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Mendel’s Experiment
Important Terminology
 P generation: purebred organisms that are used as the
parent generation
 F1 generation: offspring of the P generation
 F2 generation: offspring that arise from a F1 x F1 cross
 Gene: part of chromosome that governs expression of a
particular trait
 eg. hair colour , plant height
 Allele: alternate form of a gene
 eg. brown hair (B), red hair (b) ; tall (T), short (t)
Important Terminology
 Phenotype: the observable trait or characteristic ( what you can
see )
 Genotype: the genetic make-up of the trait ( which 2 alleles are
present? )
Types of Genotypes
 Homozygous: 2 identical alleles are present  eg. BB, tt
 Heterozygous: 2 different alleles are present  eg. Bb, Tt
 Dominant: trait that is fully expressed (visible)
 Recessive: trait that is usually masked (hidden)
 Punnett Square:
 method of predicting offspring if the genotypes of parents are known
Mendel’s First Experiment – Plant Height
 Cross-fertilized two purebred pea plants that showed
short and tall plant heights, respectively  Parent (P)
generation
 First Filial (F1) Generation offspring obtained were all
tall
 100 % tall (phenotype)
 After repeating this experiment while examining other
pairs of traits
he observed that one trait was always DOMINANT than
the other.
Important Conclusion
Principle of Dominance:
 When an organism is a hybrid (crossbred) for a pair
of contrasting traits, it will show only the dominant
trait.
Mendel’s Second Experiment
 Hybrid tall plants from F1 generation in 1st experiment
self-pollinated
 Second Filial (F2) Generation obtained were mostly tall
but some were short  75% tall and 25% short (
ratio – 3 tall : 1 short )
 After repeating this experiment while examining other
pairs of traits
 he observed the same ratio of 3 : 1 (called the Mendelian
ratio)
Important Conclusion #2
 Law (Principle) of Segregation:
 Inherited traits are determined by a pair of “factors”
that segregate during the formation of gametes.
Summary of Law – Using Genetic Shorthand
 Let the tall trait = T (dominant)
 Let the short trait = t (recessive)
Summary of Law Using Genetic Shorthand
 P
TT
male
T
 F1
x
T
tt
female
t
Tt
 only combination possible
 all plants were tall
 tall is dominant; short is masked
t
Second Filial Generation
 F2
Tt
 self-pollinated
Possible gametes: T, t
Observed:
ratio 3:1 (tall : short)
3 tall : 1 short
x
Tt
T, t
Overview of Mendel’s Model Inheritance
 Alternate forms of a gene exist  called ALLELES
 For each inherited trait, an organism has two genes (alleles)
 one allele is inherited from each parent.
 A sperm or egg carries only one allele for each inherited trait because
allele pairs segregate (separate) from each other during the production
of gametes  by meiosis
 When two alleles of a pair are different,
 one is fully expressed (DOMINANT allele) and
 the other is completely masked (RECESSIVE allele)
 NOTE: Today, we know this is not true in all circumstances !
Mendel’s principle of segregation describes the
inheritance of a single characteristic
 From his experimental
data, Mendel deduced
that an organism has
two genes (alleles) for
each inherited
characteristic

One characteristic comes
from each parent!!!
P GENERATION
(true-breeding
parents)
Purple flowers
White flowers
All plants have
purple flowers
F1
generation
Fertilization
among F1
plants
(F1 x F1)
F2
generation
Figure 9.3A
3/
of plants
have purple flowers
4
1/
4 of plants
have white flowers
 A sperm or egg carries
only one allele of each
pair
– The 2 alleles for a
gene separate
during gamete
formation, and each
gamete gets a
different one
GENETIC MAKEUP (ALLELES)
P PLANTS
Gametes
PP
pp
All P
All p
F1 PLANTS
(hybrids)
Gametes
All Pp
1/
2
1/
P
P
2
p
P
Eggs
Sperm
PP
– This is the law of
segregation
F2 PLANTS
Phenotypic ratio
3 purple : 1 white
p
p
Pp
Pp
pp
Genotypic ratio
1 PP : 2 Pp : 1 pp
Figure 9.3B
Monohybrid Cross:
Punnett Square Method
(1) Define symbols:
(2) State the cross
(3) Diagram the gametes
(4) Complete the squares
(5) Summarize the results:
Genotype
Phenotype
Phenotypic ratio: 3:1
Genotypic ratio:
1EE : 2Ee : 1ee
Example Problem
In mice, gray coat colour is dominant over white coat
colour.
What phenotypic and genotypic ratios would you
expect if:
 Heterozygous gray mouse crossed with homozygous white
mouse.
 Heterozygous gray mouse crossed with heterozygous gray
mouse.
Work with the people at your table to solve this question.