Genetics › Mendelian Inheritance

Punnett Squares

Punnett Squares

Sections

PUNNETT SQUARES

Homozygous

Having a pair of identical alleles

Heterozygous

Having two different alleles for a gene

Phenotype

Organism's traits

Genotype

Organism's genetic makeup

Punnett Squares

  • For crosses involving a single trait, used a four-squared Punnett Square
  • Use upper-case letters to denote dominant allele
  • Use lower-case letters to denote recessive allele
  • Place one allele above each column and one allele before each row
  • In the boxes, write the combination of the two alleles

Testcross

A cross that allows one to determine the genotype of an individual by crossing it with a homozygous recessive individual and looking at the offspring

Full-Length Text

  • Here we will learn about simple patterns of inheritance through the use of Punnett squares.

First, let's start a table to learn some key terminology.

  • Denote that homozygous means having a pair of identical alleles for a gene.
  • Denote that heterozygous means having two different alleles for a gene.
  • Denote that phenotype is an organism's traits.
  • Denote that genotype is an organism's genetic makeup. We will discuss the differences between these two shortly.

Let's look at pea plant flower color from Mendel's experiments.

  • Recall that during Mendel's experiments, the color of a flower could be purple or white and that the color purple was dominant.
    • To recapitulate, write that the gene that controls flower color has two alleles, one that codes for purple and one that codes for white.
  • Write that the upper case "P" describes the dominant purple allele.
  • Write that the lower case "p" describes the recessive white allele.
  • Draw a white flower.
  • Indicate that its phenotype (its visible trait) is white.
  • Regarding its genotype (its genetic makeup), recall that organisms have two copies of each gene, one from each parent.
  • In case of the white flower, write that the genotype has to be little p, little p because it is white and white is recessive (if a purple allele was present, the flower would be purple).
  • Draw a purple flower.
  • Indicate that the phenotype is purple.
  • As for its genotype, recall that Mendel's parental generation was true-breeding, which means it always produced purple flowers.
  • For this to happen, write that the genotype of this flower is big P, big P.

Now what happens when you cross these two plants? To illustrate the possibilities, let's use a Punnett square which is a device that helps us predict the allele combinations of offspring.

  • Draw a Punnett square which is a box divided into four squares.
  • Recall that according to Mendel, each allele for a gene separates into a different gamete from the other allele, so each gamete will only have one allele for a gene (the Law of Segregation).
  • For the purple flower, which is homozygous for the purple allele, write a single big P above each column.
  • Though unconventional, we underline the upper-case P so as to better differentiate it from the lower-case p.
    • This is merely used as a visual aid because of the similarities between upper-case P and lower-case p.
  • For the white flower, which is homozygous for the white allele, write a single little p to the side of each row.
    • This allows us to visualize the separation of alleles into different gametes.
  • Because offspring are given one copy of a gene from each parent, within each box write the row and column combinations of alleles.
  • In this case, each box will have a big P and a little p.
  • Write that because all of the offspring have a big P allele, they are all purple.
  • Go back to the genotype of the purple flower and write that it could also be big P, little p.

All of the offspring were heterozygotes. So what happens when you mate two heterozygotes together?

  • Draw another Punnett square.
  • Write big P, little p above each column and big P, little p to the side of each row.
  • In the upper left box, write that the offspring receives two big P alleles.
  • In the upper right box, write that the offspring receives a big P allele and a little p allele.
  • In the lower left box, write that the offspring receives a big P allele and a little p allele.
  • In the lower right box, write that the offspring receives two little p alleles.
  • All three offspring with a big P allele will be purple and the fourth offspring will be white.
  • Write that the offspring of two heterozygotes will have a 3 to 1 purple to white ratio.

So if we have a flower with a purple phenotype, how do we determine its genotype?

  • Indicate that we can perform a testcross, which is a cross where the organism in question is mated with an organism that is recessive for the trait.
  • Draw a purple flower.
  • Write that its phenotype is purple but its genotype is unknown.
  • To determine the genotype, draw two Punnett squares.
  • Write little p, little p to the side of each column.
  • Above each Punnett square, write the two possible genotypes that would provide a purple flower; big P, big P above the first Punnett square and big P, little p above the second Punnett square.

Now, stop and see if you can fill out the possible offspring genotypes. Filled out your squares?

Let's see how you did.

  • The genotypes in the first one should all be heterozygotes, phenotypes 100% purple.
  • The offspring in the second Punnett square should be big P, little p in the first column and little p, little p in the second column, with a fifty-fifty ratio of purple to white.
  • If all of the offspring of the testcross were purple, write that the parental genotype is big P, big P.
  • If the offspring of the testcross had a 50% chance of being purple or white, write that the parental genotype is big P, little p.
  • So a testcross allows us to look at the phenotype of the offspring to determine the genotype of the parent.

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