

Genetics is the study of how traits are
passed from parents to offspring.
Heredity is the actual passing of traits from
parents to offspring.


Gregor Mendel is
known as the father
of modern genetics.
Experimented with
pea plants


Because of Mendel’s experiments we now
know that inherited traits are controlled by
different versions of genes known as alleles.
Alleles can be dominant or recessive.
◦ Dominant alleles can hide alleles for other traits.
Use capital letters to represent them.
◦ Recessive alleles can be hidden by alleles for other
traits. Use lower case letters to represent them.


The results of most of Mendel’s experiments
looked the same.
The F1 generation showed all of the dominant
traits, while the F2 generation showed the
dominant and recessive traits in a ratio of
3:1.


Phenotype—an
organism’s
appearance (word
description)
Genotype—an
organism’s genetic
make-up (letter
code)
◦ Genotype for purple
flowers: PP or Pp
◦ Genotype for white
flower: pp


Homozygous—both alleles are the same
(pure) PP or pp
Heterozygous—two alleles are different
(hybrid) Pp


Monohybrid Crosses —only one characteristic is
tracked
A tool used to predict the outcome of different
types of monohybrid crosses is called a Punnett
Square
◦ Example:
TT x tt
(Pure tall plants x Pure
short plants)
t
t
T
T
Tt
Tt
Tt
Tt
Result: 100% Heterozygous tall offspring
Genotype = Tt
Phenotype = tall

Incomplete Dominance occurs when
offspring have a phenotype that is in
between that of the two parents.
RR
Rr
http://smabiology.blogspot.com/2008_11_01_archive.html
rr


Punnett Squares for incomplete dominance are
completed in the same manner. Except that the
heterozygous individuals will have the blended
phenotype.
Example: Pink Flower x Pink Flower
Rr
x
Rr
R
r
R
RR
Rr
r
Rr
rr
Results: 25% Red flowers
50% Pink flowers
25% White flowers
Genotypic Ratio: 1 RR : 2 Rr : 1 rr
Phenotypic Ratio: 1 Red : 2 Pink : 1 White



Codominance occurs when both alleles in a pair are
expressed but do not actually blend.
Human Blood Type is an example of codominance.
Three alleles (A,B and O) are involved in determining
blood type. However, you still only inherit two (one
from mom and one from dad)
◦ A and B are both dominant; O is recessive
◦ Type AB blood has one genotype: AB
◦ Type A blood has two possible genotypes: AA and
AO
◦ Type B blood has two possible genotypes: BB and
BO
◦ Type O blood has only one possible genotype: OO


Dihybrid Cross --shows two traits at the same time.
Example: RrYy
x
RrYy (heterozygous round and yellow
seeds)
R = round r = wrinkled Y = yellow y = green
RY
Ry
rY
ry
RY RRYY RRYy RrYY RrYy
Ry RRYyRRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy
Results:
9 round/yellow
3 round/green
3 wrinkled/yellow
1 wrinkled/green