Question

In a dihybrid cross, if two heterozygotes (for both genes) are crossed, what will be the expectedphenotypic ratios? (In the following order: dominant for both traits; dominant for the first trait andrecessive for the second; recessive for the first trait and dominant for the second; recessive forboth traits.)

Answer 1

In questions like this, the best thing to do is write an example

First of all you need to know that a heterozygous may contribute genotypically different alleles: SY Sy sY or sy

So, if you cross two heterozygotes:

SY Sy sY sy

SYSSYYSSYySsYYSsYy

SySSYySSyySsYySsyy

sYSsYYSsYyssYYssYy

sySsYySsyyssYyssyy

-Well, you see that dominant for both trails are: SSYY, SSYy, SsYY or SsYy

If you count them there are 9 dominant for both trails

-Dominant for the the first trait and recessive for the second can be: SSyy or Ssyy

If you count them there are 3 of them

- In recessive for the first trait and dominant for the second we have two possibilites ssYY or ssYy

So, we see that again we have 3 cases

- Finally, for recessive for both trails the only possibility is ssyy

We have just one case (the one is left in the chart)

You can write it like this: 9:3:3:1