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Rules of Genetics
Gregor Mendel
 Carried out important studies in
heredity
 Heredity  passing on
characteristics from parent to
offspring
 Characteristics that are inherited
are called traits
Mendel’s Experiment
 Mendel chose to work with pea
plants because they reproduce
sexually
 Which means they produce male
and female sex cells, or gametes
 The male gamete, pollen, unites
with the female gamete, egg, and
results in a fertilized cell (zygote)
Mendel’s Experiment Cont’d
 Mendel dusted the pollen from
one plant onto the egg of
another plant
 By doing this Mendel was sure
exactly which parents produced
the offspring he was researching
Mendel’s Experiment Cont’d
 He carefully controlled his
experiments by researching only
one trait at a time
 The tall pea plants had been tall for
many generations and had always
produced tall offspring
 The short pea plants had been short
for many generations and had
always produced short offspring
Mendel’s Experiment Cont’d
 Mendel then crossed these
parent plants to produce new
plants
 Hybrid  the offspring of parents
that have different forms of a trait
P1 (Parent
Generation)
F1 (Filial
Generation)
F2 (Filial
Generation)
Mendel’s Experiment Cont’d
 Think of your family:
 Your parents are the P1 Generation
 You are the F1 Generation
 Any children you might have are the
F2 Generation
Mendel’s Results
The Rule of Unit Factors
 The Rule of Unit Factors
 Mendel concluded that each
organism has two factors that
control each of its traits
Gene  Section of a
chromosome that determines a
specific trait of an organism
The Rule of Unit Factors
Cont’d
 Genes exist in different forms
 Allele  Alternate gene form
 EX: Gene = height
Allele = tall or short
 Offspring inherit one allele from
each parent
The Rule of Dominance
 Only one trait can be observed
at a time
 EX: You cannot be both tall and
short at the same time
 Dominant trait  the trait that
always shows up when present
 Recessive trait  the trait that is
covered up by a stronger dominant
trait
The Rule Of Dominance
Cont’d
 Dominant traits are
represented by uppercase
letters
 EX: T
 Recessive traits are
represented by lowercase
letters
 EX: t
The Rule of Dominance
Cont’d
 TT = Tall
 tt = Short
 Tt = Tall
The Law of Segregation
 Every individual has two alleles
of each gene
 When gametes are formed each
receives one of these alleles
 During fertilization these
gametes randomly pair to
produce four combinations of
alleles
The Law of Segregation
Cont’d
Parents: Tt x Tt
Gametes: T, t, T, t
Offspring:
T
T TT
t
Tt
t
Tt
tt
P1 (Parent
Generation)
TT
tt
F1 (Filial
Generation)
Tt
F2 (Filial
Generation)
TT
Tt
Tt
Tt
Tt
Tt
tt
The Law of Independent
Assortment
 Genes of different traits are
inherited independently of one
another
 EX: Plant height and seed color
 EX: Blond hair and blue eyes
Basic Terminology
 Genotype  the genetic
makeup of an organism
 EX: TT, tt
 Phenotype  the physical
appearance of an organism
 EX: Tall, Short
Basic Terminology Cont’d
 Homozygous  Members of a
gene pair are identical
 EX: TT or tt
 Homozygous Dominant 
Members of a gene pair are
both dominant
 Ex: TT
Basic Terminology Cont’d
 Homozygous Recessive 
Members of a gene pair are
both recessive
 EX: tt
 Heterozygous  Members of a
gene pair are different
 EX: Tt