Recombination Recombination between homologous pairs of chromosomes during meiosis can generate even more gamete types. The chromosome is loaded with recombinational hotspots. However, the recombination points for two different meiotic events can be different. Given that gametes such as sperm are generated by millions of different meiotic events, the number of potential gamete types from a single parent is vast. We can follow a roadmap from genotype to phenotype, but the route is rarely a one to one mapping between genes and phenotypic traits.
Environment further obscures the relationship between genotype and phenotype.
We have several hierarchical concepts that describe how genotype can be related to phenotypic traits. Thus, a gene that controls the levels of testosterone would have a pleiotropic effect on the expression of many secondary sexual traits which are morphological, as well as behavioral. Pleiotropy describes the proximate genetic source for many phenotypic trade-offs. Interactions between alleles at a single locus are termed dominance interactions. If two genes are co-dominant then the heterozygote is intermediate between the two homozygous genotypes and the alleles are additive in their effect.