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Mendelian Genetics
Ch 11
Genetics

Study of heredity

How traits are
passed on from
one generation to
the next
Gregor Mendel
Austrian monk (1800s)
 Pea plant crosses

 True-breeding
(self-
pollination)
 Fertilization (crosspollination)
Mendel’s 7 Pea Plant Traits

TRAIT (CATEGORY)
 Seed
shape
 Seed color
 Seed coat
 Pod shape
 Pod color
 Flower position
 Plant height

TYPES (ALLELES)
 ROUND
vs. wrinkled
 YELLOW vs. green
 GRAY vs. white
 SMOOTH vs. constricted
 GREEN vs. yellow
 AXIAL vs. terminal
 TALL vs. short
Hybrids

Mendel recorded the traits of different
plants and then crossed plants with
different traits.
 For
example, he crossed a tall plant with a
short plant.

The offspring produced from these
crosses are called hybrids.
Labeling Crosses

P generation =
parents

F1 generation =
offspring of P
generation

F2 generation =
offspring of F1
generation

F3…Fn
Mendel’s Findings

Genes are chemical messages that are passed on from
one generation to the next. They represent a specific
trait or category. Alleles are different types of a trait.


Example: Eye color is a trait determined by your genes. There is
an allele for brown eyes and an allele for blue eyes.
The Principle of Dominance states that some alleles
are dominant and others are recessive.


Dominant = strong; represented by a capital letter
Recessive = weak; represented by a lowercase letter
What happens to recessive alleles?

Recall that meiosis is the production of gametes
(sex cells) for sexual reproduction. A normal
diploid cell has at least 2 alleles for each trait.

The alleles separate during meiosis individually
through a process called segregation.
According to the Law of Independent
Assortment, traits segregate independently of
each other.
Phenotypes and Genotypes

Phenotypes = physical
appearance

Genotypes = alleles
coding for phenotype
 Homozygous
dominant
(AA)
 Homozygous recessive
(aa)
 Heterozygous (Aa)
Converting Genotype to
Phenotype


Trait: Eye color
Alleles:



B = Brown
b = blue
What is the phenotype of a person whose genotype is…



AA
Aa
Aa
Converting Genotype to
Phenotype

AA = Brown

aa = blue

Aa = Brown
 The
dominant B (brown) overpowers the
recessive b (blue).
Probability of Traits

Probability is the likelihood of something
happening.

We can predict the probability of seeing a trait in
the offspring using a Punnett Square.
Punnett Squares

First,
segregate
the alleles for
each parent.
Punnett Squares

Then fill in the
alleles for the
corresponding
boxes.
 NOTE:
capital letters
go in front.
Calculating Probability
(Genotypes)

What is the chance
of having a child with
the genotype…
 AA?
 Aa?
 aa?
Calculating Probability
(Phenotypes)

What is the chance
of having a child
with…
 Brown
eyes?
 Blue eyes?
Ratios

Genotypic Ratios (GR) record the number of
each genotype from the most dominant to least
dominant. In other words, AA : Aa : aa

Phenotypic Ratios (PR) record the number of
each phenotype from the most dominant to least
dominant. In other words, AA + Aa : aa.
What are the ratios for the
following Punnett Square?

GR =

PR =
More Complicated Scenarios

Sometimes there are more than two
alleles or more than one gene that
influence a phenotype.
 Incomplete
Dominance
 Codominance
 Multiple Alleles
 Polygenic
Incomplete Dominance

Incomplete
dominance occurs
when traits blend.
 For
example, if a white
rose is crossed with a
red rose, the colors
mix, forming pink
roses.
Codominance

When there is more than
one dominant allele, it is
called codominance.

For example, blood has 3 alleles
(A, B, i). A and B are equally
dominant, so when they are put
together, they make a new blood
type called AB.
Multiple Alleles

Some traits have more
than two alleles, so they
have multiple alleles.
 For
example, there are
four alleles for the color
of rabbit fur, some of
which are more dominant
than others.
Polygenic Traits

Skin color is an
example of a
polygenic trait
because there are
several genes that
work together to
determine skin
color.
Gene Linkage & Gene Maps

A gene map shows the locations of traits
on a chromosome.

Neighboring genes are more likely to be
“linked” due to crossing-over in meiosis.
This is called gene linkage.
 For
example, freckles and red hair are often
inherited together.
Gene Map
Gene Linkage & Gene Maps

Chromosomes assort independently
during meiosis, NOT individual traits

Why didn’t Mendel notice this?
 Most
of the traits were on different chromosomes or
too far apart