Chapter 3
Heredity
Heredity:
Passing of traits from
parent to offspring.
Gregor
Mendel
“Father of
Genetics”
Gregor Mendel
 Lived from July 20, 1822 – January 6,
1884)
 Austrian monk
 Worked in monastery garden
 Used pea plants to show that the
inheritance of traits follows particular
laws, which were later named after him
Trait – a characteristics of
an organism
Heredity – the study of
how traits are passed
from parents to offspring
Why Mendel used pea plants:
1. Quick reproduction
and growth
2. Easily observed traits
3. Many observable traits
4. Easy cross pollination
Self pollinating
Plants
usually
contain both
male and
female
reproductive
structures
During self pollination
• Pollen from anthers
(male) is transferred to
the stigma (female)
• Fertilization occurs when
a sperm from the pollen
travels through the
stigma and enters the egg
in the ovule.
True breeding plants – have
offspring that always show the
same form of the trait
Cross fertilization – a process in
which one plant fertilizes the egg
in a flower of a different plant
Pollen – tiny grains containing
plant sperm cells
P generation –
parental
generation
F1 generation –
offspring in
first cross =
hybrids
• Offspring from first cross are known
as first generation.
• Dominant: The trait observed when
at least one dominant allele for a
characteristic is inherited.
• Recessive: A trait that is apparent
only when two recessive alleles for
the same characteristic are inherited,
Each parent
donates one set of
instruction to an
offspring known as
genes.
ALLELES
Two forms of
the same
gene for
every
characteristic
Hybrids – the offspring of two
different true breeding plants
Monohybrid cross – a cross between
two plants that differ in only one
trait
F2 generation – the offspring that
result when 2 hybrid plants are
crossed
Mendel’s P (Parental) Cross
true breeder X
short
true breeder
long
Resulted in all long offspring (F1)
Mendel’s F1 (first filial) cross
Cross pollinated 2 of the long offspring
produced in the P generation
F1 long X F1 long
Results:
75% long and 25% short
Mendel’s conclusions:
Gene – the factor that controls traits
Allele – the possibilities of a gene
(e.g. A or a)
Simple dominance – one allele is
dominant to a recessive allele
Mendel Vocabulary
Dominant – the allele that masks any other
allele when there are 2 alleles present (A in
Aa) (symbolized by the first letter in the
dominant trait’s name, always capital)
Recessive – the allele that is masked by
another allele (a in Aa) (symbolized by the
first letter in the dominant trait’s name,
always lower case)
More Mendel Vocabulary
Homozygous – having two identical alleles for a trait (AA or
aa) (Mendel called this “true-breeding”)
Homozygous dominant – having two dominant alleles for a
trait (AA) (Mendel called this “true breeding dominant”)
Homozygous recessive – having two recessive alleles for a
trait (aa) (Mendel called this “true breeding recessive”)
Heterozygous – having two different alleles for a trait (Aa)
Phenotype – the visible
traits of an organism (e.g.
long or short)
Genotype – the alleles that
an organism carries (e.g. Aa
or AA or aa)
Punnett square – a model used
to represent crosses between
organisms
Example: What are the possible offspring of a cross between a
homozygous dominant green plant (GG) and a homozygous
recessive green plant (gg)?
Steps to doing a Punnett Square:
1. identify the gametes of the parents
Example: What are the possible offspring of a cross between a
homozygous dominant green plant (GG) and a homozygous recessive
green plant (gg)?
GG parent will produce all G gametes
gg parent will produce all g gametes
Steps to doing a Punnett Square:
2. draw a square with 4 boxes
Example: What are the possible offspring of a cross between a
homozygous dominant green plant (GG) and a homozygous recessive
green plant (gg)?
GG parent will produce all G gametes
gg parent will produce all g gametes
Steps to doing a Punnett Square:
3. put the gametes from one parent on the top of
the box and the gametes from the other parent
on the side of the box
GG parent all G gametes
gg parent all g gametes
G
g
g
G
Steps to doing a Punnett Square:
4. cross multiply to find the genotypes of
the children
G
g
g
Gg
G
Steps to doing a Punnett Square:
4. cross multiply to find the genotypes of
the children
G
G
g
Gg
Gg
g
Gg
Gg
Steps to doing a Punnett Square:
5. write the phenotypes of each child in the
boxes
G
g
Gg
Green
g
G
Gg
Green
Gg
Gg
Green
Green
Steps to doing a Punnett Square:
6. calculate genotypic and phenotypic
ratios of the offspring
G
g
g
G
Gg
Gg
Green
Green
Gg
Gg
Green
Green
List all possible Genotypes:
GG
Gg
gg
List all possible Phenotypes:
green
yellow
Steps to doing a Punnett Square:
6. calculate genotypic and phenotypic
ratios of the offspring
G
g
g
G
Gg
Gg
Green
Green
Gg
Gg
Green
Green
Count how many of each:
Genotypes:
GG = 0/4 = 0%
Gg = 4/4 = 100%
gg = 0/4 = 0%
Phenotypes:
green = 4/4 = 100%
yellow = 0/4 = 0%
Now try this one…
What are the possible offspring of a cross
between a pea plant which is heterozygous for
green peas and a pea plant which is
homozygous recessive for green peas?
Another example…
• What are the possible offspring of a cross
between a mother and father who are both
heterozygous for the ability to roll their
tongues?
Incomplete Dominance
• Sometimes, there are two dominant alleles
and no recessive alleles.
• Ex: flower color
In some flowers, red and
white are both dominant.
A red flower has the
RED
phenotype ______
and the genotype
RR.
A white flower has the
WHITE
phenotype ______
and the genotype
WW.
What if you cross a RED (RR)
flower with a WHITE (WW)
flower?
R
R
W
RW
RW
W
RW
RW
What color are the offspring?
R
R
W
RW
RW
W
RW
RW
WHITE
+
RED
PINK
What is the
phenotype of
this flower?
PINK
What is the
genotype of
this flower?
RW
So, when a trait is inherited by
incomplete dominance, there are
3 possible phenotypes
____
Red
White
Pink
3 possible genotypes.
and ____
RR
WW
RW
Multiple Alleles
In some cases, there are more than 2
possibilities.
Ex: hair color, eye color, skin color
Blood Type
• 2 Dominant alleles – A and B Genotype
• 1 recessive allele – O
AA
AO
AB
BB
BO
OO
Phenotype
A
A
AB
B
B
O
What if you cross a AO parent
with a BO parent?
A
O
B
AB
BO
O
AO
OO