MENDELIAN
GENETICS
Genetics – The Study of Heredity
Heredity – Passing traits from parents to offspring
How are genes inherited?
contain
Chromosomes
chromosome #15
produce
Genes
ATGC sequence
determine
Proteins
Traits
folded amino
acid sequence
blue eyes
But what are the rules that allow us to predict the
likelihood of inheriting certain traits over others?
In the mid-1800s, Gregor Mendel
was the first to figure it out!

Mendel was raised on a small farm in Austria.
He was always curious about nature.

Mendel was trained in mathematics and
biology and spent most of his life as a monk.

Mendel was encouraged to explore his curiosity scientifically.

Mendel chose GARDEN PEA PLANTS as his test organism.

Mendel’s findings went unrecognized until the early 1900s,
decades after his death.
1822-1884
Mendel’s Findings
The Laws of Heredity were discovered
using PEA PLANTS, but they are the
same laws for ALL living things!!!
 Law of
Unit Characters
 Law of Dominance
 Law of Segregation
 Law of Independent Assortment
Why Peas?
 Easy
to care for
 Characteristics are easy to identify.
 Short life span, so you can get several generations in a
short amount of time.
 Large number of offspring at one time, providing lots of
data.
 Easily control the mating of pea plants in a green house.
 Pea offspring can be mated with their parents and with
one another.
Pea Flower Structure


flower contains male and female reproductive organs
pea = offspring (seed)
Mendel’s Experiments
P1 Generation
(original parents)
F1 Generation
F2 Generation
X
X
Purebred tall plants
were crossed with
purebred short
All F1 offspring are
tall. Short trait
disappeared!
Short trait reappears in
a consistent ratio of
3 tall : 1 short
P1, F1, and F2 would be similar to your grandparents, your parents, and you.
Mendel’s
Results
THOUSANDS
of offspring!!!
Studied 7
different
traits!!!
All F2 offspring
3:1 ratio!!!
(3 dom :1 rec)
Important Vocabulary
Allele = A gene, but in a specific form. Each characteristic
(trait) is always determined by two alleles.
Dominant Allele = A gene that, if present, always produces a
protein (and therefore a trait).
Recessive Allele = A gene that produces a protein (and
therefore a trait) ONLY if present with
another recessive allele.
Important Vocabulary
Genotype = symbols (capital and lower case letters) that
represent genes on homologous chromosomes.
BB, Bb and bb are all genotypes.
Phenotype = the organism’s characteristic as a result of its genes.
Brown eyes and blue eyes are both phenotypes.
Homozygous = a genotype showing two of the same allele.
BB is homozygous dominant; bb is recessive.
Heterozygous = a genotype showing two different alleles.
Bb is heterozygous.
What do Mendel’s Law really mean?
Law of Unit Characters

Law of Unit Characters – Characteristics of an
individual are controlled by hereditary factors
(genes) that occur in pairs (on homologous
chromosomes)
 Example:
Height can either be TT, Tt, or tt
Law of Dominance

Law of Dominance – the dominant allele is
expressed and the recessive allele can be
hidden.

This is why offspring don’t look like a complete
blend of each of the parents.
Law of segregation
Law of Segregation – a pair of genes are separated
during the formation of gametes. You can inherit
either allele with equal probability.
Ex. T or t but not both.

The dominant allele is not passed on more often
than the recessive allele.
Law of Independent Assortment

Law of Independent Assortment – inheritance
of most traits does not depend on other traits.


The fact that you have brown hair has no effect on
the color of eyes that you have.
There are exceptions to this law, and these will be
discussed later.
Punnett Squares

