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Mendelian Genetics
Ch
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2

Animal Mating Mysteries

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3

Important genetics questions in the
early 1800’s
What traits are inherited?
How are traits inherited?
What is the role of chance in
heredity?
4

Gregor Mendel
Important Facts
Monk
Founder of Genetics
“Father of Genetics”

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Mendelian Genetics
! But first, let’s

introduce a few terms

– Mendelian factors are now called genes
– Alleles are different versions of the same gene
– An individual with two identical alleles is termed
homozygous
– An individual with two different alleles, is termed
heterozygous
– Genotype refers to the specific allelic composition
of an individual
– Phenotype refers to the outward appearance of an
individual
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!

Genotype is the genetic makeup of an organism,
a description of the genes it contains
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Phenotype is determined by interaction of genes
and environment
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1)
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11
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8

Alternative forms of traits are alleles
! Each

trait carries two copies of a unit of
inheritance, one inherited from the mother
and the other from the father
! Alternative forms of traits are called alleles

Allele 1

Allele 2

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2
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Keys to Mendel’s Experiments
!

The garden pea was an ideal organism for study because:
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Mendel analyzed traits with discrete alternative forms (one of two options)
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Vigorous growth
Self fertilization
Easy to cross fertilize
Produces large number of offspring each generation

purple vs
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green peas
round vs
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short stem length

Mendel established pure/true breeding lines to conduct his experiments
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–

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Progeny of P generation is the first filial generation,
"

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designated F1
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F2 is always F1 x F1

Subsequent interbreeding produces F3, F4, and F5
generations
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!

To determine whether both parents contribute equally to
the phenotype of a particular trait in offspring
– a set of reciprocal crosses is performed
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– This is the principle of uniformity in F1 (Figure 2
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– Complete dominance
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Fig
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6 The F2 progeny
of the cross shown in
Figure 2
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14

Monohybrid Crosses and Mendel’s Principle of Segregation

!

Mendel noted that traits that disappear in the F1
reappear in the F2
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6)
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!

Individuals with identical alleles are said to be
homozygous for that gene
– (e
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, genotypes SS and ss)
– because all their gametes will have the same allele for this trait
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g
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7)
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2
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Russell, iGenetics: Copyright © Pearson Education, Inc
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17

Monohybrid Crosses and Mendel’s Principle of
Segregation
!

When Mendel had conducted experiments for
the seven different traits in garden peas (Table
2
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Results of reciprocal crosses are always the same
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The F1 resembled only one of the parents
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The trait missing in the F1 reappeared in about 1/4
of the F2 individuals
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Write down the genotypes of both parents
– Male parent = Tt
– Female parent = Tt

!

2
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– Male gametes: T or t
– Female gametes: T or t
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Create an empty Punnett square

!

4
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! 5
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Permission required for reproduction or display

The Principle of Segregation
!

The first Mendelian law, the principle of
segregation, states:
– “Recessive characters, which are masked in the F1
from a cross between two true-breeding strains,
reappear in a specific proportion in the F2
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Confirming the Principle of Segregation:
! Mendel

observed that plants with the
recessive phenotype are all true breeding
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10)
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2
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24

Confirming the Principle of Segregation:
!

You can determine whether an individual with the
dominant phenotype is homozygous or heterozygous
by self crossing the individuals (Figure 2
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Fig
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11 Determining the genotypes of the F2 smooth progeny of Figure 2
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Russell, iGenetics: Copyright © Pearson Education, Inc
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25

Confirming the Principle of Segregation:
Testcrosses
!

A more common way to determine whether an
individual with the dominant phenotype is
homozygous or heterozygous is to perform a
testcross
– By definition you must cross the individual with one that is
homozygous recessive (Figure 2
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Testcrosses
!
!
!

A test cross allows one to determine the alleles carried by
the F1 parent:
because the test cross parent can only contribute the
recessive allele - is ALWAYS homozygous recessive!
The phenotypes of the resulting progeny allow you to
determine the genotype of the F1 parent
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If you mate a black
guinea pig with a homozygous brown guinea pig,
what proportion of the progeny will be
homozygous?
– 1
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50%
– 3
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none

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Homework Problems
! Chapter
!#

2

1, 4, 5, 6, 7, 10, 11

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