Lesson Notes By Weeks and Term v3 - Senior Secondary 3

Lesson Video

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Performance objectives

• Identify dominant and recessive characteristics.
• Understand how dominant traits mask recessive ones in the first generation.
• Recognise chromosomes as carriers of genes.
• Relate genetics to agriculture and medicine in Nigeria.

Lesson summary

Identify the dominant/recessive Characteristics. In fer that dominantcharacters mask the contribution of the recessive characters for!hefirst generation (Fl). Recognise that the dominant charactersbecome manifest in moreindividual members of apopulation than the recessive characters. Identify chromosomes in permanently prepared slides of cells. Note that chromosomescarry genes which are responsible for in heritedcharacters: (a) Explain the advantagesand disadvantages of: - Cross fertilization and self-fertilization. - Sexual reproduction and asexual reproduction.- Out and in-breeding. (b) Relate the application of the above to practices in agriculture and medicine.

Lesson notes

Heredity (Genetics): The study of how traits are passed from parents to their offspring. These traits are controlled by genes.

Gene: A segment of DNA on a chromosome that codes for a specific trait.

Allele: Alternate forms of a gene (e.g., the gene for height can have alleles for 'tall' and 'dwarf').

Dominant Characteristic/Allele: A trait that is expressed phenotypically when present in a heterozygous condition. It masks the expression of the recessive allele. Represented by an uppercase letter (e.g., T for Tallness).

Recessive Characteristic/Allele: A trait that is only expressed phenotypically when two copies of the allele are present (homozygous recessive). Its expression is masked by a dominant allele. Represented by a lowercase letter (e.g., t for dwarfness).

Phenotype: The observable physical or biochemical characteristics of an organism, resulting from the interaction of its genotype and the environment (e.g., tall, dwarf, red flower).

Genotype: The genetic makeup of an organism, referring to the specific combination of alleles for a particular trait (e.g., TT, Tt, tt).

Homozygous: Having two identical alleles for a particular gene (e.g., TT - homozygous dominant, tt - homozygous recessive).

Heterozygous: Having two different alleles for a particular gene (e.g., Tt).

Example: In pea plants, the allele for tallness (T) is dominant over the allele for dwarfness (t). A plant with genotype TT is tall (homozygous dominant). A plant with genotype Tt is also tall (heterozygous), because T masks t. A plant with genotype tt is dwarf (homozygous recessive). This concept is explained by Gregor Mendel's Law of Dominance. When two pure-breeding (homozygous) parents with contrasting traits are crossed, the offspring of the first filial (F1) generation will all express the dominant trait. The recessive trait remains hidden or 'masked'.

Worked Example (Monohybrid Cross): Consider a cross between a pure-breeding tall pea plant (genotype TT) and a pure-breeding dwarf pea plant (genotype tt).

Step 1: Parental Genotypes (P generation)

Pure Tall Parent: TT Pure Dwarf Parent: tt Step 2: Gametes produced by each parent TT parent produces only 'T' gametes. tt parent produces only 't' gametes.

Step 3: Fertilization to produce F1 generation Using a Punnett Square: | Gametes | T | T | | :------ | :- | :- | | t | Tt | Tt | | t | Tt | Tt | All F1 offspring have the genotype Tt. Since 'T' (tall) is dominant over 't' (dwarf), all F1 offspring will exhibit the tall phenotype. The dwarf characteristic (recessive) is masked in the F1 generation. Dominant characters tend to appear in more individual members of a population than recessive characters for several reasons: Expression in Heterozygotes: A dominant allele only needs one copy to be expressed (e.g., Tt genotype results in a tall phenotype). A recessive allele requires two copies (tt) for expression.

Increased Frequency: Even if the recessive allele is present in the gene pool, it will only manifest as a phenotype if an individual inherits two copies. Carriers (heterozygotes) will express the dominant trait.

Natural Selection (sometimes): In some cases, the dominant trait may confer a survival advantage, leading to its higher frequency over generations.

Example: In humans, having unattached earlobes is generally dominant over attached earlobes.

