Genetics revision notes

Basic Principles of Monohybrid Inheritance

Monohybrid inheritance focuses on the transmission of a single characteristic controlled by a single gene. Each gene has different forms called alleles. Individuals inherit two alleles for each gene, one from each parent. These alleles can be dominant (expressed even with one copy) or recessive (only expressed with two copies). The combination of alleles an individual possesses is their genotype (e.g., AA, Aa, aa), and the observable characteristic is their phenotype (e.g., tall, short).

Using Genetic Diagrams and Punnett Squares

Genetic diagrams start by identifying parental genotypes and the gametes each parent can produce via meiosis. These gametes carry one allele for the trait. Punnett squares are grids that show all possible combinations of these parental gametes, allowing for the prediction of offspring genotypes and their corresponding phenotypes, along with their probabilities. For example, crossing two heterozygous parents (Aa x Aa) yields offspring genotypes in a 1 (AA) : 2 (Aa) : 1 (aa) ratio, and phenotypes in a 3 (dominant) : 1 (recessive) ratio.

Understanding Family Pedigrees

Family pedigrees are visual charts depicting the inheritance of a specific trait across several generations of a family. Standard symbols are used: squares for males, circles for females, and shaded shapes indicate individuals expressing the trait. Lines connect parents to offspring. Pedigrees help identify patterns such as dominant or recessive inheritance, and whether a trait is autosomal or sex-linked. This makes them valuable for predicting inheritance risk in future generations and for genetic counselling.

Polygenic Traits vs. Single Gene Traits

Most phenotypic features, such as height and skin colour, are determined by the combined action of many different genes. This is known as polygenic inheritance. In contrast, some traits are determined by a single gene, like certain genetic diseases.

Role of Environment in Phenotype

While genes play a crucial role, environmental factors can also influence the expression of polygenic traits. Diet and lifestyle, for instance, can affect an individual's height even with a specific genetic predisposition.

The Genome: An Organism's Complete DNA Set

Your genome, and indeed every living organism's genome, is the full set of all the genetic information (DNA) that an organism possesses. It contains all the instructions needed to build and maintain that organism, from its earliest developmental stages through to adulthood. For humans, the genome is organised into 23 pairs of chromosomes found within the nucleus of most cells, along with a small amount of mitochondrial DNA.

What is a Gene?

A gene is a specific, functional segment of a larger DNA molecule. Its primary role is to provide the 'recipe' or code for synthesising a particular protein. These proteins are diverse in their functions, acting as enzymes, structural components, transporters, or signalling molecules, and are fundamental to cell and organism function. The sequence of nucleotide bases within a gene determines the sequence of amino acids in the protein it codes for.

Relationship between Genome, Genes, and DNA

Think of DNA as a very long instruction manual. The genome is the entire manual itself. Within that manual, a gene is like a specific chapter or page that contains instructions for making one particular component (a protein). Therefore, the genome is composed of all the DNA, and genes are specific working units or sections located along that DNA.