In plant breeding, one of the first steps toward genetic improvement is understanding the variation observed in a population. Statistically, this variation—called phenotypic variance—can be broken down into two main components:

𝗚𝗲𝗻𝗲𝘁𝗶𝗰 𝘃𝗮𝗿𝗶𝗮𝗻𝗰𝗲: The heritable part
𝗘𝗻𝘃𝗶𝗿𝗼𝗻𝗺𝗲𝗻𝘁𝗮𝗹 𝘃𝗮𝗿𝗶𝗮𝗻𝗰𝗲: the non-heritable part, caused by external factors like location, year, or field conditions.

For example, consider a trial in which you test a population of elite inbred lines and a few hybrids. You can calculate the mean value of a trait like yield, but individual plants will differ from this average. These deviations reflect phenotypic differences. Breeders aim to determine how much of this variation is due to genetic differences and how much is due to environmental influences.

This partitioning of phenotypic variance is the basis for calculating heritability. However, before learning how to estimate heritability, it’s crucial to understand that 𝗴𝗲𝗻𝗲𝘁𝗶𝗰 𝘃𝗮𝗿𝗶𝗮𝗻𝗰𝗲 itself must be further broken down into three components:

𝗔𝗱𝗱𝗶𝘁𝗶𝘃𝗲 𝘃𝗮𝗿𝗶𝗮𝗻𝗰𝗲: Caused by the average effects of alleles across loci. It is associated with homozygosity and is most prominent in self-pollinating crops. This type of variance is heritable and fixable, making it especially important for selection-based breeding strategies.

𝗗𝗼𝗺𝗶𝗻𝗮𝗻𝗰𝗲 𝘃𝗮𝗿𝗶𝗮𝗻𝗰𝗲: Results from interactions between alleles at the same locus, particularly when the heterozygote’s performance deviates from the average of the two homozygotes. It is linked to heterozygosity and underlies hybrid vigor or heterosis. However, this component is not fixable and less effective for selection.

𝗘𝗽𝗶𝘀𝘁𝗮𝘁𝗶𝗰 𝘃𝗮𝗿𝗶𝗮𝗻𝗰𝗲:Arises from interactions between genes at different loci. Though complex and more difficult to predict, it can significantly influence trait expression and response to selection, especially in polygenic traits.

Understanding these components helps breeders estimate heritability more accurately and design efficient breeding strategies. We’ll cover heritability in detail in an upcoming post.

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Breaking Down Phenotypic Variation in Breeding Populations