Genetics revision notes

What are Alleles?

Alleles are different versions of the same gene. For example, a gene for eye colour might have an allele for blue eyes and an allele for brown eyes. Individuals inherit two alleles for most genes, one from each biological parent.

Sources of Allelic Variation

The primary sources of variation in inherited characteristics are the different combinations of alleles passed down from parents during sexual reproduction. This random assortment of alleles during gamete formation and their subsequent fusion in fertilisation ensures offspring are genetically distinct from their siblings and parents. Mutations can also create new alleles, further contributing to variation.

Homozygous vs. Heterozygous

An individual is homozygous for a gene if they inherit two identical alleles (e.g., two alleles for brown eyes). They are heterozygous if they inherit two different alleles (e.g., one for brown eyes and one for blue eyes). The interaction between these alleles (dominance, recessiveness) determines the expressed trait.

Transcription: Making mRNA from DNA

Transcription starts when RNA polymerase attaches to a non-coding DNA region in front of a gene. It moves along the template strand of the gene, building a complementary mRNA strand. This mRNA is a copy of the gene’s information, with uracil (U) instead of thymine (T), and it leaves the nucleus for the next stage.

Translation: Building the Protein at the Ribosome

After leaving the nucleus, the mRNA binds to a ribosome in the cytoplasm. The ribosome reads the mRNA codons, and tRNA molecules, each with a specific anticodon and attached amino acid, bind to matching codons. The ribosome joins the amino acids together, creating a polypeptide chain that folds and may combine with other chains to form a finished protein.

Purpose of Meiotic Division

Meiosis is a specific type of cell division necessary for sexual reproduction. Its primary role is to produce gametes (sex cells like sperm and eggs) that have half the number of chromosomes of the parent cell, ensuring that when two gametes fuse during fertilisation, the resulting zygote has the correct diploid chromosome number.

Outcomes of Meiosis

Meiotic division results in four daughter cells from one parent cell. Each daughter cell is haploid, meaning it contains only one set of chromosomes. These daughter cells are also genetically distinct from each other and the original parent cell, which generates genetic variation within a species.

Haploid Gametes

The four cells produced by meiosis are called gametes. They are haploid, meaning they contain half the normal number of chromosomes found in somatic (body) cells. This reduction is essential to maintain the correct chromosome count across generations after fertilisation.

What is a Genetic Mutation?

A genetic mutation is a random change in the sequence of DNA. Mutations can happen spontaneously and can also occur more often when exposed to mutagens, which are factors like certain chemicals or radiation that increase the chance of mutation.

Effects of Mutations on Phenotype

Most mutations have no effect on an organism's phenotype. This is often because the mutation happens in a region of DNA that does not influence protein production, or the change does not alter the amino acid sequence. Some mutations have a small effect, while it is rare for a single mutation to lead to a significant observable change.