HUMAN BIOLOGY

376 Chapter 19

Figure 19.5 Animated! A Punnett square shows possible

genetic outcomes. A The four possible genotypes of gametes

from parents who are heterozygous for the chin fissure trait.

B Likely outcome of a mating between heterozygotes. Circles

are gametes. Letters represent alleles—here, for the chin fissure

trait. Genotypes of resulting offspring are inside the squares.

(© Cengage Learning)

Cc cc

C CC Cc

c

C c

CC

Cc

Cc

cc

(dominant)

(dominant)

(dominant)

(recessive)

A

A

cc

C

c

C c

female gametes (n)

(eggs)

male gametes (

n)

(sperm)

cc

cc Cc cc

CC Cc

C c

Cc

C

c

C c

C

c

C c

Cc

Cc C

c

B

B

trait, the fourth child still has only a one-in-four chance

of inheriting it.

Genes for different traits sort into gametes

independent of each other

Segregation—the separation of parental chromosomes dur-

ing meiosis—ensures that the pairs of gene alleles for a

single trait are also separated. Only one allele of each pair

ends up in each sperm or egg.

Meiosis also is when independent assortment occurs.

This mechanism ensures that most traits are inherited

independently of all others. As Figure 19.6 shows, there

are two ways the two members of each pair of homologous

Genetic outcomes: probability and

independent assortment

n    Probability—the likelihood of a given outcome—is

important in inheritance of single-gene traits.

n Genes for most traits are inherited independently of

genes for other traits.

n    Links to Meiosis 18.7, Crossing over 18.8

More than 5,000 human genetic

traits appear to be governed by a

single gene. Rules of probability—

how likely it is that a given outcome

will occur—apply to the inheritance

of such traits.

a punnett square helps

predict genetic outcomes

A grid called a Punnett square can

help predict genetic outcomes. For

instance, suppose that prospective

parents each are heterozygous for the chin fissure geno-

type—that is, both are Cc. Because half of each parent’s

gametes are C and half are c (due to segregation at meiosis),

four outcomes are possible every time a sperm fertilizes an

egg: Half the gametes will be heterozygous (Cc) and half

will be homozygous for this trait—either CC or cc (Figure

19.5A). If two homozygotes of either type (CC or cc) have a

family, their children will also be homozygous. Those who

are CC will have a chin fissure and those who are cc won’t.

Now suppose that a person who is heterozygous for the

chin fissure trait (Cc) has a family with another Cc person.

There is a 75 percent chance (three out of four) that a child

from a cross between two Cc parents will have at least one

dominant C allele and a chin fissure (Figure 19.5B).

Chance determines which sperm fertilizes which egg.

This means that:

1. The most probable outcome may not occur. Thus in
   many families, the parents have produced a string of
   boys or girls when probability says that parents with
   two or more children will have an equal number of
   girls and boys. Predicted ratios usually only turn up
   with a large number of events. When you flip a coin,
   probability predicts that heads and tails should each
   come up about half the time. Yet you may have to flip the
   coin a hundred times to end up with a ratio close to 1:1.


2. The probability that a given genotype will occur is the
   same for every child no matter how many children
   a couple has. Based on the parents’ genotypes, if the
   probability that a child will inherit a given genotype
   is one in four, then each child of those parents has
   a one-in-four (25 percent) chance of inheriting the
   genotype. If the parents have three children without the

19.3

independent assortment
Mechanism in meiosis by
which alleles of a gene
move into gametes inde-
pendently of the alleles of
other genes.

probability A measure of
the chance that a given out-
come will occur.

Punnett square A grid for
determining the probable
outcome of genetic crosses.

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