Principles of Inheritance and Variation

The One Thing

Inheritance is the transmission of distinct genetic factors, or alleles, from parents to offspring through meiosis and fertilisation. Predictable inheritance patterns arise from allele segregation, independent assortment, dominance, linkage, recombination, mutation, and the organisation of genes on chromosomes.

Reactions, Processes and Experiments

Key Terms

• Heredity: The transmission of genetic characteristics from parents to offspring.
• Variation: Differences in traits among individuals of the same species.
• Gene: A functional unit of heredity located at a specific position on a chromosome.
• Allele: An alternative form of a gene, such as T or t for plant height.
• Genotype: The genetic constitution of an organism, such as TT, Tt, or tt.
• Phenotype: The observable characteristics of an organism, influenced by genotype and environment.
• Homozygous: Having two identical alleles for a gene, such as TT or tt.
• Heterozygous: Having two different alleles for a gene, such as Tt.
• Dominant allele: An allele expressed in the phenotype even when present with a contrasting allele.
• Recessive allele: An allele expressed only when present in the homozygous condition.
• Mendel’s law of dominance: In a heterozygote, one allele may express itself and mask the effect of the other allele.
• Law of segregation: The two alleles of a gene separate during gamete formation, so each gamete receives only one allele.
• Law of independent assortment: Alleles of different genes assort independently during gamete formation when the genes are unlinked or located far apart.
• Monohybrid cross: A genetic cross involving one pair of contrasting traits.
• Dihybrid cross: A genetic cross involving two pairs of contrasting traits.
• Test cross: A cross between an individual showing a dominant phenotype and a homozygous recessive individual to determine the unknown genotype.
• Back cross: A cross between an offspring and either of its parents or a genetically similar individual.
• Incomplete dominance: A condition in which neither allele is completely dominant, producing an intermediate phenotype in the heterozygote.
• Codominance: A condition in which both alleles express themselves fully in a heterozygote, as in the AB blood group.
• Multiple alleles: The presence of more than two alternative alleles for a gene in a population, although an individual carries only two.
• Pleiotropy: The ability of one gene to influence more than one characteristic.
• Chromosomal theory of inheritance: The theory that genes are located on chromosomes and that chromosome behaviour during meiosis explains Mendelian inheritance.
• Linkage: The tendency of genes located close together on the same chromosome to be inherited together.
• Recombination: The formation of new combinations of alleles, commonly through crossing over during meiosis.
• Crossing over: Exchange of genetic material between homologous chromosomes during pachytene of prophase I of meiosis.
• Sex determination: The biological mechanism by which an organism develops as male, female, or another sex form.
• XX-XY system: A sex-determination system in which females are XX and males are XY; humans follow this system.
• Homogametic sex: The sex that produces only one type of sex-chromosome-bearing gamete, such as XX females producing only X-bearing eggs.
• Heterogametic sex: The sex that produces two types of sex-chromosome-bearing gametes, such as XY males producing X-bearing and Y-bearing sperm.
• Mutation: A sudden, stable, and heritable change in genetic material.
• Point mutation: A change involving a single base pair in DNA, such as the substitution responsible for sickle-cell anaemia.
• Frameshift mutation: A mutation caused by insertion or deletion of bases that changes the reading frame of a gene.
• Chromosomal mutation: A change in chromosome structure or number, including deletion, duplication, inversion, translocation, aneuploidy, and polyploidy.
• Pedigree analysis: The study of a family diagram to determine how a trait is inherited across generations.
• Autosomal inheritance: Inheritance of a trait controlled by a gene located on an autosome rather than a sex chromosome.
• Sex-linked inheritance: Inheritance of a trait controlled by a gene located on a sex chromosome, commonly the X chromosome.
• Carrier: A person who carries a recessive allele but usually does not show the condition, often represented as a heterozygote.

Easily Confused

• Genotype and phenotype: Genotype is the genetic constitution, whereas phenotype is the observable characteristic produced by genotype and environmental influence.
• Homozygous and heterozygous: Homozygous individuals have identical alleles, whereas heterozygous individuals have different alleles.
• Dominant and recessive: A dominant allele can be expressed in a heterozygote; a recessive allele is expressed only in the homozygous condition.
• Dominance and frequency: Dominance describes allele expression and does not mean that the allele is more common, stronger, or necessarily better.
• Incomplete dominance and codominance: Incomplete dominance produces an intermediate heterozygous phenotype, whereas codominance produces simultaneous full expression of both alleles.
• Monohybrid and dihybrid crosses: A monohybrid cross follows one pair of contrasting traits, whereas a dihybrid cross follows two pairs.
• Test cross and back cross: A test cross specifically uses a homozygous recessive individual to determine an unknown genotype; a back cross uses either parent or a genetically similar individual.
• Linkage and independent assortment: Linked genes tend to be inherited together; unlinked genes, or genes far apart, assort independently.
• Linkage and recombination: Linkage preserves parental combinations, whereas crossing over produces recombinant combinations.
• Homogametic and heterogametic sex: The homogametic sex produces one type of sex-chromosome-bearing gamete; the heterogametic sex produces two.
• Point mutation and frameshift mutation: A point mutation changes a single base pair, whereas a frameshift mutation results from insertion or deletion that alters the reading frame.
• Autosomal and sex-linked inheritance: Autosomal traits are controlled by genes on autosomes; sex-linked traits are controlled by genes on sex chromosomes.
• Carrier and affected individual: A carrier usually possesses a recessive allele without showing the condition, whereas an affected individual expresses the condition.

What Gets Asked

• Explain Mendel’s pea plant experiments and state why pea plants were suitable; marks are lost if the clear contrasting traits, short generation time, true-breeding varieties, or controlled self- and cross-pollination are omitted.
• Complete a monohybrid cross such as TT x tt or Tt x Tt; marks depend on distinguishing the F1 genotype Tt from the F2 ratios 1 TT : 2 Tt : 1 tt and 3 dominant : 1 recessive.
• Complete a dihybrid cross and identify the 9 : 3 : 3 : 1 phenotypic ratio; this ratio applies to independently assorting genes, not linked genes.
• Apply probability rules; use the product rule for two independent events that must both occur and the sum rule for mutually exclusive alternatives.
• Interpret ABO or Rh blood-group inheritance; remember that IA and IB are codominant, i is recessive, and Rh-positive is usually dominant over Rh-negative.
• Identify sex-determination and pedigree patterns; distinguish human XX-XY inheritance, bird ZW-ZZ inheritance, grasshopper XO-XX inheritance, honeybee haplodiploidy, and the absence of father-to-son transmission in X-linked traits.