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HEREDITY
What is Genetics?
Eye color, nose shape and other physical types of traits may be inherited from
your parents. These traits are controlled by genes. Heredity is the passing of traits from
parent to offspring. Genes control all traits, and are made of DNA. When pairs of
chromosomes separate during meiosis, the pairs of genes also separate. If a trait is for
hairlines, sex cells may control this form of trait. Alleles are different forms of a gene
that you may have for a trait. Genetics is the study of how these traits are inherited
through the actions of alleles.
Gregor Mendel, born 1822, in Austria was an important figure in the study of
genetics. He studied science and math, and later became a priest and teacher. In 1856,
he experimented with garden peas. His assumption was that it was possible to predict the
kinds of flowers and fruit a plant would produce. In order to do this he used the scientific
method in his research. After eight years of work on inheritance of plants, Mendel
presented his research paper to Natural Science Society of Brǜnn, Austria. This was
published in 1866 and titled “Experiments with Plant Hybrids.” It wasn’t until 1900,
that three other scientists rediscovered Mendel’s work.
Mendel’s first worked with purebred plants. A purebred is an organism that
always produces the same traits in its offspring. With this is mind, tall plants will always
produce tall plants, and short plants will always produce short plants. In addition to the
height of plants, Mendel studied six other traits of garden peas. Mendel crossed pea
plants taking pollen from the male reproductive structures of purebred tall plants placing
them on the female reproductive structures of pure short plants. The dominant Factor
was tall height, and the recessive Factor was short height, which seemed to disappear.
The result was all tall plants. Mendel then allowed the new tall plants to self-pollinate.
The recessive factor reappeared, at a ratio of 3:1, tall to short. Mendel discovered that he
could predict what plants would grow. He used probability which is a branch of
mathematics that helps you predict the chance that something will happen. Mendel’s
predictions were accurate because he worked with large numbers of plants. He counted
every plant and thousands of seeds. Mendel studied almost 30 000 pea plants over eight
years. This increased the chance of repeated patterns. Suppose Mendel had 100 second
generation pea plants. How many would he expect to be tall? How many would he
expect to be short?
Mendel used a tool to predict results in Mendelian genetics. This is called the Punnett
square. When using the Punnett square dominant alleles are represented by capital letter
(T for tall), and recessive alleles are represented by small letter (t for short). The Punnett
square will show the genotype of an organism, its genetic makeup. Most cells in your
body have two alleles for every trait. Homozygous cells have two alleles that are exactly
the same. Heterozygous cells have two different alleles. The phenotype is the physical
trait that shows as a result of a particular genotype. For example red is the phenotype for
red flowering plants, and short is the phenotype for short plants. Another phenotype is
the color of your hair. Brown hair is the phenotype for the hair color of someone having
brown hair. Finally Mendelian inheritance states the following:
1. Traits are controlled by alleles on chromosomes,
2. an allele may be dominant or recessive in form,
3. and when a pair of chromosomes separate during meiosis, the different alleles for
a trait move into separate sex cells.
HEREDITY
Genetics Since Mendel
The production of a phenotype that is intermediate to those of two homozygous
parents is called incomplete dominance. When you cross pure red four o’clock plants
with pure white four o’clocks the expected result is all red flowers. This is not the case.
When crossing these all flowers were pink.
Multiple alleles are traits that are controlled by more than two alleles. An
example is blood type. There are three blood types: A (dominant), B (dominant), and O
(recessive). AB blood type shows both alleles in his or her phenotype. A blood type is
inherited from genotype AA or AO, B blood type is inherited from genotype BB or BO,
and O blood type is inherited from genotype OO.
Polygenic inheritance is a third type of way traits are affected. Polygenic
inheritance occurs when a group of gene pairs act together to produce a single trait. The
effect may be small, but the combination produces a wide variety. Blue or brown eye
color is an example. Look at your classmate’s eyes to see if they have the same shade of
color as you do. The chances are they won’t. This is because eye color is affected by
polygenic inheritance. Finger prints are another example of polygenic inheritance. They
come in a whorl, arch, loop pattern, or a combination of these patterns. For human’s
height, weight, body build, shape of eyes, lips, and ears are all affected by polygenic
inheritance. It’s not just humans that are affected by this. Other examples are grain color
in wheat, milk production in cows, and egg production in chickens.
HEREDITY
Human Genetics
Genes sometimes undergo mutations, resulting in an unwanted trait. Not all
mutations are harmful, but some have resulted in genetic disorders in humans. There are
about 1000 simple recessive disorders known to man. These disorders are recessive
genetic disorders.
One type of recessive genetic disorder is Sickle-Cell Anemia. Sickle-cell is a
homozygous recessive disorder in which red blood cells are sickle-shaped instead of discshaped. They can’t deliver enough oxygen to cells in body, and the body cells are
damaged due to a lack of oxygen. The cells don’t move through blood vessels very
easily. Many die as children, and a few make it to adulthood. This disease can be
treated. It is commonly found in tropical areas, and affect a small percentage
of African Americans.
Cystic Fibrosis is another homozygous recessive disorder in which a thin fluid is
produced by the body to lubricate the lungs and intestinal tract. Cystic fibrosis has thick
mucus in these areas instead of thin fluid. The mucus builds up in the lungs and digestive
system. It makes it hard to breathe and causes lung damage. Cystic fibrosis also prevents
the flow of digestive enzymes. It is commonly inherited by Caucasians. 1:20 carry this
recessive allele. Cystic fibrosis is helped with antibiotics, special diets, and physical
therapy.
Scientists have discovered that fruit flies have eight chromosomes and among
these chromosomes are either two X chromosomes, or an X chromosome and a Y
chromosome. Scientists have concluded that X and Y chromosomes contain genes that
determine the sex of individuals. Females carry XX chromosomes, and males carry XY
chromosomes. When an egg from a female is fertilized from the X sperm, an XX female
is born. When and egg from a female is fertilized from the Y sperm an XY male is born
Some inherited conditions are closely linked to the X and Y chromosomes. This
brings about sex-linked disorders. A sex-linked gene is an allele that is inherited on a sex
chromosome. Color blindness is an example. More males than females are color blind.
Why?
Hemophilia is another sex-linked disorder in which blood does not clot properly.
A scrape can be life threatening. Males who inherit the X cell will have this disorder.
The ration is 1:7 000 males. Females need both the XX cells to be infected. They are
usually carriers. Scientists use a pedigree, a tool for tracing the occurrence of a trait to a
family. Normally squares are males and circles are females. Colored and filled circles
and squares show a person who has the condition. Half colored circles and squares are
carriers.
The same laws that govern inheritance of human traits also govern the traits of
watermelon, wheat, and even mice. Knowing how genes are inherited causes some
people to seek advice of genetic counselors. Genetic engineering has resulted. Genetic
engineering is when biological and chemical methods are used to change the DNA
sequence that makes up a gene. They are used to help produce large quantities of
medicines such as insulin for diabetes. They are also used to find new ways to improve
crop production. Identifying genes that are desirable traits in one plant, then inserting in
another plant is an example. Barley has the following strengths: maturity, strength,
height, and resistant to drought. Genetic engineers are planning to insert these genes in
other plants to make them more resistant and easily grown.