Understanding Reproduction and inheritance: Edexcel IGCSE Biology Explained

Reproduction and inheritance form one of the most reliably examined sections of the Edexcel IGCSE Biology specification. From flower structure to Punnett squares, from DNA to Darwin, this is where biology connects the molecular to the organismal.

Section 3 of the 4BI1 specification covers two broad topics: reproduction (both sexual and asexual, in plants and animals) and inheritance (DNA structure, protein synthesis, genetic crosses, cell division and evolution). As edexcel igcse biology reproduction and inheritance content, these edexcel igcse biology revision notes take each topic in turn, following the specification closely and highlighting the areas where candidates most frequently drop marks.

Sexual and asexual reproduction

Sexual reproduction involves the fusion of two gametes (sex cells), one from each parent, to produce a zygote. The zygote then undergoes cell division and develops into an embryo. Because two parents contribute genetic material, offspring show genetic variation.

Asexual reproduction involves a single parent and produces offspring that are genetically identical to the parent (clones). There is no fusion of gametes and no genetic variation (unless mutation occurs). Bacteria reproduce asexually by binary fission. Some plants reproduce asexually through runners (e.g. strawberry plants) or through artificial methods such as cuttings.

Reproduction in plants

Flower structure and pollination

The specification requires you to describe insect-pollinated and wind-pollinated flowers and explain their adaptations. Insect-pollinated flowers tend to have large, brightly coloured petals, nectar and a scent to attract insects. Their pollen grains are sticky or spiky to attach to insect bodies. Wind-pollinated flowers tend to have small, dull petals (or none), no nectar or scent, large feathery stigmas to catch pollen from the air, and lightweight, smooth pollen produced in large quantities.

Pollination is the transfer of pollen from an anther to a stigma. After pollination, the pollen grain grows a pollen tube down through the style to the ovule. The male gamete travels down this tube and fuses with the female gamete in the ovule (fertilisation). The fertilised ovule develops into a seed, and the ovary develops into a fruit.

Seed germination

Seeds need water, oxygen and a suitable temperature to germinate. During germination, the seed uses its stored food reserves (starch, converted to glucose by enzymes) to fuel growth until the seedling develops leaves and can carry out photosynthesis.

Reproduction in humans

The male reproductive system produces sperm in the testes. Sperm travel through the sperm duct and are released during ejaculation. The female reproductive system produces eggs (ova) in the ovaries. Each month, an egg is released from an ovary (ovulation) and travels down the oviduct (fallopian tube) towards the uterus.

The menstrual cycle

The menstrual cycle is controlled by four hormones:

1. FSH (follicle-stimulating hormone): Stimulates the development of a follicle in the ovary and the production of oestrogen.
2. Oestrogen: Causes the uterus lining to thicken and triggers the release of LH.
3. LH (luteinising hormone): Triggers ovulation (release of the egg).
4. Progesterone: Maintains the thickened uterus lining. If no fertilisation occurs, progesterone levels drop, the lining breaks down, and menstruation begins.

Pregnancy

The placenta provides the developing embryo with oxygen and nutrients (glucose, amino acids) from the mother's blood and removes waste products (carbon dioxide, urea). The mother's blood and the baby's blood do not mix directly; substances are exchanged across the thin placental membrane by diffusion. The amniotic fluid surrounding the embryo acts as a shock absorber, protecting it from physical damage.

Secondary sexual characteristics

Testosterone (produced by the testes) triggers the development of male secondary sexual characteristics: deepening of the voice, growth of facial and body hair, muscle development. Oestrogen (produced by the ovaries) triggers female secondary sexual characteristics: breast development, widening of the hips, onset of menstruation.

Inheritance

DNA and genes

The genome is the entire DNA of an organism. A gene is a section of DNA that codes for a specific protein. Genes are located on chromosomes in the nucleus of a cell.

DNA is a double helix: two strands wound around each other, linked by paired bases. The base-pairing rules are adenine (A) with thymine (T), and cytosine (C) with guanine (G). RNA is single-stranded and contains uracil (U) instead of thymine.

Protein synthesis

The specification for edexcel igcse biology requires knowledge of transcription and translation:

1. Transcription: In the nucleus, the DNA double helix unwinds. One strand is used as a template to build a complementary mRNA molecule. The mRNA carries the genetic code out of the nucleus to the ribosomes.
2. Translation: At the ribosome, the mRNA code is read in groups of three bases (codons). Each codon specifies a particular amino acid. tRNA molecules carry amino acids to the ribosome, matching their anticodons to the mRNA codons. Amino acids are linked together in the correct sequence to form a protein.