Therefore, in a random population, a higher proportion of individuals will likely have unattached earlobes because both homozygous dominant (EE) and heterozygous (Ee) individuals will show this trait, while only homozygous recessive (ee) individuals will have attached earlobes.

Chromosomes: Thread-like structures located inside the nucleus of eukaryotic cells. They are made of DNA tightly coiled around proteins called histones. Chromosomes carry genetic information in the form of genes.

Identification: In permanently prepared slides, chromosomes appear as distinct, rod-shaped or X-shaped bodies, especially visible during cell division (mitosis or meiosis) when they are condensed.

Function: Chromosomes are the physical carriers of genes. Each species has a characteristic number of chromosomes (e.g., humans have 46 chromosomes in somatic cells, 23 in gametes).

Homologous Chromosomes: Pairs of chromosomes (one inherited from each parent) that are similar in size, shape, and gene sequence.

Genes: Specific segments of DNA located on chromosomes. They contain the instructions for building and maintaining an organism.

Role in Inheritance: Genes determine inherited characteristics (e.g., eye colour, blood group, height, susceptibility to certain diseases like sickle cell anaemia). Variations in genes (alleles) lead to variations in these characteristics.

Real-life applications

Sickle Cell Anaemia Management and Genetic Counselling (Nigeria): The understanding of dominant and recessive genes, particularly heterozygous carriers, is critical for explaining sickle cell trait (AS) and disease (SS). Genetic counselling services in Nigeria use this knowledge to advise potential couples, especially those who are both AS, about the 25% risk of having a child with sickle cell anaemia. This helps families make informed decisions and manage the disease effectively. Improving Agricultural Yields and Resilience (Nigeria): Principles of cross-breeding and out-breeding are extensively applied in Nigerian agriculture. For instance, research institutes like IITA (International Institute of Tropical Agriculture) develop improved varieties of cassava, maize, and yams by crossing different strains to combine traits like high yield, pest resistance (e.g., cassava mosaic disease resistance), and drought tolerance. Asexual reproduction is crucial for propagating established, superior varieties of root crops and plantains, ensuring uniformity and rapid multiplication. Livestock Breeding for Economic Benefit (Nigeria): Genetic principles are used to improve local livestock breeds (e.g., cattle, goats, chickens). Out-breeding local breeds with exotic ones can enhance traits like meat/milk production, faster growth rates, and disease resistance, contributing to food security and economic empowerment of Nigerian farmers. For example, crossing local chickens with commercial layers/broilers to get hybrids with improved egg or meat production.

Teacher activity

• Discusses with students Mendel'sexperiment with redand white floweredpeas.
• Uses a projectorto show a slide of chromosomes.Charts can also beused to showchromosomes.
• Shows studentsthe local variety of cassava (a biennialcrop) and the improved cassava(an annual crop).
• As ks students to do some cross AA x SS (sickle cell gene)

Evaluation guide

• This section outlines strategies for assessing student understanding based on the provided evaluation guide.
• Observation: Observe students' participation in discussions, their ability to work through guided practice problems, and their engagement during microscope observation (if applicable).
• Question and Answer: Ask targeted questions throughout the lesson to check for understanding of key terms (dominant, recessive, gene, allele, chromosome), and initial comprehension of the crosses and strategies.
• Mini-Quizzes/Exit Tickets: Short questions at the end of a segment, e.g., "Define genotype and phenotype," or "What is the genotype of a heterozygous dominant individual?"
• Use the questions from the "Independent Practice" section, which directly address the evaluation guide. A possible marking scheme for selected questions:

Reference guide

• Pictures of birds, animals suchas peacock,Agama Lizard.
• Local variety of cassava (abiennial).2. Local variety of cassava (abiennial).
• Improvedvariety of cassava(an annual).
• Charts to showcross betweendifferentgenotypes e.g. AAx aa.
• Preparedslides of root tipsof on ions, testesof grasshopper1. Charts Showing crossingof AA x SS2. Charts showingproducts of selfand crossfertilisation.