW is
used
- bees are haplo-diploid / males haploid and
develop from unfertilized eggs
(parthenogenesis), females diploid and develop
from fertilized eggs / no sex chromosomes
III. Sex Chromosomes
III. Sex Chromosomes
 XX=female XY=male / very little crossing over
/ SRY codes for proteins that regulate male
characteristics
 Sex-linked traits / Duchenne Muscular
Dystrophy, Hemophilia, Color Blindness
 X inactivation – one X in females is turned off
during embryonic development
Barr body – what the inactive X condenses into
Inactivation is random after the embryo develops some
so female is a mix of active mom and dad X’s
Methylation and XIST gene causes this
IV. Errors in Chromosomal
Inheritance
1. Non-disjunction
Uneven distribution of chromosomes
Aneuploidy – abnormal chromosome
number
Trisomic – 2n + 1
Monosomic – 2n – 1
Polyploidy – more than 2 complete
chromosome sets
Ex. Down’s Syndrome, Klinefelter’s
Syndrome, Turner’s Syndrome, XXX, XYY
2. Altering Chromosome Structures
Deletion – piece of chromosome is lost
Duplication – when a fragment attaches to
a sister chromatid
Inversion – fragment occurs and flips and
reattaches Tall Oprah Boy
Translocation – fragment joins a nonhomologous chromosome
Ex. Cri du chat, Chronic myelogenous
leukemia
V. Where’d You get that gene? Mom or
Dad?
Sometimes phenotype depends on if the
alleles were passed on by mom or dad
Prader-Willi – mental retardation, obesity,
short, small hands and feet
Angelman – uncontrollable laughter, jerky
movement
Both caused by deletion on 15 / P-W if it
comes from Dad, Angelman if it comes
from Mom
V. Where’d You get that gene? Mom or
Dad?
Genomic Imprinting – one gene is silenced
while the homologous chromosome is
expressed
- These alleles have different effects depending
on if they come from sperm or egg
- Caused by methylation
V. Where’d You get that gene? Mom or
Dad?
Fragile X Syndrome – part of X hanging on
by a DNA “thread”
- methylation of abnormal allele by mom
causes the syndrome in males.
- it could come from mom or dad in
females that have the condition.
VI. Extranuclear Genes
Small circles of DNA found in mitochondria
and chloroplasts
Mom passes down mitochondrial DNA
Mutations accumulate in mitochondrial
DNA over a life time / some believe this is
part of the aging process
Problems!!
Do the following problems at the end of
chapter 15.
1, 2, 3, 4, 6, 7, 10, 15, 16