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 Morgan (early 20th century)
Drosophilia melanogaster(fruit flies)
Associated a specific gene with a specific
chromosome
Morgan’s Experiment
P1: Mated white eyed male with red eyed
female
F1: 100% red eyed
F1 generation mated
F2: 3 red : 1 white
However???
Morgan’s Experiment:
All females were red eyes:
Half the males were red
The other half were white
Morgan’s Conclusion:
1. 1. Eye color was linked to sex
2. 2. specific genes are carried on specific
chromosomes
3. 3. genes located on sex chromosomes
exhibit unique inheritance patterns
Linked Genes:
Sex-linkage: genes located on a sex
chromosome
Linked genes: genes located on the
same chromosome that tend to be
inherited together
Another Morgan Experiment:
This time he observed body color and
wing size:
Wild type = gray body (b+) and
normal wings (vg+)
Mutant type = black body (b) and
vestigial wings (vg)
First Cross: true breeding
wild type wit black vestigial
wings
b+b+vg+vg+ X bbvgvg
F1 = all wild type phenotype
(b+bvg+vg)
Second Cross: female dihybrids
vs true breeding reeessive males
b+ bvg+vg X bbvgvg
(test cross)
2300 offspring were scored
Results:
-High proportion of parental phenotypes
(965 wild type, 944 black vestigial)
-Low proportions of non- parental
phenotypes
(206 gray vestigial, 185 black normal)
Conclusion:
1. Body color and wing size are usually
inherited together
(genes must be on the same
chromosome??)
Conclusion:
2. Body color and wing size are only
partially linked:
Explaining Morgan’s Results:
Recombination of unlinked genes vs. linked
genes:
Unlinked genes = independent assortment
Linked genes = crossing over
Recombination:
Production of offspring with
combinations of traits different
from those found in either parent!
Genetic Mapping:
Genetic maps are an ordered list of the
genetic loci along a particular
chromosome
Genetic mapping:
Recombination frequencies depend on:
*Distances between genes on a
chromosome
Recombination frequency refers
to the percentage of recombinants
occurring in the offspring
Alfred Sturtevant:
*crossing over is a random event
*the farther apart the genes on a
chromosome, the higher the
probability that crossing over will
occur, so the higher the recombination
frequency
Sturtevant Reasoning:
The further apart two genes are, the more
points between them where crossing over
can occur.
Linkage Map:
Probability of crossover between two
genetic loci is proportional to the distance
separating the two loci.
*experimental crosses reveal
recombination frequencies
Example: Drosophila
Body color (b)
Wing size (vg)
Cinnabar (cn)
Map units:
Distance between genes on a
chromosome
1 map unit = 1% recombination frequency
Do not correspond to physical
frequencies
Seed and flower color in pea
plants:
Genes that are very far apart on the
chromosome
Crossing over is almost certain.
Frequency of crossing over is not
uniform over the length of the
chromosome
Map units do not portray order of genes
on a chromosome
Genetic recombination
• Crossing over
Genes that DO NOT assort
independently of each other
• 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
Human
sex-linkage
• SRY gene: gene on Y chromosome that triggers
the development of testes
• Fathers= pass X-linked alleles to all daughters
only (but not to sons)
• Mothers= pass X-linked alleles to both sons &
daughters
• 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 gene in cats)
Sex Linked Genes:
•Fathers= pass X-linked alleles to
all daughters only (but not to
sons)
•Mothers= pass X-linked alleles
to both sons & daughters
Sex Linked Disorders:
•Sex-Linked Disorders:
•Color-blindness
• Duchenne muscular dystropy
(MD)
• hemophilia
Chromosomal Errors:
•Nondisjunction: members of a pair
of homologous chromosomes do
not separate properly during
meiosis I or sister chromatids fail to
separate during meiosis II
Chromosomal Errors:
•Aneuploidy: chromosome number is
abnormal
• Monosomy: missing chromosome
• Turner Symdrome -XO
• Trisomy : extra chromosome
• Down syndrome- Trisomy- 21
• Kleinfelters Syndrome- XXY
• Polyploidy: extra sets of chromosomes
Chromosomal Errors:
• 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
Point mutations: affect
protein structure and function
•Base pair substitution: one
nucleotide pair replacing another
• Missense vs. Nonsense mutations
• Missense = altered codon still codes for an
amino acid – not necessarily the right one
• Nonsense = changes the codon to a stop codon
• Premature termination leading to malfunctional
proteins.
Insertions and Deletions:
• Adding or losing a nucleotide pair
• Disastrous effect on the protein
• Causes a Frame Shift:
• Nucleotides down stream of the mutation will
be improperly grouped into codons that will
likely produce a non- functional protein
Genomic imprinting
•a parental effect on gene
expression
• Identical alleles may have different
effects on offspring, depending on
whether they arrive in the zygote via the
ovum or via the sperm.
• Fragile X syndrome: higher prevalence of
disorder and retardation in males
Inheritance of Organelles:
•Some genes are considered extranuclear:
• That is not found in nucleus
• But, in organelles such as mitochondria and
chloroplasts
•These genes do not follow mendelian
inheritance patterns
• Randomly assorted to gametes and
daughter cells
Inheritance of Organelles:
•Organelles are inherited maternally
• Sperm only contributes genetic
information
Mutations:
•Plant variegation: due to mutations
in the genes that control plant
pigments
• Pattern of variegation is determined by the
ratio of wild type allele vs. mutant type allele
for pigmentation
Mitochondria DNA
mutations:
•Heteroplasmy: when a cell contains
both wild type and mutant type
mtDNA.
• Disorders usually affect nervous and
muscular systems
• They require the most energy from ATP
Mitochondria DNA
mutations:
• Disorders of the optic nerve (leber’s
neuropathy) and other eye defects.
• Kearns- Sayre Syndrome- abnormal heart rate
and central nervous system disorder
• Mitochondrial myopathy: muscle
deterioration, intolerance to exercise