Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Explain codominance and incomplete dominance.
• Differentiate between multiple alleles and polygenic inheritance.
• Explain sex-linked inheritance and carrier status.
• Construct and interpret pedigree diagrams.
• Explain mutations and their effects.

Lesson summary

This week, we delve deeper into the fascinating world of genetics and inheritance. Understanding how traits are passed from parents to offspring is not just an academic exercise; it has profound implications for our understanding of human health, agricultural practices, and biodiversity. For example, in South Africa, understanding the genetics of disease susceptibility can help communities make informed decisions about healthcare and preventative measures.

Furthermore, knowledge of inheritance is crucial for improving crop yields to address food security challenges in our country. We will explore more complex inheritance patterns and apply our knowledge to solve practical problems.

Lesson notes

2.1 Codominance In codominance, both alleles for a trait are expressed equally in the phenotype of the heterozygote. Neither allele is dominant or recessive. This means that the heterozygote will show both traits simultaneously.

Example: A classic example is the ABO blood group system. While blood type A and B are dominant to O, A and B are codominant to each other. An individual with the genotype I A I B will have blood type AB. They will express both A antigens and B antigens on their red blood cells. 2.2 Incomplete Dominance In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous phenotypes. Neither allele is fully dominant, so the heterozygote shows a blended phenotype.

Example: Flower color in snapdragons. If a red-flowered plant (RR) is crossed with a white-flowered plant (WW), the offspring (RW) will have pink flowers. The pink color is a blend of the red and white traits. 2.3 Multiple Alleles Multiple alleles occur when a gene has more than two possible alleles in a population. While an individual can only have two alleles for a gene, the population can have many. The ABO blood group system, mentioned earlier, is also an example of multiple alleles because there are three alleles: I A , I B , and i.

Explanation: The I A allele codes for the A antigen, the I B allele codes for the B antigen, and the i allele codes for no antigen (O blood type).

The dominance relationships are: I A and I B are dominant over i, and I A and I B are codominant to each other.