- Monohybrid inheritance is the inheritance of one characteristic controlled by one gene, which can have two or more different alleles on a homologous pair of chromosomes.
- Alleles are alternative forms of a gene that provide information for a trait (eg one allele gives black hair and one blonde hair). One allele comes from the mother and one allele from the father.
- The genetic information on the allele is the genotype. Expression of this genetic information is the phenotype.
Complete dominance
- Occurs when the information on one allele, the dominant allele, is always expressed in the phenotype. The dominant allele masks the recessive allele. The recessive alleles are expressed when both alleles are recessive. eg brown eyes (B) in human dominant to blue eyes (b). blue eyes only expressed if both alleles a b (bb). If the dominant allele is present (BB or Bb) brown eyes expressed. BB or bb are said to be homozygous individuals and are pure breeders. Bb is heterozygous and not a pure breeder.
- Expected probability of phenotype is the statistical probability (chance) of event happening. It is determined by chance as fertilisation is a chance event (eg random sperm fertilising random egg).
- Test cross can be used to find pure breeders.
- The selected individual is bred with an individual with a recessive phenotype. If offspring express the recessive phenotype the selected individual is heterozygous.
Incomplete dominance
- Occurs when neither allele dominates the other. When both alleles are present in the heterozygous genotype, they both contribute to produce a phenotype that is a blend of genetic information. Therefore 3 different phenotypes may occur (complete dominance has only 2 phenotypes)
- eg andalusian chicken dominant BB produces black feather, recessive bb produced white feather and heterozygous Bb produces blue-grey feathers
Co-dominance
- Occurs when both alleles are equally dominant, they are both expressed in the phenotype when present. eg horse hair colour BB black, WW white BW bluish-grey
Multiple alleles
- Occur when genes have more than two different alleles, though an individual will only have two of the alleles in its genotype
- eg Inheritance of human blood groups show multiple alleles, complete dominance and co-dominance. Three different alleles (Ia, Ib, i) exist for what is known as the ABO blood grouping.Ia and Ib are co-dominant, both are completely dominant to i
Lethal alleles
- A lethal allele occurs when a mutation results in an allele that produces a non-functional version of an essential protein. If an individual inherits (gets) a lethal combination of mutated alleles, it will die before or after birth. eg drosophilia mutated allele produces curly wings (C) which is dominant. homozygous dominant CC is lethal