Inheritance of Characteristics

Principles of Horticulture

Genetic information is passed from parent to offspring when material from male and female parent comes together by fusion of the sex cells. Genes from each parent can, in combination, produce an intermediate form, a mixture of the parents’ characteristics in the offspring; e.g. a gene for red flowers inherited from the male parent, combined with a gene for white flowers from the female parent, could produce pink-flowered offspring, if both conditions are equal (see below). If one of the genes, however, completely dominated the other, e.g. if the red gene inherited from the male parent was dominant over the white female gene, all offspring would produce red flowers (see Figure 1). The non-dominating (recessive) white gene will still be present as part of the genetic make- up of the offspring cells and can be passed on to the next generation. If it then were to combine at fertilization with another white gene, the offspring would be white-flowered. Figure 1 The pattern of inheritance of genes The example above considers the inheritance of one pair of genes by a single offspring (monohybrid cross). However, many sex cells are produced from one flower in the form of pollen grains and ovules, and these give rise to many seeds in the next generation. The plant breeder tries to predict the ratio (or percentage) of offspring with each option ( in this case, the ratio of red-flowered offspring to white-flowered offspring).To understand this prediction process, consider now the same example of flower colour in a little more detail.

If a red-flowered plant, containing two genes for red (described as pure), is carefully fertilized (crossed) with a “pure” white-flowered plant, the red-flowered plant supplies in this case pollen as the male parent, and the white-flowered plant supplies ovule as the female parent. As both parents are pure, the male parent can produce only one type of sex cell, containing the ‘red’ version (allele) of the gene, and the female parent only the white (allele). Since all pollen grains will carry ‘red’ genes and all ovules ‘white’, then in the absence of dominance, the only possible combination for the first generation (or F1) is pink offspring, each containing an allele for red and an allele for white (i.e. impure). Figure 2 illustrates this inheritance by using letters to describe genes, R to represent a red-inducing gene, and r to represent a white-inducing gene.

Figure 2 Simple inheritance: production of the F1 generation.

The genotype (the genetic make-up of a cell) can be represented by using letters, e.g. Rr. The phenotype (the outward appearance e.g. red, pink or white flower) results from the genotype’s action. If these plants from the F1 generation were now used as parents and crossed (or perhaps self-pollinated), then the results in the second or F2 generation would be as shown in Figure 10.8 i.e. 25 per cent of the population would have the phenotype of red flowers, 50 per cent pink flowers and 25 per cent white flowers (a ratio of 1:2:1). The plant breeder by analyzing the ratios of each colour would be able to calculate which colour genes were present in the parents, and whether the ‘colour’ was pure.

More usually, one of the alleles at the gene site exhibits dominance and the other is recessive. As a result, when offspring inherit both alleles, (dominant and recessive), there is no intermediate phenotype; only the dominant gene expresses itself (the recessive is masked). It was the monk Gregor Mendel working in a monastery in what is now Brno in the Czech Republic in the middle of the eighteenth century who laid the foundations of genetics with his breeding experiments.

He used peas and worked with single characteristics such as the height of the plant. He crossed tall with short plants and found that in the second or F2 generation the plants were either tall or short; there were no middle-sized phenotypes. They were found to be in the ratio of three tall to one short plant. Similarly, he crossed pure (homozygous) round pea parents (RR) with pure (homozygous) wrinkled pea parents (rr). When crossing the two pure parents (RR × rr), the genotype of the first generation or F1 follows the pattern as shown in Figure 3, but all the plants produced round peas, i.e. the round pea allele was dominant in peas.

Figure 3 Simple inheritance: production of the F2 generation.

When the second or F2 generation was produced the genotype followed the pattern shown in Figure 3, but there were three round peas to every one wrinkled pea type. Both RR and Rr genotypes produced the same phenotype; only a double recessive (rr) produced plants with wrinkled peas. He went on to look at other simple ‘single gene characteristics’ including seed colour (yellow dominant and green recessive) and flower colour (purple dominant and white recessive). In all these cases he found that the ratio of phenotypes in the second or F2 generation was 3:1. By observation he established the Principle of Segregation which states that the phenotype is determined by the pair of alleles in the genotype and only one allele of the gene pair can be present in a single gamete (i.e. passed on by a parent).

Dihybrid Cross. Mendel then went on to investigate the crossing of plants that differed in two contrasting characters. The results of crossing tall purple-flowered peas with short white-flowered ones produced all tall purple-flowered peas. As Mendel showed, they all had the same genotype (TtPp) and the same phenotype. The F2 generation produced from these parents is illustrated in Figure 4, where the combinations are shown in a Punnet Square (or ‘checkerboard’ – a useful way of showing the genotypes produced in a cross). Note that each gene has behaved independently; there are 12 tall to four short (ratio 3:1) and 12 purple to four white-flowered plants (3:1). Figure 4 Punnet square showing a dihybrid cross

This example illustrates Mendel’s Principle of Independent Assortment which states that the alleles of unlinked genes (i.e. genes not on the same chromosome) behave independently at meiosis. The cross involving two independent genes produces combinations of phenotypes in the ratio 9:3:3:1.

