Free Principles of Mendelian Inheritance OCR A Level Biology revision notes – covering specification point 6.1.2(bi).
Mendelian inheritance is the system of inheritance principles deduced by Gregor Mendel from his experiments with the garden pea plant, which describes how alleles pass from parents to offspring and produce predictable phenotypic ratios in their offspring.
Mendelian Inheritance: Unmodified Phenotypic Ratios
A cross between two heterozygous individuals, for traits determined by genes whose inheritance is true of these assumptions, will produce predictable phenotypic ratios:
- 3:1: Two heterozygous individuals for a single gene locus (e.g. Aa × Aa).
- 1:1: One heterozygous individual and one homozygous recessive individual, for a single gene locus (e.g. Aa × aa).
- 9:3:3:1: Two heterozygous individuals for two gene loci (e.g. AaBb × AaBb).
- 1:1:1:1: One heterozygous individual for both gene loci, and one homozygous recessive individual for both gene loci (e.g. AaBb × aabb).
If the ratios for these crosses change, then it is due to a modifying phenomenon* which breaks one of the assumptions about how genes and alleles are inherited.
*Typically, gene interactions.
Gene Interactions: Modifying Phenomena
Mechanisms which break these assumptions produce modified phenotypic ratios from what is expected, and are collectively known as ‘non-Mendelian inheritance’.
Non-Mendelian inheritance breaks the primary assumptions of Mendel’s Laws due to:
- Multiple alleles: Although diploid organisms still inherit only two alleles, more alleles produce more genotypes and phenotypes than allele pairing assumes.
- Codominance: Some alleles in a heterozygous genotype are not fully dominant over each other, and can both be expressed to produce a new phenotype.
- Disjunction: Meiosis can (rarely) fail to separate homologous chromosomes (and the alleles on them) into different gametes.
- Sex linkage: Where a gene is located on a sex chromosome, the pattern of inheritance is modified by the offspring’s sex due to their combination of sex chromosomes, which breaks the assumption of random fertilisation.
- Autosomal linkage: Where two genes are located on the same chromosome, they are inherited together as a single unit rather than assorting independently, unless separated by crossing over during meiosis.
- Epistasis: When one gene masks the expression of another gene, then the assumption that genes (and the characteristics they code for) are assorted independently is broken.