1. health and fitness
2. disease

In this lesson, you will learn how
to predict the probable genetic
makeup and appearance of
offspring resulting from specific
crosses.
Standards

Demonstrate an understanding of the key
features of DNA, genes, and chromosomes, and
the relationship that exists among them.

Make predictions concerning inheritance based
on Gregor Mendel’s laws of heredity.
How can we predict the
possible traits of an
offspring, considering the
traits of parental
generations. ?
Objectives: This is what
you are expected to know



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Explain how probability is used to predict the
results of genetic crosses.
Use a Punnett square to predict the results of
monohybrid and dihybrid genetic crosses.
Explain how a testcross is used to show the
genotype of an individual whose phenotype is
dominant.
Differentiate a monohybrid cross from a dihybrid
cross.
Before we get started…



We need to understand the meaning of
some important genetic terms. The slides
that follow will explain this.
If you want to practice, click on the link
below to go to a web site that has flash
cards of genetic words.
Flash cards
Relationship between
Gene and Allele


Gene - a segment
of DNA that
controls a specific
trait.
Allele - the
alternate (or
contrasting) form
of a gene.
Difference between
Dominant and Recessive


Dominant - refers to an
allele that masks the
expression of another
allele for the same trait.
Recessive - the allele that
is masked by the
presence of another allele
for the same trait.


Unattached (right) earlobes
is dominant to attached
earlobes (left).
A widow’s peak (left) is a
dominant trait over a
rounded face.
More examples

To see more dominant and recessive
traits, click here.
Difference between
Homozygous and Heterozygous


Heterozygous alleles that are mixed,
(dominant and
recessive) showing
the dominant trait.
Homozygous - alleles
that are the same,
dominant for the trait
or recessive for the
trait.
Difference between
Genotype and Phenotype



Genotype - genetic
makeup of an
organism; refers to the
alleles for a trait.
Phenotype - physical
or outward expression
of the alleles for that
trait. (What it looks
like)
Genotype codes ( or
determines) for
phenotype
More About Genotype

The genetic makeup of an organism is
called its genotype.
It consists of the alleles that the organism
inherits from its parents.
 Alleles are designated with letters of the
alphabet.

Dominant alleles are capital letters (P for purple
flower color).
 Recessive alleles are lower case letters (p for
white flower color).

More about Phenotype


The outward (or physical) appearance of an
organism is called its phenotype.
It is an expression of the genotype of an organism.

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There are two ways that a dominant phenotype can
be expressed.



For example purple flower color or white flower color.
heterozygous containing both the dominant and
recessive alleles (Purple flower color = Pp)
homozygous dominant containing two alleles for the
dominant trait. (Purple flower color = PP).
There is only one way that a recessive trait can be
expressed.

homozygous recessive - containing only the recessive
alleles (white flower color = pp).
Probability



Predicts the likelihood that a specific event
will occur.
May be expressed a a decimal, a
percentage, or a fraction.
Determined by the following formula:
Probability = number of times an event is expected to happen
number of opportunities for an event to happen
Why probability is important
to genetics


Mendel used probability to determine how likely
the dominant trait would appear over the
recessive trait.
The yellow pea appeared 6,022 times in the F2
generation. The green pea appeared 2,001
times.


The total number of individual was 8023 (6022+2001)
Using the formula:

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6022 ÷ 8023 = 0.75
2001 ÷ 8023 = 0.25



Percentage: 75% green peas 25% yellow peas
Ratio: 3:1 ratio of yellow to green peas
Fraction: 1/4 chance of green peas and 3/4 chance of yellow
peas
Results of the F1 generation
PP
Pp x 2
ratio: 1:2:1 or 3:1 purple to white
75% purple to 25% white
pp
F1 generation yielded 100% purple
flowers, heterozygous for the purple trait.
white (pp) x purple(PP)
yields
100% purple flowers that
are heterozygous for the
purple flower trait (Pp).
Punnett Square


