Heredity by alicejenny



    Mrs. Nell
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.
              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.
An organism that always produces the same traits in its offspring is
called a purebred. Can you recognize these purebreds?
Mendel’s Work
 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).
     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
dominant factor,
because it dominated or
covered up the short
height form. He called
the form that seemed to
disappear the recessive
      Recessive Reappears!

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.
  (example: Tt)

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
   chromosomes separates
   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
homozygous parents.
             Multiple Alleles
                                Phenotype   Genotype
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
                                   O          OO
controlled by multiple

Human blood typing is
controlled by multiple
               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.
              Human Genetics
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
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.

                            Sickle cell
                  Cystic Fibrosis
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
physical therapy.

 Cystic fibrosis is the most commonly inherited genetic disorder among Caucasians.
          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
       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. 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
explain why?

                               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
                               have hemophilia.
 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).
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
        Genetic Engineering
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.

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