New alleles arise by mutation
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Different alleles exist because any gene is subject to mutation, or change, to
a stable, heritable new form
Alleles can randomly mutate to become a different allele depending on DNA
sequence changes.
Wild type is a term used for the most common allele in the population.
Other alleles, often called mutant alleles, may produce a phenotype
different from that of the wild-type allele.
A genetic locus is called polymorphic if the wild-type allele has a frequency
less than 99% in a population (that is, if more than 1% of the alleles at that
locus are mutant alleles).
An alternate form of designating alleles. Alleles that are wild type are
expressed with a +
Ex. Red eye color (w+) is dominant to white eye color (w). The red eye is the
wild type. Don't let this confuse you, its just a different way to express
alleles.
Linkage Groups
1. Fruit flies have four pairs of chromosomes to hold thousands of genes
2. All alleles on a chromosome form a linkage group that stays together except
when crossing over.
3. Crossing-over causes recombinant gametes and at fertilization, recombinant
phenotypes.
4. Linked alleles do not obey Mendel's laws because they tend to go into the
gametes together DUE TO BEING SO CLOSE ON THE SAME
CHROMOSOME!
5. Crosses involving linked genes do not give same results as unlinked genes.
So far, your studies and practice with crosses have involved alleles located on
separate chromosomes, and crosses that follow Mendel's Law of Independent
Assortment. In actuality, many genes are located on the same chromosome, and
they do not assort independently, instead, they are inherited together. They won't
follow the normal rules of punnet squares, and the ratios obtained from crosses do
not have the normal
ratios. Consider the
following: chromosome
map of the fruit fly: All
the alleles are located on
chromosome 2 of the fruit
fly, and are inherited
together. When
performing crosses with
linkage groups, I find it
best to draw a little
picture of the
chromosomes to show
how they are inherited.
Example: A fly that is heterozygous for long wings (Ll) and heterozygous for long
aristae (Aa) is crossed with another fly of the same type. AaLl x AaLl. In both cases
the dominant allele is located on the same chromosome.
Sketch them!
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The results of the cross would change
considering the arrangement of alleles.
Show the cross that would occur if the
dominant alleles were on DIFFERENT
chromosomes.
The expected ratios are not always correct, because remember also that during
meiosis, chromosomes overlap and exchange genes. In the case above, you may get
an "odd" long wings, short aristae because the genes were exchange during meiosis.
Confusing, huh?
Speaking of crossing over, it is this frequency (the odds of the genes exchanging
during meiosis) that determines how far apart the alleles are on a chromosome.
Alleles that are farther apart, like the aristae allele and the brown eye color allele are
more likely to exchange than one closer together, like the aristae allele and the long
wing allele. In other words, alleles that are close together tend to stick together.
That being said, biologists use the percent of crossing over to determine the locus of
alleles on a chromosome. The distance between alleles is measures in MAP UNITS, or
MU. A genetic map or linkage map is based on the assumption that the probability of
a crossover between two genetic loci is proportional to the distance separating the
loci. On the diagram above the long wing allele is 13MU from the aristae allele.
Now it is timed to get confused. Read pages 272-276 now. Fill in below any gaps you
feel I might have missed. We will then do some practice problems.
Practice Questions
1. A dumpy winged (ww) fruit fly with long aristae (AA) is crossed with a long winged
(Ww) short aristae (aa). Show the cross and the phenotypic proportions.
2. A fruit fly with short legs (ll) and vestigial wings (ww) is crossed with one that is
heterozygous for both traits. Assuming the dominant alleles are on separate
chromosomes, show the cross and the expected phenotypic proportions.
3. A fruit fly with short legs (ll) and long aristae (Aa) is crossed with on that has long legs
(Ll) and long aristae (AA). Show the cross and the expected phenotypic proportions.
4. Two fruit flies that are both heterozygous for the dumpy wing and short leg traits
(WwLl) are crossed together. The resulting offspring are counted. 20 of the offspring
have long wings and long legs, 8 of the offspring have dumpy wings and short legs.
Create a chromosome map of with the above data.
5. In fruit flies, red eyes is a dominant allele located on the X chromosome. The recessive
condition results in white eyes. The tan body trait is also X-linked and is dominant to
yellow bodies. A female who is heterozygous both traits with the dominant alleles located
on the same chromosome is crossed with a white eyed, yellow bodied male. Show the
cross and the phenotypic proportions (Don't forget these traits are X-linked!)
6. Shaggy hair in dogs is dominant to its recessive allele, short hair, and black coat color
is dominant to its recessive allele, tan. When heterozygous black, shaggy-haired dogs are
crossed to short-haired tan dogs, progeny with the following phenotypes are obtained:
46% shaggy and black, 44% short and tan, 5% shaggy and tan and 5% are short and
black. What is the distance between the hair length and hair color loci?
7. Individuals with genes A, a, B, and b were mated. One thousand offspring were
counted: Aabb 474 aaBb 480 AaBb 20 aabb 26
This type of cross is known as a testcross. What are the two recombinant classes of
offspring? What is the percentage of recombination between these two loci? How many
map units apart are they?
8. The frequency of crossing-over between linked genes A and B is 35%; between B and
C is 10%; between C and D is 15%; between C and A is 25%; between D and B is 25%.
The sequence of the genes on the chromosome is: a) ADCB b) ACBD c) ABDC d)
ABCD
9. Gene A and gene B are known to be 10mu apart on the same chromosome. Individuals
homozygous dominant for these genes are mated with homozygous recessives. The
offspring are then test crossed. If there are 1,000 offspring from the test cross, how many
of the offspring would you expect to show the cross-over phenotype?