Chapter 12 -3: Complex Genetic Traits
Bellringer
List possible eye colors in humans. What colors are explained by Mendelian genetics? Are there eye colors that are not explained by Mendelian genetics?
Key Ideas
• How can mathematical probability be used in genetics?
• Are there exceptions to the simple Mendelian pattern of inheritance?
• How do heredity and the environment interact to influence phenotype?
• How can a Punnett square be used in genetics for more complex traits?
Using Probability
• A Punnett square shows the possible outcomes of a cross, but it can also be used to calculate the probability of each outcome.
• Probability is the likelihood that a specific event will occur. Probability can be calculated and expressed in many ways.
• Probability can be expressed in words, as a decimal, as a percentage, or as a fraction.
• Probability formulas can be used to predict the probabilities that specific alleles or phenotypes will be passed on to offspring.
• The possible results of a heterozygous cross are similar to those of flipping two coins at once.
• Probability = Number of one kind of outcome
Total number of all possible outcomes
Many Genes, Many Alleles
• Most patterns of inheritance are more complex than those that Mendel identified.
• The Mendelian inheritance pattern is rare in nature; other patterns include polygenic inheritance, incomplete dominance, multiple alleles, and codominance.
• A character that is influenced or affected by more than one gene is called a polygenic character.
• Eye color, height, and skin color are examples of polygenic characters. Most characters are polygenic.
• Genes that have three or more possible alleles are said to have multiple alleles.
• Multiple alleles control the ABO blood groups (blood types) in humans.
• Codominance is a condition in which both alleles for the same gene are fully expressed.
• The genetics of human blood groups is an example of codominance.
Blood Phenotypes
Determining Blood Types by Punnett Square
• While multiple alleles control blood groups, a Punnett square can be used as a model because only two alleles are expressed in each individual.
• A and B are co-dominant and O blood group is recessive.
Incomplete Dominance
• Incomplete Dominance is a phenotype intermediate between the dominant and recessive phenotypes.
• Snapdragon flowers are a good example of incomplete dominance. The heterozygous offspring, from a Red dominant flower and a white recessive flower, is pink.
• The alleles in the Punnett square are Cw or CR
Genes Affected by the Environment
• Phenotype can be affected by conditions in the environment, such as nutrients and temperature.
• In humans, many characters that are partly determined by heredity, such as height, are also affected by the environment.
• Many aspects of human personality and behavior are strongly affected by the environment, but genes also play an important role.
Summary
• Probability formulas can be used to predict the probabilities that specific alleles will be passed on to offspring.
• 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.