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Active Lecture PowerPoint® Presentation for
Essentials of Genetics
Seventh Edition
Klug, Cummings, Spencer, Palladino
Chapter 3
Mendelian Genetics
Copyright © 2010 Pearson Education, Inc.
Outline
• Mendel’s Experimental Approach
• Monohybrid cross
• Test cross
• Dihybrid cross
• Independent assortment
• Trihybrid cross
Mendel’s Experimental Approach
• Mendel chose garden pea (Pisum sativum) as
his model system because:
1. it is easy to grow
2. it can be crossbred artificially
3. it grows to maturity in one season
• Mendel used:
1. seven visible features, each with
two contrasting traits
2. true-breeding parental strains
http://www.xtec.es/~jllort1/biolegseu
ropa/mendel.jpg
Results of Mendel’s Research
(1866)
• Mendel determined discrete units of
inheritance exist and predicted their
behavior during gamete formation
http://sps.k12.ar.us/massenga
le/images/mendel2.jpg
• Mendel’s postulates eventually
accepted as basis for Mendelian or
transmission genetics
• Mendel’s findings rediscovered at the
turn of the century by three geneticists
The Monohybrid Cross
(one-factor cross)
• Monohybrid crosses involve a single pair
of contrasting traits
• Original parents are P1 generation, and
their offspring are F1 generation
• Offspring arising from selfing (selffertilizing) the F1 generation are F2
generation
The Monohybrid Cross
• In F1 generation of a monohybrid cross, all
plants have just one of two contrasting traits
• This trait is dominant over the other
recessive trait that disappeared in F1
• In F2 generation, 3/4 plants exhibit same trait
dominant trait as F1 and 1/4 exhibit
contrasting recessive trait that disappeared
in F1 generation
Traits used by Mendel
Figure 3-1
Copyright © 2006 Pearson Prentice Hall, Inc.
Mendel’s Discoveries
• To explain these results, Mendel proposed
existence of "particulate unit factors" for each
trait
• He suggested these factors:
1. serve as basic units of heredity (now called
genes)
2. are passed unchanged from generation to
generation
3. determine various traits expressed by each
individual plant
Mendel’s First Three Postulates
• Mendel proposed three postulates of
inheritance:
1. Unit factors exist in pairs
2. In a pair of unit factors for a single
characteristic in an individual, one unit factor
is dominant and other is recessive
3. Paired unit factors segregate (separate)
independently during gamete formation
Genetic Terminology
• Genes: Units of inheritance
• Alleles: Alternate forms of a single gene
• Genotype: the genetic makeup of an
individual
• Phenotype: the physical expression of
the genetic makeup
Genetic Terminology
• Homozygous: when the alleles for a
trait in an individual are the same
(eg: AA or aa)
• Heterozygous: when the alleles for a
trait in an individual differ
(eg: Aa)
The Monohybrid
Cross
Figure 3-2
Copyright © 2006 Pearson Prentice Hall, Inc.
Punnett Square
• A Punnett square allows
genotypes and
phenotypes resulting from
a cross to be visualized
easily
Mendel’s Monohybrid ratio: 3:1
Figure 3-3
Copyright © 2006 Pearson
Prentice Hall, Inc.
The Testcross
• A testcross is a test to determine whether an
individual displaying the dominant phenotype
is homozygous or heterozygous for that trait
The Dihybrid Cross (two-factor cross)
• A dihybrid cross involves two pairs of
contrasting traits
The Dihybrid
Cross
Mendel’s dihybrid ratio:
(9:3:3:1)
Mendel’s Fourth Postulate:
Independent Assortment
• Segregating pairs of unit factors assort
independently during gamete formation
• Each gamete receives one member of every
pair of unit factors. For one pair, whichever
unit factor received does not influence the
segregation of the second pair.
• Therefore, all possible combinations of
gametes will form with equal frequency
Laws of Probability and Genetics
• Product Law:
– Used to calculate probability of two
independent events occurring at the same
time
– The probability of both events occurring is
product of the probability of each
individual event
Predicted Results of a Dihybrid Cross
Mendel’s dihybrid ratio: 9:3:3:1
The Testcross: Two Characters
• Testcrosses can also be applied to individuals that
express two dominant traits but whose genotypes
are not known
• Eg: The yellow round seed phenotype in the F2
generation can result from one of the four GGWW,
GgWW, GGWw or GgWw genotypes.
• Homework: Do a test cross of above four
genotypes (cross each one separately with
homozygous recessive ggww). Find ratios for each.
The Trihybrid Cross
(three-factor cross)
• Trihybrid crosses involving 3 independent traits
show Mendel's rules apply to any # of traits
• The forked-line method is easier to use than a
Punnett square for analysis of inheritance of larger
# of traits
• Product law can be used to predict frequency with
which two independent events will occur
simultaneously
Figure 3-9
Copyright © 2006 Pearson Prentice Hall, Inc.
Predicted results of a trihybrid cross using
forked-line method
Mendel’s Laws – to remember!
• Independent segregation: Alleles of a gene
segregate (separate) independently during
gamete formation. Eg: Dd
• Independent Assortment: Alleles of two
genes assort independently during gamete
formation. Eg: Gg and Ww