Complete dominance | Incomplete dominance | Co-dominance | ||
I | F1 progeny | Resemble one of the parents | Do not resemble either of the parents | Resemble both parents |
II | F2 phenotypic ratio | 3 : 1 | 1: 2 : 1 | 1: 2 : 1 |
III | F2 genotypic ratio | 1: 2 : 1 | 1: 2 : 1 | 1: 2 : 1 |
IV | Example | Height in garden pea | Flower colour in snapdragon | ABO blood group |
V | Blending | Does not occur | Occurs | Occurs |
Statement I | If two genes are located on heterologous chromosomes they will assort independently and a dihybrid cross will yield a F2 phenotypic ratio of 9: 3: 3: 1. |
Statement II | If the two genes are linked the dihybrid F2 phenotypic ratio will vary significantly from 9: 3: 3: 1 with strength of linkage directly proportional to the distance between the genes. |
1. | 2. | ||
3. | 4. |
Sickle Cell Anaemia | Thalassemia | ||
I | Mode of inheritance | Autosomal recessive | Sex linked recessive |
II | Cause | Substitution of valine by glutamic acid due to a single base substitution at the sixth codon of the beta globin chain | Mutation or deletion of genes located on chromosomes 11 or 16 |
III | Effect on haemoglobin | Qualitative – incorrectly functioning globin | Quantitative – synthesis of too few haemoglobin molecules |
I. | Down’s syndrome | Trisomy 21; Simian palmar crease, Furrowed tongue, Mental retardation |
II. | Klinefelter’s syndrome | 47,XXY; Genetically female, gynaecomastia and sterility |
III. | Turner’s syndrome | 45, XO; genetically male, Rudimentary ovaries, lack of secondary sexual characters |