IBDP Biology (SL) Revision Notes Topic D.3.2: Inheritance

Term	Definition
Gene	A heritable factor that consists of a length of DNA and influences a specific characteristic.
Allele	One specific form of a gene, differing from other alleles by one or a few bases.
Locus	The specific position of a gene on a chromosome.
Genotype	The allele combination for a specific trait (e.g., TT, Tt, tt).
Phenotype	The observable characteristics of an organism resulting from its genotype.
Dominant Allele	An allele that is expressed when present (capital letter, e.g., “T”).
Recessive Allele	An allele that is only expressed when two copies are present (lowercase letter, e.g., “t”).
Homozygous	Having two identical alleles of a gene (e.g., TT or tt).
Heterozygous	Having two different alleles of a gene (e.g., Tt).
Carrier	An individual with one copy of a recessive allele that causes disease in the homozygous condition.
Variation	Differences in the phenotype of individuals in a population. It can be genetic, environmental, or both.
Discrete Variation	Traits that fall into distinct categories (e.g., blood group: A, B, AB, O).
Continuous Variation	Traits that show a range (e.g., height, skin color).
Polygenic Inheritance	A single characteristic controlled by multiple genes.
Box and Whisker Plot	A graphical summary of data showing the median, quartiles, and extremes.

Mendelian Genetics

Law of Segregation: Each gamete receives only one allele of each gene.
Law of Independent Assortment: Alleles of different genes are inherited independently (only
applies if genes are on different chromosomes).

Monohybrid Cross Example