Question

Using the information from problem 4, scientists do a further testcross using a heterozygote for height and nose morphology. The offspring are tall upturned snout, 40; dwarf upturned snout, 9; dwarf downturned snout, 42; tall downturned snout, 9. Calculate the recombination frequency from these data, and then use your answer from problem 4 to determine the correct order of the three linked genes.

Short answer

18% is the frequency of recombination between the genes for height tall (T) and an upturned snout (S).

The correct order of the three genes on the chromosome is T-A-S.

Step-by-step solution

Recombination frequency

Recombinant frequency measures the proportion of recombinant offspring obtained when parents with specific traits are crossed.It represents the chances of recombination that occurs between the two genes in a chromosome.

Thus, recombinant frequencies between the genes help to calculate the distance between the genes on the chromosome.

The recombinant frequency between two genes is calculated using the following formula:

$\mathrm{R}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{b}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{q}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}=\frac{\mathrm{N}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{b}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{s}}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{o}\mathrm{f}\mathrm{f}\mathrm{s}\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{s}}\times 100$

Explanation for the recombinant frequency between genes for height and nose morphology

Four types of offspring were obtained in the testcross using heterozygote for height and tall morphology. The testcross using heterozygote with tall and an upturned snout produced four types of offspring.

The tall upturned snout (40) and dwarf downturned snout (42) were wild-type offspring, while dwarf upturned snout (9) and tall downturned snout (9) were recombinant offspring.

The total number of offspring produced in the cross is $40+9+42+9=100$

The total number of recombinants is $9+9=18$.

The recombination frequency is calculated by substituting the values into the following equation:

$\begin{array}{r}l\mathrm{R}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{b}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{q}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}=\frac{18}{100}\times 100\\ \mathrm{R}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{b}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{q}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}=18\mathrm{\%}\end{array}$

Thus,the recombination frequency for the genes between tall (T) and an upturned snout (A) is 18%.

To determine the order of the genes on the chromosome  

From question four, the recombination frequency for the genes between tall (T) and antennae (A) obtained was 12%. The recombination frequency for the genes between antennae (A) and an upturned snout obtained was 5%. While the recombination frequency for the genes between tall (T) and an upturned snout (A) is 18%.

From the recombinant frequencies obtained from three crosses, it is inferred that the distance between the genes for height (T) and nose morphology (S) is the highest, that is, 18%. In comparison, the recombinant frequency between the genes for antennae (A) and nose morphology (S) is the least, that is, 5%.

The data indicate that genes for antennae and nose morphology are located nearer than the genes for height and nose morphology. This exhibits that the order of the genes on the chromosomes must beT-A-S.