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Alleles

The different forms (often dominant and
recessive) of a gene are known as alleles
(uh LEELZ). Dominant alleles are shown
with a capital letter. Recessive alleles are
shown with a lowercase letter.
Phenotype
An organism’s PHysical appearance
 In pea plants:
 characteristic = flower color



Possible phenotypes would be purple flowers
or white flowers.
characteristic = seed color

Possible phenotypes would be yellow and
green seeds
DOMINANT

A dominant allele masks or DOMINATES
the recessive allele and is noted with a
CAPITAL letter

Because the allele for purple flowers (P) is
dominant, only one P allele is needed for
the plant to have purple flowers.
RECESSIVE

To recede means “to go away or back off.”

The recessive allele will always back off
from the dominate

EX: P
p
Genotype
Both inherited alleles together form an
organism’s genotype. The genotype is
the genetic makeup of the phenotype.
 EX: PP, Pp

homozygous
A plant with two dominant or two recessive
alleles
 Remember the prefix homo means same.

EX: PP, pp
 Homozygous dominant = PP
 Homozygous recessive = pp

heterozygous
A plant that has the genotype Pp is said to
be heterozygous
 Remember hetero means different


EX: Pp
Probability

The mathematical chance that something
will happen

Most often written as a fraction or
percentage. If you toss a coin, the
probability of tossing tails is 1/2—you will
get tails half the time or 50% probability of
landing on heads.
Punnett square
25%
25%
25%
25%
G
g
G GG Gg
g Gg gg
The parents in this cross
Heterozygous
are _____________
Homozygous
Heterozygous
If G is dominant for green pods and g is recessive for
yellow pods, what percentage of the offspring will
have green pods? _______%
75
B b
B
b
The genotype of the
offspring in the blue box
is _______
bb
The offspring in the blue box is
homozygous
heterozygous
homozygous
In guinea pigs black fur (B) is dominant over
brown fur (b). The Punnett square for a cross
between a HETEROZYGOUS black guinea pig
and a PURE brown guinea pig would look like
B B
B b
B b
B b
B
b
b
B
b
b
b
B
This is the
Correct setup
Heterozygous = Bb
Pure brown = bb
homozygous
Another name for pure is _________________
Punnett square
Punnett square
Punnett square
Punnett square