Name: _____KEY_______________________________ Date: ______________________ Period: ______
Genetics & Heredity:
Gregor Mendel and Punnett Squares
Heredity:
The passing of traits from parents to offspring.
Traits- the characteristics of an organism.
Alleles: A different form of a gene.
One allele may code for dimples, and the other for no dimples.
Parents:
You get one allele from mom, and one from dad.
Genetics:
The study of how traits are inherited through the interactions of alleles.
Gregor Mendel: The first to study how traits were passed down from one generation to the
next. He studied pea plants and concluded that alleles were either dominant or recessive.
Dominant vs. Recessive:
Dominant Alleles- an allele that covers, or dominates another allele.
Recessive Allele- an allele that is covered, or hidden by another allele (a dominant allele).
Representing Alleles
(Letters!)
Upper case letters represent dominant alleles. (R)
Lower case letters represent recessive alleles. (r)
Genotype vs. Phenotype:
Genotype- an organism’s genetic makeup- a combination of 2 or more alleles for a trait.
Phenotype- outward appearance, or behavior of an organism-determined by the genotype.
Punnett Squares:
A tool to predict the probability of certain traits in an offspring.
Punnett squares show the different ways that alleles can combine.
Homozygous:
Most of your cells have two alleles for each trait- your genotype.
An organism with alleles that are the same are homozygous. (RR, rr)
Heterozygous:
An organism that has different alleles for a trait is called heterozygous. (Rr)
Example:
The tongue rolling allele is R.
*R is dominant
*r is recessive
-Your mom is homozygo us dominant for the tongue rolling gene. Her genotype is RR.
-Your dad is heterozygo us for the tongue rolling gene. His genotype is Rr.
Probability
What is the probability that you will be able to roll your tongue? Do a Punnett Square to
find out!
Making a Punnett Square:
To set up a Punnett Square, draw a large square, and then divide it into four equal sections.
R
R
RR
r
Rr
Sperm
Your genotype could be:
Your phenotype would be:

100% tongue roller

R
Eggs
RR
Rr
50% RR-homozygous dominant (genotype)
50% Rr- heterozygous
0% No tongue roller
(phenotype)
(genotype)
(phenotype)
Hybrid vs. Purebred:
Hybrids=Heterozygous- Have 2 different alleles for a gene: Rr
Purebreds=Homozygous- Have the same alleles for a gene: RR or rr
Let’s Do Another One!
The Big Nose Allele is N.
N
N
n
NN
Nn
Sperm

Big nose is N

Little Nose is n
-Mom is Heterozygous for Big Nose: Nn
-Dad is Heterozygous for Big Nose: Nn
-What is the chance you’ll have a big nose? Do the cross!
n
Nn
nn
What are the possible genotypes?
Eggs
25% NN-Homozygous dominant
50% Nn-Heterozygous
25% nn-Homozygous recessive
What are the possible phenotypes?
75 % Big Nose
25% Little Nose
Incomplete Dominance:
When the offspring of two homozygous show an “in-between” phenotype.
Example:

Purebred white flowers

Purebred red flowers
Makes “in-between” colored pink flowers