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CHAPTER 11.2
AP Biology
Fall 2010
Mendel’s Theory of Segregation
Monohybrid Experimental Predictions

Mendel suspected that every plant inherits two
“units” (genes) of information for a trait
 One
from each parent on pairs of homologous
chromosomes
Monohybrid Experimental Predictions


Mendel’s first experiments were monohybrid crosses
Monohybrid crosses have 2 parents that are truebreeding for contrasting forms of a trait
 White
vs. purple flowers
Monohybrid Experimental Predictions


One form of the trait (white) disappears in the first
generation offspring (F1), only to show up in the
second generation (F2)
We know that all members of the F1 offspring are
heterozygous (Aa) because one parent could only
produce an A gamete and the other could produce
only an a gamete
Monohybrid Experimental Predictions

Mendel had a strong mathematical background
knew about sampling error so he crossed thousands
of offspring
F2 Generation



In the F2 generation, the white flowers reappear
The numerical ratios of crossing suggested that
genes do not blend
For example:
 F2
offspring show a 3:1 phenotypic ratio of purple to
white
F2 Generation



Mendel assumed that each sperm has an equal
probability of fertilizing an egg
This can be seen most easily by using a Punnett
square
Thus, each new plant as three chances in 4 of
having at least one dominant trait
Punnett Square




Page 173, figure 11.7
True breeding homozygous recessive parent x true
breeding homozygous dominant parent
a
a
A
Aa
Aa
A
Aa
Aa
F1 Phenotypes: 4 purple plants
F1 Genotype: 4 Aa (100%)
Punnett Square




Page 173, figure 11.7
F1 x F1
A
a
A
AA
Aa
a
Aa
aa
F2 Phenotypes: 3 purple (75%), 1 white (25%)
F2 Genotypes: 1 AA (25%), 2 Aa (50%), 1 aa
(25%)
Testcrosses

Testcross: an organism shows dominance for a
specified trait but its genotype may be unknown, so
it is crossed with a homozygous recessive individual
 Test
results reveal whether it is homozygous dominant or
heterozygous
 Draw an example of a test cross….
Testcrosses


To support his concept of segregation, Mendel
crossed F1 plants with homozygous recessive
individuals to reveal whether F1 plants were
homozygous or heterozygous dominant
A 1:1 ratio of recessive and dominant phenotypes
support his hypothesis
Testcrosses


Mendel’s theory of segregation states that diploid
organisms inherit pairs of genes on pairs of
homologous chromosomes
Each gene segregates from the other during meiosis
so that each gamete will receive only one gene per
trait
Mendel’s Theory of Segregation
Diploid cells have pairs of genes, on pairs of
homologous chromosomes.
 The two genes of each pair are separated
from each other during meiosis, so they end
up in different gametes

Review
1.
2.
3.
What is a testcross?
What is Mendel’s theory of segregation?
Construct a Punnett square of testcross of F1
purple flowered plants (Aa) with true breeding
white flowered plants (aa)
Answers
1.
2.
Testcross: an organism shows dominance for a
specified trait but its genotype may be unknown,
so it is crossed with a homozygous recessive
individual
Mendel’s Theory of Segregation: Diploid cells have
pairs of genes, on pairs of homologous
chromosomes. The two genes of each pair are
separated from each other during meiosis, so they
end up in different gametes
Answers
3.
Aa x aa


a
a
A
Aa
Aa
a
aa
aa
Phenotype: 2 purple (50%), 2 white (50%)
Genotype: 2 Aa (50%), 2 aa (50%)