12.3: Modeling Mendel’s Laws
Probability
• A Punnett square shows the possible outcomes of a cross, but it can also be used
to calculate the probability of each outcome.
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Probability is the likelihood that a specific event will occur.
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Probability can be expressed in words, as a decimal, as a percentage, or as a
fraction.
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Probability formulas can be used to predict the probabilities that specific alleles
will be passed on to offspring.
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The possible results of a heterozygous cross are similar to those of flipping two
coins at once.
Using a Pedigree
• A pedigree is a diagram that shows how a trait is inherited over several
generations of a family.
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Pedigrees can be used to help a family understand a genetic disorder.
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A genetic disorder is a disease or disorder that can be inherited.
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A pedigree can help answer questions about three aspects of inheritance: sex
linkage, dominance, and heterozygosity.
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Human generation is longer than 20 years
– Must rely on family histories and medical records
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Pedigree
– diagram that follows the inheritance of a single gene thru several
generations in a family (Fig 7-4)
–  = male, O = Female
–  connect parents
–  connect parents to children
– children placed from left to right in birth order.
– Homo trait shaded in, Hetero trait 1/2 shaded
Section 12.4:
Beyond Mendelian Heredity
Objectives
• Are there exceptions to the simple Mendelian pattern of inheritance?
• How do heredity and the environment interact to influence phenotype?
• How do linked genes affect chromosome assortment and crossover during
meiosis?
Many Genes, Many Alleles
• Most patterns of inheritance are more complex than those that Mendel identified.
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The Mendelian inheritance pattern is rare in nature; other patterns include:
– polygenic inheritance
– incomplete dominance
– multiple alleles
– codominance.
Polygenic character
• A character that is influenced or affected by more than one gene
• Examples: Eye color, height, and skin color
• Most characters are polygenic.
Multiple alleles
• Genes that have three or more possible alleles
• Example: control the ABO blood groups (blood types) in humans.
Incomplete Dominance
• A condition where an offspring has a phenotype that is intermediate between the
two traits of its parents
• Example: Snapdragons: red and white with the intermediate trait being pink
Codominance
• a condition in which both alleles for the same gene are fully expressed.
• Example: The genetics of human blood groups
Genes Affected by the Environment
• Phenotype can be affected by conditions in the environment, such as nutrients
and temperature.
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In humans, many characters that are partly determined by heredity, such as
height, are also affected by the environment.
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Many aspects of human personality and behavior are strongly affected by the
environment, but genes also play an important role.
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Summary
The Mendelian inheritance pattern is rare in nature; other patterns include
polygenic inheritance, incomplete dominance, multiple alleles, and codominance.
•
Phenotype can be affected by conditions in the environment, such as nutrients and
temperature.
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During meiosis, genes that are close together on the same chromosome are less
likely to be separated than genes that are far apart.