Solutions For An Introduction to Genetic Analysis Chapter 17 Problem 49P

1. The phrase “a gene for tassel length” means that a gene specific for tassel length is present which has two alleles T and t. these two alleles control the length of the tassel. The phrase “a gene for rust resistance” indicates that a gene specific for resistance to the disease rust is present. This gene has two alleles R and r which determine whether the plant is resistant to rust or not.

2. The precise meaning of the allelic symbols of T, t, R, and r is not important in this problem. The distance between the two genes is the main focus of this problem.

3. A locus is a specific position occupied by a gene on the chromosome. The locus of a gene on two homologous chromosomes is treated as gene loci. A gene pair can consist of either same or different alleles.

4. Prior experimental evidence which would give the corn geneticist the idea that the two genes are on separate chromosomes would be previous experiments. The two genes would have shown independent assortment during meiosis and shown the corresponding phenotypic and genotypic ratios.

5. The routine crosses to a corn breeder would be F₁ crosses, F₂ crosses, backcrosses, and testcrosses. These crosses are regularly done to determine the exact genotype of a plant in the experiment.

6. The genotype T/t; R/r can be described in different terms like – double heterozygote, dihybrid or the genotype of F₁.

7. A pollen parent is the plant, which donates its pollen for mating with the female parent. The pollen parent contributes to the pollen tube nucleus, endosperm nucleus and contributes to the progeny.

8. Testcrosses are crosses where a plant is crossed with a homozygous recessive organism. When we obtain the progeny from this cross we can determine the genotype of the plant with unknown genotype. It helps the geneticist to study recombination during meiosis.

9. From the testcross, the breeder was expecting a 1:1:1:1 ratio. The types of progeny would be:

• T/t; R/r

• t/t;r/r

• T/t; r/r

• t/t;R/r

10. The observed ratio differed from the expected ratio. A 1:1:1:1 ratio was expected but we don’t see that in the results. The results show that the two genes are linked since two are parental classes having large number of progeny, while the other two classes are recombinant with less number of progeny.

The genetic distance can be calculated by the following formula:

11. The approximate equality of the first two progeny classes indicates that the genotype of the parental plant was Tt/Rr. Since the first two classes show predominance we can conclude that the parental genotype is Tt/Rr.

12. The approximate equality of the second two progeny classes indicates that these two classes represent the recombinants and not the parentals.

13. The gametes from the unusual plant along with their proportions are:

• 46.7% TR

• 49.5% tr

• 1.4% Tr

• 2.4% tR

14. The gametes which are in the majority are:

• 46.7% TR

• 49.5% tr

15. The gametes which are in the minority are:

• 1.4% Tr

• 2.4% tR

16. The progeny types which seem to be recombinant are:

• 1.4% Tr

• 2.4% tR

17. The allelic combinations which appear to be linked in some way are:

• T and R

• t and r

18. Linkage of genes is possible even if the genes are supposedly on separate chromosomes. This is possible due to the process of translocation.

19. The majority and minority classes tell us that one parent of the hybrid plant contained a translocation which linked the T and R alleles and the t and r alleles.

20. A corncob is a structure seen on the corn plant which holds the corn kernels which will become the next generation when planted.

21. A normal corncob can be shown in the following figure:

22. The corncobs from this cross would be missing half of the corn kernels:

23. A kernel is one progeny present on the surface of a corncob. There are many kernels present on one corncob.

24. Absence of half the kernels is also called semisterility. In this type, 50% of the progeny gets aborted. This condition could result from the random segregation of one normal with one translocated chromosome. This segregation would occur in a parent who is homozygous for a reciprocal translocation.

25. Yes, about half the kernels died. The female parent was responsible for these deaths. It was heterozygous for the reciprocal translocation.

a) The key features of the data which are different from the expected results is that the 1:1:1:1 ratio of the testcross progeny is absent. The given data indicates close linkage between the two genes. In addition, the corncob contained only half the usual number of kernels, which indicates semisterility.

b) A concise hypothesis that explains these results would be the translocation of material between the two chromosomes. This was responsible for the semisterility seen and the abnormal testcross ratio seen.

c) The genotypes of parents and progeny are:

Parents: TR/tr x t/t; r/r

Progeny:

• T/t; R/r 98

• t/t;r/r 104

• T/t; r/r 3

• t/t;R/r 5

d) A diagram showing the arrangement of alleles on the chromosomes is shown:

e) The origin of the two classes of progeny having three and five members is the process of recombination. In the process of recombination, crossovers occur in the region between linked genes. Since the distance between these two genes is only 3.8 m.u., the number of recombinants is very less. Recombinants are seen only in linked genes, but not in genes, which assort independently.