Unit 2: How does inheritance impact on diversity?
10 dot points, in syllabus order. Each dot point has a focused answer, with exam-style questions and worked answers where available.
How is inheritance explained?
Inheritance models. Covers autosomal dominant/recessive inheritance, codominance (ABO blood, MN), incomplete dominance (snapdragon colour), multiple alleles, and sex-linked (X-linked) inheritance such as haemophilia and red-green colour blindness.
Pedigree analysis. Covers pedigree symbols, how to identify autosomal dominant, autosomal recessive and X-linked recessive inheritance patterns from a family tree, and how to deduce genotypes and calculate probabilities.
Genes, alleles and the genome. Covers the molecular definition of a gene, the difference between an allele and a gene, the meaning of genome, locus, genotype and phenotype, and how these terms relate to inheritance.
Monohybrid and test crosses. Covers the 3:1 phenotype ratio of a heterozygote cross, the 1:1 ratio of a test cross with a recessive homozygote, and how a test cross is used to determine the unknown genotype of an organism showing the dominant phenotype.
Meiosis. Covers the two meiotic divisions (reduction and equational), the formation of haploid gametes from diploid cells, and the two main sources of genetic variation: crossing over in prophase I and independent assortment in metaphase I.
Linked and unlinked genes. Covers the 9:3:3:1 ratio of a dihybrid cross with independent assortment (unlinked), how linkage modifies the ratio by reducing recombinant gametes, and how crossing over generates a small fraction of recombinants in linked genes.
Chromosomes and karyotypes. Covers chromosome structure (DNA wound on histones into chromatin), the difference between autosomes and sex chromosomes, homologous pairs, and the use of karyotypes to diagnose chromosomal abnormalities such as Down syndrome.
How do inherited adaptations impact on diversity?
DNA manipulation. Covers the polymerase chain reaction (PCR) for amplifying DNA (denaturation, annealing, extension cycles, primers, Taq polymerase), gel electrophoresis for separating fragments by size, and how the two combine in DNA profiling.
Reproductive cloning and genetic screening. Covers somatic cell nuclear transfer (Dolly), the biological limitations and ethical issues of cloning, and the methods, uses and ethical considerations of pre-natal, newborn and carrier genetic screening.
Phenotypic variation. Covers how the same genotype can produce different phenotypes in different environments, the mechanisms of epigenetic regulation (DNA methylation and histone modification), and worked examples (Arctic foxes, Dutch Hunger Winter, identical twins).