Alleles, genotype and phenotype

Genes exist in alternative forms called alleles. A dominant allele is expressed in the phenotype when one or two copies are present. A recessive allele is only expressed when two copies are present (homozygous recessive). An organism is homozygous if it has two identical alleles for a gene (e.g. BB or bb) and heterozygous if it has two different alleles (e.g. Bb). The genotype is the combination of alleles an organism has; the phenotype is the observable characteristic produced.

Codominance occurs when both alleles are expressed in the heterozygous phenotype. Neither is dominant over the other. Most phenotypic features in real organisms result from polygenic inheritance (the combined effect of many genes) rather than single-gene inheritance.

Monohybrid crosses

The exam commonly tests your ability to complete and interpret genetic diagrams. Here is a worked example.

Sex determination

In humans, sex is determined by one pair of sex chromosomes. Females have XX and males have XY. The father's sperm determines the sex of the child: if an X-carrying sperm fertilises the egg, the child is female (XX); if a Y-carrying sperm fertilises the egg, the child is male (XY). The probability of each sex is 50%.

Family pedigrees

The specification expects you to interpret family pedigree diagrams, tracking the inheritance of a characteristic through generations. The key skill is working out whether an individual is homozygous or heterozygous based on the phenotypes of their parents and offspring.

Cell division

Mitosis

Mitosis produces two genetically identical daughter cells from one parent cell. Each daughter cell has the same number of chromosomes as the parent cell (diploid). Mitosis occurs during growth, repair of damaged tissues, asexual reproduction and cloning.

Meiosis

Meiosis produces four genetically different daughter cells, each with half the number of chromosomes of the parent cell (haploid). Meiosis is how gametes (sex cells) are formed. In humans, the diploid number is 46 and the haploid number is 23. Random fertilisation, where any sperm can fuse with any egg, adds further genetic variation to the offspring.

Variation and mutation

Variation within a species can be genetic (inherited from parents), environmental (caused by conditions such as diet, climate or injury) or a combination of both. Height in humans, for example, is influenced by genes but also by nutrition during childhood.

A mutation is a rare, random change in the genetic material that can be inherited. Most mutations have no effect on the phenotype. Some have a small effect, and rarely a mutation has a significant effect. A change in the DNA sequence can alter the amino acid sequence of a protein, which may change its shape and function. The incidence of mutations can be increased by exposure to ionising radiation (such as UV light or X-rays) and certain chemical mutagens.

Natural selection and evolution

Darwin's theory of evolution by natural selection can be summarised in these steps:

1. Within a population, individuals show genetic variation.
2. Some individuals have characteristics that make them better adapted to their environment.
3. These individuals are more likely to survive and reproduce ("survival of the fittest").
4. They pass on the alleles for the advantageous characteristics to their offspring.
5. Over many generations, the frequency of the advantageous alleles increases in the population.

A directly relevant application is antibiotic resistance in bacteria. When a population of bacteria is exposed to an antibiotic, most are killed, but any individuals that carry a mutation conferring resistance survive. These resistant bacteria reproduce, passing the resistance allele to their offspring. Over time, the proportion of resistant bacteria increases, making infections harder to control. This is why antibiotics should be used responsibly and courses should always be completed.

Self-check practice questions

Use these edexcel igcse biology practice questions to test your understanding of the reproduction and inheritance edexcel igcse content before moving on.

1. State two differences between sexual and asexual reproduction.
2. Name two adaptations of an insect-pollinated flower and explain how each aids pollination.
3. Describe the role of the placenta during pregnancy.
4. A gene has two alleles: T (tall, dominant) and t (short, recessive). Draw a Punnett square for a cross between a heterozygous tall plant (Tt) and a homozygous short plant (tt). State the expected phenotype ratio.
5. Explain the difference between mitosis and meiosis in terms of the number of daughter cells produced and their chromosome number.
6. Describe how a mutation in DNA could change the phenotype of an organism.
7. Explain how antibiotic resistance increases in a bacterial population through natural selection.

The igcse 4bi1 reproduction and inheritance section bridges the gap between molecular biology and whole-organism biology. Mastering genetic crosses, cell division and natural selection gives you the tools to tackle some of the most predictable questions on both Paper 1 (2 hours, 110 marks) and Paper 2 (1 hour 15 minutes, 70 marks). These edexcel igcse biology notes and edexcel igcse biology explained content should give you a solid foundation, but there is no substitute for working through past paper questions under timed conditions to build speed and confidence.