Inheritance, at a glance

A gene is a sequence of DNA; an allele is an alternative form of that gene. Homologous chromosomes carry the same genes at corresponding positions.

• Gene = a DNA sequence associated with an inherited function.
• Alleles = alternative forms of the same gene.
• A diploid individual normally has two alleles of an autosomal gene, one on each homologous chromosome.

Keep in mind: A chromosome carries many genes. An allele is a version of one gene at a particular position.

Continuous variation has a range of intermediate values; discontinuous variation has distinct categories with no intermediate category between them.

• Continuous: a range with intermediate values, such as height.
• Discontinuous: distinct categories, such as ABO groups.
• Graph categories chosen by a researcher are not always the natural form of a trait.

Keep in mind: The graph choice alone cannot establish the type. Identify whether the underlying characteristic has a continuous range or distinct categories.

Genotype describes the alleles present; phenotype describes the expressed characteristic. Under complete dominance, a heterozygote shows the dominant phenotype.

• RR and rr are homozygous; Rr is heterozygous.
• Genotype is the allele combination; phenotype is the expressed characteristic.
• Dominance describes expression, not frequency or fitness.

Keep in mind: It remains in the genotype and can be passed on. Complete dominance affects the phenotype expressed, not the presence of the allele.

Crossing two heterozygotes gives an expected 1:2:1 genotype ratio and, under complete dominance, a 3:1 phenotype ratio.

• F1: first offspring generation. F2: offspring produced from F1 parents.
• Gametes carry one allele of the gene being followed.
• Rr x Rr gives 1:2:1 genotypes and 3:1 phenotypes under complete dominance.

Keep in mind: The genotype ratio is 1 RR : 2 Rr : 1 rr. The 3:1 ratio groups both RR and Rr into the dominant phenotype.

Rr x rr gives an expected 1 Rr : 1 rr ratio, so the dominant and recessive phenotypes each have probability 1/2.

• Rr x rr: Rr and rr each have probability 1/2.
• A recessive offspring receives r from both parents.
• A small all-dominant sample does not prove a parent is homozygous.

Keep in mind: Both RR and Rr parents can produce a dominant offspring in a test cross. More offspring and their full pattern provide stronger evidence.

Random gamete combinations can make an observed ratio differ from its expected probability. Small samples show larger proportional fluctuations.

• A genetic ratio does not specify birth order.
• Each fertilisation is a separate chance event in the model.
• Expected number = probability x sample size; observed number is counted.

Keep in mind: The model gives the same probability at each fertilisation. Previous outcomes do not force the next outcome.

In the usual XX/XY model, eggs carry X while sperm carry X or Y. Fertilisation gives an expected 1:1 ratio of XX to XY.

• Egg: X. Sperm: X or Y in the usual model.
• X + X = XX; X + Y = XY.
• Expected proportions are probabilities, not a fixed family sequence.

Keep in mind: In the usual model eggs carry X. Sperm carry X or Y, and earlier outcomes do not force the next one.

Mutation changes genetic information. It may alter a gene sequence or chromosome number; ionising radiation and chemical mutagens can increase its rate.

• Gene mutation: altered DNA sequence.
• Chromosome-number mutation: altered chromosome count.
• Ionising radiation and chemical mutagens can increase the mutation rate.

Keep in mind: Mutations do not arise because an organism chooses a helpful change. Existing heritable variation can later be affected by natural selection.