382 Chapter 19
expLore on your own
summary
section 19.1 Genes are specific units
of inheritance that are passed to offspring.
Each gene has a specific location on a chromo-
some. Chromosomes come in pairs, so each
person has two copies of each gene. The
copies are called alleles, and they may or
may not be identical.
Someone who is homozygous for a trait (such as AA) has
two identical alleles for the trait. Having two different alleles
(Aa) is a heterozygous condition. Alleles (and traits) may be
dominant (A) or recessive (a).
The term genotype refers to the particular alleles an
individual has. Phenotype is the term used to refer to an
individual’s actual observable traits.
section 19.2 As diploid organisms,
humans have two copies (alleles) of each
gene, one on each of the two chromosomes
of a homologous pair. The two copies of
each gene segregate from each other during
meiosis, so each gamete formed ends up with
one gene or the other.
section 19.3 A Punnett square is a
grid for figuring the probable outcome of
a genetic cross. Probability expresses the
likelihood of a particular event. Matings
between two heterozygous individuals (Cc 3
Cc) produce the following combinations of
alleles in offspring:Cc ccC CC CccC cCC
Cc
Cc
cc(dominant)
(dominant)
(dominant)
(recessive)AccCcC cCcCcIdentical twins have identical genes and look alike. In addition, they have
many behaviors in common. Are these parallels coincidence, clever hoaxes, or evi-
dence that aspects of behavior are inherited characteristics, like hair and eye color?
Although the question is intriguing, clear answers have proven difficult to come by.
There is strong evidence that some basic behaviors, such as smiling to indicate
pleasure and the crying of an infant when it is hungry, are genetically programmed.
Scientists have also begun to look for links between genes and alcoholism, some
types of mental illness, violent behavior, and even sexual orientation. Such studies
raise contro versial social issues. So far their greatest impact has been to point out
how little we know about the biological basis of human behavior. To learn more about
these efforts and find links to some other fascinating and reputable human genetics
© Cengage Learning/Gary Head websites, visit the Personality Project website at personality-project.org.This results in a probability of 3/4 that any one child will have
the dominant phenotype and 1/4 that the child will have the
recessive phenotype.
The members of each pair of homologous chromosomes
sort into gametes independently of how the members of
other chromosome pairs sort. Therefore, the genes on the
chromosomes also sort into gametes independently.
section 19.4 In cases of pleiotropy, a
single gene can influence many seemingly
unrelated traits (as in sickle-cell anemia). In
codominance, two contrasting alleles of a
gene are both expressed. ABO blood types
provide an example: People who are type
AB have codominant alleles for type A and
type B blood. Someone who has two copies of the O allele has
type O blood. A gene that has three or more alleles is called a
multiple allele system.
section 19.5 Various factors can
influence the expression of genes. Penetrance
refers to the probability that someone who
inherits an allele will have the phenotype
associated with it. Polygenic traits, such
as eye color and height, are due to the
expression of several genes. For polygenic
traits we may see continuous variation in
populations. In multifactorial inheritance the
phenotype associated with a polygenic trait can be influenced
by nongenetic environmental factors.reView Questions
- Define the difference between (a) gene and allele,
(b) dominant allele and recessive allele, (c) homozygote
and heterozygote, and (d) genotype and phenotype. - State the theory of segregation. Does segregation occur
during mitosis or during meiosis? - What is probability, and how is it applied in genetic analysis?
- What is independent assortment? Does independent
assortment occur during mitosis or during meiosis?
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