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Heredity and Genetics
Heredity and Genetics
Every person inherits traits such as hair and eye color as well as the shape of their earlobes from
their parents. Inherited traits can also be influenced by the environment. Height is an inherited
trait but poor nutrition may limit growth during childhood. Other inherited traits can be
impacted by the environment including body size and the tendency for certain diseases like
diabetes.
Most of the cells in the human body contain a nucleus, the control center of a cell. Inside each
nucleus is a set of chromosomes (thread-like structures found within the nucleus of a cell that carry
the codes for inherited traits). Most of the cells in the human body contain 46 chromosomes that
are arranged in 23 pairs.
Sections of chromosomes, called genes, are the basic units of heredity. Genes occur in pairs, just
like chromosomes. One gene from each pair in inherited from each parent. You have thousands
of genes in every cell of your body.
DNA (deoxyribonucleic acid) is the chemical unit that makes up chromosomes. All living things
are made of DNA. DNA is made up of chemical building blocks arranged along a single molecule.
Several of these molecules are linked together in a strand to form a DNA sequence known as the
genetic code.
When a child is born, that child carries a combination of DNA sequencing from both parents. The
DNA contains different proteins that result in individual traits. All the characteristics you have, such
as eye color, the amount of curl in your hair, and your height are determined by your genetic code.
Only identical twins share the same DNA pattern.
Genetics and Fetal Development
Passing on traits from parent to child involves genetics. Most human cells have 46 chromosomes
or 23 pairs. Egg and sperm cells have half that number, 23 chromosomes. The resulting zygote
will have 46 chromosomes. These chromosomes carry the hereditary traits of the parents.
Dominant and Recessive Genes
Each human trait is determined by at least one pair of genes. Some genes are dominant while
others are recessive. Dominant traits usually appear in the offspring when they are present. The
traits of the recessive genes only appear when the dominant traits are not present.
2 genes (1 blue eye + 1 brown eye) = brown eyes
brown eye gene is dominant and blue eye gene is recessive
If an individual has blue eyes, that means they have two recessive genes for blue eye color.
The genotypic ratio describes the number of times a genotype (determines the expression of a
particular characteristic or trait) would appear in the offspring after a test cross.
For example, a test cross between two organisms with same genotype, Tt will result in offspring
with genotypes: TT, Tt, and tt. In this example, the predicted genotypic ratio is 1:2:1.
Phenotypic ratio pertains to the relative number of offspring manifesting a particular trait or
combination of traits.
Phenotype which is the expression of a particular trait, for example, skin color, height, behavior,
etc., according to the individual’s genetic makeup and environment.
Genes and Gender
One pair of chromosomes determines gender.
Females have two chromosomes that look exactly alike; these are called X chromosomes.
Males have two different chromosomes, one shorter than the other. The shorter chromosome is
the Y chromosome and the longer one is the X chromosome.
Since sperm and egg cells contain only half the chromosomes of other cells, these cells have only
one sex cell, not two. Since females have only X chromosomes, their eggs contain only an X
chromosome. Sperm contain either an X or a Y chromosome. Because of this, the male
determines the gender of the child. If the sperm cell carries the X chromosome, the child will be
a girl. If the sperm cell carries a Y chromosome, the child will be a boy
Genetic Disorders
A person can inherit genes that contain a mutation or abnormality. The mutation may have
Little or no effect on the person. Some of the genetic mutations may result in a birth defect or
May increase a person’s likelihood of developing a disease. These diseases are called genetic
disorder s. Genetic disorders are caused partly or completely by a defect in genes. Some
disorders may be apparent at birth where others may not show up until later in life.
Common Human Genetic Disorders
Sickle-cell anemia
Sickle cell anemia is a disease passed down through
families in which red blood cells form an abnormal sickle or crescent shape. It is
caused by an abnormal type of hemoglobin called hemoglobin S. Hemoglobin is
a protein inside red blood cells that carries oxygen. Hemoglobin S changes the
shape of red blood cells. The red blood cells become shaped like crescents or
sickles.
The fragile, sickle-shaped cells deliver less oxygen to the body's tissues.
They can also get stuck more easily in small blood vessels, as well as break into pieces that can
interrupt healthy blood flow. These problems decrease the amount of oxygen flowing to body
tissues even more.
These problems can also result in sever joint and abdominal pain, weakness, kidney disease, and
restricted blood flow.
Tay-Sachs Disease
Tay-Sachs disease is caused by a genetic mutation in the HEXA gene
on (human) chromosome 15. Destruction of nervous system, blindness,
paralysis, and death during early childhood occurs.
Cystic Fibrosis
Cystic fibrosis is a disease passed down through families that causes thick,
sticky mucus to build up in the lungs, digestive tract, and other areas of the
body. It is one of the most common chronic lung diseases in children and
young adults. It is a life-threatening disorder.
Down Syndrome
Down syndrome is a genetic condition in which a person has 47 chromosomes
instead of the usual 46. Down syndrome occurs when there is an extra copy
of chromosome 21. This form of Down syndrome is called Trisomy 21. The
extra chromosome causes problems with the way the body and brain develop.
They may be of short stature, have a round face with upper eyelids that cover the
inner corners of the eye. Down syndrome is one of the most common causes of human birth defects.
Hemophilia
Hemophilia is the failure of blood to clot. In most cases, hemophilia is
passed down through families (inherited). It most often affects males.
Technologies to Treat Genetic Disorders
Amniocentesis
A procedure in which a syringe is inserted through a pregnant female’s abdominal wall to
remove a sample of the amniotic fluid surrounding the developing fetus.
Doctors examine the chromosomes in fetal cells for genetic abnormalities. This is performed
16 to 20 weeks after fertilization.
Chorionic Villi Sampling (CVS)
A procedure in which a small piece of membrane is removed from the chorion, a layer of
tissue that develops into the placenta. The tissue can be examined for genetic disorders or to
determine the age and gender of the fetus. The procedure is done around the eighth week of
fetal development.
Battling Genetic Disorders
Genetic disorders occur when an individual is missing a functioning gene. Without the gene,
the body does not produce some of the substances it needs. One experimental treatment is
gene therapy.
Gene therapy is the process of inserting normal genes into human cells to correct genetic disorders.
Scientists feel that once a defective gene is replaced with a normal gene, the cells can then begin
producing the normal gene.
Placing parts of DNA from one organism into another is called genetic engineering.
Genetic Counseling
Genetic counselors can guide families of children with genetic disorders on treatments options.
Genetic counseling offers options to people with a family history of some diseases. They can
educate families about possible risks for certain diseases and guide families through their options.