OCR GCSE Biology A Gateway revision notes
Genes, inheritance and selection Revision Notes
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Genes, inheritance and selection
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Predicting Results of Single Gene Crosses Revision Notes
Understanding Alleles and Genotypes
Genes control characteristics, and different forms of a gene are called alleles. Alleles can be dominant or recessive. A dominant allele (represented by a capital letter, e.g., 'A') expresses its trait even if only one copy is present. A recessive allele (represented by a lowercase letter, e.g., 'a') only expresses its trait if two copies are present. An organism's genotype is the combination of alleles it possesses for a particular gene (e.g., AA, Aa, aa). An organism's phenotype is the observable characteristic resulting from its genotype (e.g., tall, short, red flowers).
Homozygous and Heterozygous Individuals
An individual is homozygous for a trait if they have two identical alleles for that gene (e.g., AA or aa). Homozygous dominant (AA) individuals will express the dominant trait. Homozygous recessive (aa) individuals will express the recessive trait. An individual is heterozygous for a trait if they have two different alleles for that gene (e.g., Aa). Heterozygous individuals will express the dominant trait because the dominant allele masks the presence of the recessive allele.
Using Punnett Squares to Predict Outcomes
A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. To construct one, write the alleles from one parent along the top and the alleles from the other parent down the side. Fill in the squares by combining the alleles from each row and column. Each box represents a possible genotype for an offspring. By counting the combinations, you can determine the probability or ratio of different genotypes and phenotypes among the offspring.
Interpreting Phenotypic and Genotypic Ratios
After completing a Punnett square, you can determine the ratios of different genotypes and phenotypes. For example, if a cross between two heterozygous parents (Bb x Bb) results in BB, Bb, Bb, and bb offspring, the genotypic ratio is 1 BB : 2 Bb : 1 bb. If 'B' is the dominant allele for black fur and 'b' is for white fur, then BB and Bb genotypes result in black fur, and bb results in white fur. The phenotypic ratio would then be 3 black fur : 1 white fur. These ratios help predict the characteristics of the next generation.
Understanding Mutations and Their Impact on Phenotype Revision Notes
Origin of Genetic Variation
All genetic variations, which are the differences observed within a species or population, arise from mutations. A mutation is a permanent change in the DNA sequence. These changes are crucial for evolution as they introduce new alleles into the gene pool, providing raw material for natural selection to act upon. Without mutations, there would be no new traits for organisms to adapt to changing environments.
Frequency of Phenotypic Effects
It is important to understand that not all mutations have a significant impact on an organism. The vast majority of mutations are considered silent or neutral. This means they either occur in parts of the DNA that do not code for proteins, or they result in a change to the DNA that does not alter the function of the protein produced. Therefore, these mutations have no observable effect on the phenotype.
Mutations Influencing Phenotype
Some mutations do have an influence on the phenotype. These mutations might lead to subtle changes in an organism's traits. For example, a mutation might slightly alter the efficiency of an enzyme, leading to a minor change in metabolic processes, or it could affect pigment production, resulting in a slightly different shade of colour. These influences can be beneficial, harmful, or have no discernible impact on survival.
Mutations Determining Phenotype
A very small number of mutations have a direct and major impact, determining a specific phenotype. These are often the mutations that lead to distinct genetic disorders, such as cystic fibrosis or sickle cell anaemia, where a single gene mutation has a profound and often severe effect on the individual. Conversely, some determining mutations can provide a significant advantage, such as antibiotic resistance in bacteria.
Understanding Single Gene Inheritance Revision Notes
Basic Principles of Inheritance
Inheritance refers to the passing of traits from parents to their offspring. These traits are controlled by genes, which are segments of DNA located on chromosomes. For many characteristics, an individual inherits two copies of each gene, one from each parent. These copies are called alleles. An allele is an alternative form of a gene. Understanding how these alleles combine and express themselves is key to understanding single gene inheritance.
Dominant and Recessive Alleles
Alleles can be dominant or recessive. A dominant allele always expresses its characteristic if it is present; it masks the effect of a recessive allele. A recessive allele will only express its characteristic if two copies of it are present (i.e., no dominant allele is present). We use capital letters to represent dominant alleles (e.g., 'T' for tall plants) and lowercase letters for recessive alleles (e.g., 't' for short plants). An individual with two identical alleles (e.g., TT or tt) is homozygous, while an individual with two different alleles (e.g., Tt) is heterozygous.
Genotype and Phenotype
The genotype is the genetic makeup of an organism, referring to the specific combination of alleles for a particular gene (e.g., 'BB', 'Bb', 'bb'). The phenotype is the observable characteristic or trait expressed by an organism, which results from its genotype and environmental interactions (e.g., brown eyes, blue eyes). For instance, a genotype of 'Bb' for eye colour would result in a brown eye phenotype, assuming 'B' is dominant for brown and 'b' is recessive for blue.
Punnett Squares
Punnett squares are a tool used to predict the possible genotypes and phenotypes of offspring from a genetic cross. To construct a Punnett square, the alleles from one parent are placed along the top, and the alleles from the other parent are placed along the side. The squares are then filled in with the combinations of alleles. This allows for the calculation of probabilities for specific genotypes (e.g., 25% homozygous dominant, 50% heterozygous, 25% homozygous recessive) and phenotypes in the offspring.
Understanding Evolution and Natural Selection Revision Notes
What is Evolution?
Evolution is defined as a gradual change in the inherited characteristics of a population of organisms over time. This means that the genetic traits passed from parents to offspring slowly alter within a group of organisms. These changes accumulate over many generations, leading to significant differences from their ancestors. It's important to remember that evolution acts on populations, not on individual organisms; an individual does not evolve during its lifetime.
The Role of Natural Selection
Natural selection is the main driving force behind evolution. It describes the process where individuals in a population vary in their inherited traits. Some individuals possess traits that give them an advantage in their specific environment, such as better camouflage, faster running, or more efficient foraging. These 'fitter' individuals are more likely to survive, reproduce, and pass on their beneficial characteristics to their offspring. Less well-adapted individuals are less likely to survive and reproduce.
Survival and Reproduction in Nature
Because organisms often produce more offspring than the environment can support, there is competition for resources like food, water, and shelter. In this competition, individuals with advantageous inherited characteristics are more successful. They not only survive longer but also have a higher chance of producing offspring. As these successful individuals reproduce, the frequency of their beneficial inherited traits increases in the population over successive generations, leading to evolutionary change.
Formation of New Species (Speciation)
When evolutionary changes accumulate over vast periods, and populations become sufficiently different from one another, they may no longer be able to interbreed successfully to produce fertile offspring. At this point, new species have formed. This process is known as speciation. It highlights how natural selection, by causing gradual changes in inherited characteristics, can lead to the incredible diversity of life we observe.
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