Punnett Squares

Terms for Genotypes

To be prepared to answer questions on genetics, understanding terminology is very important.  To begin, these are chromosomes.  The purple and white bands are the locations on these chromosomes for flower color.  Obviously, you have two chromosomes that have these genes.  You have a genotype, which is your combination of types of alleles.  If you have two dominant alleles, you are called homozygous dominant.  If you have two recessive alleles, you are called homozygous recessive.  If you have one dominant and one recessive alleleDifferent versions of a gene that determine specific traits., you are heterozygous.  These are the terms we use when describing an individual.  If you are ever asked for a genotype on a question, these three terms are how you answer.

What does this mean for what you look like or your phenotype?  That has to do with something that Mendel observed from breeding his pea plants a million times.  He noted that there was a specific relationship between the dominant and the recessive allele. Whenever a dominant allele was present, it was expressed, no matter what the second allele was.  He also noted that the recessive phenotype is only expressed when two recessive alleles are present.  These observations of Mendel’s give us some rules to follow.  The homozygous dominant and heterozygote have the dominant phenotype.  The homozygous recessive has the recessive phenotype.  Again, how do you know which phenotype is dominant or recessive?  You don’t, you are told.

Using the Shorthand

Let’s use some letters for a little shorthand.  Let’s us the letter A.  In traditional genetics notation, we designate the dominant allele with a capital letter, in this case capital A and the recessive allele as the lowercase letter, in this case lowercase a.  Let’s apply this to flower color.  A purple flower color, or phenotype, occurs when the dominant allele, capital A, is present.  A white flower only occurs when two recessive alleles, lowercase a, are present.

A plant with purple flowers could be homozygous dominant or heterozygous.  A plant with white flowers could only be homozygous recessive.  But, how do we determine if a plant is homozygous dominant or heterozygous?  We’d need more information about either the generation before or the generation after.  We’d also need to apply Mendel’s Law of Segregation.  And we’d need to use the logic of something called a Punnet Square.

Punnett squares

Punnett squares! First thing to know is that Punnett Squares show the probability of offspring.  They don’t show you what you get, they show you what you can get.  Possibilities.  Probabilities that can be calculated.  That will be calculated in the lab!!!  We know that a Punnett square usually has lots of letters in it and kinda looks like this picture at the bottom right here.  Kinda complicated.  Can you see the words sperm and egg along the left side and along the top of the square?  These are the gametes produced by each parent.  The Punnett square takes these gametes and combines them, just as in fertilization….sperm plus egg equals zygote.  Therefore, each square of the Punnett square shows you a possible zygote that could develop.   Each square answers: what kind of baba can I have?  We can then calculate probabilities or percentages as to how likely a certain type of offspring would be.  Remember, I have an entirely different powerpoint for working the different kinds of Punnett Squares.

Punnett squares

Punnett squares show the probability of offspring.  They don’t show you what you get, they show you what you can get.  Possibilities.  Probabilities that can be calculated.  That will be calculated in the lab!!!  Students generally know how to work the Punnett square, but the trick is to know what gametes the parent can make.  This is where mistakes happen.   Let’s start with a really obvious example.  Let’s say that we have a homozygous dominant parent.  So this would be capital A capital A.  During meiosis, when this parent makes their gametes, the Law of Segregation say that we can put only one of these alleles in each gamete A haploid reproductive cell (sperm or egg)..  So it could be A…..or it could be A.  Parent number 2 is homozygous recessive and the same applies for gamete making.  Now we work the Punnett Square logic like this.  Bringing the letters down and over and…viola!!  What we’ve created in each square here is the possible type of offspring these parents could have.  I mean for humans when we do this, this tells you that like 50% chance your kid will be male or female.  Here with plants that produce a whole bunch of kids, it’s like saying what percent of their multiple offspring will have any one genotype.  OK, so this gives us genotypes, and we can label them all with the right terminology: heterozygous. But what color flower would these babies have?  Well, Mendel told us that heterozygotes all have the dominant phenotype.  That’s a rule.  So.  These would all be purple and we would call them the F1 generation.

