5 Steps to a 5 AP Biology, 2014-2015 Edition

100 ❯ STEP 4. Review the Knowledge You Need to Score High

Mendel and His Peas

The person whose name is most often associated with heredity is Gregor Mendel. Mendel

spent many years working with peas. It was a very strange hobby, indeed, but it proved

quite useful to the world of science. He mated peas to produce offspring and recorded the

phenotype results in order to determine how certain characters are inherited. A character

is a genetically inherited characteristic that differs from person to person.

Before he began his work in the 1850s, the accepted theory of inheritance was the

“blending” hypothesis,which stated that the genes contributed by two parents mix as

colors do. For example, a blue flower mixed with a yellow flower would produce a green

flower. The exact genetic makeup of each parent could never be recovered; the genes would

be as inseparable as the blended colors. Mendel used plant experiments to test this hypoth-

esis and developed his two fundamental theories: the law of segregation and the law of inde-

pendent assortment.

When Mendel was observing a single character during a mating, he was doing something

called a monohybrid cross—a cross that involves a single character in which both parents are

heterozygous (Bb ×Bb). A monohybrid cross between heterozygous gametes gives a 3:1

phenotype ratio in the offspring (Figure 10.1). As you can see in Figure 10.1, an offspring

is three times more likely to express the dominant B trait than the recessive b trait.

B

B BB

b Bb

b

Bb

bb

Figure 10.1 Monohybrid cross.

Figure 10.2 Dihybrid cross.

BR Br bR br

BR BBRR BBRr BbRr

Bbrr

bbRr

bbrr

BbRR

Br BBRr BBrr BbRr

bbRR

bbRr

bR BbRR BbRr

br BbRr Bbrr

Mendel also experimented with multiple characters simultaneously. The crossing of

two different hybrid characters is termed a dihybrid cross(BbRr×BbRr). A dihybrid

cross between heterozygous gametes gives a 9:3:3:1 phenotype ratio in the offspring

(Figure 10.2).

From his experiments, Mendel developed two major hereditary laws: the law of segregation

and the law of independent assortment.

The law of segregation.Every organism carries pairs of factors, called alleles,for each trait,

and the members of the pair segregate (separate) during the formation of gametes. For

example, if an individual is Bb for eye color, during gamete formation, one gamete would

receive a B, and the other made from that cell would receive a b.

The law of independent assortment.Members of each pair of factors are distributed indepen-

dently when the gametes are formed. Quite simply, inheritance of one trait or characteristic