Biology Chapter: Variation and Heredity

Variation During Reproduction

• Reproductive processes create new individuals similar yet subtly different
• Variation occurs even in asexual reproduction
• Sexual reproduction maximizes successful variations
• Sugarcane fields show little variation; animals, including humans, show distinct variations
• Variations arise from inheritance and newly created differences during reproduction
• Asexual reproduction (e.g., bacteria) produces very similar offspring due to minor DNA copying errors
• Sexual reproduction creates greater diversity

Accumulation of Variation

• Inheritance from previous generations provides a common body design with subtle variations
• Subsequent generations inherit these differences and new variations
• This process creates more diversity over time

Heredity

• The primary outcome of reproduction is creating individuals with similar characteristics

• Heredity rules determine the inheritance of traits

• Human populations exhibit numerous variations, although basic human features are present in children

• Trait Inheritance: Children inherit traits from both parents

• Each trait has two versions (from each parent's DNA)

• Mendel's experiments show traits in offspring are not always a mixture of parental traits

Mendel's Contributions

• Mendel used contrasting pea plant characteristics (round/wrinkled seeds, tall/short plants, etc.)
• Progeny of crossed plants showed only one parental trait in the first generation
• The second generation showed a mixture of parental traits, indicating that both parental traits were retained but not always expressed
• Mendel proposed that organisms have two copies of factors (genes) that control traits, which may be identical or different
• Dominant traits (e.g., tallness) express themselves, even when paired with recessive traits (e.g., shortness)

Independent Inheritance

• Breeding plants with two different characteristics (e.g., tall/short and round/wrinkled seeds) creates new combinations in the second generation
• Tallness and seed shape traits are independently inherited
• Independent inheritance creates new variations

Trait Expression

• Genes provide instructions for making proteins in cells
• Proteins control characteristics, influencing factors like hormone production.
• A gene's effect on a trait (like plant height) depends on the efficiency of the process
• Enzyme efficiency linked to a plant hormone determines plant height

Gene and Chromosome Structure

• Parents contribute equally to offspring's DNA during sexual reproduction

• Each plant/organism must possess two sets of genes (one from each parent)

• Germ cells (egg and sperm) have only one gene set

• Genes aren't single threads of DNA; they reside on separate, independent pieces called chromosomes

• Each cell has two copies of each chromosome (one from each parent)

• Germ cells receive one chromosome from each pair, potentially of maternal or paternal origin

• Restoring the chromosome number in progeny ensures species stability

Sex Determination

• Sex determination varies; some animals rely entirely on environment (temperature) or change sex
• Human sex is largely determined by inherited genes (X and Y chromosomes)
• Women have two X chromosomes (XX), while men have one X and one Y chromosome (XY)
• Children inherit an X chromosome from their mother, and either X or Y from their father, determining the sex