Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Define and apply terms like monohybrid cross, dihybrid cross, incomplete dominance, codominance, and multiple alleles.
• Explain and solve problems related to sex-linked inheritance.
• Describe mutations and their impact.
• Interpret and construct pedigree diagrams.

Lesson summary

Genetics and inheritance are fundamental to understanding the diversity of life around us. In South Africa, understanding these principles is crucial for addressing challenges related to inherited diseases, improving agricultural practices, and conserving our unique biodiversity. From understanding the prevalence of genetic conditions in different South African populations to developing drought-resistant crops, genetics plays a vital role. This week, we will delve deeper into specific inheritance patterns and focus on applying our knowledge to real-world scenarios. We'll explore how genes are passed from parents to offspring and how these genes contribute to our individual traits.

Lesson notes

2.1 Monohybrid Crosses & Dihybrid Crosses (Revisited): Monohybrid Cross: A cross involving only one trait. Remember Mendel's work with pea plants! For example, let's consider seed colour. Suppose yellow seed colour (Y) is dominant over green seed colour (y).

Example: A homozygous yellow seed plant (YY) is crossed with a homozygous green seed plant (yy).

Parental Genotypes: YY x yy Gametes: Y and y F1 Generation Genotype: Yy (all heterozygous)

F1 Generation Phenotype: All yellow seeds.

F2 Generation (Yy x Yy): Punnett Square: | | Y | y | | :---- | :-: | :-: | | Y | YY | Yy | | y | Yy | yy | Genotypic Ratio: 1 YY : 2 Yy : 1 yy Phenotypic Ratio: 3 Yellow : 1 Green Dihybrid Cross: A cross involving two traits. For example, seed colour (Y/y) and seed shape (Round (R) dominant over wrinkled (r)).

Example: A plant homozygous for yellow, round seeds (YYRR) is crossed with a plant homozygous for green, wrinkled seeds (yyrr).

Parental Genotypes: YYRR x yyrr Gametes: YR and yr F1 Generation Genotype: YyRr (all heterozygous)

F1 Generation Phenotype: All yellow, round seeds.

F2 Generation (YyRr x YyRr): Requires a larger Punnett Square.

Gametes from each parent: YR, Yr, yR, yr F2 Generation Phenotypic Ratio: 9 Yellow, Round : 3 Yellow, Wrinkled : 3 Green, Round : 1 Green, Wrinkled 2.2 Incomplete Dominance: Neither allele is completely dominant over the other. The heterozygous phenotype is a blend of the two homozygous phenotypes.

Example: Flower colour in snapdragons. Red (R) and White (W) alleles. RR = Red flowers WW = White flowers RW = Pink flowers If a pink flower (RW) is crossed with a white flower (WW): Parental Genotypes: RW x WW Gametes: R, W and W Punnett Square: | | R | W | | :---- | :-: | :-: | | W | RW | WW | | W | RW | WW | Genotypic Ratio: 2 RW : 2 WW Phenotypic Ratio: 2 Pink : 2 White 2.3 Codominance: Both alleles are equally expressed in the heterozygote. The heterozygote displays both phenotypes.

Example: Coat colour in cattle. Red (R) and White (W) alleles. RR = Red coat WW = White coat RW = Roan coat (both red and white hairs are present) If a roan cow (RW) is crossed with a white cow (WW): Parental Genotypes: RW x WW Gametes: R, W and W Punnett Square: | | R | W | | :---- | :-: | :-: | | W | RW | WW | | W | RW | WW | Genotypic Ratio: 2 RW : 2 WW Phenotypic Ratio: 2 Roan : 2 White 2.4 Multiple Alleles: More than two alleles exist for a particular gene, although an individual can still only inherit two alleles (one from each parent).

Example: Human blood types (A, B, O). The alleles are I A , I B , and i (where 'i' represents the O allele). I A and I B are codominant, and both are dominant over i.

Genotypes and Phenotypes: I A I A or I A i = Blood type A I B I B or I B i = Blood type B I A I B = Blood type AB ii = Blood type O If a person with blood type A (I A i) has a child with a person with blood type B (I B i): Parental Genotypes: I A i x I B i Gametes: I A , i and I B , i Punnett Square: | | I A | i | | :------ | :----------: | :--: | | I B | I A I B | I B i | | i | I A i | ii | Genotypic Ratio: 1 I A I B : 1 I B i : 1 I A i : 1 ii Phenotypic Ratio: 1 AB : 1 B : 1 A : 1 O 2.5 Sex-Linked Inheritance: Genes located on the sex chromosomes (X and Y). Most sex-linked traits are X-linked because the X chromosome is much larger than the Y chromosome.

Example: Haemophilia, a bleeding disorder, is a recessive X-linked trait. Let X H represent the normal allele and X h represent the haemophilia allele.

Females: X H X H (normal), X H X h (carrier, usually normal), X h X h (haemophiliac)

Males: X H Y (normal), X h Y (haemophiliac) If a carrier female (X H X h ) has a child with a normal male (X H Y): Parental Genotypes: X H X h x X H Y Gametes: X H , X h and X H , Y Punnett Square: | | X H | Y | | :---- | :----------: | :-: | | X H | X H X H | X H Y | | X h | X H X h | X h Y | Genotypic Ratio: 1 X H X H : 1 X H X h : 1 X H Y : 1 X h Y Phenotypic Ratio: 1 Normal female : 1 Carrier female : 1 Normal male : 1 Haemophiliac male 2.6 Mutations: A change in the nucleotide sequence of DNA. Mutations can be spontaneous or induced by mutagens (e.g., radiation, chemicals). Mutations can be beneficial, harmful, or neutral.

Types of Mutations: Point Mutations: Changes in a single base pair.

Substitution: One base is replaced by another (e.g., A to G). Can be silent (no change in amino acid), missense (change in amino acid), or nonsense (change to a stop codon).

Insertion/Deletion (Frameshift): Addition or removal of one or more bases, shifting the reading frame and altering the amino acid sequence drastically.

Chromosomal Mutations: Changes in the structure or number of chromosomes.

Deletion: Loss of a part of a chromosome.

Duplication: Segment of a chromosome is repeated.

Inversion: Segment of a chromosome is reversed.

Translocation: Segment of a chromosome moves to another chromosome.

Non-disjunction: Failure of chromosomes to separate properly during meiosis, leading to aneuploidy (abnormal number of chromosomes).