Genetics
The study of Heredity
Mendel and His Peas
Gregor Mendel 1860’s
monk that discovered
the fundamental
principles of genetics by
breeding garden peas.
Mendel crosspollinated pea
plants
 He
cut away the
male parts of
one flower, then
dusted it with
pollen from
another
Mendel’s Conclusions

He concluded that factors are passed
from one generation to the next.
Genes and Alleles
Genes
Sequence of DNA that
codes for a protein and
determines a trait.
Alleles
Alternate forms of genes.
One comes from your
mom and one from your
dad.
Dominant and Recessive alleles
Dominant alleles – represented by
upper-case letters (B)
trait will always appear, only one
dominant allele needed.
(BB or Bb)
Recessive alleles – represented by
lower case letters(b)
trait will not appear UNLESS
both recessive alleles are
present (bb).
Recessive traits show up about 1/4th
of the time.

Because there is only a
25% chance that two
recessive alleles will be
paired together.
eye color locus
B = brown eyes
The Principle of
Dominance
Some alleles are dominant
and others are recessive. Paternal Maternal
Recessive alleles are not
expressed in the organism,
but may be passed on to
future generations.
eye color locus
b = blue eyes
Types of Allele pairing
Homozygous
(purebred)
both alleles are
the same
(BB – Brown eyes)
(bb – blue eyes)
Heterozygous
(hybrid)
two different
alleles
(Bb – Brown eyes)
Genotype


Arrangement of
genes that
produces the
phenotype
Example:
1. BB, Bb
2. bb
Phenotype
Outward appearance
 Physical
characteristics


Examples:
1.Brown eyes
2.blue eyes
Punnett Square

A Punnett square is
used to show the
possible
combinations of
gametes and allow us
to determine the
probability of an allele
combination.
Monohybrid Cross

Example:
Cross between two heterozygotes
brown eyes (Bb)
BB = brown eyes
Bb = brown eyes
bb = blue eyes
B
B
b
BB
Bb
Bb
bb
Bb x Bb
b
female gametes
male
gametes
Monohybrid Cross
BB= homozygous dominant; 1 of 4 = 25%
Bb= heterozygous; 2 of 4 = 50%
bb= homozygous recessive; 1 of 4 = 25%
Chances of Brown Eyes: 3 of 4 = 75%
Chances of Blue Eyes: 1 of 4 = 25%
B
b
B
BB
Bb
b
Bb
bb
Bb x Bb
Dihybrid Cross: RrYy x RrYy
CROSSES WITH TWO DIFFERENT TRAITS
Here’s your game plan:
FIRST: FOIL the alleles just like you
do for polynomials in math class
(Rr) (Yy) x (Rr) (Yy)
First:
RY
RY
Outer: Ry
Ry
Inner:
rY
rY
Last:
ry
ry
Dihybrid Cross: RrYy x RrYy
SECOND: take your results from your
FOILing and place them on the sides of your
Punnett square! Add like letters to like letters
in the boxes.
RY
RY
Ry
rY
ry
RRYY
Ry
rY
ry
Dihybrid Cross
RY
RY RRYY
Ry
Ry
RRYy
rY
RrYY
ry
RrYy
Smooth/Yellow: 9 of 16
Smooth/green: 3 of 16
RRYy
RRyy
RrYy
Rryy
wrinkled/Yellow: 3 of 16
rY RrYY
RrYy
rrYY
rrYy
wrinkled/green: 1 of 16
ry
Rryy
rrYy
rryy
9:3:3:1 phenotypic ratio
RrYy
Incomplete
Dominance

One allele is not
completely dominant
over another and the
traits combine, or
blend R
R
W
RW
RW
W
RW
RW
Incomplete
Dominance
W
W
R
R
RW
RW
RW
RW
a. List the possible genotypes and
phenotypes
genotypes – RW
phenotypes – Pink
b. How many of the plants would have
red flowers?
0%
c. How many would have white
flowers?
0%
d.
How many would have pink
flowers?
100%
Codominance
Black cow + white cow =
spotted cow
Blood Types: A, B, AB, O
Codominance
Example:


Both alleles are expressed
Example: blood
1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
Blood Type O (ii) X Blood Type A (IAIA )
i
i
IA
IA i
IA i
IA
IA i
IA i
Codominance







GENOTYPE
IAIA = ______ of ______=____% = Type________
IAi = 4 of 4 = 100% = Type A blood
IBIB = ______ of ______=____% = Type________
IBi = ______ of ______=____% = Type________
IAIB = ______ of ______=____% = Type________
ii = ______ of ______=____% = Type________
Karyotype Analysis
To analyze chromosomes, we cut up and
assemble photographs of chromosomes in
pairs.
 There are 44 autosomal chromosomes
and 2 sex chromosomes in humans.
