This final lesson explains how traits are passed down from parents to offspring. It covers the groundbreaking work of Gregor Mendel, the basics of genetics, and the inheritance of human traits and disorders.

1. Short Notes: Core Concepts

A. Gregor Mendel & His Experiments

• Who: Gregor Mendel is called the "Father of Genetics". He studied pea plants to discover the basic rules of heredity.

• Key Findings:

  • Traits are controlled by "factors" (now called genes) that are passed from parents to offspring.

  • Each trait is controlled by a pair of genes.

  • Dominant & Recessive: One gene in a pair can mask the effect of the other. The expressed trait is dominant, and the hidden trait is recessive.

  • Mendel's Cross: When he crossed pure-tall (TT) and pure-short (tt) pea plants, all the offspring (F1 generation) were tall. When he crossed these tall offspring, the short trait reappeared in a 3:1 ratio (3 tall to 1 short) in the next generation (F2).

B. Key Genetic Terms

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

• Alleles: Different forms of the same gene (e.g., the allele for tallness 'T' and the allele for shortness 't').

• Genotype: The genetic makeup of an organism, represented by letters (e.g., TT, Tt, tt).

• Phenotype: The observable physical characteristic of an organism (e.g., tall, short).

• Homozygous: Having two identical alleles for a trait (e.g., TT or tt). Also called "pure-breeding".

• Heterozygous: Having two different alleles for a trait (e.g., Tt). Also called a "hybrid".

C. DNA, Chromosomes, and Genes

• Chromosome: A thread-like structure in the nucleus made of DNA. Humans have 46 chromosomes (23 pairs).

• DNA: The molecule that carries the genetic instructions. A gene is a specific section of this DNA.

• Homologous Chromosomes: A pair of chromosomes (one from each parent) that have genes for the same traits at the same locations.

D. Human Inheritance

• Sex Determination:

  1. Human sex is determined by the 23rd pair of chromosomes (sex chromosomes).

  2. Females have two X chromosomes (XX).

  3. Males have one X and one Y chromosome (XY).

  4. The father's sperm determines the sex of the child. If the sperm carries an X, the baby is a girl (XX). If it carries a Y, the baby is a boy (XY). The probability is always 50/50.

• Genetic Disorders:

  1. Sex-Linked Disorders: Caused by recessive genes on the X chromosome. They are more common in males because males only have one X chromosome.

     • Haemophilia: A blood-clotting disorder.

     • Red-Green Colour Blindness: Difficulty distinguishing between red and green.

     • Females can be "carriers" (e.g., XᶜXᶜ) without having the disease but can pass the gene to their children.

  2. Autosomal Disorders: Caused by genes on the other 22 pairs of chromosomes (autosomes).

     • Albinism: Lack of melanin pigment in skin, hair, and eyes. It is a recessive disorder.

     • Thalassemia: A blood disorder affecting haemoglobin production. It is a recessive disorder.

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2. Tips & Tricks for the Exam

• Punnett Squares are Your Best Friend: Use Punnett squares to predict the outcomes of genetic crosses.

  1. Write the alleles of one parent across the top.

  2. Write the alleles of the other parent down the side.

  3. Fill in the boxes to find all possible genotypes of the offspring.

• Genotype vs. Phenotype Ratio: Be careful what the question asks for. A cross between two heterozygous parents (Tt x Tt) gives a genotype ratio of 1 TT : 2 Tt : 1 tt, but a phenotype ratio of 3 tall : 1 short.

• Sex-Linked Crosses: When doing Punnett squares for sex-linked traits, always write the alleles as superscripts on the X chromosome (e.g., Xᴴ, Xʰ). Remember that the Y chromosome has no allele for these traits.

3. Important Points & Common Exam Questions

• Monohybrid Cross: "A heterozygous tall pea plant (Tt) is crossed with a short pea plant (tt). What is the probability of the offspring being short?"

  • Answer: Create a Punnett square. | | T | t | |---|---|---| | t | Tt| tt| | t | Tt| tt|

  • The genotypes are 2 Tt and 2 tt. The phenotypes are 2 tall and 2 short. The probability of being short (tt) is 2 out of 4, which is 50%.

• Sex Determination: "What is the probability that a couple's next child will be a boy?"

  • Answer: The probability is 50% for every birth, regardless of previous children.

• Sex-Linked Inheritance: "A man who is colour-blind (XᶜY) has a child with a woman who is a carrier for colour blindness (XᶜXᶜ). What is the chance they will have a colour-blind daughter?"

  • Answer: Create a Punnett square. | | Xᶜ | Y | |---|---|---| | Xᶜ | XᶜXᶜ| XᶜY| | Xᶜ | XᶜXᶜ| XᶜY|

  • The possible offspring are:

    • XᶜXᶜ (Carrier daughter)

    • XᶜXᶜ (Colour-blind daughter)

    • XᶜY (Healthy son)

    • XᶜY (Colour-blind son)

  • The chance of having a colour-blind daughter (XᶜXᶜ) is 1 out of 4, or 25%.