Genetic control of traits

Most of the important characteristics of apples, including size, colour and flavour, are inherited quantitatively. In other words they are polygenic traits, meaning they are controlled by many genes. In these cases, most of the control is usually exerted by relatively few genes, each having a large effect, with the remaining genes having minor effects.


For polygenic traits, the progeny phenotypes of crosses between two varieties usually exhibit a wide and continuous range of variation for the trait - they don’t segregate into discrete categories. The statistical frequency distribution for the progeny phenotype of a polygenic trait usually approximates to a normal distribution curve. According to Janick et al. (1996), the range of variation is related to the expression of the trait in the parents, and the progeny mean is always related to the parental mean. For traits where the genes are additive in their effects, and there is no dominance, the progeny mean will equal the parental mean. For other traits the progeny mean can be above or below the parental mean, the extent and direction depending on (1) dominance or epistasis (where expression of a gene is affected by that of one or more independently inherited genes), and (2) the difference between the dominant and recessive phenotypes. This occurs, for example, with traits such as fruit size that have been subjected to extreme selective pressure over many generations for the largest fruit. In these cases, the progeny mean tends to be much lower than the parental mean.


According to Janick et al. (1996), many traits are distorted by metrical bias. For example, crosses between large and small apples often give progeny closer in size to the small parent, suggesting dominance for small size. However, a plot of fruit size on a log scale will often show that the progeny mean is at the midpoint, suggesting that the genes for size work on a logarithmic increase rather than on an arithmetic increase.


The table below summarises characteristics of the frequency distribution of progeny phenotypes associated with a range of polygenic traits, based on information in Janick et al, (1996).


Not all characteristics are polygenic; many are controlled by single genes, their inheritance obeying simple Mendelian genetics. Frequently associated with disease resistance, many are listed by Janick et al. (1996). Transfer of a character controlled by a single dominant gene is achievable in a single generation. However, mutation-like characters are often controlled by a single recessive gene. In these cases, when a cultivar expressing the character (i.e. homozygous for the recessive gene) is crossed with a ‘normal’ cultivar (i.e. homozygous for the dominant gene) the F1 generation will all have ‘normal’ phenotypes. The recessive character will reappear in a quarter of the F2 generation resulting from sibling crossing, or in half of the backcross to the homozygous recessive parent.   


Janick J, Cummins JN, Brown SK and Hemmat M. 1996. Apples. In: Janick J, Moore JN (eds), Fruit Breeding, Volume I: Tree and Tropical Fruits. John Wiley & Sons, Inc. 1-77.