Biology Now, 2e

Dog Days of Science ■ 123

and brown offspring that would result if two

heterozygous (Bb) black-coated dogs were

mated. It is important to understand that the

predicted ratios simply give the probability that

a particular offspring will have a certain pheno-

type or genotype; the actual ratio will vary

(see “What Are the Odds?” on page 124).

Peas in a Pod

Mendel’s research on pea seeds led to his second

law, the law of independent assortment.

This law states that when gametes form, the

two alleles of any given gene segregate during

meiosis independently of any two alleles of

other genes. For example, pea seeds can have

a round or wrinkled shape, and they can be

yellow or green. Two different genes control the

two different traits—the R gene, with alleles R

(round) a nd r (wrinkled), controls seed shape,

while the Y gene, with alleles Y (yellow) and

y (green), controls the color of the seed—but

neither gene affects the inheritance of the

other.

Mendel tested the idea of independent assort-

ment in the set of experiments illustrated in

Figure 7.6. He tracked seed shape, the trait

controlled by the R and r alleles, and seed color,

controlled by the Y and y alleles. The test of his

hypothesis came when he examined the pheno-

types of the offspring produced by crossing the

heterozygous F 1 plants (RrYy). As predicted by

the hypothesis, two new phenotypic combina-

tions were found among the F 2 offspring: plants

with round, green seeds (RRyy or Rryy) and

plants with wrinkled, yellow seeds (rrYY or

rrYy). Figure 7.6 summarizes the ratios of the

two parental phenotypes and the two novel,

nonparental phenotypes.

Traits controlled by a single gene and

unaffected by environmental conditions are

called Mendelian traits. But when Mendel

described his laws of inheritance, he had no

idea what genes were made of, where they

were located within a cell, or how they segre-

gated and independently assorted. Today we

know that genes are located on chromosomes

and that these chromosomes are the basis

for all inheritance. We call these assertions

the chromosome theory of inheritance,

which explains the mechanism underlying

P generation

F 1 generation

F 2 generation

Eggs

Sperm

RrYy

RRYY rryy

Phenotypes of

Mendel’s F 2 plants

appear in a ratio

close to 9:3:3:1. The

presence of round,

green and wrinkled,

yellow offspring

showed that the two

traits—seed shape

and seed color—are

inherited

independently.

...and when these

four types of

gametes are used to

produce the F 2

generation, a 9:3:3:1

phenotype ratio

should result.

Assuming the two traits are

inherited independently, RrYy

individuals should produce

equal numbers of RY, Ry, rY,

and ry gametes...

The phenotype of the F 1 hybrids

confirmed that round seed

shape is dominant over

wrinkled, and that yellow seed

color is dominant over green.

Mendel crossed plants with

round, yellow seeds (RRYY) and

plants with the contrasting

phenotype: wrinkled seed shape

and green seed color (rryy).

RRYY RRYy

RRYy RRyy

RrYY RrYy

RrYy Rryy

RrYY RrYy

RrYy Rryy

rrYY rrYy

rrYy rryy

RY Ry

RY Ry rY ry

RY ry

rY ry

RY

Ry

rY

ry

×

Summary of Mendel’s results

9/16 Round, yellow (315 plants)

3/16 Round, green (106 plants)

3/16 Wrinkled, yellow (101 plants)

1/16 Wrinkled, green (32 plants)

Figure 7.6

Independent assortment of pea color and shape

Mendel used two-trait breeding experiments, called dihybrid crosses, to

test the hypothesis that the alleles of two different genes are inherited

independently from each other. M

Q1: List all the possible offspring genotypes and phenotypes.

Q2: What is the offspring phenotype ratio?

Q3: Complete a Punnett square for a genetic cross of two true-

breeding Portuguese water dogs—one with a black, wavy coat

(homozygous dominant, BBWW) and one with a brown, curly coat

(homozygous recessive, bbww). What is the phenotype ratio of

their offspring (F 1 )? Now fill out another Punnett square, crossing

two of the offspring. What is the phenotype ratio of the

F 2 generation?