The Chromosomal Basis of
Inheritance
21 and 24 October, 2005
Text Chapter 15
Genes and
Chromosomes
The behavior of
chromosomes in
meiosis and
fertilization explains
Mendel’s rules of
inheritance.
Genes on different
chromosomes assort
independently.
Morgan’s Mutant
Morgan’s experiments showed that
some genes are inherited along with
the X chromosome.
Genes that are on the same
chromosome are linked. Those on the
X chromosome are X-linked.
Crossing over allows linked genes to appear to assort independently.
Remember, the
purpose of crossing
over is to generate
diversity in offspring
arising from sexual
reproduction.
Crossing over results in recombinant gametes.
Recombinant gametes lead to
offspring.
Recombination Distance
Genes that are far apart are more likely to have a crossing-over event
occur between them than genes that are closer together on the
chromosome.
Mapping
Knowing the recombination
frequencies between several
linked genes allows the researcher
to construct a genetic map. Note
that the maximum genetic distance
discernable in a cross is 50%
(indistinguishable from
independent assortment.) Larger
genetic distances are determined
by adding.
Sex Determination
Sex-linked genes (carried on the X chromosome) show unique
patterns of inheritance. Since the Y chromosome has few genes,
recessive sex-linked alleles are not masked in males.
A father with an xlinked trait will
transmit the allele to
all daughters, but no
sons.
A carrier mother will
pass the allele to 1/2
of her offspring.
Sons will be affected,
daughters will be
carriers.
Carrier female x
affected male
produces affected
females
X Inactivation
Nondisjunction
Meiotic nondisjunction occurs when
homologues or sister chromatids fail
to separate in anaphase. This error
leads to gametes that have too many
or too few chromosomes.
If these gametes are involved in
fertilization, the resulting offspring
will have an abnormal number of
chromosomes (aneuploidy). They
may have one (monosomy) or three
(triploidy) copies of a chromosome.
This changes the gene dosage of the
genes on the affected chromosome
Alterations of Chromosome Structure
Gene dosage can also be changed by structural alterations within a
chromosome, including deletion, duplication, translocation, and
inversion.
Down Syndrome