Non-Chromosomal
Let's have a look at the non-chromosomal deviations from Mendel's theory. Although his particulate theory of inheritance and his laws of segregation and independent assortment still hold true, the phenotypic expression of these genes are not as simple as what Mendel observed with his peas. His pea experiments are examples of complete dominance, meaning that one allele has full dominance over the other. However, not all characters are inherited this way.
Codominance
In codominance, multiple alleles have equal effect on the expression of certain traits. For example, human blood types are inherited through the ABO system. If the individual has the A allele, then the A antigen will appear; the same is true for the B allele, while the O allele means no antigens. Neither A nor B are dominant over each other. If a person inherits both alleles (blood type AB), then both A and B antigens will appear on red blood cells.
Incomplete DominanceIncomplete dominance means that the heterozygous phenotype is an intermediate between the homozygous phenotypes. For example, when homozygous red-flowered and homozygous white-flowered snapdragons are crossed, their offspring will all be heterozygotes with pink flowers. This may appear to support the blending hypothesis of inheritance. However, if one were to mate pink heterozygotes with each other, they will find the reappearance of the red and white phenotypes. This is because the alleles undergo segregation and fertilization again, and using a punnett square, we find that the ratios of red:pink:white is 1:2:1. This occurs because the heterozygotes do not produce enough pigment to make their flowers completely red.
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Epistasis
Epistasis is when one gene can affect the expression of another gene. A common example is albinism. If an individual is albino, then his/her hair colour will be white regardless of their genotype for hair colour. In other words, the hair colour gene is overridden by the albino gene.
PleiotropyPleiotropy is when one gene affects multiple phenotypic characters. This is common because a gene can usually code for one enzyme, and that enzyme may serve a critical function in multiple metabolic pathways. Therefore, one faulty gene can adversely affect multiple characters. Cystic fibrosis and sickle-cell disease are examples of pleiotropy.
Polygenic InheritanceIn polygenic inheritance, multiple genes affect one character. This is the opposite of pleiotropy, whereas one gene affect multiple characters. In general, when a character is quantitative (along a continuum), then it is polygenic. For example, height and skin colour are quantitative and polygenic.
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Environmental Factors
Phenotypes can depend on both the organism's genotype and the environment it lives in. For humans, our height depends on nutrition and exercise as well as our inherited genes. Another example is the flower colour of hydrangea plants, where acidic soil will produce blue flowers and basic soil will lead to pink flowers. The range of traits that can be expected from a certain genotype due to environmental factors is called the norm or reaction. For example, the norm of reaction of a hydrangea plant's flowers ranges from blue to pink. Characters with multiple factors are called multifactorial.
Others
Genomic Imprinting: Certain traits depend on which parent the allele is inherited from. Imprinting "silences" alleles of certain genes during gamete formation. If the alleles are silenced in the sperm, then the offspring will express the mother's allele. It will still inherit both alleles, but the paternal version is not expressed. On the other hand, if the alleles are silenced in the ova, then the offspring will express the father's allele. When the offspring produces gametes, the imprints are removed and the chromosomes are re-imprinted according to the gender of the offspring.
Extranuclear Genes: Not all of an organism's genes are located within its chromosomes. In fact, a considerable amount is found outside of the nucleus within the organelles. For example, mitochondria and plastids have their own DNA which they use for protein synthesis. In addition, they reproduce by themselves. However, the pattern of organelle gene inheritance is vastly different from nuclear genes. That is, all of the offspring's mitochondrial and plastid DNA are inherited from their mother. The reason is that all of the zygote's mitochondria and plastids come from the cytoplasm of the egg, so as the zygote develops, all of its organelle DNA will be the same as the mother's.
Extranuclear Genes: Not all of an organism's genes are located within its chromosomes. In fact, a considerable amount is found outside of the nucleus within the organelles. For example, mitochondria and plastids have their own DNA which they use for protein synthesis. In addition, they reproduce by themselves. However, the pattern of organelle gene inheritance is vastly different from nuclear genes. That is, all of the offspring's mitochondrial and plastid DNA are inherited from their mother. The reason is that all of the zygote's mitochondria and plastids come from the cytoplasm of the egg, so as the zygote develops, all of its organelle DNA will be the same as the mother's.