12 Unit 1 Reaching Your Potential
Heredity is a major factor in your personal
growth and development. Your heredity infl uences
your rate of development and the patterns in which
you grow and develop. Some of these patterns
infl uence the way you feel, think, and behave and
become part of your personal identity. (This topic
is covered in-depth in Chapter 2.) Knowing about
the characteristics you inherit will give you some
insight to your own potential development. You
will also pass on to your children part of what you
inherit from your parents.
Genes
The science that studies heredity is called
genetics. Each human being begins life with his
or her own set of genetic material. A person
normally inherits 23 rod-shaped structures
called chromosomes from each biological parent.
Chromosomes carry hereditary information from
each parent. The father and mother each contribute
23 chromosomes, so the child inherits a total of
46 chromosomes. Each chromosome contains many
genes, which determine all inherited characteristics,
or traits. This genetic “blueprint” exists in the nucleus
of every cell in the body.
Because of inherited genes, your personal
traits may be similar to a brother, sister, or another
family member. The genes that come from both
parents combine in different patterns in their
children. As a result, brothers and sisters have
different traits.
Dominant and Recessive Genes
Genes are classifi ed as either dominant or
recessive. Dominant genes, when present, deter-
mine how a certain trait is expressed in a person.
A dominant gene always overrules a recessive
gene. Recessive genes determine the nature of a
trait only when two of them are present. The child
must receive one recessive gene from each parent
for the recessive characteristic to be expressed.
Figure 1.6 lists some examples of physical traits
that are dominant and recessive.
Scientists use letter combinations such as DD or
dd to identify a person’s genotype for various traits.
A genotype is the genetic makeup of an individual
or group. A capital letter signifi es a dominant trait,
such as dimples in one’s cheeks. A lowercase letter
signifi es a recessive trait, such as no dimples on
the cheeks. A person with two dominant genes,
genotype DD, will have dimples. A person with
one dominant and one recessive gene, genotype
Dd, will also have dimples because the dominant D
gene will be expressed. Only a person with two
recessive genes, genotype dd, will have no dimples.
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Figure 1.5 What physical characteristics do these
family members have in common? For each shared
trait, identify if it is dominant or recessive.
Figure 1.6 This chart shows some of the inherited traits that are dominant or recessive. Can you recognize any
of your personal traits as dominant or recessive?
Examples of Dominant and Recessive Traits
Dominant Traits Recessive Traits
Dimples in cheeks Freckles No dimples in cheeks Lack of freckles
Free earlobes Widow’s peak hairline Attached earlobes Straight hairline
Cleft chin Hitchhiker’s thumb Chin with no cleft Straight thumb
Blood types A and B Blood type O Cystic fi brosis
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