Download Genetic Crosses

Genetic Crosses
How to predict the probable
genetic makeup and appearance
of offspring resulting from
specified crosses.
Genotype
The genetic makeup
of an organism.
Consists of the
alleles that the
organism obtains
from its parents.
Ex. Blue eyes is bb
and brown eyes
could be BB or Bb.
Phenotype
The appearance of
an organism as a
result of its
genotype. What
the characteristic
of the organism
looks like. i.e.
does it have blue
eyes or brown
eyes.
Homozygous
When both alleles
of a pair are
alike. An
organism may be
homozygous
dominant (BB)
or homozygous
recessive (bb).
Heterozygous
When the two
alleles in the
pair are
different.
Ex. Bb
Predicting Results
Monohybrid cross: A
cross between
individuals that
involves 1 pair of
contrasting traits. Ex.
A cross between a
blue eyed person and
a brown eyed person.
A Punnett square is
used.
Homozygous X Homozygous
A monohybrid
cross between
a person
homozygous
for brown eyes
and a person
homozygous
for blue eyes.
Homozygous X Heterozygous
A monohybrid
cross between
a person
homozygous
for blue eyes
and a person
heterozygous
for brown eyes.
Heterozygous X Heterozygous
A monohybrid
cross between
a person
heterozygous
for brown eyes
and a person
heterozygous
for brown eyes.
Genotypic Ratio
The ratio of
the
genotypes
that appear
in offspring.
Homozygous X
homozygous: 4 Bb
Homozygous X
Heterozygous:
2 Bb: 2bb
Heterozygous X
heterozygous:
1BB:2Bb:1bb
Phenotypic Ratio
The ratio of
the
offspring’s
phenotypes
or what they
look like.
Homozygous X
homozygous: 4 brown
Homozygous X
Heterozygous:
2 brown and 2 blue
Heterozygous X
heterozygous: 3
brown and 1 blue
Incomplete Dominance
When offspring
have a phenotype
that is in between
that of the
parents. Ex. Four
o’clock flowers.
RR is red, Rr is
pink and rr is
white.
Four O’Clock Flowers
Codominance
Occurs when both alleles for a
gene are expressed in a
heterozygous offspring. Neither
allele is dominant or recessive nor
do they blend in the phenotype.
Ex. The color roan coat color. The
genes for both white coat and red
coat are expressed.
Roan Coat Color
Dihybrid Crosses
A cross between
individuals that
involves two pairs of
contrasting traits.
Homozygous X Homozygous
This Punnett square
shows a dihybrid
cross between a pea
plant that is
homoozygous for
green pods and
yellow seeds (GGYY)
and a pea plant that is
homozygous for
yellow pods and
green seeds (ggyy).
Pod color
G (green) and
g (yellow)
Seed color
Y (yellow) and
y (green)
Dihybrid Cross
All plants will be have
the same genotype.
Heterozygous for both
traits. (GgYy).
All plants will have the
same phenotype. All
will be green pods
and yellow seeds.
Heterozygous X Heterozygous
This Punnett square
shows a dihybrid
cross between a pea
plant that is
heterozygous for
green pods and
yellow seeds (GgYy)
and a pea plant that is
heterozygous for
green pods and
yellow seeds (GgYy).
Pod color
G (green) and
g (yellow)
Seed color
Y (yellow) and
y (green)
Dihybrid Cross
9 different genotypes:
9/16 green pods yellow
seeds. 3/16 green pods,
green seeds. 3/16 yellow
pods, yellow seeds. 1/16
yellow pods green seeds.
4 different phenotypes.
9 Green pods, yellow seeds
3 Green pods, green seeds
3 Yellow pods,yellow seeds
1 Yellow pods, green seeds