HUMAN BIOLOGY

How to Calculate Probability

Step 1. Actual genotypes of parental gametes

In the cross Cc × Cc, gametes have a 50–50 chance of

receiving either allele (C or c) from each parent. Said

another way, the probability that a particular sperm or

egg will be C is 1/2, and the probability that it will be c

is also 1/2:

probability of C:

probability of c:

1/2

1/2

Step 2. Probable genotypes of offspring

Offspring receive one allele from each parent. Three

different combinations of alleles are possible in this

cross. To figure the probability that a child will receive

a particular allele combination, simply multiply the

probabilities of the individual alleles:

probability of CC:

probability of Cc:

probability of cC:

probability of cc:

1/2 × 1/2 = 1/4

1/2 × 1/2 = 1/4

1/2 × 1/2 = 1/4

1/2 × 1/2 = 1/4

1/2





Step 3. Probable phenotypes

Chin fissure:

(CC,Cc,cC)

Smooth chin:

(cc)

1/4 + 1/4 + 1/4 = 3/4

1/4

Probability in a Mating Where Both

Parents Are Heterozygous at Two Loci

If both parents are heterozygous for two single-gene traits,

a mating produces the following 9:3:3:1 phenotype ratio:

9/16 or 9 chin fissure, freckles

3/16 or 3 chin fissure, no freckles

3/16 or 3 smooth chin, freckles

1/16 or 1 smooth chin, no freckles

Individually, these phenotypes have the following

probabilities:

Trait combination

Chin fissure, freckles

Chin fissure, no freckles

Smooth chin, freckles

Smooth chin, no freckles

3/4 × 3/4

3/4 × 1/4

1/4 × 3/4

1/4 × 1/4

probability of chin fissure (12 of 16)

probability of freckles (12 of 16)

probability of smooth chin (4 of 16)

probability of no freckles (4 of 16)

= 3/4

= 3/4

= 1/4

= 1/4

To figure the probability that a child will show a particular

combination of phenotypes, multiply the probabilities of

the individual phenotypes in each possible combination:

Probability

9/16

3/16

3/16

1/16

Figure A.11 Probability rules also apply to the inheritance

of two single-gene traits, which assort independently into

forming gametes during meiosis. (© Cengage Learning)

Figure A.10 Simple multiplication lets you figure the
probability that a child will inherit alleles for a given
phenotype that is a single-gene trait. (© Cengage Learning)

Appendix iV A-11

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