Genes, inheritance and selection Study Guide

Genetic variations, or 'variants', are the raw material for evolution and are crucial for the diversity of life. All variants in an organism arise from mutations, which are changes in the DNA sequence. These changes can occur spontaneously during DNA replication or be induced by environmental factors such as radiation or certain chemicals. Mutations can happen in somatic cells (body cells) or germ cells (cells involved in reproduction). Only mutations in germ cells can be passed on to offspring.

The impact of mutations on an organism's phenotype (the observable characteristics) can vary significantly. Most mutations have no discernible effect on the phenotype. This is often because regions of DNA do not code for proteins, or redundant genetic code means a change in a single base pair may not alter the amino acid sequence of a protein. If the amino acid sequence is changed, it might still not affect the protein's function if the new amino acid has similar properties or if the change occurs in a non-critical region of the protein.

Some mutations, however, do influence the phenotype. These might cause slight alterations in a characteristic, such as a subtle change in eye colour, or affect an organism's susceptibility to certain conditions. For example, a mutation might make an individual slightly more resistant or susceptible to a disease. Such mutations contribute to the range of variation seen within a population.

A very few mutations can determine the phenotype entirely, leading to significant and often visible changes. These are typically mutations that affect critical genes involved in development or fundamental biological processes. An example in humans is cystic fibrosis, caused by a mutation in a single gene that leads to the production of abnormal mucus. Similarly, certain genetic disorders like albinism, which affects pigment production, are directly determined by specific mutations. These impactful mutations often have clear and profound consequences for the organism's survival or reproduction.

When we talk about single gene crosses, we are looking at the inheritance of a characteristic determined by just one gene. Each gene has different forms called alleles, and these alleles can be dominant or recessive. A dominant allele expresses its characteristic even if only one copy is present (represented by a capital letter, e.g., 'A'), while a recessive allele only expresses its characteristic if two copies are present (represented by a lowercase letter, e.g., 'a').

Organisms inherit two alleles for each gene, one from each parent. The combination of these alleles is called the genotype (e.g., AA, Aa, aa). The observable characteristic that results from the genotype is called the phenotype (e.g., tall, short). To predict the outcomes of a cross, we use a Punnett square. This grid helps us to systematically determine all possible genotypes and phenotypes of the offspring from the genotypes of the parents.

Here’s how to construct a Punnett square:

1. Identify the genotypes of the two parents.
2. Determine all possible alleles each parent can pass on to their offspring.
3. Draw a grid. Write the alleles from one parent along the top and the alleles from the other parent down the side.
4. Fill in the squares by combining the alleles from the top and side. Each square represents a possible genotype for an offspring.
5. From the completed Punnett square, calculate the ratios of expected genotypes and phenotypes among the offspring. For example, if a cross between two heterozygous parents (Aa x Aa) results in AA, Aa, Aa, and aa, the genotypic ratio is 1:2:1 (AA:Aa:aa), and if 'A' is dominant for a trait, the phenotypic ratio would be 3:1 (dominant trait:recessive trait).