A diagram used to predict
the probability of certain
traits by offspring.
The following examples
will illustrate the outcome
of different types of
crosses.
How to set up and work a
Punnett square


Draw a four-square
box.
Place one set of
alleles on the side of
the box as shown at
right.
How to set up and work a
Punnett square

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One set of alleles for
a trait go on top of the
box (usually male)
and the other set of
alleles go on the side
of the box.
Each letter from the
set of alleles is placed
on top of the square.
Filling in the boxes…


Fill in the top left box
with the alleles from
top left and upper left.
The dominant letter is
placed first.
Filling in the boxes…

The second box gets
the top left and
bottom left allele
Filling in the boxes…


The third box gets the
top right and the top
left letters
Remember that the
capital letter goes
first.
Filling in the boxes…

The fourth box gets
the top right and the
lower left letter.
Ok. So what does this mean?


Each box represents a
possible zygote.
The alleles are for a
single trait, in this case T
is tall and t is short.



Tt is the genotype for a
heterozygous tall.
tt is the genotype for
homozygous recessive
short.
From this cross, 50% of
the offspring will be tall
and 50% will be short.
This is the phenotype.
Let’s apply this to Mendel’s
experiment.

Two homogeneous
parental generations
were crossed to yield
the F1 generation.
 The results were
100% purple
flowers,
heterozygous for
the trait (Pp).
Let’s apply this to Mendel’s
experiment.

Two heterozygous F1
generations were selfpollinated.
 The results were 25%
heterozygous purple
flowers; 50%
homozygous purple
flowers, and 25% white
flowers (homozygous
recessive)

This 3:1 ratio hold true for all
heterozygous monohybrid
crosses!
Genetic Crosses


Genetic crosses are used to predict the
probability of offspring resulting from the union
of sperm and egg.
Types of crosses:

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Monohybrid cross - cross between one pair of
contrasting traits.
Dihybrid cross - cross between two pairs of
contrasting traits.
Test cross - an unknown genotype is crossed with a
homozygous recessive individual.
Examples of Monohybrid
Genetic Crosses
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Homozygous x Homozygous
Heterozygous x Heterozygous
Homozygous x Heterozygous
Testcross
Incomplete Dominance
Codominance
Homozygous x Homozygous
pp
x
PP

This Punnett represents
Mendel’s P1 generation

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The recessive alleles for
white flowers (pp) are
crossed with the
homozygous dominant purple
flower (PP)
All of the offspring are
heterozygous (Pp) and show
the dominant trait of purple.
Genotype: 100% Pp
Phenotype: 100%purple
flower color
Heterozygous x Heterozygous
Bb
x
Bb


This is an example of
Mendel’s F2 generation that
shows 75% dominant and
25% recessive trait (3:1
ratio).
This cross represents a
cross between two
heterozygous black haired
rabbits (brown hair is the
recessive trait).
 Genotype: 25% BB; 50%
Bb; 25% bb or 1:2:1 ratio.
 Phenotype: 75% black
hair and 25% brown hair
(3:1 ratio).
Homozygous x Heterozygous
BB
x
Bb

This cross represents a
homozygous dominant allele for
black coat (BB) crossed with a
heterozygous allele for black
coat (Bb)

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Genotype: 50% of the
offspring are homozygous
dominant (BB) and 50% are
heterozygous (Bb)
Phenotype: 100% black coat.
Test Cross


Useful when you want to
determine whether a trait
is homozygous or
heterozygous for the
trait.
An unknown genotype is
crossed with a
homozygous recessive
individual.


Left; If no recessive traits
appear, then the unknown
genotype if most likely
homozygous for the trait.
Right: If any of the
offspring show the
recessive trait, then the
unknown genotype is likely
Incomplete Dominance

Occurs when two or more
of the alleles influence
phenotype, resulting in a
phenotype intermediate
between the dominant and
recessive trait.


