3b Inheritance¶

Part of 3 Reproduction and Inheritance.

DNA stores the instructions for making proteins, and those instructions pass from parents to offspring in gametes. The proteins an organism makes, together with its environment, decide its phenotype. This topic follows that chain from DNA to genetic crosses, then through cell division, variation, mutation and natural selection.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

How DNA codes for proteins (with extra detail for Biology-only students), how alleles are inherited and how to predict the results of a cross, how mitosis and meiosis differ, where variation comes from, and how mutation and natural selection produce evolution.

Biology-Only Content: DNA, Genes and Protein Synthesis¶

This content is required for Biology-only students and is not required for Combined Science students.

DNA structure:

• DNA (deoxyribonucleic acid) is a double helix — two strands wound around each other.
• It is a polymer made of repeating units called nucleotides. Each nucleotide contains a sugar molecule, a phosphate group and one of four organic bases: adenine (A), thymine (T), cytosine (C) or guanine (G).
• The two strands are joined by complementary base pairing: A pairs with T; C pairs with G.
• A sequence of three bases (a triplet) codes for one amino acid.

Use the interactive below to move from one nucleotide to a DNA strand and then to the full double helix. It keeps the repeating sugar-phosphate backbone and complementary base pairing in one view. Open full interactive.

From gene to protein — protein synthesis:

Transcription is the nucleus stage of protein synthesis. DNA unwinds and unzips at the gene, one strand acts as the template, and free mRNA nucleotides pair with the exposed bases to make an mRNA copy. Use the interactive below to focus on that copying stage and to notice that RNA uses uracil (U) instead of thymine. Open full interactive.

After the mRNA leaves the nucleus, it attaches to a ribosome in the cytoplasm. The ribosome reads the bases in groups of three (codons), and tRNA molecules with complementary anticodons bring the correct amino acids. The amino acids are joined in the correct order to make the protein.

The order of bases in DNA therefore determines the order of amino acids in a protein, which in turn determines the protein's shape and function.

DNA, Genes and Protein Synthesis¶

The genome is the entire DNA of an organism. Chromosomes are structures in the nucleus made of long DNA molecules. Genes are sections of a chromosome that code for a specific protein. Humans have 23 pairs of chromosomes — 22 pairs control characteristics, and the 23rd pair determines sex.

Alleles and Inheritance Patterns¶

An allele is an alternative form of a gene. A gene is the section of DNA carrying the instruction for a protein, while an allele is one version of that instruction. Most body cells are diploid, containing two alleles for each gene, one inherited from each parent.

Important distinctions:

• Dominant allele: when the allele is present in the genotype, it is expressed in the phenotype. One dominant allele in a heterozygous genotype is enough. Represented by a capital letter (e.g. A).
• Recessive allele: only expressed when both alleles are recessive, so one recessive allele in a heterozygous genotype is not enough. Represented by a lowercase letter (e.g. a).
• Homozygous: both alleles are the same (AA or aa).
• Heterozygous: two different alleles are present (Aa).
• Genotype: the combination of alleles an individual has (e.g. Aa).
• Phenotype: the observable characteristic produced (e.g. brown eyes).
• Codominance: when neither allele is dominant over the other, so both contribute to the phenotype (e.g. a speckled chicken from crossing a white and a black chicken).

Monohybrid crosses use Punnett square diagrams to predict the probability of offspring having particular genotypes and phenotypes. Dominant alleles are represented by capital letters; recessive by lowercase. Most phenotypic features result from multiple genes interacting rather than a single gene.

Sex determination: Humans have 23 pairs of chromosomes. The 23rd pair consists of the sex chromosomes, X and Y. Females have two X chromosomes (XX); males have one X and one Y (XY). During meiosis, eggs always contain an X chromosome, while sperm contain either X or Y. The sex of offspring depends on which sperm fertilises the egg, giving a 50% chance of each sex.

Mitosis, Meiosis and Variation¶

Mitosis produces two genetically identical daughter cells, each with the same diploid (46) chromosome number as the parent. It occurs during growth, repair, cloning and asexual reproduction.

Stages of the cell cycle leading to mitosis:

1. Interphase: the cell grows, organelles replicate, proteins are synthesised and all chromosomes are copied (producing the characteristic X shape).
2. Chromosomes line up at the cell's equator and spindle fibres pull each chromatid to opposite poles.
3. The cytoplasm and cell membrane divide, producing two identical daughter cells.

Meiosis produces four non-identical haploid cells (gametes), each with 23 chromosomes. It occurs in the reproductive organs.

1. The cell copies all its chromosomes.
2. The cell divides once to produce two cells, each with the normal 46 chromosomes (diploid).
3. Each cell divides again to produce four cells, each with 23 chromosomes (haploid).

Chromosomes are shuffled randomly during meiosis, so all four gametes are genetically different from one another and from the parent cell. Random fertilisation (the fusion of any egg with any sperm) further increases genetic variation in offspring.

Sources of variation:

• Genetic variation: differences in DNA sequences between individuals. Examples include eye colour and blood group, which are determined by inherited alleles.
• Environmental variation: differences caused by factors such as diet, climate or lifestyle. For example, a plant that could grow tall genetically may remain stunted if it lacks nutrients.
• Both: many characteristics — including height — are influenced by a combination of genetics and environment.

