Chapter 6, Problem 18EQ

Three recessive traits in garden pea plants are as follows: yellow pods are recessive to green pods, bluish green seedlings are recessive to green seedlings, creeper (a plant that cannot stand up) is recessive to normal. A true-breeding normal plant with green pods and green seedlings was crossed to a creeper with yellow pods and bluish green seedlings. The ${\text{F}}_{\text{1}}$ plants were then crossed to creepers with yellow pods and bluish green seedlings. The following results were obtained:  

2059 green pods, green seedlings, normal   

151 green pods, green seedlings, creeper   

281 green pods, bluish green seedlings, normal    

15 green pods, bluish green seedlings, creeper  

2041 yellow pods, bluish green seedlings, creeper   

157 yellow pods, bluish green seedlings, normal   

282 yellow pods, green seedlings, creeper    

11 yellow pods, green seedlings, normal

Construct a genetic map that indicates the map distances between these three genes.

Summary Introduction

To review:

A genetic map indicating map distances between the genes for pod color, seedling color, and plant stature.

Introduction:

Thepair of alleles ofa gene determines the protein encoded by the genes, and results in the phenotype of the trait. True breeders have the monomorphic condition for the alleles of a trait. A genetic map helps in describing the arrangement of the genes on a particular chromosome.

Explanation of Solution

A normal pea plant, which is truebreeding for green seedling and green pods is crossed with a creeper pea plant, having bluish-green seedlings and yellow pods. The F₁ offspring are then crossed with a creeper having bluish-green seedlings and yellow pods.

Different characters of the pea plant can be denoted as follows:

Dominant characters: G for green pods, S for green seedling, and C for normal plants

Recessive characters: g for yellow pods, s for bluish green seedlings, and c for the creeper.

Given, the true breeding normal plant, having green seedlings as well as green pods is crossed with true breeding creeper, having bluish green seedling and yellow pods. The genotype of true breeding plants is GGSSCC and ggsscc. Thus, the gametes will be GSC and gsc. The plants in the F₁ generation obtained by crossing these true breeders will be GgSsCc.

When three genes are linked then G, S, and C alleles will be linked together whereas g, s, and c alleles will be linked together, on a homologous chromosome. Given, the F₁ plants GgSsCc are crossed with ggsscc (creepers having yellow pods and bluish green seedlings) and following results were obtained:

Number of plants	Phenotype
2059	Green pods, green seedlings, normal
151	Green pods, green seedlings, creepers
281	Green pods, bluish green seedlings, normal
15	Green pods, bluish green seedlings, creepers
2041	Yellow pods, bluish green seedlings, creepers
157	Yellow pods, bluish green seedlings, normal
282	Yellow pods, green seedlings, creepers
11	Yellow pods, green seedlings, normal

The genetic map for the three genes can be constructed by using the below-mentioned formula for map distance:

$\text{Map distance = }\frac{\text{Number of recombinant offspring}}{\text{Total number of offsping}}\times 100\text{mu (map units)}$

The distance between the three genes can be measured by separating the data of the offspring and phenotypic categories into gene pair, and then calculating the map distance between the two genes.

For the map distance between the genes for plant stature and pod color,

the number of offsprings is calculated for each pair of plant stature and color of pods.

$\begin{array}{c}\text{green pods, normal = 2059+281}\\ \text{= 2340}\\ \text{green pods, creeper = 151+15 }\\ \text{= 166}\end{array}$

$\begin{array}{c}\text{yellow pods, normal = 157+11}\\ \text{= 168}\\ \text{yellow pods, creeper = 2041+282 }\\ \text{= 2323}\end{array}$

Non-recombinant offspring are 2340 normal, green pods, and 2323 creeper, yellow pods.

Recombinant offspring are 166 creeper, green pods, and 168 normal, yellow pods.

Therefore, the map distance will be calculated as,

$\begin{array}{l}\text{Map distance =}\frac{166+168}{2340+2323+166+168}\times \text{100 mu}\\ \text{Map distance =}\frac{334}{4997}\times \text{100 mu}\\ \text{Map distance = 6}\text{.7 mu}\end{array}$

For the map distance between the genes for plant stature and seedling color, the number of offspring having genes for plant stature and color of the seedlingis given as follows:

$\begin{array}{c}\text{green seedling, normal = 2059+11 }\\ \text{= 2070}\\ \text{green seedling, creeper = 151+282 }\\ \text{= 433}\end{array}$

$\begin{array}{c}\text{bluish green seedling, normal = 281+157 }\\ \text{= 438}\\ \text{bluish green seedling},\text{ creeper =15+2041 }\\ \text{= 2056}\end{array}$

Nonrecombinant offspring are 2070 normal, green seedlings, and 2056 creeper, bluish green seedlings, and the recombinant offspring are 433 creeper, green seedlings, and 438 normal, bluish green seedlings.

Therefore, the map distance will be calculated as.

$\begin{array}{l}\text{Map distance = }\frac{433+438}{2070+2056+433+438}\times 100\text{mu}\\ \text{Map distance = }\frac{871}{4997}\times 100\text{mu}\\ \text{Map distance = }17.4\text{ mu}\end{array}$

For the map distance between the genes for seedling color and pod color, the number of offspring having genes for the color of seedling and pods is given as follows:

$\begin{array}{c}\text{green seedlings, green pods = 2059+151}\\ \text{= 2210}\\ \text{bluish green seedlings, green pods = 15+281 }\\ \text{= 296}\end{array}$

$\begin{array}{c}\text{green seedlings, yellow pods = 282+11}\\ \text{= 293}\\ \text{bluish green seedlings, yellow pods = 2041+157 }\\ \text{= 2198}\end{array}$

Non-recombinant offspring are 2210 green seedling, green pods, and 2198 bluish green seedling, yellow pods, and the recombinant offspring are 296 bluish green seedling, green pods, and 293 green seedling, yellow pods.Therefore, the map distance will be calculated as,

$\begin{array}{l}\text{Map distance = }\frac{296+293}{2210+2198+296+293}\times 100\text{ mu}\\ \text{Map distance =}\frac{589}{4997}\times 100\text{ mu}\\ \text{Map distance = }11.8\text{ mu}\end{array}$

Thus, the distance between the genes for seedling and pods colors is 11.8 mu, and that between the genes for plant stature and seedling color is 17.4 mu and for the genes for plant stature and pod color, it is 6.7 mu. Therefore, the order of the three genes, according to the map distance in-between them will be seedling color, pod color, plant stature. Gene for pod color is present in the middle. Genetic map is shown below:

Conclusion

Therefore, it can be concluded that according to the map distance calculated in-between the genes for pod color, plant stature, and seedling color, the order of the three genes will be seedling color, pod color, plant stature(or the opposite order). The distance between the genes responsible for seedling and pods colors is 11.8 mu, and between the genes for plant stature and pod color, it is 6.7 mu. The genes that code for plant stature and seedling color are 17.4 mu apart.