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

• Apply Mendel's laws to solve genetic cross problems.
• Explain incomplete dominance, co-dominance, and multiple alleles.
• Solve problems related to sex-linked inheritance.
• Relate genetics to real-life South African scenarios.

Lesson summary

Genetics and inheritance are fundamental to understanding the diversity of life and how traits are passed down from one generation to the next. In South Africa, understanding genetics is crucial for addressing various health challenges like inherited diseases, understanding genetic diversity within our diverse populations, and improving agricultural practices. For example, understanding genetic predisposition to diseases like Type 2 Diabetes, prevalent in many South African communities, can inform preventative healthcare strategies.

Lesson notes

2.1 Mendelian Genetics: Laws of Segregation and Independent Assortment Mendel's Law of Segregation: This law states that each individual possesses two alleles for a particular trait, and these alleles separate (segregate) during gamete formation, so each gamete receives only one allele.

Think of it like this: you have two copies of the gene for eye color, one from your mother and one from your father. When you produce eggs or sperm, each of those cells only gets one of those eye color genes. This ensures that when the sperm fertilizes the egg, the offspring gets the normal complement of two alleles.

Example: Consider a flower with alleles for flower color: R (red) and r (white). A plant with genotype Rr will produce gametes containing either R or r, but not both in the same gamete.

Mendel's Law of Independent Assortment: This law states that alleles of different genes assort independently of one another during gamete formation. In simpler terms, the allele a gamete receives for one gene does not influence the allele it receives for another gene. This applies when the genes are located on different chromosomes or are far apart on the same chromosome.

Example: A plant with genotype RrYy (R = round seeds, r = wrinkled seeds; Y = yellow seeds, y = green seeds) will produce gametes in roughly equal proportions of RY, Ry, rY, and ry. Note that the alleles for seed shape (R/r) are assorting independently from the alleles for seed color (Y/y).

Monohybrid Cross: A cross involving only one trait (e.g., flower color). We use a Punnett square to predict the genotypes and phenotypes of the offspring.

Example: Crossing two heterozygous tall pea plants (Tt x Tt), where T = tall and t = short.

The Punnett square would be: ``` | T | t | --|----|----| T | TT | Tt | --|----|----| t | Tt | tt | --|----|----| ``` The genotypic ratio is 1 TT : 2 Tt : 1 tt.

The phenotypic ratio is 3 tall : 1 short.

Dihybrid Cross: A cross involving two traits (e.g., seed shape and seed color). A 4x4 Punnett square is used to predict the genotypes and phenotypes.

Example: Crossing two heterozygous plants for seed shape and color (RrYy x RrYy), where R = round, r = wrinkled, Y = yellow, y = green. The expected phenotypic ratio is 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. 2.2 Non-Mendelian Inheritance Patterns Incomplete Dominance: Neither allele is completely dominant over the other. The heterozygous phenotype is a blend of the two homozygous phenotypes.

Example: In snapdragons, a cross between a red-flowered plant (CRCR) and a white-flowered plant (CWCW) produces pink-flowered plants (CRCW). ``` | CR | CR | --|-----|-----| CW| CRCW| CRCW| --|-----|-----| CW| CRCW| CRCW| --|-----|-----| ``` The phenotypic ratio is 100% pink. If you cross two pink flowers, the ratio will be 1 red : 2 pink : 1 white.

Co-dominance: Both alleles are expressed equally in the heterozygous phenotype. Both traits appear simultaneously.

Example: In cattle, coat color can be red (RR), white (WW), or roan (RW). Roan cattle have both red and white hairs. Another important example is the ABO blood group system in humans. Individuals with IAIB genotype express both A and B antigens on their red blood cells, resulting in blood type A

B. Multiple Alleles: More than two alleles exist for a particular gene in the population.

However, each individual can still only have two alleles.

Example: The ABO blood group system in humans has three alleles: IA, IB, and i. IA and IB are co-dominant, while i is recessive. This results in four different blood types: A (IAIA or IAi), B (IBIB or IBi), AB (IAIB), and O (ii). This is significant in South Africa due to the diversity of the population and the need for compatible blood transfusions. 2.3 Sex-Linked Inheritance Sex-linked genes: Genes located on the sex chromosomes (X and Y in humans). Most sex-linked genes are found on the X chromosome because it's much larger than the Y chromosome.

X-linked recessive traits: Females have two X chromosomes, so they can be homozygous or heterozygous for X-linked traits. Males have only one X chromosome, so they are hemizygous; if they inherit the recessive allele on their X chromosome, they will express the trait.

Example: Hemophilia is an X-linked recessive disorder. Let XH represent the dominant allele for normal blood clotting and Xh represent the recessive allele for hemophilia. A carrier female (XHXh) and a normal male (XHY) can have the following offspring: ``` | XH | Y | --|-----|----| XH| XHXH| XHY| --|-----|----| Xh| XHXh| XhY| --|-----|----| ``` The probabilities are: 25% normal female (XHXH), 25% carrier female (XHXh), 25% normal male (XHY), and 25% affected male (XhY). 2.4 Pedigree Diagrams Pedigree diagrams are used to trace the inheritance of traits through generations. They use specific symbols to represent individuals and their relationships: Squares represent males, and circles represent females. Filled shapes indicate individuals with the trait.