New Alleles Arise by Mutation

New alleles arise by mutation

 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! 

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 crossover phenotype?