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TRAITS, GENES, AND
PUNNETT SQUARES
WHAT IS A TRAIT?
 A trait is something about you that makes you "you." When
your mother says that you get all your best traits from her,
she means you have the same charming smile and the same
brilliant mind as she has. In science, trait refers to a
characteristic that is caused by genetics .
WHAT IS A GENE?
 a unit of heredity that is transferred from a parent to
of fspring and is held to determine some characteristic of the
of fspring.
WHAT DOES GENETICS MEAN?
 Genetics is the study of genes, heredity, and genetic variation
in living organisms. It is generally considered a field of
biology, but it intersects frequently with many of the life
sciences and is strongly linked with the study of information
systems.
WHAT IS GENETIC VARIATION?
Genetic variation can refer to differences between
individuals or to differences between populations.
WHAT IS A PUNNETT SQUARE?
 The Punnett square is a diagram that is used to predict an
outcome of a particular cross or breeding experiment. It is
named after Reginald C. Punnett, who devised the approach.
The diagram is used by biologists to determine the probability
of an of fspring having a particular genotype.
WHAT IS A GENOT YPE AND A
PHEONT YPE?
A genotype is an individual’s collection of genes. The genotype for BLUE eyes is
Bb, while the genotype for brown eyes is Bb or BB. The phenotype is what you see.
You see blue eyes and brown eyes. Those are the phenotypes.
HOW DO YOU FILL IN A PUNNETT
SQUARE?
 Let’s try filling in a punnett square together. Please draw a
box on your notes. Give it four squares, so it looks like this!
FILLING OUT THE PUNNETT SQUARE
 Along the sides, you will put the genotypes of each parent.
Pictured below, you can see the genotype of the father will go
across the top, and the genotype of the mother will go up and
down.
LET’S START!
We will do eyes. The father has a genotype of Bb, which means he
has brown eyes. Even if he has a lower case letter be in his
genotype, if there is a capital letter in the combination, the
dominant trait is going to show. The mother has a genotype of bb,
which means she has blue eyes.
FILLING IN THE SQUARE
 Now we are going to fill in the square. We take the letter
from each first box and get Bb. If there is any capital letter, it
always has to be first in the combination of two genotypes.
Bb
FILLING IN THE SQUARE
 Next, we will continue across. We are still using the first
small b from the mom, but we move over and use the small b
from the dad now.
Bb
bb
FILLING IN THE SQUARE
Now, we are going to move down to the bottom row. We will use the
mom’s second small b, and start with the dad’s big B. Remember,
the capital B always comes first, no matter what box you are doing!
Bb
Bb
bb
LAST SQUARE!
Now, we will fill in the last box. We are working with the father’s
small b and the mother’s small b.
Bb
bb
Bb
bb
WHAT DO WE DO WITH THE PUNNETT
SQUARE?
Look at the genotypes we have here. We have 2 Bb’s, and we
have 2 bb’s. If there is any capital letter in the combination of
the genotype, we know that represents BROWN eyes. If it is
bother lower case, we know it represents BLUE eyes.
USING THE SQUARE
There are 4 squares, so the likelihood that this mother and father have a
child with BROWN eyes is 2:4. The likelihood that this mother and father
have a child with BLUE eyes is 2:4. That is how a punnett square works!