This study note covers the topic of Genetics and Evolution within the Higher Level (HL) syllabus for the International Baccalaureate (IB) Biology course. The document will delve into the intricate processes of meiosis, inheritance patterns, gene pools, and speciation, providing a comprehensive understanding of these advanced biological concepts.
Meiosis is a type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell. This process is crucial for sexual reproduction and genetic diversity.
Example:
For instance, during crossing over in Prophase I, sections of DNA are exchanged between homologous chromosomes, leading to new combinations of alleles.
Unlinked genes are located on different chromosomes or far apart on the same chromosome, and they assort independently during meiosis.
A dihybrid cross involves individuals heterozygous for two traits. The phenotypic ratio in the F2 generation of a dihybrid cross is typically 9:3:3:1 if the genes are unlinked.
Gene linkage occurs when genes are located close together on the same chromosome and tend to be inherited together.
Note:
Linked genes do not follow Mendel's law of independent assortment.
Recombinants are offspring with new combinations of traits not found in either parent, resulting from crossing over.
The chi-squared test is used to determine if there is a significant difference between observed and expected frequencies in genetic crosses.
$$ \chi^2 = \sum \frac{(O - E)^2}{E} $$
Where:
Tip:
Use the chi-squared test to analyze the results of genetic crosses and test for linkage.
Variation can be continuous (e.g., height) or discontinuous (e.g., blood type), influenced by genetic and environmental factors.
A gene pool is the total collection of genes and their alleles in a population.
Evolution is the change in allele frequencies in a population over time, driven by mechanisms such as natural selection, genetic drift, and gene flow.
Speciation is the process by which new species arise. It can occur through mechanisms like allopatric speciation (geographic isolation) and sympatric speciation (reproductive isolation).
Polyploidy, the condition of having more than two sets of chromosomes, can lead to speciation, especially in plants.
Comparing allele frequencies between populations can provide insights into evolutionary processes and the impact of selective pressures.
Common Mistake:
A common misconception is that evolution always leads to more complex organisms. In reality, evolution is about adaptation to the environment, which can lead to simplification in some cases.
Understanding the principles of genetics and evolution is fundamental to the study of biology. Meiosis, inheritance patterns, gene pools, and speciation are key concepts that explain the diversity of life and the mechanisms behind genetic variation and evolutionary change.