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Ch 15: The Chromosomal
Theory of Inheritance
 Genes have
specific loci on
chromosomes
and
chromosomes
undergo
segregation
and
independent
assortment
Discovery of Chromosomal
Linkage
 Thomas Hunt
Morgan
observed the
occasional
white-eyed
(blind) fruit fly
So he mated the flies…
…and discovered half the males
had white eyes and NONE of the
females had white eyes!
Conclusion:
 The gene for
eye color is
located on
the X
chromosome
Follow-Up question
 Why were all of Morgan’s F1 flies red-eyed
and only males in F2 were white-eyed?
Why were there no white-eyed females in
F2?
Human sex-linkage
 SRY gene: on Y chromosome, triggers
development of testes
 Fathers: pass X-linked alleles to all
daughters (but not to sons)
 Mothers: pass X-linked alleles to both
sons & daughters
Sex determination in different species
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)
Genetic recombination
 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 (# CO / total ) * 100 = %CO;
m.u.=%CO / 2
 Linkage maps: Genetic map based on
recombination frequencies
Follow-Up
 What is the MAXIMUM number of map units
possible for two unlinked genes?
 How would one explain a testcross involving F1
dihybrid flies in which more parental-type
offspring are produced than recombinant-type
offspring?
 What can cause new combinations of linked
genes?
Follow-Up
 Why are all females a “mosaic” of X-linked
genetic information?
 In cats, black fur is caused by an X-linked allele;
the other allele at this locus causes orange fur.
Heterozygotes have tortoiseshell color. What
kinds of offspring would you expect from crossing
a black female and an orange male?
Chromosomal errors: nondisjunction
& aneuploidy
Other chromosomal errors:
Leukemia: reciprocal
translocation
Philadelphia c-some  CML, translocation moves a protooncogene under control of an active gene so the cell-cycle
activating gene is now abnormally activated, resulting in
leukemia (BCR-ABL)
Interesting note – the successful drug Gleevec ® prevents ATP
from binding the active site of the mutant ABL protein, thus
stopping the cancer cells from undergoing the cell cycle!!!
Genomic
imprinting
 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
Genomic imprinting in mice:
Follow-Up, again!
 A woman is found to have genotype 47, XXX.
What is her expected karyotype?
 In mammals, we use the X-Y system to identify
gender. What system is used in birds? In
ants/bees?
 The genotype for gene “d” on the X c-some in
homozygous dominant females is written XDXD.
What is the reason for writing the second locus as
a superscript? How would you write the genotype
for a female carrier at this locus?
More Follow-Up…
 What is the result of a nonreciprocal cross
over?
Follow-Up – at-home study!
 Research and understand the genetic causes,
treatments, and methods of detection/prevention
for the following conditions:
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PKU
Sickle-cell anemia
Down’s syndrome, Edward’s syndrome
Cretinism
Erythroblastosis fetalis
Blue-baby condition
Tay-Sachs
Klinefelter (XXY), Turner (X0)