B5 Genes, inheritance and selection — OCR Gateway Biology (J257/02)

DNA structure and the genetic code

DNA (deoxyribonucleic acid) is a double-stranded helix made of nucleotides. Each nucleotide has three parts:

1. Deoxyribose sugar.
2. Phosphate group.
3. Nitrogenous base (A, T, C, or G).

Base pairing rules (complementary):

• Adenine A pairs with Thymine (T).
• Cytosine C pairs with Guanine (G).

The two strands are held together by hydrogen bonds between complementary base pairs.

DNA → Protein (gene expression):

1. Transcription (nucleus): one strand of DNA serves as template; RNA polymerase builds a complementary mRNA strand (using RNA bases: A, U, C, G — uracil replaces thymine).
2. Translation (ribosome): mRNA read in triplets (codons); each codon specifies an amino acid; tRNA brings amino acids; polypeptide chain assembled.

Gene: a sequence of DNA bases that codes for a specific protein. Genome: the entire genetic information of an organism. Allele: an alternative version of a gene (different base sequence).

Sexual vs asexual reproduction

Meiosis (reduction division) produces 4 haploid (n) gametes:

• Chromosome number halved: diploid (2n = 46) → haploid (n = 23).
• Genetic variation created by: crossing over (prophase I) and independent assortment of chromosomes (metaphase I).

Fertilisation: gametes fuse → zygote (2n) restored.

Mendelian genetics

Key definitions:

• Genotype: the allele combination an organism has (e.g. Bb).
• Phenotype: the observable characteristic.
• Dominant: allele that is expressed in both homozygous BB and heterozygous (Bb) forms. Written as a capital letter.
• Recessive: allele only expressed when homozygous (bb). Written as a lower-case letter.
• Homozygous: two identical alleles (BB or bb).
• Heterozygous: two different alleles (Bb).

Monohybrid crosses (Punnett squares):

Example: Cystic fibrosis (autosomal recessive). Let F = normal allele, f = CF allele.

Parents: Ff × Ff (both carriers)

    F        f

+--------+--------+ F | FF | Ff | +--------+--------+ f | Ff | ff | +--------+--------+ Ratio: 1 FF : 2 Ff : 1 ff = 3 normal phenotype : 1 CF phenotype. Probability of CF child = 1/4 = 25%.

Sex determination:

• Human sex determined by X and Y chromosomes.
• Female: XX; Male: XY.
• Father determines sex of child (Y chromosome from sperm).

Sex-linked inheritance:

• Genes on X chromosome (but not Y) show sex-linked inheritance.
• Example: colour blindness (recessive, X-linked). Carrier female: X^B X^b.
• Males are more often affected (only one X — one defective copy is enough).

Codominance:

• Both alleles expressed equally in heterozygote. Use I^A, I^B, I^O notation for blood groups.
• ABO blood groups: I^A and I^B are codominant; I^O is recessive.
• Blood type AB genotype: I^A I^B.

Variation

Continuous variation: characteristic measured on a scale; influenced by many genes (polygenic) + environment. Example: height, weight. Shows normal distribution.

Discontinuous variation: distinct categories; controlled by one or few genes, little environmental influence. Example: blood group, tongue rolling.

Sources of genetic variation:

• Mutation (random change in DNA base sequence).
• Meiosis: crossing over + independent assortment.
• Fertilisation: random fusion of gametes.

Mutation types:

• Substitution: one base replaced by another → one amino acid may change (or silent if same amino acid encoded).
• Insertion/deletion: one or more bases added/removed → frameshift → all downstream amino acids changed → usually non-functional protein.

Evolution by natural selection

Darwin's theory of natural selection (four conditions):

1. Variation: individuals in a population show variation in heritable traits.
2. Overproduction: more offspring produced than the environment can support.
3. Competition/struggle for survival: individuals compete for food, mates, habitat.
4. Survival of the fittest: individuals with advantageous adaptations are more likely to survive and reproduce.
5. Inherited: survivors pass advantageous alleles to offspring → allele frequency increases over generations.

Example: antibiotic resistance in bacteria:

• Random mutations produce bacteria resistant to an antibiotic.
• When antibiotic is applied, non-resistant bacteria die.
• Resistant bacteria survive, reproduce → resistant allele frequency increases.
• Natural selection acts rapidly in bacteria (short generation time).

Speciation: when populations of the same species become reproductively isolated (e.g. by a geographical barrier) → each accumulates different mutations by natural selection → gene pools diverge → eventually cannot interbreed → new species.

Common OCR examiner traps

1. Natural selection does NOT cause mutations — mutations are random. Natural selection acts on existing variation.
2. Phenotype ratios are probabilities — 3:1 is an expected ratio, not guaranteed for a family of 4.
3. Carrier = heterozygous for a recessive condition — phenotypically unaffected.
4. Meiosis produces 4 cells; mitosis produces 2 — and meiosis halves chromosome number.
5. Codominance ≠ incomplete dominance (blending) — codominance means both alleles are fully expressed (e.g. AB blood group), not a blend.