Genetics revision notes

Genetic Sources of Variation

Genetic variation comes from differences in DNA. Mutations create new alleles, sometimes resulting in new traits. Sexual reproduction mixes genes from both parents, producing unique genetic combinations in offspring. These inherited features, like eye colour or blood type, are passed through generations.

Environmental Sources of Variation

Environmental variation results from external factors such as nutrition, temperature, disease, or experience. These acquired traits are not inherited and develop as an organism interacts with its surroundings. For example, identical twins may differ in height or weight if raised with different diets or exercise routines.

Non-coding DNA and Gene Regulation

Non-coding DNA does not code for proteins but plays vital roles in regulating gene expression. These regions often contain regulatory sequences such as promoters, enhancers, or silencers that control when and how much a gene is transcribed. Genetic variants in these regions can disrupt or modify these regulatory elements, thereby changing gene activity.

Impact on RNA Polymerase Binding

RNA polymerase is the enzyme responsible for initiating transcription by binding to specific sites, typically promoters, within the non-coding region of a gene. A genetic variant in these binding sites can either strengthen or weaken the interaction between RNA polymerase and the DNA. Stronger binding leads to more frequent and efficient transcription, while weaker binding reduces it.

Altered Protein Quantity and Phenotype

Changes in RNA polymerase binding directly influence the rate of transcription, subsequently affecting the amount of messenger RNA (mRNA) produced. More mRNA typically translates into more protein, and less mRNA into less protein. This alteration in the quantity of a specific protein can have a profound effect on the cell's function and, ultimately, on an organism's observable characteristics, its phenotype.

Genetic Variation within Populations

Within any given population of a species, there is typically a large amount of genetic variation. This means that individuals are not genetically identical; they have differences in their DNA sequences. This variation is a fundamental aspect of life and is essential for the long-term survival and evolution of species. For example, some individuals might be taller, some might have different coloured fur, or some might be more resistant to certain diseases than others within the same species group.

Origin of Genetic Variation: Mutations

The primary source of this genetic variation is mutations. A mutation is a random change in the DNA sequence. These changes can occur spontaneously during DNA replication or be caused by external factors like radiation or certain chemicals (mutagens). While many mutations have no noticeable effect or can even be harmful, some mutations can introduce new traits or alter existing ones. These new traits, if advantageous, can be passed on to offspring and increase in frequency in the population over time.

Importance of Genetic Diversity

Extensive genetic variation is vital because it provides the raw material for natural selection. When environmental conditions change, individuals with certain advantageous traits (arising from mutations) are more likely to survive and reproduce. Without a diverse gene pool, a population would have a reduced capacity to adapt to new challenges, making it more vulnerable to extinction. This variation allows species to evolve and persist through changing times.

Multiple Alleles in ABO Blood Groups

Blood group inheritance involves multiple alleles, meaning there are more than two possible alleles for a single gene in the population. Specifically, there are three alleles responsible for the ABO blood types: Iᔧ (produces A antigen), Iᔥ (produces B antigen), and i (produces no antigen).

Codominance and Dominance in ABO Blood Groups

The relationship between these alleles demonstrates both codominance and simple dominance. Alleles Iᔧ and Iᔥ are codominant, meaning that if an individual inherits both, both antigens (A and B) are expressed, resulting in blood type AB. Both Iᔧ and Iᔥ are dominant over the i allele, meaning that if i is paired with either Iᔧ or Iᔥ, only the A or B antigen will be expressed, respectively. The i allele is recessive, so individuals with genotype ii have blood type O.

Genotypes and Phenotypes of ABO Blood Groups

The possible genotypes and their corresponding phenotypes are: IᔧIᔧ or Iᔧi result in blood type A. IᔥIᔥ or Iᔥi result in blood type B. IᔧIᔥ results in blood type AB. ii results in blood type O.