Punnett Squares:

 

Reginald C. Punnett is probably best remembered today as the creator of the Punnett square, a tool still used by biologists to predict the probability of possible genotypes (genetic makeup) of offspring.  A Punnett Square is used to determine the probability of getting a specific genetic makeup and visable characteristics in offspring.  Knowing the genotypes (genetic makeup) gives us more information than just knowing the phenotype (visible characteristics).

 

Lets start with some Vocabulary: 

 

We've already been introduced to three important vocabulary words for thsi unit:

 

Punnett Square: A tabular tool used to determine the gentic makeup of offspring

 

Genotype:   The genetic makeup of an individual.  The Genotype consiste of two copies of each gene.

 

Phenotype: The visable characteristics of an individual.  Phenotype is determined by the genetic makeup

                            in the cells of the individual.

 

There are a still few more words that will be introduced in the next few pages - and it is important that we understand all of these, as they will be used in class.

 

Other more frequently used words and terms are presented here in an order that uses one to help define the next.

 

You will need to memorize and be comfortable using these words when discussing concepts in class.

A gene is said to be dominant when it expresses itself even if there is only one copy present in a cell.  We have already learned that DNA is coiled into chromosomes in our cells, and that each somatic cell has two copies of each chromosome (one copy from each parent).  Each of these chromosomes contains a copy of the genes associated with that chromosome. Therefore, each cell has two copies of each gene.  The dominant gene will express itself even if there is a copy of the recessive gene present. Dominant alleles are shown as upper case letters.

DNA:

 

 

 

 

Proteins:

 

 

 

 

 

 

Chromosome: 

 

 

 

 

 

 

Gene: 

         

 

 

 

Allele:

 

 

 

 

 

Gametes:

 

 

 

 

 

Characteristic: 

 

 

 

Dominant:

 

 

 

 

 

Recessive:

 

 

We know that each parent provides one copy of an allele to its offspring.  In Giraffes, the gene for albinism is recessive to normal pigment.  Only giraffes with two copies of the albino allele will be albino.  We can use a Punnet Square to determine the likelyhood of an albino giraffe being born.

 

We need to start by drawing a table and add in the allele contributed by the female at the top of the table.  We'll seperate the two alleles and place one atop each box:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DNA is the genetic code.  This is the part of the cell that contains all of the information that the cell uses to live.  DNA is a molecule contains the instructions for the cell to build proteins.  Proteins serve many functions in the cell; they are the building blocks of living things; they control what portions of DNA is activated; they make chemical reactions possible in the cell.  Ultimately, proteins control how an organism looks.  Proteins control the characteristics of an organism.

Proteins are macromolecules (Macro = very big) that are made up of many small components called Amino Acids.  The sequence of Amino Acids determines the shape and function of a protein.  Proteins are the molecules that build our muscles, and are a major structural component of an organism - but proteins also perform other imprtant functions. 

Enzymes: Specialized proteins that make chemical reactions possible in the cell.   Enzymes make the cell function.

 

Chromosomes form from tightly coiled DNA molecules.  The DNA molecule is a very long molecule, and it is catostrophic to the cell should it become damaged.  The DNA is coiled to preven tangles and breaks in the molecule.  Most of the time, this coiling is moderate, and the DNA is called Chromatin.  When the cell is ready to divide, there is a real danger that it will become damaged, andthat is when DNA is coiled tightly into Chromosomes.  Every cell has two copies of the same chromosome - one is inherited from the mother, and one inherited from the father.  Two copies of the same genetic instructions are therefore present in each cell.

A gene is a section of DNA that codes a specific protein.  These proteins control the function of the cell, and determine the phenotype of the individual. Each cell has two copies of each Chromosome, and each chromosome contains a copy of the genes.  Therefore, there are two copies of each gene coded in the DNA of an individual.  There may be more than one version of the gene for height  - a "tall" version of the height gene and a "short" version.  These versions are called Alleles.

The trait or way that the organism looks.  Some charateritics are easily visible, like eye color or height, while some require closer examination, like blood type or handedness.  Charactristics are controlled by DNA.