Punnett squares

Let’s do a cross of two of these plants from the F1 generation.  What color flowers would their babies have?  And what percent of their babies would have each color?  Let’s make our gametes, let’s do our cross. Let’s determine the colors…Viola! 

This is what is called a heterozygote cross.  We have two parents, both heterozygotes.  So, they each make gametes, some with the dominant allele noted by capital A and they make some gametes with the recessive allele noted by lowercase a.  Now that we have the gametes, let’s fertilize them and make offspring.  Filling in the squares, we seem to have all three genotypes here.  Now we use Mendel’s rules to interpret the phenotypes and viola!  We can see that these two plants will produce a bunch of offspring that all three genotypes and both phenotype.  They are all possible, but how probable, or how likely are they?

Let’s add some percents.  Each square here is 25%.  So, how many will be white?  25%, one square.  How many will be purple?  Three squares, 25+25+25=75%.  That’s probability that is reported in percent.  You ask: what’s the probability that they’d have white offspring? And you say 25%.

You can also report this as a ratio.  You would report the ration by stating the number of squares for the dominant phenotype, then a colon, then the number of squares for the recessive phenotype.  Here we’d have 3:1.  You read this 3 to 1, meaning there are 3 purple flowers for every one white flower. 

What we just did there are phenotype probabilities and phenotype ratios.  I’m a fan of the probability.  I don’t connect to the ratio statements.

We can do the same here with genotypes.  Let’s do the probability for each genotype.

Homozygous dominant has one square and is 25% likely to appear in the offspring.

Heterozygous has two square and is 50% likely to appear.

Homozygous recessive has one square and is also 25% likely. When we report genotype ratios, we list the dominant homozygous, heterozygous, recessive homozygous.  Here we’d have 1:2:1.  Again, not my thing.  I like the percents.  I mean, humans don’t have litters.  Most of them.  And, if you are doing genetic counseling you get a percent probability of your expectation for a trait in your child.  People with cystic fibrosis in their family want to know if there’s a 50% chance their kid will have it or a

Punnett squares

Let’s do a cross of two of these plants from the F1 generation.  What color flowers would their babies have?  And what percent of their babies would have each color?  Let’s make our gametes, let’s do our cross. Let’s determine the colors…Viola! 

This is what is called a heterozygote cross.  We have two parents, both heterozygotes.  So, they each make gametes, some with the dominant allele noted by capital A and they make some gametes with the recessive allele noted by lowercase a.  Now that we have the gametes, let’s fertilize them and make offspring.  Filling in the squares, we seem to have all three genotypes here.  Now we use Mendel’s rules to interpret the phenotypes and viola!  We can see that these two plants will produce a bunch of offspring that all three genotypes and both phenotype.  They are all possible, but how probable, or how likely are they?

Let’s add some percents.  Each square here is 25%.  So, how many will be white?  25%, one square.  How many will be purple?  Three squares, 25+25+25=75%.  That’s probability that is reported in percent.  You ask: what’s the probability that they’d have white offspring? And you say 25%.

You can also report this as a ratio.  You would report the ration by stating the number of squares for the dominant phenotype, then a colon, then the number of squares for the recessive phenotype.  Here we’d have 3:1.  You read this 3 to 1, meaning there are 3 purple flowers for every one white flower. 

What we just did there are phenotype probabilities and phenotype ratios.  I’m a fan of the probability.  I don’t connect to the ratio statements.

We can do the same here with genotypes.  Let’s do the probability for each genotype.

Homozygous dominant has one square and is 25% likely to appear in the offspring.

Heterozygous has two square and is 50% likely to appear.

Homozygous recessive has one square and is also 25% likely.

When we report genotype ratios, we list the dominant homozygous, heterozygous, recessive homozygous.  Here we’d have 1:2:1.  Again, not my thing.  I like the percents.  I mean, humans don’t have litters.  Most of them.  And, if you are doing genetic counseling you get a percent probability of your expectation for a trait in your child.  People with cystic fibrosis in their family want to know if there’s a 50% chance their kid will have it or a 75% chance. 

