HUMAN BIOLOGY

Even with the availability of high-tech genetic analysis,
geneticists may still use a Punnett square to determine the
probability that a couple’s offspring will inherit a particu-
lar single-gene trait. Punnett squares, like those shown in
Section 19.3 and in this Appendix, show the probable out-
comes of genetic crosses.

You may recall that Section 19.3 considered the geno-

types for the chin fissure trait likely to result from a mating

between two heterozygous parents—that is, parents who

are both Cc (see Figure 19.5B). In that case, remember, there

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

inherit at least one dominant C allele and so have a chin

fissure. It’s not unusual, however, for a geneticist or genetic

counselor to work with couples in which one parent is

thought (or known) to be homozygous for the dominant

form of a trait of interest (in our example, CC), while the

other parent is thought or known to be homozygous for

the recessive version of the trait (cc). In such a scenario, in

theory each of the couple’s children will inherit at least one

dominant version of the allele, as shown in the upper por-

tion of Figure A.9. As you may notice there, the practice in

genetics is to call the first generation to result from a mat-

ing the F 1 generation.

The lower portion of Figure A.9 illustrates the possible

genotypes for the trait of interest that would result from

a mating between a second generation (denoted F 2 ) of

offspring. Scientists who study genetic outcomes in non-

human organisms use this type of cross, called a testcross,

to track outcomes over several generations. For ethical rea-

sons we don’t do testcrosses with human subjects, but the

genetic equivalent is a mating between individuals who

both are heterozygous for a given trait. As you can see in

the lower portion of Figure A.9, the theoretical result is the

same as in Figure 19.5B. Each child of such parents has a

75 percent chance of having at least one dominant allele.

Figure A.10 uses the chin fissure example to explain the

three basic steps for calculating the probability that parents

will pass a particular single-gene trait to offspring.

Section 19.3 also discussed the mechanism of indepen-

dent assortment, which occurs as meiosis forms gametes

(sperm or eggs). Independent assortment explains why a

particular single-gene trait generally is inherited indepen-

dently of other single-gene traits. Figure 19.7 showed how

this works when two single-gene traits (the chin fissure

and freckles) are considered. The sixteen possible combina-

tions of phenotypes among offspring of such a cross tend

to occur in a 9:3:3:1 ratio, on average. Figure A.11 explains

how the rules of probability operate in this sort of scenario.

A Closer Look at Probability and Independent Assortment

Figure A.9 With a testcross performed to learn parental
genotypes for a single trait, a different set of genetic results
is possible in the second generation. (© Cengage Learning)

Cc

C

c c

Parent:

homozygous

recessive

CcCc

Parent:

homozygous

dominant

CC

C

cc

cc

C C Cc

F 1

phenotypes

Cc

Cc

Cc

Cc

C

C c

F 1

offspring:

CC Cc

F 1

offspring:

Cc

C

C c

Cc

c c Cc

F 2

phenotypes

CC

Cc

Cc

cc

alleles segregate

3 dominant (CC,Cc,Cc)

1 recessive (cc)

cc

Appendix IV

A-10 Appendix iV

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