See overhead notes.
parent
genotype is
heterozygous
is a symbol to represent a sequence of
nitrogen bases (gene) that will make a
protein to produce the dominant trait.
DNA
replication
is a symbol to represent a sequence of
nitrogen bases (gene) that will make a
protein to produce the recessive trait ONLY
if present with another one ( ).
meiosis
I
meiosis
II
1. The gamete used for each fertilization is randomly chosen.
2. The probability of inheriting either or from one parent is
50/50 each time an offspring is born.
3. This Punnett square predicts a 3:1 ratio in the offspring, but this
is not guaranteed, unless. . .
you are looking at a LARGE NUMBER of offspring!!!
What is the difference between a gene
and a trait?
A gene is a sequence of nitrogen bases on
a chromosome that codes for a protein.
ATGCTTTCCCAGAGCTGCGT
A trait is a physical feature, due to the
presence of specific proteins.
green eyes
• What do the letters Bb represent?
B
b
Bb is a genotype that
represents genes on
homologous
chromosomes.
homologous chromosomes
• Give an example of an allele.
B is a dominant allele.
b is a recessive allele.
What is the difference between a
genotype and a genotypic ratio?
Tt
Two genes that
an organism has
for any given
trait.
1:2:1
A way to express the chances
that the offspring will have a
certain genotype.
25% chance the offspring will be TT.
50% chance they will be Tt.
25% chance they will be tt.
Explain what is meant by P1, F1, and F2.
P1: The original parents, always a purebred (homozygous)
dominant crossed with a purebred recessive.
EE x ee
F1: the offspring from the above cross (first generation).
all Ee (heterozygous)
F2: the second generation of offspring, achieved by mating
two F1 offspring with each other.
Ee x Ee will produce the F2 offspring
In mice, brown color is dominant over white. Two brown mice are
mated several times. They have a total of 20 offspring, 14 brown, and
6 white. Determine the genotypes of the parents and all the offspring.
B= brown
b = white
Bb
brown (x)
Bb
brown
14 brown and 6 white
B?
bb
Genotypes of parents: Bb (x) Bb
Genotypes of offspring:
B-
bb
In humans, tongue-rolling is controlled by a dominant allele. A non-tongue
roller man (whose parents are both tongue rollers) marries a woman who can
roll her tongue. The woman’s father was a tongue roller but her mother was
not. The couple have one child who is a non-tongue roller. Determine the
genotypes of all the people in the problem (7 total).
T= tongue roller
tt
t = non tongue roller
TTt
Tt
non tongue roller (x) tongue roller
tt
Tt
Genotypes :
tt
man= _____
Tt
Tt
His dad ______
His mom _____
Tt
woman = _____
Ttt
Her dad ______
Her mom _____
tt
Child = ________
Skipping a Generation


When a trait disappears (not seen in parents) and
then reappears in offspring generation
Proof that trait is controlled by a recessive gene.
 Ex.
Two white sheep produce a black sheep.
Determine the genotypes of the parents and offspring.
B= white
b= black
Parents: Bb (x) Bb
Offspring:
bb
Incomplete Dominance

When there is no dominant allele for a trait and
blending occurs in the heterozygous condition
Example: Flower in Four- O’Clocks and Snapdragons
Red (RR), Pink (RW), White (WW)
Pink Flower (x) Pink Flower
RW
(x)
RW
Phenotypic Ratio:
Genotypic Ratio:
R
W
R
RR
RW
W
RW
WW
1 : 2 : 1
Red : Pink : White
1
:
2
:
1
RR
:
RW
:
WW
Phenotypic & Genotypic Ratios are the same!
Pedigree Chart
Show relationships in a family (family tree)
 Shaded individuals have different phenotype from
unshaded
 Circles – females
Squares- males

bb
B-
Bb
bb
b = blue eyes
Bb
bb
bBbb
Bb
B = Brown eyes
Bb
Bb
B
bbb
WRONG!
Bbb
-
bb
B
-
B
bb-
DIHYBRID CROSSES – a cross involving two traits

