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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Understanding Key Genetic Terms Revision Notes
Basic Units of Inheritance: Genes, Alleles, and Chromosomes
Genetic information is stored in chromosomes, which are found within the nucleus of cells. Each chromosome is a long strand of DNA. Along these chromosomes are specific segments called genes. A gene is a unit of heredity that codes for a particular characteristic, such as hair colour. For each gene, there can be different versions, known as alleles (or variants). For example, the gene for flower colour in a plant might have an allele for red flowers and an allele for white flowers. These alleles are what contribute to the diversity of traits visible in a population.
Dominant and Recessive Alleles
Alleles interact to determine an organism's traits. A dominant allele is one that will always express its characteristic whenever it is present in the genotype. This means if an individual has even one copy of a dominant allele, the corresponding trait will be shown. In contrast, a recessive allele will only express its characteristic if two copies of it are present in the genotype. If a dominant allele is also present, the recessive allele's trait will be masked. For example, if 'B' represents a dominant allele for brown eyes and 'b' represents a recessive allele for blue eyes, an individual with 'Bb' genotype will have brown eyes because 'B' is dominant.
Homozygous and Heterozygous Genotypes
The combination of alleles an individual has for a specific gene is called their genotype. If an individual inherits two identical alleles for a particular gene, whether two dominant alleles (e.g., BB) or two recessive alleles (e.g., bb), they are said to be homozygous for that gene. If an individual inherits two different alleles for a gene (e.g., Bb), they are said to be heterozygous for that gene. These terms are crucial for predicting the inheritance patterns of traits.
Genotype vs. Phenotype and Gametes
The genotype is the genetic makeup of an organism, representing the specific set of alleles it possesses for a trait. The phenotype, however, is the observable physical or biochemical characteristic that results from the genotype and environmental influences. For instance, the genotype 'BB' or 'Bb' might both lead to a brown eye phenotype, while 'bb' leads to a blue eye phenotype. Gametes are reproductive cells (sperm in males, egg in females) that carry half the genetic information of a normal body cell. Each gamete contains only one allele for each gene, meaning when two gametes fuse during fertilisation, the offspring receives one allele from each parent to form its genotype.
Genetic Variation within a Species Revision Notes
What is Genetic Variation?
Genetic variation refers to the differences in the DNA sequences among individuals within a population of a single species. It means that not all individuals in a species are genetically identical; they exhibit a range of traits and characteristics. This variation is usually extensive, implying that a large number of different genetic combinations exist, making each individual largely unique. Think about how diverse humans are, despite all belonging to the same species.
Sources of Genetic Variation: Mutations
One primary source of genetic variation is mutation. Mutations are random changes in the DNA sequence. These changes can be small, like a single base pair alteration, or larger, involving whole sections of chromosomes. If these mutations occur in the reproductive cells (gametes), they can be passed on to the next generation. This introduces new alleles (different forms of a gene) into the gene pool of the population, thereby increasing genetic diversity.
Sources of Genetic Variation: Sexual Reproduction
Sexual reproduction is another major contributor to genetic variation. It involves the combination of genetic material from two parents. During the formation of gametes (sperm and egg) through a process called meiosis, there's independent assortment of chromosomes and crossing over. These processes shuffle and recombine genetic material, leading to unique combinations of alleles in each gamete. When these gametes fuse during fertilisation, they create offspring with a novel combination of genes, ensuring genetic distinctiveness.
Importance of Genetic Variation
Extensive genetic variation within a species is incredibly important for its survival and evolution. It provides the raw material for natural selection. If a population encounters a change in its environment (e.g., a new disease, climate change), individuals with certain advantageous genetic variations are more likely to survive, reproduce, and pass on those beneficial traits. Without variation, a species would be less able to adapt and could face extinction if environmental conditions change significantly.
The Role of Meiosis in Gamete Formation Revision Notes
What is Meiosis?
Meiosis is a specific type of cell division that produces gametes (sex cells) in sexually reproducing organisms. Unlike mitosis, which produces two genetically identical diploid daughter cells, meiosis produces four genetically different haploid daughter cells. This reduction in chromosome number is crucial for maintaining the species' characteristic chromosome count across generations after fertilisation. Meiosis involves two distinct rounds of cell division.
Halving the Chromosome Number
Organisms that reproduce sexually have a diploid number of chromosomes (2n) in their body cells, meaning they have two sets of chromosomes, one from each parent. For example, humans have 46 chromosomes (23 pairs). Gametes, however, must be haploid (n), containing only one set of chromosomes (23 in humans). Meiosis ensures this halving. During Meiosis I, homologous chromosomes (pairs of chromosomes, one from each parent) separate, reducing the chromosome number by half. Meiosis II then separates sister chromatids, resulting in four haploid cells.
Importance for Sexual Reproduction
The halving of the chromosome number during meiosis is essential for successful sexual reproduction. When a haploid sperm cell (n chromosomes) fertilises a haploid egg cell (n chromosomes), the resulting zygote is diploid (2n chromosomes). This restoration of the diploid number ensures that the offspring has the correct number of chromosomes characteristic of the species. Without meiosis, each generation would have double the number of chromosomes, leading to unsustainable genetic material.
Gamete Formation
Meiosis is the cellular process specifically responsible for creating gametes. In males, it produces sperm cells through spermatogenesis, and in females, it produces egg cells (ova) through oogenesis. Each of these gametes carries half the genetic information of the parent organism. This genetic contribution from both parents through haploid gametes allows for genetic variation in offspring, which is beneficial for adaptation and survival of a species.
Impact of Developments in Biology on Classification Systems Revision Notes
Early Classification Systems (Linnaean System)
Historically, organisms were classified primarily based on observable physical characteristics, such as their anatomy, morphology, and behaviour. The Linnaean system, devised by Carl Linnaeus, established a hierarchical ranking from Kingdom down to Species. This system was vital for organising biodiversity but had limitations. Sometimes, organisms with similar appearances but different evolutionary origins were grouped together, while organisms with different appearances but close evolutionary ties were placed in separate groups. This was due to the reliance solely on surface-level traits.
Impact of Molecular Biology on Classification
Significant developments in molecular biology, particularly DNA sequencing and protein analysis, have revolutionised classification. By comparing the genetic material (DNA) and proteins of different organisms, scientists can determine their evolutionary relatedness with much greater accuracy. Organisms that share more similar DNA sequences are considered more closely related, reflecting a more recent common ancestor. This molecular evidence often provides clearer insights into evolutionary history than physical appearance alone.
Emergence of Phylogenetic Classification
The information gained from molecular biology has led to the development of phylogenetic classification. This approach groups organisms based on their actual evolutionary relationships and common ancestry, rather than just superficial resemblances. Phylogenetic trees are often used to visually represent these relationships. This shift has resulted in reclassifications, where some species have been moved to new groups based on genetic evidence, even if their traditional morphological classification was different. It provides a more accurate and robust understanding of life's diversity.
Other Biological Developments and Their Contributions
Beyond molecular biology, advancements in other biological fields have also contributed. Cytology, the study of cells, has allowed comparison of cell structures and organelles. Biochemistry has enabled the analysis of metabolic pathways and biochemical compounds. Embryology, the study of embryonic development, can also reveal shared ancestry. These areas provide additional evidence to support or refine evolutionary relationships, collectively making classification systems more comprehensive and reflective of the true biological interconnectedness of organisms.
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