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Mendel and the Gene Idea
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White flowers

PP

P

pp

p

F1 Generation

Gametes:

Genetic makeup:

Appearance:

Purple flowers

Pp

P

p

1/2

1/2

F2 Generation

Sperm

Eggs

P

P

PP

Pp

p

p

Pp

pp

3

1

Slide 27

Useful Genetic Vocabulary

Useful Genetic Vocabulary

An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character

An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character

Unlike homozygotes, heterozygotes are not true-breeding

Slide 28

Because of the different effects of dominant and recessive alleles, an organismís traits do not always reveal its genetic composition

Because of the different effects of dominant and recessive alleles, an organismís traits do not always reveal its genetic composition

Therefore, we distinguish between an organismís phenotype, or physical appearance, and its genotype, or genetic makeup

In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes

Slide 29

Fig. 14-6

Fig. 14-6

Phenotype

Purple

Purple

3

Purple

Genotype

1

White

Ratio 3:1

(homozygous)

(homozygous)

(heterozygous)

(heterozygous)

PP

Pp

Pp

pp

Ratio 1:2:1

1

1

2

Slide 30

The Testcross

The Testcross

How can we tell the genotype of an individual with the dominant phenotype?

Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous

The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual

If any offspring display the recessive phenotype, the mystery parent must be heterozygous

Slide 31

Fig. 14-7

Fig. 14-7

TECHNIQUE

RESULTS

Dominant phenotype,

unknown genotype:

PP or Pp?

Predictions

Recessive phenotype,

known genotype:

pp

If PP

If Pp

or

Sperm

Sperm

p

p

p

p

P

P

P

p

Eggs

Eggs

Pp

Pp

Pp

Pp

Pp

Pp

pp

pp

or

All offspring purple

1/2 offspring purple and

1/2 offspring white

Slide 32

Fig. 14-7a

Fig. 14-7a

Dominant phenotype,

unknown genotype:

PP or Pp?

Predictions

Recessive phenotype,

known genotype:

pp

If PP

If Pp

or

Sperm

Sperm

p

p

p

p

P

P

P

p

Eggs

Eggs

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