* Punnett square
* homozygous and heterozygous genes
* dominant and recessive alleles
* phenotype and genotype
-> And note the special way that we use lowercase and uppercase letters 🙂
From Ask-A-Biologist: Punnett Squares
Punnett squares are a useful tool for predicting what the offspring will look like when mating plants or animals.
Reginald Punnett, a mathematician, came up with these in 1905, long after Mendel’s experiments.
Let’s take a look at how Punnet squares work using the yellow and green peas example from Mendel’s garden experiments.
For every gene, different versions called alleles exist.
Alleles control things like pea color or the presence of dimples on your face.
Children inherit two alleles for each gene from their parents: one from the mother and one from the father.
The genotype refers to which two alleles an organism has.
Sometimes both alleles are the same, and sometimes they are different.
The phenotype refers to the visible trait that results from the combination of alleles that are present.
Mendel began his experiments with true breeding strains: plants that have offspring of only one phenotype when mated.
In true breeding strains, both alleles are the same for a gene.
Since there is only one kind of allele present, mating two plants from the same strain will produce offspring that have the same phenotype and genotype as their parents.
Plants or animals with two identical alleles for a gene are said to be homozygous.
Mendel first crossed two different true breeding strains together:
One that produced yellow peas and one that produced green peas.
We’ll use letters to represent alleles.
Capital “A” will represent the yellow pea allele
Lowercase “a” will represent the green pea allele.
The yellow pea phenotype has a genotype of AA.
The green pea phenotype has a genotype of aa.
When Mendel looked at the results of this mating, he saw that all of the offspring had yellow seeds.
How did this happen?
If one of the parent plants had green peas, why didn’t a single one of the children plants have green peas?
We can answer these questions and understand what’s happening to the alleles in this crossing with the help of a Punnet Square.
Begin by writing the parents’ genotypes along the top and side of the Punnet square.
Next, fill in each cell with two alleles:
one from the parent along the top and one from the parent along the side.
The letters in the middle show you all possible combinations of alleles that can happen from mating these two genotypes.
In this case, all offspring have the same genotype and phenotype.
The order of the letters doesn’t make a difference in the phenotype:
aA is the same as Aa
The capital letter is usually written before the lowercase one.
These offspring are said to be heterozygous, meaning that they have two different alleles for pea color.
Despite the fact that both alleles are present in the offspring, the traits did not blend together to result in yellowish-green peas.
Instead, only one phenotype was visible and all peas were yellow.
Because of this, the yellow pea phenotype is said to be dominant, meaning that it is visible in the heterozygous individual.
For the second generation, Mendel mated the heterozygous offspring from the first generation together.
Mendel looked at the offspring from this mating:
He noticed that 1/4 of the children plants had green seeds.
Why did this happen?
How was it possible for some of the offspring to have green seeds when both of the parent plants had yellow seeds?
Let’s once again use a Punnet square to answer these questions and understand what’s happening to the alleles in this crossing.
We see that there are three possible genotypes that could result from this crossing:
AA, Aa, aa.
The genotypes AA and Aa will result in the yellow pea phenotype because A is dominant.
Only aa will produce the green pea phenotype.
Now we see how it was possible for the green pea phenotype to skip a generation.
The green pea allele was present in the F1 generation, but the phenotype was hidden by the yellow pea allele.
The green pea phenotype is said to be <b>recessive</b>:
it is only visible in the homozygous individual <i>when the yellow allele is not present.</i>
In the F2 generation, only 1 of the 4 boxes produced green peas.
In other words, 25% of the offspring had green peas.
This tells you the probability, or likelihood, that an offspring will produce green or yellow peas.
We can use the probability to predict how many offspring are likely to have certain phenotype when mating plants or animals with different traits.
Just take the probability of a phenotype and multiply it by the total number of offspring.
Let’s imagine there were 160 total offspring in Mendel’s F2 generation.
How many peas are likely to be green?
25% green peas x 160 total offspring = 40 green pea offspring
(Don’t forget that 25% = 0.25)
Pixton Comic about genotypes, phenotypes and heredity
<h3>Another Punnett Square lesson:</h3>
A. Dihybrid Crosses
- A dihybrid cross is an experimental cross between two parent organisms that are true-breeding for different forms of two traits; produces offspring heterozygous for both traits.
- Mendel observed that the F1 individuals were dominant in both traits.
B. Plants to Self-Pollinate
- Mendel observed four phenotypes among F2 offspring; he deduced second law of heredity.
- Mendel’s law of independent assortment states members of one pair of factors assort independently of members of another pair; all combinations of factors occur in gametes.
C. Dihybrid Genetics Problems
- Laws of probability indicate a 9:3:3:1 phenotypic ratio of F2 offspring resulting in the following:
- 9/16 of the offspring are dominant for both traits;
- 3/16 of the offspring are dominant for one trait and recessive for the other trait;
- 3/16 of the offspring are dominant and recessive opposite of the previous proportions; and
- 1/16 of the offspring are recessive for both traits.
- The Punnett Square for Dihybrid Crosses
- A larger Punnett square is used to calculate probable results of a dihybrid cross.
- A phenotypic ratio of 9:3:3:1 is expected when heterozygotes for two traits are crossed and simple dominance is present for both genes.
- Meiosis explains these results of independent assortment.
D. Two-Trait Test Cross
- A dihybrid test cross tests if individuals showing two dominant characteristics are homozygous for both or for one trait only, or is heterozygous for both.
- If an organism heterozygous for two traits is crossed with another recessive for both traits, expected phenotypic ratio is 1:1:1:1.
- In dihybrid genetics problems, the individual has four alleles, two for each trait.
Finding the hidden assumptions
Genetics isn’t always this simple.
It’s only easy to do with these ‘squares’, if the chromosomes get assorted independently.
That’s not always the case.
When genes get sorted with each other (“not independently) then the situation is more complex.
The law of segregation, and the law of independent assortment, illustrated