7th Life Science
What is heredity?
Heredity is the passing of traits from parents to
offspring. These traits are controlled by genes.
Genes are found on chromosomes
and are made up of DNA. 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 trait.
The different forms a gene may
have for a trait are its alleles. The
study of how traits are inherited
through the actions of alleles is the
science of genetics.
Widow’s Peak Straight hairline
The Father of Genetics
• The first recorded scientific
study of how traits pass from
one generation to the next
was done by Gregor Medel,
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.
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.
An organism that always produces the same traits in its offspring is
called a purebred. Can you recognize these purebreds?
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
Dominant or recessive?
Mendel discovered that
tall plants crossed with
short plants produced all
tall plants. Mendel
called the tall height
form that appeared the
because it dominated or
covered up the short
height form. He called
the form that seemed to
disappear the 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.
Using a Punnett Square
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.
Homozygous or Heterozygous?
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.
The physical trait that shows a a result
of a particular genotype is its
phenotype (example: tall or short)
Summary: How Traits are Inherited
1) Traits are controlled by
alleles on chromosomes.
2) An allele may be dominant
or recessive in form.
3) When a pair of
during meiosis, the different
alleles for a trait move into
separate sex cells.
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’clocks 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
Mendel studied traits in peas A AA or AO
that were controlled by just
B BB or BO
two alleles. However, many
traits are controlled by more AB AB
than two alleles- they are
controlled by multiple
Human blood typing is
controlled by multiple
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.
Sometimes a gene undergoes mutation that results in an unwanted
trait. Some mutations aren’t harmful, while others result in genetic
disorders in humans, like sickle-cell anemia and cystic fibrosis.
Sickle-cell anemia is a homozygous recessive disorder in which red blood
cells are sickle-shaped. These cells can’t deliver oxygen to the cells in the
body and don’t move through blood vessels easily. Often the patient will die
as children. Sickle cell patients can be treated with drugs to increase
oxygen in the blood or by transfusions of blood containing normal cells.
Cystic fibrosis is another homozygous recessive disorder. Patients
have a thick mucus in their lungs and intestinal tract instead of a thin
fluid. Mucus makes it hard for cystic fibrosis patients to breathe and
causes lung damage. Mucus in the digestive tract damages the
pancreas. Patients are helped with antibiotics, special diets, and
Cystic fibrosis is the most commonly inherited genetic disorder among Caucasians.
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
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
What number do you see?
An allele inherited on a sex
chromosome is a sex-
linked gene. Another sex-
linked gene disease is
hemophilia, a disorder in
which blood does not clot
properly. Females are
usually carriers of sex-
linked diseases… can you
Normal Mother + Father with Hemophilia
Each pregnancy has a 50% chance of resulting in a female
carrier and a 50% chance of resulting in a normal male.
Sons of hemophiliac fathers and normal mothers will not
A Closer Look at Sex-Linked Traits
Carrier Mother + Normal Father Carrier Mother + Father with Hemophilia
Each pregnancy has a 25% chance of resulting in a Each pregnancy has a 25% chance of resulting in a female
normal female, a 25% chance of resulting in a female carrier, a 25% chance of resulting in a female with
carrier, a 25% chance of resulting in a normal male, and hemophilia, a 25% chance of resulting in a normal male,
a 25% chance of resulting in a male with hemophilia. and a 25% chance of resulting in a male with hemophilia.
A Closer Look at Sex-Linked Traits
Mother with Hemophilia + Father with Hemophilia Mother with Hemophilia + Normal Father
Each pregnancy has a 50% chance of resulting in a Each pregnancy has a 50% chance of resulting in a
female with hemophilia and a 50% chance of resulting female carrier and a 50% chance of resulting in a male
in a male with hemophilia (actual occurrence is with hemophilia (actual occurrence is extremely rare).
A pedigree is a tool for tracing the occurrence of a trait in a
family. Males are represented by squares and females by
Why is Genetics Important?
Knowing how genes are
inherited is important when a
couple decides to have a child.
Couples who discover genetic
traits that could lead to
disorders in their children may
elect to adopt healthy children
Through genetic engineering, scientists are experimenting with
biological and chemical methods to change the DNA sequence that
makes up a gene. It is presently being used to make medicines (like
insulin for diabetics) and to improve crop production.