Biology (Holt)

Objectives

Describethe four
major hypotheses Mendel
developed.

Definethe terms homozy-
gous, heterozygous, genotype,
andphenotype.

CompareMendel’s two laws
of heredity.

Key Terms

allele

dominant

recessive

homozygous

heterozygous

genotype

phenotype

law of segregation

law of independent

assortment

Section # A Head 1-line

A Theory of Heredity

Before Mendel’s experiments, many people thought offspring were a

blendof the characteristics of their parents. For example, if a tall

plant were crossed with a short plant, the offspring would be medium

in height. Mendel’s results did not support the blending hypothesis.

Mendel correctly concluded that each pea has two separate “heritable

factors” for each trait—one from each parent. As shown in Figure 4,

when gametes (sperm and egg cells) form, each receives only one of

the organism’s two factors for each trait. When gametes fuse during

fertilization, the offspring has two factors for each trait, one from

each parent. Today these factors are called genes.

Mendel’s Hypotheses

The four hypotheses Mendel developed were based directly on the

results of his experiments. These four hypotheses now make up the

Mendelian theory of heredity—the foundation of genetics.

1.For each inherited trait, an individual has two copies of

the gene—one from each parent.

2.There are alternative versions of genes.For example, the

gene for flower color in peas can exist in a “purple” version

166 CHAPTER 8Mendel and Heredity

Section 2 Mendel’s Theory

YY = Gene for

yellow seeds

yy = Gene for

green seeds

Parent Parent

Gametes

Meiosis

Fertilization

1. During gamete
   formation (meio-
   sis), the two
   genes separate.
   2. During fertilization,
   each offspring
   receives one version
   of each gene (allele)
   from each parent.

Each parent has two separate “factors,”

or genes, for a particular trait.

Figure 4 Mendel’s factors

3F

3F

6D