Mendel and the Gene Idea

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The hybrid offspring of the P generation are called the F1 generation

When F1 individuals self-pollinate, the F2 generation is produced

Slide 11

The Law of Segregation

When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple

When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white

Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

Slide 12

Fig. 14-3-1

EXPERIMENT

P Generation

(true-breeding

parents)

Purple

flowers

White

flowers



Slide 13

Fig. 14-3-2

EXPERIMENT

P Generation

(true-breeding

parents)

Purple

flowers

White

flowers



F1 Generation

(hybrids)

All plants had

purple flowers

Slide 14

Fig. 14-3-3

EXPERIMENT

P Generation

(true-breeding

parents)

Purple

flowers

White

flowers



F1 Generation

(hybrids)

All plants had

purple flowers

F2 Generation

705 purple-flowered

plants

224 white-flowered

plants

Slide 15

Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids

Mendel called the purple flower color a dominant trait and the white flower color a recessive trait

Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits

What Mendel called a “heritable factor” is what we now call a gene

Slide 16

Table 14-1

Slide 17

Mendel’s Model

Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring

Four related concepts make up this model

These concepts can be related to what we now know about genes and chromosomes

Slide 18

The first concept is that alternative versions of genes account for variations in inherited characters

For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers

These alternative versions of a gene are now called alleles

Each gene resides at a specific locus on a specific chromosome

Slide 19