Chapter Vocabulary
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How to do a One-Factor Punnet Square
Step 1: Determine the genotypes of the parent organisms.
Genotypes - the alleles of the organisms (written as letters of the alphabet); sexually reproducing
organisms have two alleles (one inherited from their mother and one from their father).
Homozygous = the alleles will be identical
Example: homozygous dominant = DD
homozygous recessive = dd
Heterozygous = the alleles will be different (one dominant and one recessive)
Example: Dd
Step 2: Write down your "cross" (mating) by writing the genotypes of the parents with an "X" between them.
Example: In pea plants, yellow peas are dominant to green peas. Cross a homozygous dominant yellow pea plant with a homozygous recessive (green peas).
The parents (P generation) would look like this:
Step 3: Draw the Punnett Square, separate the parent alleles and write them on either side.
Genotypes - the alleles of the organisms (written as letters of the alphabet); sexually reproducing
organisms have two alleles (one inherited from their mother and one from their father).
Homozygous = the alleles will be identical
Example: homozygous dominant = DD
homozygous recessive = dd
Heterozygous = the alleles will be different (one dominant and one recessive)
Example: Dd
Step 2: Write down your "cross" (mating) by writing the genotypes of the parents with an "X" between them.
Example: In pea plants, yellow peas are dominant to green peas. Cross a homozygous dominant yellow pea plant with a homozygous recessive (green peas).
The parents (P generation) would look like this:
Step 3: Draw the Punnett Square, separate the parent alleles and write them on either side.
Step 4: Fill in each cell with two allele, 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.
Step 5: Summarize the results. Give genotypes and phenotypes of offspring with the probabilities. In this example, there would be a 100% chance of having plants with yellow peas (phenotype) and their genotypes would be heterozygous (Aa).
two factor crosses / dihybrid crosses
These punnett squares are for looking at two traits at a time.
Biology Corner has a good tutorial. Click here.
Biology Corner has a good tutorial. Click here.
Suppose you wanted to investigate two traits with the alleles "A" and "a" with "B" and "b".
To cross two heterozygous individuals for both traits, use the genotypes AaBb for both male and female.
1) Create the possible gametes, by using the "FOIL" method as taught in class.
To cross two heterozygous individuals for both traits, use the genotypes AaBb for both male and female.
1) Create the possible gametes, by using the "FOIL" method as taught in class.
You may also see dihybrid punnett squares in a diamond pattern. Don't let it confuse you. It's still set up and carried out in the same way.
This is a cross of two fish with two traits being studied. The genes being studied are f (fancy tail) and (e) funny eye.
Modern Genetics: Going beyond dominant and recessive alleles
Other patterns of inheritance include codominance, incomplete dominance, multiple alleles, and polygenic traits.
Incomplete dominance results in a blending of the two alleles in the heterozygous. Ex: Red + White = Pink
Incomplete dominance results in a blending of the two alleles in the heterozygous. Ex: Red + White = Pink
Codominance results in seeing both alleles in the heterozygous. Ex: Red coat cow + white coat cow = Red & White Coat
Some genes have more than two alleles for a trait. This is known as multiple alleles. Rabbit fur color and human blood type are examples.
Mendel thought that one gene controlled one trait. We now know that sometimes a trait is caused by many genes, not just one. This is known as polygenic traits. These traits have a wide range of phenotypes. Examples are eye color, hair color, height and skin color.
Pedigree Analysis
One of most powerful tools in human genetic studies is pedigree analysis. When human geneticists first began to publish family studies, they used a variety of symbols and conventions. Now there are agreed upon standards for the construction of pedigrees. Here are some basic symbols that you should be aware of:
Circles/Squares=female/male
Black/white=affected/non-affected
Carriers (Heterozygotes) are sometimes shown w/internal dot or half shaded
Deceased/unavailable phenotype sometimes as a slash
Circles/Squares=female/male
Black/white=affected/non-affected
Carriers (Heterozygotes) are sometimes shown w/internal dot or half shaded
Deceased/unavailable phenotype sometimes as a slash
Click on this link to watch a video lesson about pedigree: Education Portal Pedigree Analysis
Zero Biology has a drag and drop pedigree practice. Read the directions carefully.
Online Resources for GeneticsGREAT explanation of how to do a Punnett Square and what it means.
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