Mendel and the Gene Idea

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3

Fig. 14-4

Allele for purple flowers

Homologous

pair of

chromosomes

Locus for flower-color gene

Allele for white flowers

Slide 20

The second concept is that for each character an organism inherits two alleles, one from each parent

Mendel made this deduction without knowing about the role of chromosomes

The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation

Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

Slide 21

The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance

In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

Slide 22

The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism

This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

Slide 23

Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses

The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

Slide 24

Fig. 14-5-1

P Generation

Appearance:

Genetic makeup:

Gametes:

Purple flowers

White flowers

PP

P

pp

p

Slide 25

Fig. 14-5-2

P Generation

Appearance:

Genetic makeup:

Gametes:

Purple flowers

White flowers

PP

P

pp

p

F1 Generation

Gametes:

Genetic makeup:

Appearance:

Purple flowers

Pp

P

p

1/2

1/2

Slide 26

Fig. 14-5-3

P Generation

Appearance:

Genetic makeup:

Gametes:

Purple flowers