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Chapter 15
The Chromosomal Basis of
Inheritance
Early Days of Genetics
 Pre 1900, geneticists and cytologists
studied mitosis and meiosis.
 In 1900, scientists in the two fields
began to see parallels between the
behavior of chromosomes and Mendel’s
“heredity factors” during the sexual life
cycle.
Early Days of Genetics
 Walter Sutton and Theodore Boveri and
others working independently began to
recognize the parallels.
 The things they noticed were:
 Chromosomes and genes are present in pairs in
diploid cells.
 Homologous chromosomes separate and alleles
segregate during the process of meiosis.
 Fertilization restores the paired conditions for both
chromosomes and genes.
 The chromosomal theory of inheritance
began to take shape.
The Chromosomal Theory of
Inheritance
 This theory suggested that:
 Mendelian genes have specific loci on
chromosomes.
 Chromosomes undergo segregation and
independent assortment.
Thomas Hunt Morgan
 Initially, Morgan was skeptical about
the chromosomal theory of inheritance
and Mendelism. However, his results
proved otherwise.
 Around 1906, Morgan began to piece
together the first solid evidence
associating specific genes with specific
chromosomes.
Morgan’s Organism of Study
 Morgan chose the
fruit fly (Drosophila
melanogaster) for its
ease of use,
heartiness, and
prolific breeding
habits.
Drosophila melanogaster
 Another thing making the fruit flies an
ideal tool for study is that they only
have 4 chromosomes (3 pairs of
autosomes and 1 pair of sex
chromosomes--Females XX; Males XY).
 One drawback: there were no suppliers
of fruit flies so he had to capture and
breed his own.
White Eyed “Mutants”
 After about a year of breeding and studying,
Morgan found a white eyed fly (normal eye
color is red).
 This eye color was due to a mutation and is
known as the mutant phenotype.
 Morgan and his students invented a notation
that is still used today to denote a mutant--a
lowercase letter, w.
 Writing the lowercase letter with a “+,” (w+)
superscript denotes the wild type phenotype.
Experiments With the Mutant
 Morgan immediately
mated the white
eyed male with a
red eyed female and
found all of the F1
offspring have red
eyes--suggesting
that red is the
dominant allele.
Experiments With the Mutant
 He also found that
when mating the F1
generation, the F2
exhibited the 3:1 ratio
of red eyes to white
eyes, but only males
had white eyes, and
they were present in a
50/50 ratio of red to
white.
Morgan’s Conclusion:
 Somehow the eye color of the fly is linked to
its sex. (If not, 1/2 of the white eyed
offspring would have been male, the other
half would have been female).
 Since females are XX and males are XY, he
concluded that the gene for eye color must
be located on the X chromosome, with no
corresponding gene on the Y chromosome.
Morgan’s Reasoning and
Analysis:
 In a male fly, having a
single copy of the
mutant allele would
give the mutant trait,
white eyes. Since
females have 2 X
chromosomes and all of
the F1 males have red
eyes, there is no way
for the females to have
white eyes in this
generation.
In Support of Sex Linkage
 This finding lent support to the
chromosomal theory of inheritance--a
specific gene is carried on a specific
chromosome.
 It also provided data regarding sex
linkage. That is, genes located on the
sex chromosomes exhibit unique
inheritance patterns and unique ratios
in the offspring.
Sex-Linkage Continued…
 Now that you know a little about sex
linkage, most sex-linked genes are only
found on the X chromosome.
 Females can pass such a disorder
(gene) on to both male and female
offspring.
 Males can only pass a disorder (gene)
on to daughters only.
Sex-Linkage Continued…
 Sex linked recessives are only displayed
in females when they are inherited in
the homozygous condition.
 Males display the trait when they inherit
one copy of the gene (said to be
hemizygous).
 Color blind example.
Linkage
 Some genes are said to be sex linked.
 Others are simply said to be linked.
They are on autosomes.
 They are inherited together with other
genes and the results of breeding
experiments lead to results different
from those predicted by Mendel’s law of
independent assortment.
Morgan’s Evidence of Linkage
 Morgan used more mutant traits that he
discovered.
 Normal fruit flies have gray bodies and
normal wings.
 2 mutants he noticed had black (b) bodies
and vestigial wings (vg).
 It was known that these mutations are
autosomal and recessive.
 He didn’t know if the traits were on the same
or different chromosomes, however.
To Determine Where the
Alleles for these Traits Were…
 Morgan crossed flies until he got truebreeding wild-type flies and truebreeding double mutant flies for black
bodies and vestigial wings.
 He could now perform a series of
crosses to see if the alleles for these
traits were on the same chromosomes
or were on different ones.
His First Cross…
 He crossed the
homozygous wild-type
fly with the double
mutant (homozygous
recessive) and got a
heterozygote.
 Remember, both true
breeding flies produce
only one type of
gamete, so a
heterozygote in the F1
is assured.
