Chapter 12 -4: Complex Genetic Traits

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.