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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.
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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 generations are 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
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through several generations in a family
 = Male, O = Female
 connects parents
 connects parents to children
children are placed from left to right in birth order
Homozygous trait shaded in
Heterozygous trait 1/2 shaded
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Are there exceptions to the simple Mendelian
pattern of inheritance?
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How do heredity and the environment
interact to influence phenotype?
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How do linked genes affect chromosome
assortment and crossover during meiosis?
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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:
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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
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 R- gene for red flowers, W- gene for white flowers:
 RR- red, WW- white, RW- pink
RR = Red
WW = White
All RW = Pink
Codominance
 a condition in which both alleles for the same gene
are fully expressed.
 Example: The genetics of human blood groups
 Another example- Chicken feathers
 B- gene for black feathers, W- gene for white feathers:
 BB- black, WW- white,
BW- “erminette” Black and White!
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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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The Mendelian inheritance pattern is rare in nature;
other patterns include polygenic inheritance,
incomplete dominance, multiple alleles, and
codominance.
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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.