Punnett Squares

Punnett Squares

You have all this independent assortment and cross over. Yet for each gene there are only 2 alleles - one from your mum and one from your dad. 

Because gametes have only one of each chromosome then for each gene they can only have one allele

This one allele might have been from your dad, or from your mum

Thus your offspring inherit alleles from their grandparents

 Because the gamete can only carry one allele, we can use a single letter to represent that allele

Because of independent assortment and meiosis, we know that another gamete that you make will have the other allele

Then because of random fertilization, one or the other egg will be released from the female ovary. And one or the other sperm will win the race. 

There are 2 options for the female and 2 options for the male. This gives a total of 4 possible outcomes

These possible outcomes can be shown on a grid, a square... a punnett square

But we remember from meiosis that from each 'germ cell' we will end up with 4 sex cells - e.g. 1 germ cell makes 4 sperm cells

However, because their are only 2 allele possibilities due to your own parents then the sperms will either have one allele or the other allele

So we can simplify it down to just 2 cells 

For example, you might have a blue eyed mum and a brown eyed dad. Your eye colour is Brown. This is your phenotype, Brown. 

Brown is dominant and Blue is recessive, so you will have inherited one recessive Blue eye allele from your mum, with the genotype b. And one dominant Brown eye allele from your dad, with the genotype B

So your phenotype is Brown eyes. And your Genotype is B,b (heterozygous)

Because your genotype is B,b you sex cells can have only one of these 2 alleles.

IF you are a male, then you 4 sperm cells will be: B, B, b, b

To simplify this, as there are only 2 choices, we would write on the that the sperm choices are either B or b

We can then put these on the top of the punnett square

Then you mate with some one who also has Blue eyes, you can put her alleles on the side of the punnett square 

Because her phenotype is blue eyed, which is recessive her genotype needs to be b,b

So her egg options are b or b

Then you can pair up your B,b options with her b,b to find out the probability of the childs eye colour

If your B meets with her b then the child will have a genotype of B,b and a phenotype of Brown

If your b meets with her b then the child will have a genotype of b,b and a phenotype of blue

You can see in the punnett square that there is a 50% chance of either eye colour

This is all explained in the "Learn Biology: How to Draw a Punnett Square" YouTube clip below

If you decide to mate with a different person and you both have the phenotype of brown eyes, and you are both have the heterozygous genotype of B,b

Then you can put that into a punnet square and you will get ratios or percentages:

You will have a 

You can see how well the recessive allele hides. As if we just look at the phenotypes then:

Punnett Squares from Textbook

Punnett Squares for Sex Determination

The easiest example of a Punnett Square is for sex determination

Because woman are XX, their eggs (Ova) are all X 

Because males are XY, their sperm are either X or Y

The winner of the race (random fertilization) will determine the sex of the baby

We can put this into a Punnett Square

If Sperm X meets with egg X then the baby will have XX and be a female. 

This occurs 2/4 times in the Punnett square, thus there is a 50% probability of a girl.

If Sperm Y meets with egg X then the baby will have the genotype XY and the phenotype Male. 

This occurs 2/4 times thus there is a 50% probability of a male child due to random fertilization

Punnett Squares Ratios

In the example above and below we are looking at the gene for Hairy Toes

'Hairy Toes' is dominant = T

'Not hairy toes' is recessive = T

Harry has Hairy Toes. His parents are "Hairy Toes" and "not hairy toes" so he must be Tt

Harry meets Sally. Sally also has Hairy Toes. Her parents are "Hairy Toes" and "Not hairy toes". So her genotype is also Tt

We know from Meiosis that Harry's gametes will either be T or t

Likewise, Sally's gametes will either be T or t

We can put them into the Punnett Square below to find out what the probability of them having a child that has "Hairy Toes" and "Not hairy toes"

We Can then combine the alleles to get each of the 4 possible zygotes

There are 4 options


So 3 of the children will have Hairy Toes and only one will have "No hair toes" Furthermore two are the children are carriers for the "No hair toes" allele


We can express the Genotypic Ratio as:

1TT: 2Tt: 1tt

Then the Phenotypic Ratio as

3 Hairy Toes : 1 No hair toes

If we do it as percentages, then 

When drawing up your Punnett Square, it doesn't matter who is on top, the mother or the father

Remember mother = maternal, father = paternal

With our Punnett Squares we do "Monohybrid crosses" that is, we look at just one gene and the 2 alleles possible for that gene

 We use a big letter for Dominant and a small letter for recessive

If we get two different alleles, then the baby will be heterozygous

If we get two of the same alleles, then the baby will be homozygous

If the two alleles are Dominant, then it will be Homozygous Dominant

If the two alleles are Recessive, then it will be Homozygous Recessive

If we 'cross' two Heterozygous parents, then there are 3 options:

Punnett Squares with Amoeba Sisters

The Amoeba Sisters explain Punnett Squares really well

Monohybrid crosses are Punnett squares that just look at one gene. Mono = one (like mono-brow)

They also cover: 

Have a watch. 