A gene is said to be recessive when it does not express itself if there is a copy of a dominant gene present in a cell.  We have already learned that DNA is coiled into chromosomes in our cells, and that each somatic cell has two copies of each chromosome (one copy from each parent).  Each of these chromosomes contains a copy of the genes associated with that chromosome. If one copy of the gene is DOminant, and the other is recessive, only the dominant gene will express itself.  The only way for a recessive gene to express itself is for there to be two copies of the recessive gene (one from the mother, and one from the father). Recessive alleles are shown as lower case letters.

Alleles are verisons of a gene.  Genes can have more than one version.  For instance, the gene that controls if your ear lobe is connected to your head or not has two versions; one version (allele) is detached, while the other version of the allele is attached.  These versions determine the way an individual looks. (See Pheonotype and Genotype for more information.

Has  one copy of the Normal Pigmentation allele and one copy of the allele for albinism.

Two copies of the allele are present; one copy is dominant (normal pigment - A) and one is recessive (albino - a)

The sex cells of an organism.  Males produce sperm, and females produce eggs.  These cells contain only one copy of each chromosome (this process will be discussed at length later on during the marking period).

On to the Punnett Square:

We will add in the allele contributed by the male at the right side of the table.  Again, we'll seperate the two alleles and place one next to each box.  This seperating of alleles is called segreigation (which means to seperate):

Two copies of the allele are present; one copy is dominant (normal pigment - A) and one is recessive (albino - a)

Now, we populate the table with the allele located at the top of each column, and then with the allele at the end of each row:

Two copies of the allele are present; one copy is dominant (normal pigment - A) and one is recessive (albino - a)

Two copies of the allele are present; one copy is dominant (normal pigment - A) and one is recessive (albino - a)

Two copies of the allele are present; one copy is dominant (normal pigment - A) and one is recessive (albino - a)

Two copies of the allele are present; one copy is dominant (normal pigment - A) and one is recessive (albino - a)

a

a

A

a

a

A

A

A

A

A

a

a

A

a

A

a

A

a

The Punnett Square is complete - we can see the Genotypes of each of the possible offspring.  We will get the following distribution of alleles from these parents:

 

1  Dominant/Dominant offspring

 

 

2 Dominant/Recessive offspring

 

1 Recessive/Recessive offspring

 

normal pigmentation because there is at least one dominant allele for pigmentation present)

 

normal pigmentation because there is at least one dominant allele for pigmentation present)

 

albino pigmentation because both alleles are recessive, and there is no dominant allele present)

We can see this in our Punnett Square if we show the phenotype along with the genotype:

 

Two copies of the allele are present; one copy is dominant (normal pigment - A) and one is recessive (albino - a)

Two copies of the allele are present; one copy is dominant (normal pigment - A) and one is recessive (albino - a)

a

a

a

A

A

A

A

A

a

A

A

a

Additional Vocabulary:

There are words for when there are two of the same alleles present in a cell, as well as when the allele pair are different.  These words are important, and we'll be using them from now on:

 

Homozygous:  The condition when both alleles in the pair code for the same version of the characteristic.  This is indicated when we show the allele pair by showing two alleles that are both upper case, or both lower case:
    AA is a Homozygous genotype for the Dominant allele trait,

          and will show the dominant characteristic

   aa is a Homozygous genotype for the Recessive allele trait

          and will show the recessive characteristic

 

Heterozygous:  The condition when both alleles in the pair code for a different version of the same characteristic.  This is indicated when we show the allele pair by showing two alleles that are mixed case (It really doesn't matter which comes first, but by convention, we usually show the dominant characteristic first).

    Aa is a Heterozygous genotype, and will show the dominant characteristic

   aA is the same genotype as above, and will show the dominant characteristic

So that's the basics of a Punnett Square.  Geneticists make more complicated Punnett Squares for genetic tracing and pedigrees, but that's whet we need to know at our leve in High School.  You can always ask your math teacher for additional information about Punnett Squares and Statistics if you are interested.