Chapter 15
The Chromosomal Basis of
Inheritance
Chromosome Theory of Inheritance
• Created by Sutton, Boveri, et al in 1902
• States that Mendelian genes have specific loci
on chromosomes, and it is the chromosomes
that undergo segregation and independent
assortment
Genes are Traced to Specific
Chromosomes
• Thomas Hunt Morgan was the first to
associate a specific gene with a specific
chromosome, early in the 20th century
• Experiments provided convincing evidence
that chromosomes are indeed the location of
Mendel’s heritable factors
Step One: Choose an Experimental
Organism
• Used Drosophila, a type of fruit fly
• Chosen because a single mating will produce
hundreds of offspring, and a new generation
can be bred every two weeks
• Only has four pairs of chromosomes, which
are easily distinguishable with a light
microscope
• Bred these flies for a year before a mutant
phenotype had come about
– The wild type has red eyes, where as the mutant
had white
– Found later that it is a Sex Linked gene (found
only on X chromosome)
• Further supported the chromosome theory of
inheritance
Some Genes Are Linked
• Morgan found that wing size and body color
were linked genes found on the same
chromosome
– Instead of crosses showing Mendelian outcomes,
they were either dominant or recessive, and
hardly ever heterozygous
Recombination of Unlinked
• Parental types are offspring who share a
phenotype with the parent generation
• Recombinants are offspring that do not share
a phenotype with the parent generation
– Blame it on recombination, not the mailman!
Recombination of Linked Genes
• Crossing over accounts for the recombination
of linked genes
• Nonsister chromatids break and switch
fragments
– Allows for recombinants to show in linked genes
as well
Genetic Map
• An ordered list of the genetic loci along a
particular chromosome
• The recombination frequencies calculated reflect
the distances between genes on a chromosome
– The farther the two genes are apart, the more likely
that crossover will occur between them and therefore
the higher the recombination frequency
• Came up with a linkage map, a genetic map
based on recombination frequencies
•Red Rover Example!
Practice Example
Practice Problem
• A-B = 78.9%, or 78.9 map units
• B-C = 1.7%, or 1.7 centimorgan
• A-C = 18.3%
Cytological Maps
• Locate genes with respect to chromosomal
features, such as stained bands, that can be
seen with a microscope
– Linkage maps don’t actually paint a true picture,
they give a rough idea of how a chromosome is
aligned
Sex Chromosomes
• It is quite simple, in humans, if you receive
two X chromosomes, then you are female, and
if you receive one X and one Y, then you are
male
• However, if you do get a Y chromosome, that
does not mean you are male
– SRY genes (sex determining region of Y) is
required for the development of testes
• In the absence of SRY, ovaries develop
• Bugs: there is the X-0 system
– There is only 1 sex chromosome, if you receive two
copies you are female, only one means you are male
• Birds, fish, and some insects: Z-W system
– Female is what determines sex of offspring, males are
ZZ, females are ZW
• Bees and Ants: haplo-diploid system
– No sex chromosomes, females develop from fertilized
ova and are diploid, males develop from unfertilized
eggs and are haploid (they have no father)
Interesting Fact
• Sperm containing a Y chromosome are lighter,
and therefore more likely to reach the target
(egg)
– At fertilization, there is a ~56% likelihood of a
male being produced
– However, most fertilizations don’t make it to
pregnancy, (more often male than female) so the
odds are still 50/50
Sex Linked Disorders
• Duchenne Muscular Dystrophy
– One of 3,500 males
– Rarely live past early 20’s
– Progressive weakening of muscles and loss of
coordination
– Traced the disorder to the absence of a key
muscle protein at a specific locus on the X
chromosome
• Hemophilia is the absence of one or more of
the proteins required for blood clotting
– Early Jewish people had an idea of how
hemophilia was passed, and sons born to women
with a family history of the disorder were exempt
from circumcision
– Passed along through Royal Families in Europe
• Consanguenios mating
X-Inactivation in Female Mammals
• One X is completely inactivated during embryonic
development
– It is random
• The inactive X in each cell of a female condenses into a
compact object called a Barr body
• Females consist of a mosaic of two types of cells; those
with the active X from mother, an those with the active
X from father
– Shown in a calico cat, which has an allele for orange and
black
– In humans, sometimes appears as a prevention of
developing sweat glands
• Will have patches of normal skin and patches without sweat
glands
Alterations to Chromosomes
• Change in number
• Change in structure
Change in Number
• Nondisjunction: the members of a pair of
homologous chromosomes do not move apart
properly during meiosis
• Some gametes will receive two of the same,
and some will receive none
– Zygote will have abnormal number of
chromosomes, called aneuploidy
• If a chromosome is present three times, it is
called trisomy
– 2n + 1
• If a chromosome is missing, it is called
monosomy
– 2n – 1
• Some organisms have more than two complete
sets of chromosomes, they are called polyploidy
– Common in plants
Change in Structure
• Deletion: a chromosomal fragment lacking a
centromere is lost during cell division
• Insertion: fragment may become attached as
an extra segment to a sister chromatid
• Inversion: reattaches in the reverse order
• Translocation: fragment joins a
nonhomologous chromosome
• Duplication: just what the name says
Human Disorders- Numbers
• Most aneuploidy human zygotes are naturally
aborted
– Chromosome alterations are too disasterous
• Those who survive end up with disorders such
as:
– Down’s Syndrome
– Klinefelter Syndrome
– Turner Syndrome
Down Syndrome
• 1:700
• Extra chromosome 21 (Trisomy 21)
• Characteristic facial features, short stature,
heart defects, respiratory infections, mental
retardation, more apt to have leukemia and
Alzheimers
• Has a correlation to age of mother
Klinefelter Syndrome
• 1:2,000 births
• An extra X chromosome in males: XXY
• Have male sex organs, but testes are small and
sterile
• Breast enlargement and other feminine body
characteristics
Turner Syndrome
• 1:5,000 births
• Monosomy X
• Phenotypically female, sex organs do not
mature, sterile
Human Disorder- Structure
• Genetics Library
Disorders that are from Mom or Dad
• Prader-Willi syndrome
– Mental retardation, obesity, short stature, small
hands and feet
– Deletion on chromosome 15 from father
• Angelman syndrome
– Spontaneous laughter, jerky movements, other
motor and mental symptoms
– Deletion on chromosome 15 from mother
• Genomic imprinting
Genomic Imprinting
• A gene on one chromosome is somehow
silenced, while its allele on the homologous
chromosome is left free to be expressed
• Same alleles may have different effects on
offspring, depending on if they originated in the
ovum or sperm
– In offspring, all imprints are erased and gone for next
generation
• Happens because of methyl groups
– epigenetics
Amendment
• Not all genes are located on chromosomes, or
even in the nucleus
• Extranuclear genes are found in mitochondria
and chloroplasts
– Tell them when to reproduce
– Based on endosymbiotic theory
• Mitochondrial DNA comes from the mother
because the mitochondria passed on by the
zygote all come from the cytoplasm of the ovum