Download 2-Mohybrid Crosses

• Genes are segments of
DNA on chromosomes
• Genes occur in pairs
because chromosomes
occur in pairs.
• We refer to each of
several alternative
forms of the same
gene as ALLELES.
Alleles are the
equivalent of Mendel’s
“factors”.
How do we identify Alleles in
Genetic Problems?:
• Letters are used to refer to
dominant and recessive alleles.
– Capital letters are used to
refer to dominant alleles
– Lowercase letters are used to
refer to recessive alleles.
• One example would be the allele
for height in pea plants.
– The letter T would represent
the dominant allele for tallness
in pea plants
– While the letter t would
represent the recessive allele
for shortness in pea plants.
Genetic Crosses:
• The genetic makeup of an
organism is its genotype.
• The external appearance of an
organism is its phenotype.
• A plant that has a genotype that
is pure for Tallness would have be
represented by the letters TT .
• A plant that is pure for shortness
would be represented by tt.
• When both alleles of a pair are
the same, the organism is said to
be homozygous. For example, a
plant that is homozygous for
tallness would have the genotype
TT.
• When two alleles are not the
same, for example Tt, the
organism is said to be
heterozygous for that
characteristic.
• Mendel was fortunate enough to
study characteristics that had
only two alleles.
• Today, we know that many traits
are controlled by three or more
alleles and are said to have
multiple alleles.
Blood Type:
• The trait for blood type is an
example of multiple alleles. A , B,
and O are the alleles that affect
blood type.
• No matter how many alleles affect a
characteristic one individual can only
have 2 alleles at one time,
– for example, ABO are all alleles for
blood types but an individual can only
have two of these alleles at one time
(AO, BO, OO, AB).
Probability:
• The degree of chance that an event will
occur is referred to as probability.
• Probability = Number of one kind of event
Number of all events
Fill in the formula on the handout!
With only a few tosses you
may not get this 50% ratio.
Probability is the
expected ratio of
occurrence when
there are many
trials, for example,
when there are
many coin tosses
the expected yield
would be heads
50% and tails 50%
of the time.
• When one examines a cross between
one pair of contrasting traits is
referred to as a monohybrid cross.
In 1905 Reginald Punnett, an English
biologist devised a shorthand way of
finding the expected proportions of
possible genotypes of a monohybrid
cross.
• If you know the genotype of the
parents you can use a Punnett square to
predict the possible genotype of their
offspring.
• In mice the phenotype for black hair color is
dominant over brown.
• Predict the genotypic and phenotypic ratios
of parents that are heterozyogous for the
trait of black hair.
B
b
B
B B
B b
b
B b
b b
Genotypic Ratio: 1/4 BB, 2/4 Bb, 1/4 bb
Phenotypic Ratio: 3/4 Black 1/4 brown
Test Cross:
• Sometimes scientist use a test
cross procedure to determine the
unknown genotype of a parent by
crossing it with a known
homozygous recessive parent.
Predict the results of a test cross to
determine the genotype for a rabbit
that is homozygous for a black coat.
(BB)
B
B
b
B b
Bb
b
B b
B b
GR: 4/4 Bb
PR: 4/4 (100%) Black
• Notice the results.. If all the offspring of a
test cross have a dominant phenotype then
the individual in question MUST be
homozygous dominant.
Suppose the rabbit in question
were heterozygous.
Suppose the rabbit in
question were heterozygous
B
b
b
Bb
bb
b
Bb
bb
Genotypes: 2/4 Bb 2/4 bb
Phenotypes: 2/4 (50%) black
2/4 (50%) brown
Incomplete dominance:
• Mendel was able to restrict is research
to traits in which one allele dominated
completely over the other.
• This is not true
in all genetic
crosses. In
some instances
two or more
alleles influence
the phenotype.
• Japanese Four O’clock flowers have alleles
that are not completely dominate. We call
this incomplete dominance.
• The allele for Red flowers is R .
• The allele for white flowers is W.
• When these alleles are mixed R & W the
result is PINK flowers. Parents: RW x RW
R
W
R
RR
RW
W
RW
WW
Genotypes:
1/4 RR
Phenotypes:
2/4 RW 1/4 Red
1/4 WW 2/4 Pink
1/4 white
Codominance:
• Is a situation in which both alleles are
equally strong and both alleles are visible in
the hybrid genotype.
• An example of codominance is found in
chickens. When white chickens are crossed
with black chickens, the result is NOT a
grey chicken, but a chicken with both black
and white feathers.
• When expressing codominant alleles, both
alleles are written as superscripts capital
letters placed above the letter for the trait.
• The allele for Black feathers is FB.
• The allele for White feathers is FW.
• Parents: FBFW x FBFW…….Then…..
FB
FB FW
FBFB FBFW
FW FBFW FWFW
Genotypes:
1/4 FBFB
2/4 FBFW Phenotypes:
1/4 FWFW 1/4 Black
2/4 Black & White
1/4 White