Are they on the Same
Chromosome?
 Next, he performed a
cross of the hybrid F1
offspring with another
double mutant.
 If the genes were on
different chromosomes,
then 4 different types of
offspring would be seen
in a 1:1:1:1 ratio.
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If they are on the Same
Chromosome…
 If they are on the
same chromosome
and no crossing over
occurs, then we
should see a 1:1
ratio of the parental
phenotypes.
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If they are on the Same
Chromosome…
 …and you see some
crossing over and
the genes are very
close together, most
of the offspring will
look like the parents
but some will be
recombinants.
 This is exactly what
Morgan saw!
Crossing Over…
 The whole idea of crossing over came from
these experiments that Morgan performed.
From the results, he reasoned that since the
numbers didn’t fit what was supposed to be
happening, something else must be
occurring.
 He reasoned that somehow a physical
breakage must be occurring between the
homologous chromosomes, something we
now call “crossing over.”
Additional Things to Come
From These Experiments…
 The idea of a linkage map using the
recombination frequencies of genes.
 Determination of the order of genes.
 Gene mapping to determine where the
genes were located.
Crossing Over and Gene Mixing
 Crossing Over
Sex Determination
 Whether or not a person is male or
female is determined from their
genotype: XX is female; XY is male.
 In humans, the father determines the
sex of the baby.
 The chance of being a male or female is
50/50. Half of the sperm will inherit a
Y, the other half will inherit the X.
Sex Determination and the Y
Chromosome
 The Y chromosome contains a region
(SRY gene) which codes for proteins
that induce the gonads to form testes.
 In the absence of this protein, the
gonads form ovaries.
Sex Determination and the X
Chromosome
 Inheriting an X chromosome from dad
will give a female 2 X chromosomes.
 Only one functions within the cell, the
other is inactivated.
 It becomes a Barr body.
 The Barr body becomes reactivated in
gametes so all of them have an active X
chromosome when produced.
X Inactivation
 This process is a totally random event
and occurs independently in embryonic
cells at the time of X inactivation.
 Females consist of a mosaic of active X
genes--those derived from the father
and those derived from the mother.
X Inactivation
 As the embryo continues to divide
mitotically, then we now have groups of
cells with active X chromosomes
derived from the mom, and active X
chromosomes derived from dad.
 If a female is heterozygous for a sex
linked trait, approximately 1/2 of the
cells will express one gene, and 1/2 will
express the other gene.
X Inactivation and Mosaicism
 X inactivation can be seen in calico cats.
 It can also be seen in a sweat gland disorder.
Nondisjunction
 Normally, in meiosis, the chromosomes
are distributed without fail and the
numbers of chromosomes remains the
same throughout the generations.
 Occasionally, chromosomes don’t get
separated properly in meiosis I or II.
 Some gametes fail to receive a copy of
a chromosome; others receive 2 copies.
Aneuploidy
 When nondisjunction occurs and is
followed by fertilization, a situation
arises where an abnormal number of
chromosomes are present in the
developing organism.
Aneuploidy
 A cell with triple the number of a
chromosome is known as trisomy.
 Trisomy 21 or Down Syndrome is an
example.
 Having only one copy of the cell
produces a situation known as
monosomy.
Nondisjunction and Mitosis
 Nondisjunction occurs in mitosis too.
 If it occurs very early on, then the
organism, if it survives, will likely have a
large number of phenotypic
abnormalities.
Chromosomal Alterations
 1. Deletion--a gene or base pair is lost.
 2. Duplication--a segment of DNA gets
repeated.
 3. Inversion--occurs when a
chromosomal fragment flip-flops and
reattaches to the original chromosome.
 4. Translocation--occurs when a
fragment from one chromosome is lost
and becomes attached to another
chromosome.
Deletions and Duplications
 Occur most often during meiosis
because of crossing over.
Duplications and
Translocations
 These don’t alter the balance of genes,
but the order on the chromosome is
disrupted.
 This affects the neighboring genes and
the expression of the
duplicated/translocated genes.
 They are usually lethal.
Non-Lethal Disruptions
 Aneuploidy--Down Syndrome. It isn’t a
duplication event, but essentially is like
a duplication event.
Aneuploidy
 Klienfelter Syndrome-XXY males. Have
male sex organs but their testes are
small and they are sterile.
 Also have other feminine characteristics
such as large breasts.
 They can be of normal intelligence, but
some often exhibit some mental
impairments.
Aneuploidy
 XXX females cannot be distinguished
from any other female except by
karyotype.
 XO are females with Turner’s syndrome.
It is the only known monosomy in
humans. They are sterile because their
sex organs don’t mature, can develop
2°sex characteristics with hormone
treatment.
Extranuclear Genes
 These are the genes found on the
chromosomes of organelles such as
mitochondria and chloroplasts.
 These are derived from the mother and
replicate themselves.
 They code for the proteins and RNA
that they use to perform their particular
functions.