Mutation and Natural Selection¶

A mutation is a rare, random change in DNA sequence that can be inherited. All alleles originally arose through mutation. Most mutations occur in non-coding DNA and have no effect on phenotype; some affect phenotype slightly; and a very few (occurring in coding regions) can have a major effect.

Mutations can occur more frequently when organisms are exposed to:

• Ionising radiation (e.g. gamma rays, X-rays, ultraviolet light).
• Chemical mutagens (e.g. chemicals in tobacco smoke).

Some mutations cause uncontrolled cell division, resulting in tumours. Mutagens that cause cancer are called carcinogens.

Evolution and natural selection:

• Evolution is the gradual change in the inherited characteristics of a population over many generations.
• Natural selection is the mechanism by which evolution occurs: a process by which advantageous alleles become more frequent in a population over time.

For unfamiliar examples, follow this chain:

1. Variation already exists within a population because individuals carry different alleles. Random mutations can create new alleles.
2. A selection pressure such as predation, disease, competition or climate change makes some phenotypes more successful than others.
3. Individuals with an advantageous phenotype are more likely to survive and reproduce.
4. They pass the advantageous alleles to their offspring.
5. Repeated over many generations, those alleles become more common in the population. That population-level change is evolution.

A classic example is the peppered moth. At an unpolluted Cambridgeshire site, the frequency of the dark melanic form fell from 0.12 in 2001 to 0.01 in 2007, consistent with selection against that form when it was less well camouflaged.

Use the interactive below to see this as a population-level pattern. In this model the pale green dots are food. Individuals that gather enough food are more likely to survive and reproduce, so the key thing to watch is the change in allele frequency across the whole population, not the fate of one organism. Open full interactive.

Antibiotic resistance in bacteria: Bacteria reproduce very rapidly by binary fission, so advantageous mutations can spread quickly. When bacteria are exposed to antibiotics, susceptible bacteria die while any that carry a resistance allele survive and reproduce. Over many generations, the resistance allele becomes more common in the population. This is an example of natural selection. This is also why antibiotics should be taken for the full prescribed course: stopping early may leave the most resistant bacteria alive, allowing them to multiply and making the infection harder to control. MRSA ('superbug') is resistant to many antibiotics and spreads in hospitals when healthcare workers move between patients.

Speciation occurs when a population of a species becomes so genetically different from others that they can no longer interbreed to produce fertile offspring.

Common Confusions¶

• Gene vs allele: A gene is the DNA instruction for a protein. An allele is one version of that gene. Natural selection changes allele frequencies in populations.
• Evolution vs natural selection: Evolution is the long-term change in inherited characteristics. Natural selection is the mechanism by which evolution occurs.
• Mitosis vs meiosis: Mitosis makes two identical diploid cells. Meiosis makes four non-identical haploid gametes.
• Dominant vs common: A dominant allele is expressed over a recessive one, but it does not necessarily become more common in a population over time.
• Probability in crosses: The Punnett square gives the chance for each offspring. It does not guarantee the ratio in a small family.

Check Yourself¶

1. State the difference between a gene and an allele.
2. State what is meant by a dominant allele and a recessive allele.
3. Two heterozygous brown-eyed parents (Bb) have a child. Brown (B) is dominant to blue (b). Calculate the probability that the child has blue eyes.
4. Describe two differences between mitosis and meiosis.
5. A student writes: "The moths changed their colour to match the trees." Explain why this is wrong and rewrite it accurately using natural selection.
6. Explain why a patient should finish a full course of antibiotics, even if they feel better.

Key Terms¶

• Genome: the entire DNA of an organism.
• Gene: a section of DNA that codes for the production of a specific protein.
• Allele: an alternative form of a gene.
• Genotype: the combination of alleles an organism possesses.
• Phenotype: the observable characteristics of an organism.
• Codominance: when two different alleles are both expressed in the phenotype.
• Homozygous: having two identical alleles of a gene.
• Heterozygous: having two different alleles of a gene.
• Dominant: describing an allele that is expressed when it is present in the genotype.
• Recessive: describing an allele expressed only when both alleles are recessive.
• Diploid: having two sets of chromosomes.
• Haploid: having one set of chromosomes.
• Mitosis: cell division producing two genetically identical diploid cells.
• Meiosis: cell division producing four genetically non-identical haploid cells.
• Mutation: a random change in DNA sequence that may be inherited.
• Natural selection: the process by which advantageous alleles become more frequent over time.
• Evolution: the gradual change in the inherited characteristics of a population over generations.
• Speciation: the formation of a new species when populations become so different they can no longer interbreed to produce fertile offspring.
• Carcinogen: a substance or agent that causes mutations leading to cancer.
• Transcription: the production of an mRNA molecule from a DNA template.
• Translation: the assembly of a protein at a ribosome using the sequence of codons on mRNA.

Connected Pages¶

• 2b Cell Structure
• 3a Reproduction
• 5b Selective Breeding
• DNA Structure
• Transcription
• 5c Genetic Modification
• IGCSE Biology Key Terms Glossary