Download Probability Notes

Probability
and
Inheritance
Probability
►The
likelihood that a particular
event will occur.
Percentages
 One way you can express a probability is as a
percentage.
 Suppose that 3 out of 5 tossed coins landed
with heads up. Here’s how you can calculate
what percent of the coins landed with heads
up:
1. Write the comparison as a fraction.
Specific events vs possible events
3 out of 5 = 3/5
2. Multiply the fraction by 100% to express
it as a percentage.
3/5 x 100%/1 = 60%
How is probability associated with
genetics?
► Much
like the flipping of a quarter,
the likelihood of alleles is
completely random.
What is a PUNNETT SQUARE?
►A
tool to predict the probability of certain
traits in offspring that shows the different
ways alleles can combine
► A way to show phenotype (the trait) &
genotype (the alleles)
► A chart that shows all the possible
combinations of alleles that can result
when genes are crossed
Genotype and Phenotype
► Genotype
refers to genetic makeup, or what
alleles the organism has
► Phenotype refers to the physical appearance of
an organism
 Organisms with the same phenotype may not
have the same genotype
Dominant and Recessive
Traits
► When
making Punnett squares, capital
letters are used to represent dominant
traits
 P= Purple Flower
 always shows up when present
► Lower case letters are used to represent
recessive traits
 p= white flower
 hidden when dominant allele is
present
Homozygous & Heterozygous
► Organisms
that have two identical
alleles for a particular trait (TT or tt) are
said to be homozygous
 True-breeding
► Organisms
that have two different
alleles for the same trait Tt are
heterozygous
 Hybrid
A Punnett Square
In this cross, both parents are heterozygous for
the trait of seed shape. R represents the
dominant round allele, and r represents the
recessive wrinkled allele.
Punnett Square #1:
breeding 2 homozygous animals
B
Brown= BB
b
Bb
B
Bb
100% Bb =
hybrid brown
What is the probability?
Daddy
white= bb
b
Mommy
Genotype
Bb
Bb
Phenotype
100% All brown
offspring
Punnett Square #2:
breeding 1 homozygous animal and 1
heterozygous animal
B
Brown= Bb
b
Bb
b
bb
Genotype
50% Bb =
hybrid brown
50% bb= white
Daddy
white= bb
b
Mommy
Bb
bb
Phenotype
50% brown
offspring
What is the probability? 50% white
offspring
Punnett Square #3:
breeding 2 heterozygous animals
B
Brown= Bb
b
25% BB= brown
B
BB
Bb
Daddy
50% Bb =
hybrid brown
25% bb= white
Brown= Bb
b
Mommy
Genotype
Bb
bb
Phenotype
75% brown
offspring
What is the probability? 25% white
offspring
Codominance
In codominance, the alleles are neither
dominant nor recessive. As a result, both alleles
are expressed in the offspring.
Incomplete Dominance
When one allele is NOT completely dominant
over another (they blend)
► Example: In carnations the color red (R) is
incompletely dominant over white (W). The
hybrid color is pink.
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