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What is heredity?

Heredity - the passing on
of characteristics (traits)
from parents to offspring.
Genetics is the study of
heredity.
•
Gregor Mendel

Modern genetics is based on Mendel’s
explanations for the patterns of
heredity in garden pea plants.
• Gregor Mendel – a monk in the 1800s
who studied pea plants and their
traits.
Study of Heredity

Characters- physical features
that are inherited.
Traits – several possible forms of
a character.
Gametes – haploid male or female
cell
Mendel crossed the peas

Fertilization - the uniting of male
and female gametes.
Cross - combining gametes from
parents with different traits.
What Did Mendel Find?

He discovered different
laws and rules that explain
factors affecting heredity.
Explaining mendel’s
results

The Mendelian theory of heredity
forms the foundation of modern
genetics.
Mendelian theory explains simple
patterns of inheritance.
Different traits result from
different versions of genes.
Mendel’s Results

Each organism has two alleles
for each trait
Alleles - different forms of the
same gene
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)

Dominant = strong
Recessive = weak, “recedes”
Law of Independent
Assortment

The genes for different
traits are inherited
independently of each other.
Equal chance to get an allele
Questions...

What do we call the trait that is
observed?
What case (upper or lower) is it
written in?
What about the one that
disappears?
What case is it written in?
Phenotype & Genotype

Phenotype - the way an
organism PHYSICALLY looks
red hair or brown hair
Genotype - the gene
combination of an organism
AA or Aa or aa

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)


Dihybrid vs Monohybrid

Monohybrid Cross crossing parents with only
one trait (AA with aa)
Dihybrid Cross - crossing
parents with two traits
(AAEE with aaee)
Questions...

What is the phenotype?
What is the genotype?
What is homozygous?
What is heterozygous?
What is monohybrid
crossing?
Using Punnett Squares

A Punnett square is a model that predicts likely
outcomes of a genetic cross.
A Punnett square shows all of the genotypes
that could result from a cross.
The simplest Punnett square consists of a
square divided into four boxes.
The letter combination in each box represents
one possible genotype in the offspring.
Codominance

In codominance,
neither allele is
recessive and the
phenotypes of both
alleles are expressed.
Dihybrid
Cross
Inheritance
of 2
different
traits.
FOIL!

Using Probability

A Punnett square shows the possible
outcomes of a cross, but it can also be used
to calculate the probability of each outcome.
Probability - the likelihood that a specific
event will occur.
Probability can be calculated and
expressed in many ways
1:2:1 or 25% 50% 75%
Using a pedigree
A pedigree is a diagram that shows how a
trait is inherited over several
generations of a family.
Pedigrees can be used to help a family
understand genetic disorders.
A genetic disorder is a disease or
disorder that can be inherited.
A pedigree can help answer questions
about three aspects of inheritance: sex
linkage, dominance, and heterozygosity.
Using a pedigree (cont.)

A sex-linked gene is located on either
an X or a Y chromosome.
Traits that are not expressed equally
in both sexes are commonly sex-linked
traits.
Colorblindness is an example of a sexlinked trait that is expressed more in
males than in females.
Hemophilia

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