Heredity

The One Thing

Heredity is the transmission of genetic information from parents to offspring through DNA, genes and chromosomes. Variation results from differences in genetic material and environmental effects, providing the basis for adaptation and evolution.

Reactions, Processes and Experiments

Key Terms

• Heredity: The passing of genetic characteristics from parents to their offspring.
• Variation: Differences in traits among individuals of the same species or between parents and offspring.
• DNA: Deoxyribonucleic acid, the molecule that stores hereditary information in cells.
• Gene: A specific segment of DNA that carries information for a particular characteristic or protein.
• Chromosome: A thread-like structure made of DNA and proteins that contains many genes and is found in the nucleus.
• Trait: A distinct characteristic of an organism, such as height, seed shape or eye colour.
• Genotype: The genetic makeup of an organism for one or more traits.
• Phenotype: The observable appearance or expression of a trait, influenced by genes and sometimes the environment.
• Dominant trait: A trait that appears in the phenotype when at least one copy of its allele is present.
• Recessive trait: A trait that appears only when both copies of the relevant allele are recessive.
• Allele: An alternative form of a gene controlling the same characteristic.
• Homozygous: Having two identical alleles for a particular trait, such as TT or tt.
• Heterozygous: Having two different alleles for a particular trait, such as Tt.
• Mendel: Gregor Johann Mendel, who used pea plants to study the inheritance of traits and proposed basic laws of heredity.
• Monohybrid cross: A genetic cross that studies the inheritance of one pair of contrasting traits.
• Law of dominance: In a pair of contrasting alleles, the dominant allele expresses itself in a heterozygous condition while the recessive allele remains unexpressed.
• Law of segregation: The two alleles of a gene separate during gamete formation, so each gamete receives only one allele.
• Sex determination: The process by which the sex of an offspring is determined by the combination of sex chromosomes.
• Sex chromosomes: Chromosomes involved in determining sex; in humans, females generally have XX and males generally have XY.
• Autosomes: Chromosomes other than the sex chromosomes.
• Mutation: A sudden change in DNA that can produce a new variation; it may be useful, harmful or neutral.
• Asexual reproduction and variation: Asexual reproduction usually produces genetically similar offspring, but small DNA-copying errors can still introduce variation.

Easily Confused

• Genotype and phenotype: Genotype is the genetic makeup, whereas phenotype is the observable expression of a trait.
• Dominant and recessive traits: A dominant trait appears when at least one dominant allele is present; a recessive trait appears only when both alleles are recessive.
• Homozygous and heterozygous: Homozygous organisms have identical alleles, such as TT or tt; heterozygous organisms have different alleles, such as Tt.
• Inherited and environmental variation: Genetic variations can be inherited, whereas changes caused only by environmental conditions generally are not passed to offspring.
• Genes and chromosomes: A gene is a specific segment of DNA; a chromosome is a DNA-and-protein structure containing many genes.
• Sex chromosomes and autosomes: Sex chromosomes are involved in determining sex; autosomes are the other chromosomes.
• Sexual and asexual reproduction: Sexual reproduction combines genetic material from two parents and produces greater variation; asexual reproduction usually produces genetically similar offspring.
• Law of dominance and law of segregation: The law of dominance concerns which allele is expressed; the law of segregation concerns the separation of alleles during gamete formation.
• XX and XY sex determination: Eggs always contribute X, while sperm contribute either X or Y; therefore, the sperm determines whether the combination is XX or XY.

What Gets Asked

• Defining the relationship between DNA, genes and chromosomes: Marks are lost when genes and chromosomes are treated as identical rather than recognising that chromosomes contain many genes and genes are segments of DNA.
• Interpreting a monohybrid cross such as Tt × Tt: The required results are the genotypic ratio TT:Tt:tt = 1:2:1 and the phenotypic ratio tall:dwarf = 3:1; these ratios should not be confused.
• Applying dominance to TT, Tt and tt: TT and Tt are tall, whereas only tt is dwarf; describing Tt as dwarf ignores the law of dominance.
• Explaining sex determination in humans: Eggs carry only X, while sperm may carry X or Y; the mother therefore does not determine the sex of the child.
• Distinguishing inherited from environmental variation: Environmental changes can affect phenotype without changing inherited DNA and are generally not passed to offspring.
• Explaining the importance of variation: Inherited variations provide the raw material for natural selection, adaptation and evolution over many generations.