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The Chromosomal Basis of
Inheritance
25 October, 2002
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 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
Genomic
Imprinting
In a plant, leaf color and leaf shape are controlled by two linked
genes. Leaves of the wild-type plant are red. A recessive mutation
in this gene causes white leaves. Wild-type leaves are pointed, and
a recessive mutation in this gene causes them to be smooth. The
following crosses were performed: pure breeding white, smooth X
pure breeding wild type gives F1: all red, pointed
Now, the next cross: red, pointed X pure breeding white, smooth
gives F2:
40 white, smooth
36 red, pointed
10 white, pointed
14 red, smooth
What is the recombination frequency between the gene for color
and the gene for leaf shape?
Parental types: 76
Recombinants: 24
Total is 100
Recombinants / Total is 100 / 24 = 0.24
If a man and a non-color blind woman jointly have three sons,
two of whom are color blind, which is the most likely
explanation?
a) The mother is homozygote for color blindness.
b) The mother is heterozygote for color blindness.
c) The father is color blind.
d) None of the above.
A recessive sex-linked lethal mutation is generated in
Drosophila. If a female heterozygous for the lethal allele is
crossed to a wild-type male, the ratio of males to females
expected among the living
progeny would be:
(a) 0:1
(b) 1:1
(c) 2:1
(d) 1:2
Hemophilia in humans is due to an X-chromosome mutation. What
will be the results of mating between a normal (non-carrier) female
and a hemophiliac male?
A. half of daughters are normal and half of sons are hemophilic.
B. all sons are normal and all daughters are carriers.
C. half of sons are normal and half are hemophilic; all daughters are
carriers.
D. all daughters are normal and all sons are carriers.
E. half of daughters are hemophilic and half of daughters are
carriers; all sons are normal.