Punnett Squares

Question

S= DOMINANT

s= Recessive

each square is 25%

You don't have to solve them all but i'm trying to study from this, how would i go about solving it using a punnett square, one or two of the questions solved would be helpful, with an explanation.

1. state the probability that a homozygous dominant pea plant passes down an S allele to its offspring? What about an s allele?
2. state probability that, when crossing a true-breeding smooth pea plant with a true-breeding wrinkled pea plant, that we get a heterozygous offspring?
3. If we did an experiment and cross two heterozygous pea plants, what is the probability of a heterozygous offspring?
4. If we did an experiment and cross two heterozygous pea plants, what is the probability of a homozygous dominant OR a homozygous recessive offspring?

Answer 1

Hi Jeremiah!

Let's try and use the Punnet square method to solve the first problem here.

Since we know that the parent is a homozygous dominant pea plant, we know that the parent has two of the same alleles, and they are both dominant. In other words, the parent has SS.

We don't know what the other parent might have, and cannot speak to that in this question, but we can determine the probability of the parent we do know about passing down their alleles.

So, while it might not seem so necessary in this case, since the parent has 100% chance of passing an S allele and a 0% chance of passing a s allele (since it doesn't have one), let's look at this in a punnet square anyway.

? ? The question marks are because we don't know what the other parent is.

S | S? | S? |

-------------------------

S | S? | S? |

I hope my makeshift punnet square is clear here, we are crossing the known S alleles from the dominant homozygous parent with the ? alleles of the unknown parent. We see that in every case, each square is ensured it has a dominant S allele that it received from our dominant homozygous parent, and no square has received a recessive s allele from its dominant homozygous parent. Does that help?

Now, let's look at question 3. We are told we have two heterozygous pea plants, which means both parents have one dominant S allele, and one recessive s allele. So let's set up our punnet square and cross those alleles!

S s

S | SS | Ss

-----------------------

s | Ss | ss

So how many total cases do we have for the offspring? We have 4 total cases, and each case is equally likely, which is why each square represents 25%. But, as we can see in the punnet square, we have one case yielding SS, one case yielding ss, and two cases yielding the heterozygous Ss. Which means 2 out of the 4 total possible cases will yield the heterozygous Ss. That means that there is a 50% chance that the offspring will be heterozygous Ss.

You can apply this method to the other problems as well! Hope this helped! :)