Genetica Ejercicios De Practicas

Ejercicios de práctica sobre genes enlazados
Biol 3305: Genética

*13. In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (BB) is recessive to the allele for unbanded shell (BO). Genes at a different locus determine the background color of the shell; here, yellow (CY) is recessive to brown (CBw). A banded, yellow snail is crossed with a homozygous brown, unbanded snail.The F1 are then crossed with banded, yellow snails (a testcross).
a. What will be the results of the testcross if the loci that control banding and color are linked with no crossing over?
b. What will be the results of the testcross if the loci assort independently?
c. What will be the results of the testcross if the loci are linked and 20 map units apart?

14. In silkmoths (Bombyx mori),red eyes (re) and white-banded wing (wb) are encoded by two mutant alleles that are recessive to those that produce wild-type traits (re+ and wb+); these two genes are on the same chromosome. A moth homozygous for red eyes and white-banded wings is crossed with a moth homozygous for the wild-type traits. The F1 have normal eyes and normal wings. The F1 are crossed with moths that have red eyes andwhite-banded wings in a testcross. The progeny of this testcross are:
wild-type eyes, wild-type wings 418
red eyes, wild-type wings 19
wild-type eyes, white-banded wings 16
red eyes, white-banded wings 426
a. What phenotypic proportions would be expected if the genes for red eyes and white-banded wings were located on different chromosomes?
b. What is the genetic distance between thegenes for red eyes and white-banded wings?

*15. A geneticist discovers a new mutation in Drosophila melanogaster that causes the flies to shake and quiver. She calls this mutation spastic (sps) and determines that spastic is due to an autosomal recessive gene. She wants to determine if the spastic gene is linked to the recessive gene for vestigial wings (vg). She crosses a fly homozygous forspastic and vestigial traits with a fly homozygous for the wild-type traits and then uses the resulting F1 females in a testcross. She obtains the following flies from this testcross:

Are the genes that cause vestigial wings and the spastic mutation linked?


*17. In tomatoes, tall (D) is dominant over dwarf (d), and smooth fruit (P) is dominant over pubescent (p) fruit, which is covered withfine hairs. A farmer has two tall and smooth tomato plants, which we will call plant A and plant B. The farmer crosses plants A and B with the same dwarf and pubescent plant and obtains the following numbers of progeny:

a. What are the genotypes of plant A and plant B?
b. Are the loci that determine height of the plant and pubescence linked? If so, what is the map distance between them?
c.Explain why different proportions of progeny are produced when plant A and plant B are crossed with the same dwarf pubescent plant.

18. Alleles A and a occur at a locus that is located on the same chromosome as a locus with alleles B and b. Aa Bb is crossed with aa bb, and the following progeny are produced:
Aa Bb 5
Aa bb 45
aa Bb 45
aa bb 5
What conclusion can be made about thearrangement of the genes on the chromosome in the Aa Bb parent?

19. Daniel McDonald and Nancy Peer determined that eyespot (a clear spot in the center of the eye) in flour beetles is caused by an X-linked gene (es) that is recessive to the allele for the absence of eyespot (es+). They conducted a series of crosses to determine the distance between the gene for eyespot and a dominant X-linked genefor stripped (St), which acted as a recessive lethal (is lethal when homozygous in females or hemizygous in males). The following cross was carried out (D. J. McDonald and N. J. Peer. 1961. Journal of Heredity 52:261–264).


es St+ Y

a. Which progeny are the recombinants and which progeny are the nonrecombinants?
b. Calculate the recombination frequency between es and St....