Question

In the tomato, three genes are linked on the same chromosome.Tall is dominant to dwarf, smooth skin is dominant to peachy skin, and round fruit shape is dominant to oblate shape (flattened-looking). A plant that is true-breeding for the dominant traits is crossed to a dwarf, peachy, oblate tomato plant. The F1 plants are heterozygous for all three traits and dominant phenotype for all. The F1 plants were testcrossed to a dwarf, peachy, oblate tomato plant (homozygous recessive for all three genes). The following results were obtained: (dominant traits are boldfaced for emphasis) 148 tall, smooth, round fruit 35 tall, smooth, oblate fruit 12 tall, peachy, oblate fruit 3 tall, peachy, round fruit 151 dwarf, peachy, oblate fruit 32 dwarf, peachy, round fruit 14 dwarf, smooth, round fruit 1 dwarf, smooth, oblate fruit Determine the mapped distance between two genes at a time Round your answer to the nearest whole map unit. Type just a number in. Distance between height and skin texture genes: _________ mu (Other distances are on subsequent questions)

Answer 1

Distance between height and skin texture genes: 8 mu

Step-by-step explanation:

The genes are:

T_ Tall , tt dwarf

S_ smooth skin, ss peachy skin

R_ round shape, rr oblate shape

The first cross was:

P TSR/TSR x tsr/tsr

F1 TSR/tsr

The test cross was:

TSR/tsr x tsr/tsr

The homozygous recessive individual only produces 1 type of gamete (tsr) so the observed phenotypes of the offspring depend on the gametes produced by the F1 individual.

The following offspring was observed:

• 148 TSR
• 151 tsr
• 35 TSr
• 32 tsR
• 12 Tsr
• 14 Tsr
• 3 TsR
• 1 tSr

Total 396

The parental gametes always appear in the highest frequency, and the double crossover gametes in the lowest frequency.

• Parentals: TSR, tsr
• DCO: TsR, tSr

To get those DCO gametes the skin gene must be in the middle of the other two.

Now we can determine that the other gametes are recombinant and arose after a single crossover between the different genes:

• SCO [s-r]: TSr, tsR
• SCO [t-s]: Tsr, tSR

To determine the distance between the height and skin texture genes, we need to use the formula:

`Distance[t-s]=(Number\ of\ Recombinants\ [t-s])/(total\ number \ of\ individuals ) * 100`

The number of recombinants is the sum of the SCO between those genes and the DCO.

`Distance[t-s]= (12+14+3+1)/(396) * 100`

`Distance[t-s]= 7.58 \ map \ units`

Rounded to the nearest whole, the answer is 8 map units.

The distance between the the skin texture and fruit shape genes is:

`Distance[s-r]=(Number\ of\ Recombinants\ [s-r])/(total\ number \ of\ individuals ) * 100`

`Distance[s-r]= (35+32+3+1)/(396) * 100`

`Distance[s-r]= 17.9 \ map \ units`

Rounded to the nearest whole, the answer is 18 map units.