Section 6.5
Traits and Probability
Objectives
• SWBAT describe what a monohybrid and
dihybrid crosses are.
• SWBAT explain how heredity can be
illustrated mathematically
Vocabulary
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Punnett square
Monohybrid cross
Dihybrid cross
Testcross
Law of independent assortment
Probability
Punnett Squares
• 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
of each parent.
– The boxes show the
possible genotypes
of the offspring.
• The Punnett square yields the
ratio of possible genotypes and
phenotypes.
Punnett Squares
A monohybrid cross involves one trait.
• Monohybrid crosses
examine the inheritance
of only one specific
trait.
– homozygous dominanthomozygous recessive:
all heterozygous, all
dominant
Punnett Squares
Heterozygous X
heterozygous cross
results in 1:2:1 genotypic
ratio.
Phenotypic ratio is 3:1
dominant to recessive
Punnett Squares
• Heterozygous X
homozygous recessive
results in a genotypic
ratio of 1:1
• Heterozygous X
homozygous recessive
results in a phenotypic
ratio of 1:1 dominant to
recessive
Dihybrid Cross: 2 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.
Heredity patterns calculated with probability.
• Probability =
number of ways a specific event can occur
number of total possible outcomes
• Probability is the likelihood
that something will happen.
• Probability predicts an
average number of
occurrences, not an exact
number of occurrences.
• Probability applies to
random events such as
meiosis and fertilization.
Probability
Phenotype can depend on interactions of alleles.
• In incomplete dominance, neither allele is
completely dominant nor completely recessive.
– Heterozygous phenotype is intermediate between
the two homozygous phenotypes
– Homozygous parental phenotypes not seen in F1
offspring
Many genes have more than two alleles.
Polygenic Traits
Polygenic traits are the
result of two or more
genes interacting.
They are expressed as
gradations.
Human height is a
polygenic trait.
Polygenic Traits
Skin color is a
polygenic
trait.