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As students learn about the various possible combinations of robot alleles and how they combine, they will observe, e.g., that when fire and ice alleles are paired, neither is dominant. Instead, the resulting water bot is a combination of the two traits. When this pattern occurs in nature, such as with blood groups, it is called incomplete dominance, a complex pattern of genetic inheritance.
A teacher explains to the class that there are many traits that are inherited in ways more complex than a simple dominant/recessive relationship. She explains that one such example is human blood types, where there are more than two possible alleles —A, B, and O—and, while some combinations display a simple dominant/recessive relationship (AO and BO display only the dominant allele, A and B, respectively), in one pair, AB, both alleles are expressed: This is called incomplete dominance. The teacher puts up the Robopedia entries and asks the students to identify which alleles have a simple relationship and which have a complex relationship.
Slides 11 and 12 describe a case where the simple dominant/recessive relationship does not apply. The slides describe the case of co-dominance using human blood types as an example. There are three alleles possible for human blood types: A, B, and O. Alleles A and B are dominant over O, but are co-dominant, meaning that when an individual has both the A and B allele, BOTH are expressed at the same time.
What the Textbooks Say
A section of text usually explains the exceptions to the Mendelian model.
- Some traits result from various combinations of dominant-recessive alleles (“multiple alleles”), e.g., human blood type (A, AB, B, or O), which is determined by combinations of three different alleles.
- Some traits result from a combination of many genes, some with incomplete dominance (“polygenetic inheritance”), e.g., eye color.
- When crossbred, some plants express intermediate traits, rather than dominant traits (“incomplete dominance”); e.g., when a red flowering plant is bred with a white flowering plant, and the offspring has pink flowers.
A common misconception is that single genes are the cause of most traits and inherited diseases.