LA FIESTA DE JESUCRISTO REY

A cultivar is a plant (or grouping/population of plants in a species such as maize (Zea mays)) which has been selected due to the presence of desirable characteristics or traits which can then be propagated to create plants with identical characteristics to their parents. Most commercially grown plants are cultivars and the selection and/or breeding process allows uniformity, leading to plants with predictable phenotypes suitable for commercial production. This stability, and indeed homogeneity and predictability, can be attributed to the inbreeding that routinely happens in commercial varieties whereby heterozygous individuals through a process of selfing, or through alternative genetic engineering processes such as the creation of doubled haploids (whereby haploid cells undergo artificial chromosome doubling) to create homozygous individuals with predictable characteristics. While inbreeding has

detrimental effects in humans and can be the cause of many congenital birth defects due to the increase in chance of expressing recessive detrimental alleles, in commercial crop varieties the ability to predict phenotype with a degree of accuracy allows a product which can be sold commercially owing to its desirable characteristics. Indeed, since the majority of flowering plants are hermaphroditic, inbreeding or autogamy represents a breeding strategy that exists and is common in flowering plants. Subsequently, many crop species reproduce vegetatively and apomictically whereby the produced seeds have the same genotypic composition as the mother plant resulting in offspring that are genetic clones.

Problems such as inbreeding depression whereby there is a detrimental effect through mating with close relatives are also more easily tolerated in non-human systems and in plants tend to be most apparent in phenotypes such as pollen quantity, seed generation and growth rate (Keller and Waller 2002).

The experimental data used in this work deals with inbred barley varieties which are homozygous. Commercial varieties (and those developed as part of breeding programmes) are assigned a name to define a population of genetically identical plants derived from homozygous seed. These names are often the preferred names by the originating breeder and include examples in barley such as ‘Optic’, ‘Golden Promise’ and ‘Tipple’. As can be seen there is often a link between the targeted use of the plant line (commonly brewing and distilling) and its varietal name. It is important to remember that when a plant variety is referred to as a name it is in fact a population of genetically identical individuals as opposed to, for example in humans, an individual unique genotype.

2.3 Pedigrees

A pedigree (Figure 2.5) is a representation of how genetically discrete individuals are related (usually but not exclusively) in time to one another. It is therefore a representation of the genetic relationship between individuals, their parents and progeny (predecessors/ancestors and successors/descendants). Pedigrees are often used in human contexts to show the transmission of alleles responsible for genetic conditions of medical importance or for the display of traits and phenotypes of medical, or research importance.

Figure 2-5 Typical barley pedigree (Fishbeck 2003)

Pedigrees in humans and other animals

Pedigree charts are most commonly used to show relatedness within species, in particular, and most commonly, in humans, dogs, birds and racehorses. The word pedigree is said to have come from a derivation of the French word ‘pied de grue’ which crudely translates to ‘cranes foot’ in recognition of the physical appearance of early pedigree diagrams (Oxford English Dictionary 2002).

Most people will be familiar with the concept of pedigree animals where individuals are of known genetic descent and breeding stock. In animals, pedigree charts have been used for over a hundred years to show ancestry of successful breeds. They are also used to select individuals with specific desirable traits which can be used in subsequent crosses. An early example showing the first known horse pedigree (Figure 2-6) shows the tree-like structure represented horizontally for six generations.

The main function of pedigree charts, as in any successful visualization is to take complex relationships and present them in a way that is easy to comprehend and understand. In humans, pedigrees use standard nomenclatures to show both male and female members. Generations are traditionally represented using Roman numerals (I, IV, V and so on) and individuals within a generation by standard numbering (1, 2 and 3). In human pedigrees the base individual, insofar as the individual which is selected

as the focus or root node of any diagram is referred to as the proband; however this is not seen in plants. In medical circumstances pedigrees are commonly used to determine an individual’s chances of showing a particular genetic disorder or working out the chances of progeny inheriting such conditions where the condition has a hereditary component such as diabetes or hypertension. In a human context pedigrees can also be used to determine the genetic basis for disease, autosomal or x-linked based on the percentage of individuals displaying the phenotype in the pedigree. Whether a condition is dominant or recessive can also be derived from such diagrams. It should be noted that in a human context the numbers of individuals in any chart is relatively low, again quite different from most plant pedigrees (Figure 2-7).

Figure 2-7 Human pedigree showing insulin resistance (Savage et al. 2002)

An adaptation of the common human family tree, the genogram, was described by Jolly (Jolly, Froom, and Rosen 1980) as a means of including additional information on to the family tree diagram. The genogram is used in a medical context and allows the inclusion of additional information such as relationship type (Figure 2-8) and the status of progeny. Additional information is also included on such charts such as dates of birth and death and disease states.

Genograms have a specific purpose in family medicine but detract from specific genetic relatedness and so can be discounted when looking at data in a specific genetic context. They aim to show the relationships that exist within human familial contexts where there may be same-sex couples or where divorced individuals have got back together. They also include dates of birth and death. In essence they are a description of the processes that go on in families that may or may not have a genetic basis.

Figure 2-8 Genogram diagrams used in human pedigree representations