Summary and Exam Tips for Chromosomes and Genes
Chromosomes and Genes is a subtopic of Inheritance, which falls under the subject Coordinated Science in the Cambridge IGCSE curriculum. Chromosomes are thread-like structures located in the cell nucleus, composed of DNA that carries genetic information in the form of genes. Each gene is a segment of DNA that codes for a specific protein, influencing characteristics by determining the proteins that cells produce. Humans have 23 pairs of chromosomes, with diploid cells containing full pairs and haploid cells containing half. Sex determination is based on the XX or XY chromosome pair, with the father determining the child's sex. Mitosis and meiosis are crucial cell division processes, with mitosis producing genetically identical diploid cells for growth and repair, while meiosis produces genetically diverse haploid gametes. Understanding alleles—different forms of a gene—is essential for predicting inheritance patterns, such as dominant and recessive traits. Phenotypic variation arises from genetic and environmental factors, with continuous and discontinuous variations explaining differences in traits. Selective breeding and natural selection illustrate how traits are passed and optimized across generations.
Exam Tips
- Genotype Notation: Always write the dominant allele first (e.g., Bb, not bB).
- Chromosome Numbers: Remember, haploid cells have half the normal number of chromosomes.
- Mitosis vs. Meiosis: Be clear on differences; mitosis results in identical cells, while meiosis increases genetic diversity.
- DNA and RNA: Don't confuse amino acid sequences with base sequences; know which applies to DNA or RNA.
- Punnett Squares: Practice using them to predict genetic outcomes, focusing on dominant and recessive allele interactions.
