Inheritance

The One Thing

Genes are sections of DNA located on chromosomes, and alleles are separated during meiosis and recombined at fertilisation. Monohybrid crosses use this inheritance pattern to predict possible genotypes, phenotypes and their probabilities for one characteristic.

Reactions, Processes and Experiments

Key Terms

• DNA: A molecule that carries genetic information and has two strands made from repeating units called nucleotides.
• Nucleotide: A unit of DNA made of a sugar, a phosphate group and one nitrogen-containing base.
• Base: One of four chemicals in DNA: adenine, thymine, cytosine and guanine. Adenine pairs with thymine, while cytosine pairs with guanine.
• Gene: A section of DNA that codes for a particular protein or influences a specific characteristic.
• Chromosome: A long, coiled DNA molecule associated with proteins and found in the nucleus; it contains many genes.
• Genome: The complete genetic material of an organism.
• Allele: An alternative form of a gene, such as an allele for tall plants or an allele for short plants.
• Homologous chromosomes: A matching pair of chromosomes carrying the same genes at the same positions, with one chromosome inherited from each parent.
• Diploid: A cell containing two sets of chromosomes, one set from each parent.
• Haploid: A cell containing one set of chromosomes, as in gametes.
• Gamete: A reproductive cell, such as a sperm or egg cell, containing one allele of each gene.
• Genotype: The allele combination possessed by an organism, such as TT, Tt or tt.
• Phenotype: The observable features of an organism, determined by its genotype and sometimes influenced by the environment.
• Dominant allele: An allele expressed in the phenotype when at least one copy is present.
• Recessive allele: An allele expressed in the phenotype only when two copies are present, if the dominant allele is also present.
• Homozygous: Having two identical alleles for a gene, such as TT or tt.
• Heterozygous: Having two different alleles for a gene, such as Tt.
• Locus: The position of a gene on a chromosome.
• Mutation: A change in the DNA base sequence that can create a new allele.
• Mitosis: Cell division producing two genetically identical diploid cells for growth, repair and replacement.
• Meiosis: Cell division producing genetically different haploid gametes, with chromosome number reduced by half.
• Fertilisation: The fusion of male and female gamete nuclei to form a diploid zygote.
• Monohybrid cross: A genetic cross that follows the inheritance of one pair of contrasting characteristics.
• Punnett square: A diagram used to show possible combinations of alleles from two parents.
• Codominance: A situation in which both alleles are expressed in a heterozygote, producing a phenotype showing both effects.

Easily Confused

• Gene and allele: A gene is a section of DNA controlling or influencing a characteristic; an allele is an alternative form of that gene.
• Genotype and phenotype: Genotype is the allele combination; phenotype is the observable characteristic produced by genotype and sometimes the environment.
• Dominant and recessive: A dominant allele is expressed when at least one copy is present; a recessive allele is expressed only when two copies are present, if the dominant allele is also present.
• Homozygous and heterozygous: Homozygous means having two identical alleles, such as TT or tt; heterozygous means having two different alleles, such as Tt.
• Diploid and haploid: Diploid cells contain two sets of chromosomes; haploid cells contain one set, as in gametes.
• Mitosis and meiosis: Mitosis produces two genetically identical diploid cells; meiosis produces genetically different haploid gametes.
• Dominant and common: Dominance describes expression in the phenotype, not whether an allele is more common.
• Dominant and beneficial: Dominance describes expression in the phenotype, not whether an allele is more beneficial.
• Expected ratio and actual ratio: A Punnett square predicts probabilities and expected ratios, whereas observed ratios may differ because fertilisation is random and sample sizes may be limited.
• Recessive phenotype and dominant phenotype: A recessive phenotype indicates a homozygous recessive genotype, whereas a dominant phenotype may result from either a homozygous dominant or heterozygous genotype.
• Genotype and appearance: Genotype provides genetic potential, but environmental factors such as nutrition, temperature and light can also affect phenotype.
• Codominance and complete dominance: In codominance, both alleles are expressed in a heterozygote; in complete dominance, the dominant phenotype is expressed in a heterozygote.

What Gets Asked

• Define and relate DNA, genes and chromosomes. Marks are lost by treating a gene as a whole chromosome rather than as a section of DNA, or by omitting that chromosomes are found in the nucleus and contain many genes.
• Explain how meiosis and fertilisation produce inheritance. The essential points are that each gamete receives one allele of each gene and that fertilisation restores the diploid chromosome number with one allele from each parent.
• Complete a monohybrid cross such as Tt × Tt. The required genotypes are TT, Tt, Tt and tt, giving the expected genotype ratio 1 TT : 2 Tt : 1 tt and, under complete dominance, the phenotype ratio 3 dominant : 1 recessive.
• Complete a cross such as Tt × tt. The expected genotype ratio is 1 Tt : 1 tt and the expected phenotype ratio is 1 dominant : 1 recessive under complete dominance; confusing this with Tt × Tt costs marks.
• Infer genotype from phenotype. A recessive phenotype must be homozygous recessive, but a dominant phenotype may be homozygous dominant or heterozygous.
• Calculate probabilities and percentages. Use the number of favourable outcomes divided by the total number of equally likely outcomes, then calculate expected percentage as probability × 100; do not present a predicted ratio as a guarantee for a small family or group.