Download Section 6.5: Traits and Probabilities

Section 6.5
Traits and Probability
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Objectives
• SWBAT describe what a monohybrid and
dihybrid crosses are.
• SWBAT explain how heredity can be
illustrated mathematically
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Vocabulary
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Punnett square
Monohybrid cross
Dihybrid cross
Testcross
Law of independent assortment
Probability
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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.
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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
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Punnett Squares
Heterozygous X
heterozygous cross
results in 1:2:1 genotypic
ratio.
Phenotypic ratio is 3:1
dominant to recessive
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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
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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.
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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.
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Probability
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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
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Many genes have more than two alleles.
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Polygenic Traits
Polygenic traits are the
result of two or more
genes interacting.
They are expressed as
gradations.
Human height is a
polygenic trait.
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Polygenic Traits
Skin color is a
polygenic
trait.
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