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Exploring Mendelian Genetics
11-3
Independent Assortment

Mendel wondered if the segregation of one
pair of alleles affect the segregation of
another pair of alleles

To find the answer, Mendel did an
experiment to follow two different genes as
they passed from one generation to the next
Two-Factor Cross

Mendel crossed true-breeding plants that
produced only round yellow peas with a
plant that produced wrinkled green peas

All of the F1 offspring were round and yellow
–
Proving round and yellow are dominant

While the cross didn’t prove if segregation
happened, it provided the hybrid plants needed
for the F2 generation

To get the F2 generation, Mendel crossed two of
the F1 offspring
–

RrYy x RrYy
556 seeds were produced, with 4 different
characteristics

315 seeds were yellow and round
–

32 were green and wrinkled
–

Dominant traits
Recessive traits
209 had a combination of traits
–
~1/2 were green and round

–
~1/2 were yellow and wrinkled


Recessive and dominant
Dominant and recessive
Phenotype ratio 9:3:3:1

Mendel discovered the principle of
independent assortment
–
Genes for different traits can segregate
independently during the formation of gametes
–
Accounts for many genetic variations among
organisms
Summary of Mendel’s Principles

Inheritance of biological characteristics is
determined by genes. Genes are passed
from parents to their offspring

When there are 2 or more alleles, some
forms are dominant and others recessive

In sexually reproducing organisms, each
adult has 2 copies of each gene – genes are
segregated when gametes form

Alleles for different genes usually segregate
independently of one another
Beyond Dominant and Recessive
Alleles

Despite the importance of Mendel’s work,
there are exceptions
–
Some alleles are neither dominant or recessive
–
Some traits are controlled by multiple alleles or
multiple genes
Incomplete Dominance

One allele is not completely dominant over
another allele

Heterozygous individual is a blend of the two
homozygous phenotypes

Ex: red (RR) x white (WW) = pink (RW)
Codominance

In heterozygous individuals, both traits appear at the
same time

They are not blended together like in incomplete
dominance

Ex: heterozygous chickens that have erminette
coloration (black and white feathers)
Multiple Alleles

Genes that more than two alleles are said to
have multiple alleles

Individuals still only have 2 copies of a gene,
but there is variation in the alleles they can
inherit

Ex: rabbit fur and blood types
Polygenic Traits

Traits controlled by two or more genes

Show a wide range of phenotypes

Ex: human skin color and eye color
Applying Mendel’s Principles

Thomas Hunt Morgan wanted to test
Mendel’s principles on organisms other than
plants

He needed an animal that produced large
numbers of offspring, in a short amount of
time

Used Drosophila melanogaster – common
fruit fly

Soon, other biologists tested all of Mendel’s
principles on a variety of organisms and
discovered that the principles applied to them
as well
Genetics and the Environment

Characteristics of any organism are not based only
on the genes they inherit

Also based on the interaction of their genes with the
environment

Genes provide a plan for development, but how that
plan unfolds also depends on the environment