Linked genes

1. In the high-pressure, deep, dark depths of the ocean's abyssal plains lives an organism called a viper fish. These fish resemble transparent eels with overlarge heads and very long, pointy teeth. Because of where they live, very little is know about these fish but for the sake of this problem, we shall assume that some of them can produce biochemical lights along the length of their bodies and some cannot. Also, some have very large eyes while some have almost no eye at all. Further, we shall assume that these two traits are each controlled by single locus genes with the production of lights (L) and large eyes (E) being the dominant alleles for each. Lastly, assume that these genes are linked. It is very difficult in the vast expanses of the deep ocean for two viper fish to find each other, but suppose that a female heterozygous for both traits (the two dominant alleles are on one chromosome and the two recessive are on its homolog in this individual) happens to meet a male that is homozygous dominant for lights and homozygous recessive for eyes. What are the possible genotypes of their offspring (choose all that are correct)?

What are the possible phenotypes of the F1 generation (again, choose all that are correct)?

2. Snow leopards live in the high reaches of the Himalayas and have several adaptations to deal with living most of the time in snow. They have wide paws with hair between the toes to allow them to run on top of the snow crust. Assume that wide paws (W) and hair between the toes (T) are both dominant traits and are on the same chromosome. Suppose that two snow leopards, heterozygous for both traits, mate. Suppose further that in the female, the two dominant alleles are on the same chromosome of the homologous pair, while in the male, each chromosome of the pair has one dominant and one recessive allele. What percentage of each of the following genotypes will be produced?


How many of the offspring (assume 4 cubs) will have narrow paws with hair between the toes?

3. Suppose you have a bunch of white mice, some that are only white and some with black spots. There are also some of your mice with black eyes and some with pink eyes. Assume that the genes for hair color and eye color are linked and that plain white (W) is dominant to black spots and that black eyes (B) are dominant to pink. You decide to cross a female that is homozygous dominant for hair color (you just happen to know the genotype of each mouse) and heterozygous for eye color with a male that is heterozygous for both traits. In the male, each homologous chromosome contains a dominant and recessive allele from the two genes. What genotypic ratio do you expect in the F1 generation (assume 16 pups)?

What percentage of the offspring will have black spots and pink eyes?

4. A group of spiders known as orb weavers often construct patterns with threads visible in the ultraviolet spectrum into the centers of their webs. These patterns mimic the centers of flowers and provide a lure for unwary flying insects that can see in the ultraviolet range. Suppose that in a particular species of orb weavers, the gene controlling the type of pattern placed in the center of the web is linked to a gene controlling whether the spider has green spots. In this case, the allele for a solid circle (P) of UV pattern is dominant to the allele for concentric rings forming a target pattern and the allele for green spots (G) is dominant to no spots. Suppose a female heterozygous for both traits and with the dominant alleles of the two traits on one homologous chromosome and the recessive alleles on the other homolog mates with a male that is homozygous recessive for pattern type and heterozygous for spots. How many of each phenotype will be found in the F1 generation (assume 4 offspring)?

Circle / spots:
Circle / no spots:
Target / spots:
Target / no spots:

Assuming 16 offspring in this brood, how many will have the PpGg genotype?

How many will have the ppgg genotype?

5. A farmer who wants to get into the milk business has bought a small beginning herd of 12 cows all of which produce high quantities of milk (M, dominant over low quantities) and have high fertility (F, also dominant over low fertility). However, she knows that all of her cows are heterozygous for both these traits and that the genes are linked. In her cows, one homolog of the pair contains the dominant alleles for each trait and one contains the recessive allele of each of the traits. Two of the farmer's nearby neighbors own bulls that are also heterozygous for these traits. The first neighbor's bull has the same alleles linked as her cows but the second neighbor's bull has one dominant and one recessive allele from each trait on each homolog. Baring in mind that the farmer would like to obtain the maximum number of calves with high milk production and high fertility, which of the two available bulls should she choose to introduce her cows to?

What percentage of the offspring resulting from the above match will have the most desirable phenotype of high milk production and high fertility?

What percentage of the offspring resulting from the above match will have the most desirable genotype of MMFF?

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