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HEREDITY
Heredity is the passing on
of traits from parents to
offspring. In this family
tree, traits such as hair color
and poor eyesight are
passed from generation to
generation.
HEREDITY
GENETICS
Genetics is the study of how traits are passed
from parent to offspring. Individual animals
and plants differ from other individuals of the
same species in all sorts of ways, even when
they are offspring of the same parents. These
differences between individuals of the same
species are called variations.
Height is a
variation
THE FATHER OF GENETICS
The first recorded scientific
study of how traits pass from
one generation to the next
was done by Gregor Mendel,
a Monk. In 1856, he began
experimenting with garden
peas. He found that it was
possible to predict the kinds of
flowers and fruit a plant would
produce. He realized that
information about the parents
of the plants was needed
before such a prediction could
be made.
SCIENTIFIC METHOD
Mendel made extremely careful use of scientific methods in his
research. He studied pea plants for eight years! He eventually
became known as the Father of Genetics.
In nature, bees and other insects pollinate
plants as they go from flower to flower.
Mendel cross-pollinated the pea plants he
studied (he took pollen from the male
reproductive structure of flowers of purebred
tall plants and placed it on the female
reproductive structure of flowers of pure short
plants).
MENDEL’S WORK
GENES
Chromosomes contain genes and are made
up of DNA. Genes are small sections of
chromosomes that determine traits
(physical characteristics of an organism).
When pairs of chromosomes separate into
sex cells during meiosis, pairs of genes also
separate from one another. Each sex cell
ends up with one form of a gene for each
The study of how traits are inherited
through the actions of alleles is the science
of genetics.
ALLELES
The different forms that a
gene may have for a trait are
called alleles. For instance,
in pea plants flowers may be
purple or white. Purple is
dominant. Dominance is
when one trait covers over
or masks another form of the
trait. White is recessive. A
trait that is hidden or masked
if the dominant form of the
trait is present is called a
recessive trait.
DOMINANT VS. RECESSIVE
In humans, freckles is the dominant trait,
whereas no freckles is the recessive trait.
Mendel discovered that tall
plants crossed with short
plants produced all tall
plants. Mendel called the
tall height form that
appeared the dominant
factor, because it
dominated or covered up
the short height form. He
called the form that seemed
to disappear the recessive
factor.
DOMINANT OR RECESSIVE?
Mendel allowed the new tall
plants to self-pollinate. When
he planted the new seeds he
found that the recessive form
had reappeared… both tall and
short plants grew. He found
that for every three tall plants
there was one short plant, a 3:1
ratio.
RECESSIVE REAPPEARS!
PUREBRED
When an organism has two
identical alleles for a trait, it is
said to be a pure bred (ex- 2
purple flower alleles). When
an organism contains different
alleles for a trait it is said to
be a hybrid (ex- 1 purple and
1 white flower allele).
A handy tool used to predict results like
Mendel did for genetics is called the
Punnett square. In a Punnett square
dominant and recessive alleles are
represented by letters. A capital letter
represents the dominant allele and a small
letter represents the recessive allele for a
trait. The letters show the genetic makeup,
or genotype, of an organism.
USING A PUNNETT SQUARE
Most cells in your body have two
alleles for a trait.
An organism with two alleles for a trait
that are exactly the same is called
homozygous. (example: TT or tt)
An organism with two different alleles
for a trait is called heterozygous.
(example: Tt)
The physical trait that shows the result
of a particular genotype is its
phenotype (example: tall or short)
HOMOZYGOUS OR HETEROZYGOUS?
GENETICS SINCE MENDEL
While Mendel’s results were true for
pea plants, different plants
sometimes varied from Mendel’s
predictions. When scientists crossed
pure red four o’clock plants with
pure white four o’clock they
expected to get all red flowers… but
all the flowers were pink! Neither
allele for the flower color seemed
dominant. Incomplete dominance is
the production of a phenotype that
is intermediate to those of the two
homozygous parents.
Mendel studied traits in
peas that were controlled by
just two alleles. However,
many traits are controlled
by more than two allelesthey are controlled by
multiple alleles.
Phenotype
Genotype
A
AA or AO
B
BB or BO
AB
AB
O
OO
Human blood typing
is controlled by multiple
alleles.
MULTIPLE ALLELES & GENES
MULTIPLE GENES
Some traits are produced by a combination of many genes.
Polygenic inheritance occurs when a group of gene pairs act
together to produce a single trait. Eye color and fingerprints
are examples. Height, weight, body build, hair, and skin color,
are as well.
SEX DETERMINATION
Females produce eggs that
have only an X
chromosome. Males
produce sperm that contain
either an X or a Y
chromosome. If an egg is
fertilized by an X sperm the
offspring is XX- a female.
If the egg is fertilized by a Y
sperm the offspring is XY- a
male.
SEX-LINKED DISORDERS
Some inherited conditions
are closely linked with the X
and Y chromosomes that
determine the sex of an
individual. Individuals who
are red-green color blind
have inherited an allele on
the X chromosome that
prevents them from seeing
these colors.
What number do you see?
SEX-LINKED GENE
An allele
inherited on a sex
chromosome is a
sex-linked gene.