Worksheet for problem set #7
February 27 2006
1.
a) how
are these traits inherited?
Treat each trait separately-
add up the proportions for eye color: rosy and red. What are the observed
proportions of offspring for each eye color?
F2 ratio of _________=
predictable ____________________ dominant:recessive.
Therefore, the trait
____________ eyes is ___________________ recessive.
Do the same for body color.
F2 ratio of _________=
____________________.
Therefore, the trait
___________________ body color is ___________________ recessive.
Note: with a sex-linked F2
cross, if the male carries the recessive trait, expect a ratio of
________________; if the
male carries the dominant trait, expect a ratio of
__________.
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2.
Testcross: an individual of
unknown genotype is crossed with another individual with a homozygous recessive
genotype for the trait in question.
Three-point testcross: a
cross between an individual heterozygous at three linked loci with an
individual homozygous recessive at the three loci.
With a three-point cross,
the order of the three genes can be determined, plus double crossovers (DCO)
can be detected, allowing the calculation of map distances.
Parental genotypes always
have the _____________ progeny (nonrecombinant progeny), DCOs always have the
_____________.
figure 7.12 here
Note that in the DCOs, the
outer two alleles are the same as in the
____________________, but
the middle allele is different. From this we can figure the order of the genes.
In a triple cross, two kinds
of progeny are produced: those that are heterozygous,
displaying the _____________ trait, and those that are
homozygous, displaying
the ________________ trait (you get these two classes
for each locus or trait- so that you expect 23=8 classes of progeny.
Shortcut: usually, the
testcross progeny are not represented by the complete genotype, e.g.
e+z rather, we
list only the alleles expressed in the phenotype, e.g. e+z
eyz
To make a map of the genes,
we need information about where and how often crossing over occurs. The info
that we need for mapping comes entirely from the
_________________ parent (the _______________ parent may cross over, but since
the alleles are the same, we don’t gain any relevant information from this).
Map distances are based on
the ______________________________. Add up the number of recombinant progeny
(including both SCO and DCO), divide by the total number of progeny, and
multiply by 100.