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Genetics
What is Genetics?
► Genetics
is the study of heredity.
► Heredity
is the passing of characteristics
from parents to offspring (children).
► Traits
► Traits
are inherited characteristics.
are controlled by genes found on
chromosomes.
Who is Mendel?
► Gregor
Mendel is called the father of
genetics.
► He
was the first person to successfully
predict how traits are passed on from one
generation to the next.
► Mendel
did the majority of his experiments
with pea plants.
Why Pea Plants?
► Pea
plants reproduce sexually, producing
male and female gametes (sex cells).
► Mendel
could control breeding
(called crossing) of plants.
 Mendel removed the male
organs.
 He then dusted the female
organ with pollen (male
gamete) from a chosen plant.
► Using
this process, called cross pollination,
Mendel could be control the parents in the
cross.
What did Mendel study?
► He
studied only one trait at a time to control
variables.
 For example flower color, seed shape, or plant
height.
► He
selected plants that were considered
true breeding for the particular traits.
 The pea plants were from populations of plants
that the particular trait many generations and
had always produced that trait.
What were Mendel’s experiments?
► He
performed monohybrid crosses,
crossing two plants that differed from
each other by a single trait, such as
height.
► He
cross-pollinated a tall plant and a short
plant and the results were all tall plants
for the first generation.
► Then
he allowed the offspring to selfpollinate and the results were ¾ of the
plants were tall and ¼ were short in the
second generation.
► The
original true
breeding parents are
called the P1-generation.
P1
Short pea plant
► The
offspring of the
parent plants are called
the F1 generation.
Tall pea plant
F1
All tall pea plants
► The
offspring of the F1
plants are called the F2
generation.
F2
3 tall : 1 short
What did Mendel find?
► He
did many monohybrid crosses with
different traits following the same
procedure.
►
In every case, he found that one trait of a
pair seemed to disappear in the F1
generation, only to reappear unchanged in
one-fourth of the F2 plants.
► He
called the observed trait dominant and
the trait that disappeared recessive.
Traits in pea plants
Seed Seed
shape color
Flower
color
Flower
position
Pod
color
Pod
shape
purple
axial
(side)
green
inflated
white
terminal
(tips)
yellow
constricted
Plant
height
Dominant
trait
round yellow
Recessive
trait
wrinkled
green
short
What did Mendel conclude?
► Mendel
concluded that each organism has
two factors that control each of its traits.
► These
factors are now know to be genes
and that they are located on chromosomes.
► Genes
exist in alternative forms. Each
alternate form is called an allele.
 An organism’s two alleles are located on
different chromosomes, one inherited from the
female parent and one from the male parent.
What are Mendel’s Laws?
The Law of Dominance
►
Dominant traits mask recessive traits and
are represented with a capital letter

►
For example T= tall
Recessive traits are hidden by dominant
traits and are represented by the same
letter as the dominant, only lower case.

For example t=short.
The Law of Segregation
► Every
individual has two alleles of each
gene and when gametes are produced, each
gamete receives one of these alleles.
 During fertilization, these gametes randomly
pair to produce four combinations of alleles.
The Law of Independent Assortment
► Genes
for different traits are inherited
independently; they are not linked.
 For example a pea plant can be tall with purple
or white flowers.
Other important terms
Genotype
► Genetic makeup of an organism or alleles.
 For example Tt in pea plants.
Phenotype
► The physical characteristics or traits of an
organism determined by the genotype.
 For example tall in pea plants.
Heterozygous
► An organism with two different alleles for a
trait, also called hybrid.
 Tt (a capital T and a lower case t)
Homozygous
► An organism with two of the same alleles
for a trait, also called pure.
Homozygous Dominant
 T T (two capital T’s)
Homozygous Recessive
 tt (two lower case t’s)