Human Biology 101: Human Development and Genetics Basic Principles of Inheritance (Dominant, Recessive, Incomplete Dominance, Polygenic)

Concept Summary

• Dominant alleles will always be expressed over recessive alleles in a heterozygous individual.
• Recessive alleles will only be expressed if an individual is homozygous recessive.
• Incomplete dominance occurs when a heterozygous individual expresses a combination of both parental traits.
• Polygenic traits are influenced by multiple genes, resulting in a wide range of possible phenotypes.
• The interaction between genes and environment can affect the expression of inherited traits.

Questions

WHAT (definitional)

• What is a dominant allele?
• Answer: A dominant allele is a version of a gene that will always be expressed over a recessive allele in a heterozygous individual.
• Real-world example: The gene for brown eye color is dominant over the gene for blue eye color.
• Misconception cleared: A dominant allele does not always mean the trait is more common or desirable.
• What is incomplete dominance?
• Answer: Incomplete dominance occurs when a heterozygous individual expresses a combination of both parental traits.
• Real-world example: The flower color of a heterozygous snapdragon is a combination of red and white, resulting in a pink color.
• Misconception cleared: Incomplete dominance is not the same as codominance, where both alleles have an equal effect.
• What is a polygenic trait?
• Answer: A polygenic trait is influenced by multiple genes, resulting in a wide range of possible phenotypes.
• Real-world example: Human height is a polygenic trait, influenced by multiple genes and environmental factors.
• Misconception cleared: Polygenic traits are not determined by a single gene, but rather by the interaction of multiple genes.

WHY (causal reasoning)

• Why do dominant alleles always express over recessive alleles?
• Answer: Dominant alleles have a stronger effect on the phenotype, resulting in the expression of the dominant trait.
• Real-world example: The gene for sickle cell anemia is recessive, but individuals with one copy of the gene are less likely to develop the disease.
• Misconception cleared: Dominant alleles do not always mean the trait is more common or desirable.
• Why do polygenic traits result in a wide range of possible phenotypes?
• Answer: The interaction of multiple genes and environmental factors results in a wide range of possible phenotypes.
• Real-world example: Human height is influenced by multiple genes and environmental factors, resulting in a wide range of possible heights.
• Misconception cleared: Polygenic traits are not determined by a single gene, but rather by the interaction of multiple genes.
• Why do incomplete dominance and codominance result in different phenotypes?
• Answer: Incomplete dominance occurs when one allele has a weaker effect on the phenotype, resulting in a combination of both parental traits. Codominance occurs when both alleles have an equal effect on the phenotype, resulting in the expression of both traits.
• Real-world example: The flower color of a heterozygous snapdragon is a combination of red and white, resulting in a pink color. In contrast, the shell color of a heterozygous abalone is a combination of brown and white, resulting in a mottled color.
• Misconception cleared: Incomplete dominance is not the same as codominance.

HOW (process/application)

• How do you determine the genotype of an individual with a dominant trait?
• Answer: You can determine the genotype of an individual with a dominant trait by looking at the phenotype of their parents or offspring.
• Real-world example: If a parent has a dominant trait, such as brown eye color, and their child has the same trait, it is likely that the child is homozygous dominant.
• Misconception cleared: You cannot determine the genotype of an individual with a dominant trait by looking at their phenotype alone.
• How do you determine the genotype of an individual with a polygenic trait?
• Answer: You can determine the genotype of an individual with a polygenic trait by looking at the interaction of multiple genes and environmental factors.
• Real-world example: Human height is influenced by multiple genes and environmental factors, resulting in a wide range of possible heights.
• Misconception cleared: Polygenic traits are not determined by a single gene, but rather by the interaction of multiple genes.
• How do you predict the phenotype of an individual with a heterozygous genotype?
• Answer: You can predict the phenotype of an individual with a heterozygous genotype by looking at the effect of the two alleles on the phenotype.
• Real-world example: If an individual has a heterozygous genotype for a trait, such as brown eye color, and one allele is dominant and the other is recessive, the individual will express the dominant trait.
• Misconception cleared: You cannot predict the phenotype of an individual with a heterozygous genotype by looking at their genotype alone.

CAN (possibility/conditions)

• Can a dominant allele be recessive in certain environments?
• Answer: Yes, a dominant allele can be recessive in certain environments, such as in the presence of a dominant allele from a different gene.
• Real-world example: The gene for sickle cell anemia is recessive, but individuals with one copy of the gene are less likely to develop the disease in certain environments.
• Misconception cleared: Dominant alleles do not always mean the trait is more common or desirable.
• Can a polygenic trait be influenced by a single gene?
• Answer: No, a polygenic trait is influenced by multiple genes and environmental factors.
• Real-world example: Human height is influenced by multiple genes and environmental factors, resulting in a wide range of possible heights.
• Misconception cleared: Polygenic traits are not determined by a single gene, but rather by the interaction of multiple genes.
• Can a heterozygous individual express a combination of both parental traits?
• Answer: Yes, a heterozygous individual can express a combination of both parental traits, resulting in incomplete dominance.
• Real-world example: The flower color of a heterozygous snapdragon is a combination of red and white, resulting in a pink color.
• Misconception cleared: Incomplete dominance is not the same as codominance.

TRUE/FALSE (misconception testing)

• Statement: Dominant alleles always express over recessive alleles in a heterozygous individual.
• Answer: TRUE
• Real-world example: The gene for brown eye color is dominant over the gene for blue eye color.
• Misconception cleared: Dominant alleles do not always mean the trait is more common or desirable.
• Statement: Polygenic traits are determined by a single gene.
• Answer: FALSE
• Real-world example: Human height is influenced by multiple genes and environmental factors, resulting in a wide range of possible heights.
• Misconception cleared: Polygenic traits are not determined by a single gene, but rather by the interaction of multiple genes.
• Statement: Incomplete dominance occurs when both alleles have an equal effect on the phenotype.
• Answer: FALSE
• Real-world example: The flower color of a heterozygous snapdragon is a combination of red and white, resulting in a pink color.
• Misconception cleared: Incomplete dominance is not the same as codominance.