Pearson Science Double Award Biology 3: Reproduction and Inheritance

Reproduction passes biological information to a new generation. Sexual reproduction creates variation through meiosis and random fertilisation, inheritance connects alleles to phenotype, and natural selection changes populations across generations.

These notes cover the Double Award points in Pearson 4SD0 Issue 4 and keep separate-Biology additions outside this qualification.

Sexual and asexual reproduction

Sexual reproduction involves fusion of male and female gametes at fertilisation, producing a zygote that divides and develops into an embryo. Asexual reproduction uses one parent and no gamete fusion, producing genetically identical offspring unless mutation occurs. It can be rapid but supplies less inherited variation when conditions change.

In flowering plants, insect-pollinated flowers commonly have conspicuous petals, scent or nectar and sticky pollen. Wind-pollinated flowers expose anthers and stigmas and produce abundant light pollen. Pollination transfers pollen to a stigma. A pollen tube grows to the ovule, fertilisation occurs, and seeds and fruit develop. Germinating seeds use stored food until leaves photosynthesise. Runners and artificial cuttings allow asexual reproduction.

Human reproductive organs produce, transport and receive gametes and support development. Oestrogen and progesterone coordinate the menstrual cycle. The placenta permits exchange between maternal and fetal blood supplies without normally mixing them directly, while amniotic fluid protects the embryo. Oestrogen and testosterone contribute to secondary sexual characteristics.

Genes, chromosomes and cell division

The genome is an organism's entire DNA. A gene is a section of DNA coding for a specific protein. Genes occupy positions on chromosomes in the nucleus, and alternative versions are alleles.

A genotype is the allele combination; a phenotype is the observable feature produced by genotype and environment. A dominant allele affects the phenotype in a heterozygote, while a recessive phenotype normally requires two recessive alleles. Dominant does not mean common, stronger or beneficial. Many characteristics are polygenic and do not follow a one-gene pattern.

Mitosis produces two genetically identical diploid cells for growth, repair, cloning and asexual reproduction. Meiosis produces four genetically different haploid gametes. Humans have 46 chromosomes in diploid body cells and 23 in haploid gametes. Random fertilisation combines gametes and increases variation.

Genetic diagrams and pedigrees

Define allele symbols and parental genotypes before forming gametes. Combine gametes in a Punnett grid, then state genotype and phenotype probabilities. Probabilities apply to each independent conception; four expected categories do not guarantee one of each in four children.

Pedigrees record phenotype across a family. Use the key, generations and relationships. Affected children of two unaffected parents can support recessive inheritance, but test proposed genotypes against the whole pedigree.