Mendel’s Laws of
Heredity
Why we look the way
we look...
What is heredity?
The
passing on of
characteristics (traits)
from parents to offspring
Genetics is the study of
heredity
Mendel used peas...
They
reproduce sexually
They have two distinct,
male and female, sex cells
Their traits are easy to
isolate
What Did Mendel Find?
He
discovered different
laws and rules that explain
factors affecting heredity.
Rule of Unit Factors
Each
organism has two
alleles for each trait
–Alleles - different forms of
the same gene
–Genes - located on
chromosomes, they control
how an organism develops
Rule of Dominance
The
trait that is observed
in the offspring is the
dominant trait (uppercase)
The trait that disappears
in the offspring is the
recessive trait (lowercase)
Heterozygous & Homozygous
Heterozygous
- if the two
alleles for a trait are
different (Aa)
Homozygous
- if the two
alleles for a trait are the
same (AA or aa)
Heredity
The
two alleles for a trait
must separate when the
sex cells are formed
A parent randomly passes
only one allele for each
trait to each offspring
Heredity
The
genes for different
traits are inherited
independently of each
other.
Phenotype & Genotype
Phenotype
- the way an
organism looks
– red hair or brown hair
genotype
- the gene
combination of an organism
– AA or Aa or aa
Incomplete Dominance
Neither allele is dominant so
there is a blending of traits
when two different alleles for
the same trait occur together.
The offspring have a mix of
their parents.
Incomplete Dominance
Heterozygous individuals =
3rd phenotype
Incomplete Dominance
In Four O’ Clocks,
if you cross a red
(which is always pure)
with a white (that is
also always pure),
you get a pink
(which is always
a hybrid).

Incomplete Dominance
Cross of two pink flowers
What are the possibilities?
genotype ratio
phenotype ratio
Multiple Alleles
Multiple Alleles
 Some
human traits are controlled
by a single gene that has two or
more alleles.
 Three or more forms of a gene
that code for a single trait.
 In this pattern of inheritance, the
genes have more than two alleles
controlling them.
Multiple Alleles
Ω Even though a gene may have
multiple alleles, a person can
only carry two of those alleles.
Ω Chromosomes exist in pairs –
Each chromosome in a pair
only carries one allele for each
gene
Multiple Alleles
Blood type in humans is an example of
this inheritance pattern.
 The four different
blood groups:
 A, B, O, and AB
 Are produced
by three different
alleles:
 A, B, and O

Multiple Alleles
Examples of Blood type crosses
Practice Problems:
A mother is AB and a father is O. Draw the Punnett
square.
A
B
O
O
What is the probability their offspring will have A type
blood? ____
Can they have an offspring with O blood?
POLYGENIC TRAITS
Polygenic Traits
Some human traits show a large
number of phenotypes because the
traits are controlled by many
genes.
The genes act together as a
group to produce a single trait.
Polygenic Traits
Traits controlled by two or more genes
 Show a wide range of phenotypes
 The phenotype
is produced by
the interaction
of more than
1 pair
of genes.

Polygenic Traits
In humans,
eye color,
skin color, hair
color are a few
controlled by
many genes.
Polygenic Traits
ΩHeight is controlled by four genes
working together.
Polygenic Traits
ΩSkin color is
controlled by at least
three genes, each one
containing two
different alleles.
Various combinations
of alleles produce the
many skin colors in
humans.
Genetic Disorders
Genetic Disorder
ΩAn abnormal condition that a
person inherits through genes or
chromosomes.
ΩGenetic disorders are caused
by mutations.
ΩDown syndrome and cystic fibrosis
are two examples of genetic
disorders.
Genetic Recessive
Disorders
 Many
human genetic disorders are
caused by recessive genes.
 Occur when both parents have the
recessive allele for the disorder.
 Parents may be heterozygous and
have no symptoms and pass the
trait onto any offspring.
Sex-linked Genetics
Ex. Colorblindness
Boy or Girl?
ΩThe gender of a baby is determined
by genes on chromosomes.
ΩThere are 23 pairs of chromosomes
in each of our cells. One of pair of
chromosomes are called sex
chromosomes.
Boy or Girl?
ΩThe sex chromosomes determine
whether a person is
male or female.
ΩThe sex chromosomes are the
only pair of chromosomes that do
not always match.
ΩFemales
Two sex chromosomes match XX
Since both chromosomes are X, all eggs
carry one X chromosome.
ΩMales
Two sex chromosomes don’t match XY,
so sperm cells will either carry an X or
a Y chromosome.
Sex-Linked Genes
 Some
human traits occur more
often in one gender than the other.
 Sex-Linked
Genes: Genes on the X
and Y chromosomes, whose alleles
are passed from parent to offspring
on sex chromosomes
Sex Linked Gene
 In
females, a dominant allele on one
X chromosome will mask a recessive
allele on the other X chromosome.
 In males, there is no matching
allele on the X and Y chromosome.
As a result, any allele on the X
chromosome will produce the trait
in a male who inherits it.
Sex-Linked Genes
ΩBecause males have only one X
chromosome, males are more likely
than females to have a sex-linked
trait that is controlled by a
recessive allele.
Sex-Linked Punnett Square
 Let
C = Normal Vision c = Colorblind
Cross: Normal Male (
) x Carrier
Female (
)
Pedigree
Pedigree
 Pedigree:
A chart or “family tree”
that tracks which members of a
family have a particular trait