Lecture #6 Date - Cloudfront.net
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Transcript Lecture #6 Date - Cloudfront.net
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)