Question #61451

1 Answer
Nov 28, 2017

See punnett squares below:


First, let's go ahead and restate all the traits we're dealing with:

  • Leaf Color; namely Dominant Purple (P) vs. Recessive Green (p)
  • Leaf Size; namely Dominant Broad (B) vs. Recessive Thin (b)

This converts a lot of complicated sentences into some data that we can work with.

Part 1

Now, the first part of the prompt asks us to "cross a pure breeding (homozygous) purple and pure breeding broad leaved tulip with a pure breeding green and pure breeding thin leaved tulip "

When you cross a homozygous dominant with a homozygous recessive, your resulting offspring will always be heterozygous.

Why? Well, think about it: one parent can only donate homozygous dominant alleles (i.e. PB) through its gametes, and the other can only donate homozygous recessive alleles (i.e. pb) through its gametes. This means that no matter which two gametes come together, the resulting offspring will always be heterozygous.

We can prove this with a Punnett square:

By Darshan Senthil

So that's where you'd get your heterozygous offspring. Notice that since there's no possible variations in the alleles that each parent can contribute, we only need a 1x1 punnett square!

Part 2

This is slightly trickier to do, namely since each heterozygous offspring has a larger number of alleles it can contribute. Our first course of action, therefore, is to list out all the possible alleles you could get from the heterozygous PpBb:

  • PB
  • Pb
  • pB
  • pb

And this is what goes on the side of our (4x4) punnett square. Note that since we're crossing 2 of the same organism, we just have the same thing on both edges fo the square:

By Darshan Senthil

This is all the possible allele combinations, or genotypes you could get from a cross of these two organisms.

Another question you could be asked would be about the phenotype, or what each of the offspring would actually look like on the outside. To determine this, we just use the following rules of thumb:

  • Wherever you have a P, you have purple.
  • Wherever you have a B, you have broad.

So, we'd have:

By Darshan Senthil

For additional help, check out this great video by Bozeman Science (from 6:47 for info specific to your problem):

Hope that helped :)