events happening. Te probability of getting heads in one coin
toss is 1/2, so the probability of getting two heads in a row is 1/2
1/4. Remember that the production of each child, like
each coin toss in a series, is an
that does not
inﬂuence the production of any other child by the same couple.
If you get heads on the ﬁrst toss, the chance of getting heads the
second time is still 1/2. Likewise, if our couple’s ﬁrst child is a
they still have a 1/4 chance of getting a
the next time.
Human traits dictated by dominant alleles include widow’s
peaks, dimples, and freckles.
Disorders caused by dominant genes are uncommon because
lethal dominant genes
are almost always expressed and result in
the death of the embryo, fetus, or child. However, in some domi-
nant disorders the person is less impaired or at least survives long
enough to reproduce. One example is
, a fatal
nervous system disease involving degeneration of the basal nu-
clei. It involves a
that is expressed when the
aﬀected individual is about 40. Oﬀspring of a parent with Hunt-
ington’s disease have a 50% chance of inheriting the lethal gene.
(Te parent is heterozygous, because the dominant homozygous
condition is lethal to the fetus.) Many informed oﬀspring of such
parents are opting not to become parents themselves.
Tese and some other dominant gene–determined traits are
from independent assortment and random fertilization, any oﬀ-
spring represents one out of the close to 72 trillion (8.5 million
8.5 million) zygotes possible. Te additional variation introduced
by crossovers increases this number exponentially. Perhaps now
you can understand why brothers and sisters are so diﬀerent, and
marvel at how they can also be so alike in many ways.
Check Your Understanding
We said that genetic variability is introduced by independent
assortment. Just what is assorting independently?
How does crossover increase genetic variability?
For answers, see Appendix H.
Types of Inheritance
A few human phenotypes can be traced to a single gene pair (as
we will describe shortly), but most such traits are very limited
in nature, or reﬂect variation in a single enzyme. Most human
traits are determined by multiple alleles or by the interaction of
several gene pairs.
Compare and contrast dominant-recessive inheritance with
incomplete dominance and codominance.
reﬂects the interaction of
dominant and recessive alleles. A simple diagram, called the
, is used to ﬁgure out, for a single trait, the possi-
ble gene combinations that would result from the mating of par-
ents of known genotypes
. In the example shown,
both parents can roll their tongue into a U because both are
heterozygous for the dominant allele (
) that confers this ability.
In other words, each parent has the genotype
. Te alleles of
one parent are written along one side of the Punnett square, and
the alleles for the other parent are shown along an adjacent side.
Te alleles are then combined down and across to determine
the possible gene combinations (genotypes) and their expected
frequency in the oﬀspring of these two parents.
As the completed Punnett square shows, the probability of
these parents producing a homozygous dominant child (
25% (1 out of 4); of producing a heterozygous child (
), 50% (2
out of 4); and of producing a homozygous recessive child, 25%
(1 out of 4). Te
oﬀspring will be tongue rollers. Only
oﬀspring will not be able to roll their tongues.
Te Punnett square predicts only the
of a particular
genotype (and phenotype). Te larger the number of oﬀspring,
the greater the likelihood that the ratios will conform to the pre-
dicted values—just as the chances of getting heads half the time
and tails half the time increase with the number of tosses of a
coin. If we toss only twice, we may well get heads both times.
Likewise, if the couple in our example had only two children, it
would not be surprising if both children had the genotype
What are the chances of having two children of the same ge-
notype? ±o determine the probability of two events happening
in succession, we must multiply the probabilities of the separate
Genotype and phenotype probabilities resulting
from a mating of two heterozygous parents.
square shows all possible combinations of a set of alleles in the
zygote. In this example, the
allele is dominant and determines
tongue-rolling ability; the
allele is recessive.