For the cross TtPp × TtPp, the genotypes produced are shown in Figure 4 and the phenotypes are as follows: 9 Tall, purple-flowered plants 1 TTPP 2 TTPp 2 TtPP 4 TtPp 3 Tall, white-flowered plants 2 TTpp 1 Ttpp 3 Short, purple-flowered plants 2 ttPP 1 ttPp 1 Short, white-flowered plants 1 ttpp

Not all genes act independently in the way shown above. Some are linked by being on the same chromosome and so they will usually appear together. The number of grouped genes is equal to the number of chromosome pairs for the species (seven for peas). F1 Hybrid Breeding

Breeders may choose to produce seeds for the commercial market which are all F1 offspring or sometimes called F1 hybrids. F1 hybrid seeds are important to the grower since, given a uniform environment, all plants of the same cultivar will produce a uniform crop because they are all genetically identical (see Figure 2). Crops grown from F1 hybrid seed such as cabbage, Brussels sprouts and carrots can be harvested at one time and they have similar characteristics of yield. Similarly, F1 hybrid flower crops will have uniformity of colour and flower size.

Another feature of F1 hybrids is hybrid vigour. Plants crossed from parents with quite different characteristics will display the feature to a marked extent, giving outstanding growth, especially in good growing conditions.

The desirable characteristics of the two parents, such as disease resistance, good plant habit, high yield and good fruit or flower quality, may be incorporated along with established characteristics of successful commercial cultivars by means of the F1 hybrid breeding programme.

F1 hybrid seed production first requires suitable parent stock, which must be pure for all characteristics. In this way, genetically identical offspring are produced, as described in Figure 2. The production of pure parent plants involves repeated self-pollination (selfing) and selection, over eight to twelve generations, resulting in suitable inbred parent lines. During this and other self-pollination programmes, vigour is lost (the parent plants do not look impressive) but, of course, the vigour is restored by hybridization.

The parent lines must now be cross-pollinated to produce the F1 hybrid seed. It is essential to avoid self-pollination at this stage, therefore one of the lines is designated the male parent to supply pollen. The anthers in the flowers of the other line, the female parent, are removed, or treated to prevent the production of viable pollen. The growing area must be isolated to exclude foreign pollen, and seed is collected only from the female parent. This seed is more expensive than most other commercial seed, due to the complex breeding programme requiring intensive labour.

Seed collected from the planted commercial F1 hybrid crop represents the F2, and will produce plants with very diverse characteristics (Figure 3).

Some F2 seed, however, is deliberately produced by breeders for flowering plants such as geraniums and fuchsias where a variety of colour and habit is required for bedding plant display.

Other Breeding Programmes

In addition to F1 hybrid breeding, where specific improvements are achieved, plant breeders may wish to bring about more general improvements to existing cultivars, or introduce characteristics such as disease resistance. Programmes are required for crops which self- pollinate (inbreeders), or those which cross-pollinate (outbreeders), and two of these strategies are described. Pedigree breeding is the most widely used method in plant breeding by both amateurs and professionals. Two plants with different desirable characteristics are crossed to produce an F1 population. These F1 plants of very similar genotype are then crossed (called “selfed“) and any offspring with useful characteristics are selected for further selfing to produce a line of desirable plants. After repeated selfing and selection, the characteristics of the new lines are compared with existing cultivars and assessed for improvements. Further field trials will determine a new type’s suitability for submission and possible registration as a new cultivar. If a plant breeder wishes to produce a strain adapted to particular conditions, (such as a cultivar with hardiness), exposure of plants of a selected cultivar to the desired conditions will eliminate unsuitable plants, thus allowing the hardy plants to set seed. Repetition of this process gradually adapts the whole population. Selection for other characteristics such as earliness may be selected by harvesting seed early.

Disease resistance breeding . A genetic characteristic enables the plant to combat fungal attack. The disease organism may itself develop a corresponding genetic capacity to overcome the plant’s resistance by mutation. The introduction of disease resistance into existing cultivars often requires a backcross breeding programme. This involves a commercial cultivar lacking resistance crossed with a wild plant which exhibits resistance. For example, a lettuce cultivar may lack resistance to downy mildew (Bremia lactucae), or a tomato cultivar lack tomato mosaic virus resistance. This commercial cultivar is crossed with a resistant wild plant to produce an F1, then an F2. From this F2, plants having both the characteristics of the commercial cultivar and also disease resistance are selected. The process continues with backcrossing of these selected plants with the original commercial parent to produce an F1, from which an F2 is produced. More commercial characteristics may be incorporated by further backcrossing and selection over a number of generations, until all the characteristics of the commercial cultivar are restored, but with the additional disease resistance.