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The heterozygous individual
(Rr) shows the intermediate
trait of pink.
RR (homozygous dominant)
is red flower
rr (homozygous recessive) is
white flower color.
Codominance


Occurs when both
alleles for a gene are
expressed in
heterozygous
offspring.
Neither the dominant
or recessive allele is
dominant, nor do they
blend in phenotype.
Dihybrid Crosses

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
Cross between individuals that involves two pairs of
contrasting traits
Four alleles allows for 16 possible combinations of
alleles. (16 box Punnett square)
Four combinations of alleles can be determined by
using the “foil” method of distribution. YyTt
 First pair of alleles = YT (dominant )
 Outer pair of alleles = Yt (heterozygous)
 Inner pair of alleles = yT (heterozygous)
 Last pair of alleles = yt (recessive)
Dihybrid Crosses
homozygous x homozygous



The example at right
crosses two homozygous
monohybrid traits
This is representative of a
dihybrid cross of Mendel’s
P generation
Notice that all of the
offspring are heterozygous
(RrYy) for the dominant
trait-- yellow (R) and
smooth (Y)
Dihybrid Cross
heterozygous x heterozygous

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RrYy is a heterozygous trait
for yellow, smooth peas
This represents Mendel’s cross
of the F1 generation, with two
traits.
Using the foil method to
determine possible gametes,
the choices are RY, Ry, rY, and
ry
After placing the allele
combinations along the top
and side, you follow the basic
rule for combining alleles,
remembering to place capital
letters first, and like
combinations of alleles
together.
Dihybrid Cross
heterozygous x heterozygous

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Yellow color ( R ) is dominant to
green ( r ) : = yellow; = green
Round ( Y ) is dominant to
wrinkled ( y ):
The possible combinations are:

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9/16 - round and yellow seeds
(genotype: RRYY, RRYy, RrYY,
RrYy)
3/16 - round, green seeds
(genotype: Rryy, Rryy)
3/16 wrinkled,yellow seeds
(genotype: rrYY, rrYy)
1/16 wrinkled, green seeds
(genotype: rryy)
The ratio of 9:3:3:1 holds true for
every dihybrid heterozygous
cross!
Nine different genotypes and four
different phenotypes.
Study Questions

Explain the difference between the following terms:
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self-pollination, cross-pollination
F1,F2 generation
pure, hybrid
dominant, recessive
law of segregation, law of independent assortment
gene and allele
genotype and phenotype
homozygous and heterozygous
monohybrid, dihybrid cross
Complete dominance, incomplete dominance,
codominance
Genetic Problem

Assume that black hair is dominant to
brown hair. Cross a heterozygous black
haired trait with a homozygous recessive
brown hair trait.
Draw a Punnett square and predict the
offspring.
 Give the percentages and ratio’s of the
phenotype and genotypes of the offspring.

Questions
1.
What is the ratio of a dihybrid cross between two heterozygous
traits?
2.
In an dihybrid cross between two heterozygous parents, what is
the probability of obtaining an offspring that is homozygous for
both traits?
3.
What is the most likely explanation for two parents with dominant
phenotypes producing offspring with a recessive phenotype?
4.
You cross a red-flowering plant with a yellow-flowering plant and
notice that some of the offspring have orange flowers. What is the
most likely explanation for this occurrence?
5.
Explain the difference between the P generation, F1 generation,
and F2 generation.
Questions
6.
When the dominant and recessive traits are known, why is it not
necessary to use the term homozygous when referring to the
genotype of an individual with a recessive phenotype?
6.
In pea plants, smooth texture is dominant over wrinkled texture. A
gardener has a pea plant that produces smooth seeds. How can
the gardener determine whether the plant is homozygous or
heterozygous for allele that determines seed texture?
7.
In rabbits, the allele for black coat color (B) is dominant over the
allele for brown coat color (b). Predict the results of a cross
between a rabbit homozygous for black coat color (BB) and a rabbit
homozygous for brown coat color (BB).