Demonstrates Law of Independent Assortment
TtRr – Tall and Round
outside
first
( T x t )
( R
x
r)
inside
last
F O I L
First, Outside, Inside, Last
Key 1
T= Tall
t = Short
R = Round
r = Wrinkled
Key 2
T-R- = Tall, Round
T-rr = Tall, Wrinkled
ttR- = Short, Round
ttrr = Short, wrinkled
P1 Pure Tall, Pure Round (x) Pure Short, Pure Wrinkled
P1
TTRR
Possible Gametes:
(x)
ttrr
TR, TR, TR, TR (x) tr, tr, tr, tr
Possible Gametes:
TR (x) tr
tr
TR
Phenotypic Ratio:
1
TtRr
:
0
F1
:
0
:
0
Tall, Round : Tall, Wrinkled : Short, Round : Short, Wrinkled
1st dominant trait :
2nd dominant trait :
1st dominant trait
2nd recessive trait
:
:
1st recessive trait
2nd dominant trait
:
:
1st recessive trait
2nd recessive trait
We will not be doing genotypic ratios for dihybird problems.
Find the F2 phenotype ratio: F1 (x) F1
Heterozygous Tall,
Heterozygous Round
Possible
Gametes:
TtRr
TR, Tr, tR, tr
(x)
Heterozygous Tall,
Heterozygous Round
(x)
TtRr
(x)
TR, Tr, tR, tr
Possible Gametes:
TR
TR, Tr, tR, tr (x) TR, Tr, tR, tr
Tr
tR
tr
TR TTRR
TTRr
TtRR
TtRr
Tr TTRr
TTrr
TtRr
Ttrr
tR TtRR
TtRr
ttRR
ttRr
tr TtRr
Ttrr
ttRr
ttrr
Phenotypic Ratio:
Tall, Round : Tall, Wrinkled : Short, Round : Short, Wrinkled
Possible Gametes:
TR, Tr, tR, tr (x) TR, Tr, tR, tr
TR
Tr
tR
tr
TR TTRR
TTRr
TtRR
TtRr
Tr TTRr
TTrr
TtRr
Ttrr
tR TtRR
TtRr
ttRR
ttRr
tr TtRr
Ttrr
ttRr
ttrr
Phenotypic Ratio:
9
:
3
:
3
:
1
Tall, Round : Tall, Wrinkled : Short, Round : Short, Wrinkled
1st dominant trait :
2nd dominant trait :
1st dominant trait
2nd recessive trait
:
:
1st recessive trait
2nd dominant trait
:
:
1st recessive trait
2nd recessive trait
NOW, if we
consider
another set of
genes (on different
chromosomes)
we have two
possible
in
metaphase I of
meiosis!
gametes with genes gametes with genes
AB
ab
gametes with genes gametes with genes
Ab
aB
AABBAABbAaBBAaBb
AABbAAbbAaBb Aabb
AaBBAaBbaaBB aaBb
AaBbAabb aaBb aabb
AABBAABbAaBBAaBb
AABbAAbbAaBb Aabb
AaBBAaBbaaBB aaBb
AaBbAabb aaBb aabb
Another dihybrid example:
The genotype must always have 4 letters!
 In bunnies, black hair is dominant to white hair,
and long hair is dominant to short
Key 1
Key 2
 B = Black
B_H_ = Black, long hair
B_hh = Black, short hair
 b = White
bbH_ = white, long hair
 H = Long Hair
bbhh = white, short hair
 h = Short Hair

P1: A homozygous black, homozygous long haired
bunny is crossed with a white, short haired one.
P1
BBHH
(x)
bbhh
Determine all possibilities for gene combinations in gametes (FOIL),
cross off any duplicates, and make a Punnett Square.
P1
BBHH
(x)
BH, BH, BH, BH (x) bh, bh, bh, bh
Expected Phenotypic Ratio:
(use Key 2 order)
bh
bbhh
1: 0: 0: 0
BH
BbHh
Key 2
Black, Long
Black, short
white, Long
white, short
A heterozygous black, heterozygous long haired
bunny is crossed with a white, short haired one.
BbHh
(x)
bbhh
Determine all possibilities for gene combinations in gametes (FOIL),
cross off any duplicates, and make a Punnett Square.
BbHh
BH, Bh, bH, bh
(x)
(x) bh, bh, bh, bh
BH
bh
bbhh
BbHh
Bh
Bbhh
Expected Phenotypic Ratio:
(use Key 2 order)
bH
bbHh
1: 1: 1: 1
bh
bbhh
TRIHYBRID CROSSES – a cross involving three
traits
1)
2)
3)
4)
Genotype
TTRRYY
TTRrYY
TTRrYy
TtRrYy
1)
2)
3)
4)
Possible Gamete Combinations
TRY
TRY, TrY
TRY, TRy, TrY, Try
TRY, TRy, TrY, Try, tRY, tRy, trY, try
How many Punnett squares are needed if you cross:
1 and 2?
1 x 2 = 2 Punnett Squares
2 and 3?
2 x 4 = 8 Punnett Squares
3 and 4?
4 x 8 = 32 Punnett Squares
Show what the Punnett square would look like if you
crossed the following:
TTRrYY (x) TTRRYy
TRY
TRy
TRY TTRRYY TTRRYy
TrY TTRrYY TTRrYy