Answer:
----Yellow-------------White----------------------Ruby---------------------------Singed---
Step-by-step explanation:
Available data:
- Yellow gene is 1.4cM apart from White gene
- White gene is 19.6 cM apart from Singed gene
- Ruby gene exhibits 7.5% recombination with Yellow gene
- Ruby gene exhibits 13.5% recombination with Singed gene
First, you need to know that
- The genetic distance = recombination frequency x 100 expressed in map units (MU).
- The map unit is the distance between the pair of genes for which every 100 meiotic products, one results in a recombinant one.
- 1% of recombination = One centiMorgan (cM) = one map unit (MU).
Knowing this, we can assume that
- the distance from Ruby to yellow equals 7.5 cM
- the distance from Ruby to Singed equals 13.5 cM.
So we have four genes: yellow, white, singed, and ruby. And we need to put them in order in the chromosome, from left to right.
What we need to do here is to start from the most distant genes. So first, we will place these two genes in the extremes of a segment. The second step is to find the second most distant pair and place it. And so on, until we get to order all the genes.
Distances:
- yellow-white→ 1.4cM
- white-singed→ 19.6 cM
- ruby-yellow→ 7.5 cM
- ruby-singed→ 13.5 cM
1) The most distant pair is white-singed, so these two genes are places for now in the extremes of the chromosomal segment
----White---------------(19.6 cM)-------------------Singed---
2) The second most distant pair is ruby-singed. So ruby is placed somewhere between white and singed.
----White-------------------------Ruby-------------------Singed---
║---------------------------19.6 cM-----------------------║
║-----------13.5 cM-----║
We could assume that the distance between white and ruby is the difference between white-singed distance and ruby-singed distance. This is
white-ruby distance = white-singed distance - ruby-singed distance
white-ruby distance = 19.6 - 13.5
white-ruby distance = 6.1 cM
3) The third most distant pair is ruby-yellow. So now we need to place yellow. We know that the distance between white and yellow is 1.4 cM. We also know that the distance between white and ruby is 6.1 cM. If yellow was between white and ruby, the distance between white-yellow plus the distance yellow-ruby should equal the distance white ruby. But this is not true:
distance between white-yellow + distance yellow-ruby = 1.4 + 7.5 ≠ 6.1.
So yellow is not in the middle between white and ruby.
According to the given information, yellow precedes white in the order from left to right. This is coherent, as the addition of distances Yellow-white + white-ruby equals the distance yellow-ruby.
the distance yellow-ruby = distances Yellow-white + white-ruby
7.5 cM = 1.4 cM + 6.1 cM
So yellow is in fact before white.
According to this order, the distance from yellow to singed equals the addition of all the other distances.
yellow-singed = yellow-white + white-ruby + ruby-singed
yellow-singed = 1.4 + 6.1 + 13.5 = 21 cM
The correct order is
----Yellow-------------White----------------------Ruby---------------------------Singed---
║----------------------------------------------(21cM)--------------------------------║
║-----------------------(19.6cM)------------------------------║
║--------------(13.5cM)---------║
║------------------------------(7.5cM)----------║
║---------(6.1cM)--------║
║------(1.4cM)----║