Transcript Ch. 15 Chromosomal Inheritance

Chapter 15
The Chromosomal Basis of
Inheritance
Mendelian inheritance has its
physical basis in the behavior
of chromosomes
 In
the early 1900s biologists
began to see parallels between
the behavior of chromosomes
and the behavior of Mendel’s
“factors.”
Chromosome Theory of
Inheritance
 Mendelian
genes have specific
loci on chromosomes, and it is
the chromosomes that undergo
segregation and independent
assortment.
Law of Segregation
*Two alleles for
each gene separate
during meiosis.
Law of Independent
Assortment
*Alleles of genes on nonhomologous assort independently during meiosis
Linked Genes
 Tend
to be inherited together
because they are located near each
other on the same chromosome.

The results are visible in crosses
where the outcome deviates from
those expected (from Mendel’s law of
independent assortment)
Evidence for linkage
Gene Maps
 An
ordered list of the genetic loci
along a particular chromosome.

The further apart two genes are, the
higher the probability that a crossover
will occur between them.
• Such map is called a linkage map
Linkage Mapping Using
Recombination Data
A Partial
Linkage Map
for Fruit Flies
Gene Maps
 These
distances are relative to each
other and are not actual physical
distances.
 A linkage map portrays the order of
genes along a chromosome no the
actual precise locations.

Cytogenetic maps…
Chromosomal
basis of sex
Inheritance of Sex-Linked
Genes
 Sex-linked

versus X-linked
Often spoke of
interchangeably…though it is not
h
X – hemophilia
d
 X – Duchenne muscular

dystrophy
X
X
Y
X
X inactivation in female
mammals
 Although
female mammals,
including humans, inherit two X
chromosomes, one X
chromosome in each cell becomes
almost completely inactivated
during embryonic development.

Barr body
Calico Cats: Example of Barr Body
http://www.bio.mi
ami.edu/dana/do
x/calico.html
Abnormal Chromosome #
 Nondisjunction

Leads to aneuploidy:
• Aneuploidy is the condition of having
less than or more than the normal
diploid number of chromosomes, and
is the most frequently observed type
of cytogenetic abnormality.
The most common type of
aneuploidy
 Monosomy
Turner's syndrome is the most common
sex chromosome abnormality of human
females. Its incidence in newborns is
approximately 1 in 2,500
 Trisomy

–
A common autosomal trisomy in humans in
Down syndrome, or trisomy 21.
Meiotic nondisjunction
Alterations of chromosome
structure
The Key Points in Review
 Mendelian
inheritance has its
physical basis in the behavior of
chromosomes during sexual life
cycles.
 Morgan
traced a gene to a specific
chromosome.
Key Points in Review
 Linked
genes tend to be inherited
together because they are located
on the same chromosome.
 Independent
assortment of
chromosomes and crossing over
produce genetic variation
(recombinants)
Key Points in Review
 Geneticists
can use
recombination data to map a
chromosomes genetic loci.
 Chromosomal
basis for gender is
dependent upon the organism.
Key Points in Review
 Sex-linked
genes have unique
patterns of inheritance.
 Alterations
of chromosome
numbers or structure cause some
genetic disorders.
Morgan’s Fruit Flies
A
tutorial