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
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 organism’s traits do not always reveal its genetic composition
Therefore, we distinguish between an organism’s 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
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
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
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
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