Download 6.5 Traits and Probability

6.5 Traits and Probability
KEY CONCEPT
The inheritance of traits follows the rules of
probability.
6.5 Traits and Probability
Punnett squares illustrate genetic crosses.
• The Punnett square is a grid system for predicting all
possible genotypes resulting from a cross.
– The axes represent
the possible gametes (alleles)
of each parent.
– The boxes show the
possible genotypes
of the offspring.
• The Punnett square
yields the ratio of
possible genotypes and
phenotypes.
6.5 Traits and Probability
A monohybrid cross involves one trait.
• Monohybrid crosses examine the inheritance of only one
specific trait.
– homozygous dominant-homozygous recessive: all
heterozygous, all dominant
Recall: What did
Mendel call a
homozygous plant
such as FF or ff?
6.5 Traits and Probability
– heterozygous-heterozygous—1:2:1 homozygous
dominant: heterozygous:homozygous recessive; 3:1
dominant:recessive
Apply: How did
the white flower
appear among the
offspring if
neither parent
had white
flowers?
6.5 Traits and Probability
• heterozygous-homozygous recessive—1:1
heterozygous:homozygous recessive; 1:1
dominant:recessive
• A testcross is a cross between an organism with an
unknown genotype and an organism with the recessive
phenotype.
Apply: From an FF
x Ff cross, what
percent of offspring
would have purple
flowers?
6.5 Traits and Probability
A dihybrid cross involves two traits.
• Mendel’s dihybrid crosses with heterozygous plants yielded
a 9:3:3:1 phenotypic ratio.
• Mendel’s dihybrid crosses
led to his second law,
the law of independent
assortment.
• The law of independent
assortment states that
allele pairs separate
independently of each
other during meiosis.
Explain: Would the Punnett square
represent the expected results if the
genes for the two traits were inherited
together?
6.5 Traits and Probability
Heredity patterns can be calculated with probability.
• Probability is the likelihood that something will happen.
• Probability predicts an average number of occurrences, not
an exact number of occurrences.
number of ways a specific event can occur
• Probability =
number of total possible outcomes
• Probability applies to
random events such as
meiosis and fertilization.
Apply: Explain how
Mendel’s laws relate to
probability.
6.5 Traits and Probability
Mendelian Inheritance Reflects Rules of Probability
• Rule of Multiplication: The probability that
independent events will occur simultaneously
is the product of their individual probabilities.
What is the probability that you will roll
a 6 and a 4?
1/6 x 1/6 = 1/36 chance
6.5 Traits and Probability
Mendelian Inheritance Reflects Rules of Probability
• Question: In a Mendelian cross between pea
plants that are heterozygous for flower color
(Pp), what is the probability that the offspring will
be homozygous recessive?
• Answer:
 Probability that an egg from the F1 (Pp) will
receive a p allele = ½
 Probability that a sperm from the F1 will receive
a p allele = ½
 Overall probability that 2 recessive alleles will
unite at fertilization: ½ x ½ = ¼
6.5 Traits and Probability
Mendelian Inheritance Reflects Rules of Probability
Works for Dihybrid Crosses:
• Question: For a dihybrid cross, YyRr x YyRr,
what is the probability of an F2 plant having the
genotype YYRR?
• Answer:
 Probability that an egg from a YyRr parent will
receive the Y and R alleles = ½ x ½ = ¼
 Probability that a sperm from a YyRr parent will
receive the Y and R alleles = ½ x ½ = ¼
 Overall probability of an F2 plant having the
genotype YYRR: ¼ x ¼ = 1/16
6.5 Traits and Probability
Mendelian Inheritance Reflects Rules of Probability
• Rules of Addition: The probability of an
event that can occur in two or more
independent ways is the sum of the separate
probabilities of the different ways.
What is the probability that you will roll
a 6 or a 4?
1/6 + 1/6 = 2/6 or 1/3 chance
6.5 Traits and Probability
Mendelian Inheritance Reflects Rules of Probability
• Question: In a Mendelian cross between pea
plants that are heterozygous for flower color
(Pp), what is the probability that the offspring will
being a heterozygote?
• Answer:
 There are 2 ways in which a heterozygote may
be produced: the dominant allele may be in the
egg and the recessive allele in the sperm, or the
dominant allele may be in the sperm and the
recessive allele in the egg.
6.5 Traits and Probability
Mendelian Inheritance Reflects Rules of Probability
• Probability that the dominant allele will be in the
egg with the recessive in the sperm is ½ x ½ = ¼
• Probability that the dominant allele will be in the
sperm with the recessive in the egg is ½ x ½ = ¼
• Therefore, the overall probability that a
heterozygote offspring will be produced is ¼ + ¼
=½