Section 1:2 Heredity
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Physical Characteristics
The genes you inherit affect your body. Your
physical appearance may be similar to a brother,
sister, or another family member because of
inherited genes. However, the genes that come
from both parents combine in different patterns
in their children. As a result, brothers and sisters
have different traits.
Are you tall, short, or average height? Are
you male or female? What color are your eyes,
hair, and skin? These are some of your inherited
traits.
Dominant and Recessive
Genes
Some genes are dominant while others are
recessive.
‚ When present, dominant genes determine
the nature of a certain trait in a person. A
dominant gene always overrules a recessive
gene. Chart 1-5 lists some examples of
physical traits that are dominant.
‚ Recessive genes determine the nature of the
trait only when two of them are present. The
child must receive one recessive gene from
each parent.
Scientists use letters such as DD and rr to
identify a person’s genotype for various traits. A
genotype is the genetic makeup of an individual
or group. Capital D signifies a dominant trait
such as brown hair. A lowercase r signifies a
recessive trait such as blonde hair. A person
with two dominant genes, genotype DD, will
have brown hair. A person with one recessive
and one dominant gene, genotype rD, will also
have brown hair because the dominant D gene
is present. A person with two recessive genes,
genotype rr, will have blonde hair. In the case of
eye color, a person with two recessive genes will
have blue eyes.
Scientists use a Punnett square, invented by
R.C. Punnett, to determine what possible gene
pairs may result from combining two genes, 1-6.
The genes of the parents are identified along two
adjacent sides of the square—one parent per
side. Each possible combination is recorded in
an inner box. A total of four combinations are
possible. Consequently, each offspring will have a
25 percent chance of inheriting the characteristic
indicated in each box.
Many traits are influenced by more than one
pair of genes. For example, height seems to be
determined by at least four pairs of genes. When
several genes influence a trait, more variations
may result in offspring.
Inherited Traits
Dominant Traits Recessive Traits
Black or brown hair
Full lips
Free earlobes
Dimples in cheeks
High and narrow
nose
Brown eyes
Long full lashes
Curly hair
Freckles
Feet with normal
arches
Farsightedness and
astigmatism
Blond hair
Thin lips
Attached earlobes
No dimples in
cheeks
Broad nose
Blue eyes
Short thin lashes
Straight hair
Lack of freckles
Flat feet
Normal vision
1-5
This chart shows some of the inherited traits that dominate over recessive traits.