# Unit Review: Heredity - Inheritance and Variation

This review covers everything we learned about genetics and heredity. Use it to prepare for your unit assessment.

## Topic 1: Heredity, Traits, and Genes

Heredity is the passing of traits from parents to offspring through genetic information.

Two Types of Traits: - Inherited traits: Determined by genes, passed from parents (eye color, blood type, tongue rolling). CAN be passed to offspring. - Acquired traits: Develop during life through environment or choices (scars, muscles from exercise, learned skills). CANNOT be passed to offspring.

The Genetic Hierarchy (smallest → largest): - Gene → segment of DNA with instructions for a trait (~20,000-25,000 in humans) - Chromosome → organized structures of DNA (46 in humans, 23 pairs, 23 from each parent) - Nucleus → cell's control center containing chromosomes - Cell → fundamental unit of life

DNA is a double-helix molecule, a twisted ladder whose rungs spell out genetic instructions.

## Topic 2: Alleles, Genotype, and Phenotype

Alleles are different versions of the same gene. You have two alleles for each gene, one from each parent.

- Dominant allele (CAPITAL letter, B): Only needs ONE copy to show the trait - Recessive allele (lowercase, b): Needs TWO copies to show the trait

Genotype = the actual alleles (BB, Bb, or bb) Phenotype = the observable trait (what you see)

| Genotype | Name | Phenotype | |----------|------|-----------| | BB | Homozygous dominant | Dominant trait shows | | Bb | Heterozygous | Dominant trait shows | | bb | Homozygous recessive | Recessive trait shows |

Homozygous = same alleles (AA or aa). Heterozygous = different alleles (Aa).

## Topic 3: Punnett Squares

A Punnett square is a grid that shows all possible offspring genotypes from two parents.

Key Crosses to Know: - BB × bb → 100% Bb → 100% dominant phenotype - Bb × Bb → 25% BB : 50% Bb : 25% bb → 75% dominant : 25% recessive - Bb × bb → 50% Bb : 50% bb → 50% dominant : 50% recessive - bb × bb → 100% bb → 100% recessive

Important: Punnett squares show probability, not certainty. Each offspring's outcome is determined independently, like flipping a coin.

## Topic 4: Reproduction and Genetic Variation

Sexual Reproduction: Two parents → unique offspring → HIGH genetic variation Asexual Reproduction: One parent → clones (identical) → LOW genetic variation

Methods of Asexual Reproduction: - Binary fission (bacteria split) - Budding (yeast) - Fragmentation (starfish) - Vegetative propagation (strawberry runners, onion bulbs)

Why Genetic Variation Matters: - Provides raw material for natural selection - Some individuals may survive diseases others can't - Allows species to adapt to change - Acts as 'insurance' against threats

The Banana Warning: All Cavendish bananas are clones, so no genetic variation means no resistance to Panama Disease (TR4).

## Topic 5: Environment, Lifestyle, and Inheritance

Three ways environment matters: 1. Changes gene expression - sunlight increases melanin, exercise activates muscle genes (DNA NOT changed) 2. Can cause mutations - UV or chemicals damage DNA - Body cell (somatic) mutation → NOT inherited - Egg or sperm (germ cell) mutation → CAN be inherited 3. Changes survival outcomes - same trait can be helpful or harmful depending on environment

Key examples: - PKU: inherited gene, but diet reduces harmful effects - Sickle cell: one allele helps in malaria regions

Remember: Most lifetime changes (muscles, scars, tans, skills) are NOT inherited.