Genetics study guide

Genetics involves a specific vocabulary to describe how traits are inherited. At the core are chromosomes, structures found in the nucleus of most living cells, carrying genetic information in the form of DNA. Segments of these chromosomes are called genes, which code for specific characteristics. Variations of a gene are known as alleles. An allele can be dominant, meaning it expresses its trait even if only one copy is present, or recessive, meaning it only expresses its trait if two copies are present. An individual's genetic make-up for a particular trait is its genotype, while the observable expression of that trait is its phenotype. If an individual has two identical alleles for a trait, they are homozygous; if they have two different alleles, they are heterozygous. Gametes are reproductive cells (sperm or egg) that carry one set of chromosomes, and when two gametes fuse, they form a zygote, the first cell of a new organism.

Genetic inheritance can be predicted and analysed using monohybrid crosses and pedigree charts. A monohybrid cross examines the inheritance pattern of a single gene with two alleles. This involves using Punnett squares to determine the genotypes and phenotypes of offspring from specific parental crosses, expressing outcomes as probabilities, ratios, or percentages. Dominant traits are expressed even if only one copy of the dominant allele is present, while recessive traits are only expressed when two copies of the recessive allele are inherited. Pedigree analysis extends this by charting the inheritance of a trait across multiple generations within a family, using standardised symbols to represent individuals, their sex, and whether they express the trait. This allows for the inference of genotypes, prediction of offspring outcomes, and identification of the mode of inheritance (e.g., dominant or recessive).