Punnett Squares for Pigeons

Have a play with the Pigeon genetics interactive below - it takes a bit of getting used to so it is a good cognitive challenge

Punnett Squares and Test crosses for Pure Breeding

Your Hobbit has the phenotype Hairy Feet

However, you do not know his genotype

Is he TT or is he Tt. You can't tell as the phenotype is the same - Hairy Toes or Hairy Toes

What you could do though, is breed him with a Hobbit that has "No hair toes"

As "No hair toes" is recessive, we will know that the "No hair toes" female Hobbit must be: tt

So she is "No hair toes", tt

And he is "Hairy Toes", TT or Tt

We can find out what he is by looking at their children

The options are below

If all the children have "Hairy Toes" then his genotype must be TT

Due to random fertilization though, they should have more than just 4 children to be certain

As soon as one of them is born with "No hairy toes" Then he must actually be Tt

Of course, ethics approval means that you probably shouldn't test cross your Hobbits and make them have lots of children just so you know what the genotype is

However, with Farms you can do this with Cows, sheep, chickens etc

The children become meat and then if your Cow, sheep, chicken has the Genotype you want, you can send it off to breed with other Farmers animals so that they can also get the animals that have that Genotype - and there is a lot of money in breeding

Punnett Squares and Your Cousin

We have a protein called Major Histone Compatibility Complex (MHC)

It is small, it can float in their air. It is part of your Body odor

It is unique to you, but it is similar to your family members

It tells your brain that these people are related to you, so don't mate with them. - Its mentioned in Hanks short clip on being Sexy.

This is why you do not find your siblings attractive, but you might your step siblings - they are not related to you so its more a moral thing than a biological thing

Often your recessive alleles are bad. This is because they are often for a protein that no longer works.

But, that's OK because they are hidden by your Dominant alleles. The Dominant alleles will make more protein to do the job of the one that no longer works 

If you just let mate selection do its thing and marry a random person, then chances are high that they won't have the same recessive alleles as you

So, in your children, your dominant alleles will hide any recessive alleles that they inherited from your partner

Likewise, in your children, your recessive alleles are hidden by the dominant alleles that they inherited from your partner

 However, if you marry your  cousin. Then because you have the same grandparents, it is far more likely that you are heterozygous for a few of the same bad recessive alleles. 

For you and your partner, the dominant alleles that you've got keep those recessive alleles hidden might not be passed on, rather you might both pass on the recessive alleles

As seen in the previous Punnett Squares, when two heterozygous individuals meet, there is a 25% chance, or a 1 in 4 chance, of the child being Homozygous Recessive. That's could be a 1 in 4 chance of passing on a disease.

And this chance is for each gene that you both carry a recessive allele and there are 25,000 genes! So chances are high for disease

So marry someone who is far away from your family, that way you get some new dominant alleles into the mix.

Cousins are a bit further away biologically, so the MHC is a bit 'weaker'

In ancient times of people living is very small villages or in small tribes they might have little choice but to marry a cousin.

In more recent times, the other end of the wealth spectrum, extremely rich and powerful families like the Habsburg Royal Family, have wanted to 'keep the wealth in the family' and married their cousins. 

So incest is bad for humans

Sadly some animals are forced to reproduce like this to get 'desired' recessive alleles to create a 'breed' this however can also cause a range of health disorders 

You are far better getting a mongrel. They are less inbred that way. Also mongrels are cool. 

And you help more if you get them from Humane Society or the SPCA

Punnett Squares For Mendel and his peas

Inbreeding is bad for animals, but seem to be less so for Plants

In fact, plants often inbreed with themselves - self-fertilization

This leads into Pea Plants and Gregor Mendel

What Gregor did was take a brush and pollenate his pea plants

But he noticed something weird

Sometimes when he crossed a yellow plant with a green plant, all of the first generation of peas (F1) were yellow

But then when he inbreed these Yellow peas, then the second generation (F2) had offspring with the ratio of 3 yellow peas for one green

So the green pea was able to skip a generation

He thought about how is it possible for 'traits' to skip a generation

He figured that there must be something that has the instructions for these traits

He decided that Yellow seemed to dominate the crosses, so he called this a Dominant trait and thus he decided to call Green Recessive

He then needed to figure out why the trait of Green could be hidden and skip a generation

He ultimately came up with the ideas

He did this without knowing anything about chromosomes or DNA, as Scientists didn't know about that

But they did know that things are inherited. 

He figured that there must be two traits that are always inherited, one from each parent!

He also figured out that which of the two traits that each parent has they will then pass onto the offspring - he called this the Law of Segregation - we call it Meiosis 

He also found that different genes can be inherited separately from each other - he called this the law of Independent Assortment.

By running with his knowledge of 'traits' and the traditional knowledge of animal and plant breeding he was able to figure out things that nobody else was aware of. 

In fact, his experiments were so advanced, that it took another 35 years before other Scientists started to agree with him

But then it was still another 80 years before people discovered chromosome and all of his discoveries finally made sense.

Punnett Squares Lecture with Mr Cowley