Understanding
Inheritance
Chapter 5 Lesson 2
Part 1
Understanding Inheritance
• What determines the expression
of traits?
• How can inheritance be
modeled?
• How do some patterns of
inheritance differ from Mendel’s
model?
What controls traits
Mendel concluded that two factors—one
from each parent—control each trait.
 Mendel’s “factors” are part of
chromosomes which exist as pairs—one
chromosome from each parent.
 Each cell in an offspring organism
contains chromosomes from both
parents.

What controls traits

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A gene is a section on a chromosome
that has genetic information for one
trait.
The different forms of a gene are called
alleles. Each chromosome has one
allele for every gene on it.
The two chromosomes in an offspring
cell may have the same or different
alleles.
Important Genetic Terms
An allele is one member of a pair or
series of different forms of a gene
(the different
forms of a trait
that a gene
may have)
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What controls traits
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An example is the gene for blossom color in
many species of flower — a single gene
controls the color of the petals, but there may
be several different versions (or alleles) of the
gene. One version might result in red petals,
while another might result in white petals. The
resulting color of an individual flower will
depend on which two alleles it possesses for
the gene and how the two interact.
Important Genetic Terms

Geneticists call how a
trait appears, or is
expressed, the trait’s
phenotype.
• The two alleles that
control the phenotype
of a trait are called the
trait’s genotype.
Important Genetic Terms
Genotype - genetic makeup or allele combinations
 when writing
a genotype, the
dominant trait is
always listed first
 Scientists use uppercase and lowercase
letters as symbols to
represent the alleles in a genotype.
RR or Rr

Important Genetic Terms

Phenotype - the physical traits of the
organism (its physical appearance or
visible traits)
Red Rose
Genotypes:
GG
Gg
GG
Phenotypes:
purple-grained
white-grained
Important Genetic Terms

When the two alleles
of a gene are the
same, the genotype
is homozygous.
• If the two alleles of
a gene are different,
the genotype is
heterozygous.
Homozygous (purebred)

an organism usually the result of many
generations of such breeding (both
alleles the same) (having identical
factors)
Heterozygous (hybrid)
an offspring that was given different
genetic information for a trait from each
parent (alleles different)
 (having factors that are different)
(has two different alleles for a trait)
 a hybrid was the result of a cross between
two different homozygous (purebred)
organisms

Important Genetic Terms

cross – mating of organisms to test how
they inherit traits
Important Genetic Terms
first filial generation (F1) - the 1st
generation of offspring of a genetic cross
 The word filial
comes from filia
and filius, the
Latin words for
“daughter” and
“son.”
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