Recessive Autosomal Disorders

Take a quick look at the Punnett Square.  It’s a heterozygote cross that we just did.  OK, let’s interpret.  Cystic Fibrosis is a recessive disease meaning that only the homozygous recessive genotype will have the recessive phenotype that expresses the disease.  Therefore, we know from Mendel’s rules on dominant/recessive relationships that the heterozygote and the dominant homozygous genotype will also have the dominant phenotype of no disease.  Look at how this Punnet Square is already filled in with the genotypes and phenotypes of the possible offspring.  So, if two heterozygote parents were to have a baby, what are the chances the baby will have the disease? 25%.  What’s the chance that the baby won’t have the disease but will be a carrier?  50%

The last question asks about phenotype…what’s the chance the baby won’t have the disease?  75%

Recessive Autosomal Disorders

You can do the same, answering the questions here on this cross of a homozygous dominant individual with a heterozygous individual.  Remember that 0% is an acceptable answer.

Dominant Autosomal Disorders

Diseases like Huntington’s disease are dominant diseases.  This would mean that the homozygous dominant and heterozygous individuals would have the disease and that the only people without the disease would be homozygous recessive.  The reality is that a zygote receiving two dominant alleles is not viable and therefore homozygous dominant people do not exist in the population.  Let’s talk about the first possible mating of people concerned about Huntington’s.  We might have a homozygous recessive individual, with no trace of the disease in their family.  We might also have an individual with evidence of it in their family, leading us to realize that this person is heterozygous.  Huntington’s is a disease that hits in the 3^rd or 4^th decade of life so reproductionThe process of producing offspring. is possible if you do it before the disease hits.  Looking at the genotypes of this cross, we have to remember to interpret them differently for the phenotypes than we did for a recessive trait.  Remember, here, the homozygous recessive individual doesn’t have the disease.  You can see in this cross that there is a 50% chance that the baby will have the disease.  This is an excellent topic in this class that I encourage students to talk about and then step back and watch the opinions fly.  So, if you knew you had Huntington’s and still had enough time to have a baby, would you have a baby?  Knowing these odds, would you?  There’s no right answer to that. 

I encourage you to do a cross between two heterozygotes here and realize that their probability of having a child with Huntington’s would be 75%.  Would this change your answer about having a baby?  Does the percent matter?  Again…no right answer.

Dihybrid Cross

Let’s do a cross of two of these plants from the F1 generation.  What color flowers would their babies have?  And what percent of their babies would have each color?  Let’s make our gametes, let’s do our cross. Let’s determine the colors…Viola! 

This is what is called a heterozygote cross.  We have two parents, both heterozygotes.  So, they each make gametes, some with the dominant allele noted by capital A and they make some gametes with the recessive allele noted by lowercase a.  Now that we have the gametes, let’s fertilize them and make offspring.  Filling in the squares, we seem to have all three genotypes here.  Now we use Mendel’s rules to interpret the phenotypes and viola!  We can see that these two plants will produce a bunch of offspring that all three genotypes and both phenotype.  They are all possible, but how probable, or how likely are they?

Let’s add some percents.  Each square here is 25%.  So, how many will be white?  25%, one square.  How many will be purple?  Three squares, 25+25+25=75%.  That’s probability that is reported in percent.  You ask: what’s the probability that they’d have white offspring? And you say 25%.

You can also report this as a ratio.  You would report the ration by stating the number of squares for the dominant phenotype, then a colon, then the number of squares for the recessive phenotype.  Here we’d have 3:1.  You read this 3 to 1, meaning there are 3 purple flowers for every one white flower. 

What we just did there are phenotype probabilities and phenotype ratios.  I’m a fan of the probability.  I don’t connect to the ratio statements.

We can do the same here with genotypes.  Let’s do the probability for each genotype.

Homozygous dominant has one square and is 25% likely to appear in the offspring.

Heterozygous has two square and is 50% likely to appear.

Homozygous recessive has one square and is also 25% likely. When we report genotype ratios, we list the dominant homozygous, heterozygous, recessive homozygous.  Here we’d have 1:2:1.  Again, not my thing.  I like the percents.  I mean, humans don’t have litters.  Most of them.  And, if you are doing genetic counseling you get a percent probability of your expectation for a trait in your child.  People with cystic fibrosis in their family want to know if there’s a 50% chance their kid will have it or a 75% chance.