Inheritance, variation and crossing cacti

Genotype, phenotype and observation conditions

Genotype means genetic constitution considered at a particular level. We may describe variants of one gene, a larger marker set or the whole genome. Phenotype comprises observed and measured traits. It results from gene action under specific developmental conditions. The same plant can produce different growth after changes in lighting, temperature or nutrition without a DNA-sequence change. [Clark et al. 2018]

An allele is a variant of a gene or particular genomic site. Locus denotes the position being compared. In a diploid, we typically consider two copies of a given autosomal locus, one inherited from each parent. If the variants match, it is homozygous. If they differ, it is heterozygous. These terms concern a particular locus. A plant need not be uniformly homozygous or heterozygous throughout the genome. [Clark et al. 2018]

In a collection, compare phenotypes in similarly aged plants under similar conditions. Spine length on shaded young growth is not directly comparable with length on a mature stem developed in strong light. Without this organization, an environmental difference may easily be attributed to inheritance. [Clark et al. 2018]

Meiosis and combining parental information

In sexual reproduction, meiosis reduces the number of chromosome sets and enables genetic-variant segregation. Fertilization combines the material of two gametes. In angiosperms, gametophytes also develop between meiosis and gamete formation. A pollen grain is therefore not simply a single male gamete. It is a structure in which cells involved in fertilization develop. [Clark et al. 2018]

In the simplest diploid model, an Aa heterozygote passes allele A or a to its gametes. With equal segregation, each has a probability of one half. An offspring does not receive both alleles of one parent as a complete set, but combines variants from both. This separation explains how a trait absent from the parents' appearance may emerge. [Clark et al. 2018]

A and a are conventional model symbols. An uppercase letter does not mean a stronger, more frequent or more beneficial gene. Without data on a particular trait, these symbols cannot be assigned to any flower colour or spine form and its inheritance considered established. [Clark et al. 2018]

A single-locus model and the meaning of probability

Crossing two Aa heterozygotes produces four equally probable gamete combinations. In this model, one quarter of offspring are AA, half Aa and one quarter aa. Aa includes both directions of allele origin. A from the mother plant and a from the pollen donor give the same allele set for this locus as the reverse arrangement. [Clark et al. 2018]

If A shows complete dominance for the trait analysed, AA and Aa share the phenotype being distinguished. We then expect three quarters of plants with one phenotype and one quarter with the other. This is the expected ratio in repeated, numerous observations. It does not require one recessive plant in every group of four seedlings. [Clark et al. 2018]

For four independent offspring, the probability that none is aa is about 31.6 percent. This follows from multiplying three quarters by itself four times. Absence of a recessive seedling in such a small sample is therefore entirely consistent with the model. Inheritance assessment must account for sample size and whether all offspring classes had similar survival chances. [Clark et al. 2018]

Incomplete dominance and codominance

With complete dominance, the selected heterozygote trait matches one homozygote. With incomplete dominance, the heterozygote phenotype is intermediate. Codominance means simultaneous detectable expression of both variants. Intermediate appearance and coexistence of two products do not describe the same mechanism. [Clark et al. 2018]

Konrad Szychowski's archival “Odrobina genetyki” noted hybrids with intermediate traits but called this codominance. The current account separates these concepts. It retains the author's central point that simply equating a hybrid with a copy of one parent often fails. [Clark et al. 2018]

An intermediate hybrid habit does not even establish incomplete dominance of a single gene. Shoot shape, spot intensity or spine length may depend on many loci and their interactions. Identifying the mechanism requires data on successive generations, trait measurements and, where possible, marker analysis. Visual effect alone describes phenotype but does not resolve its cause. [Clark et al. 2018]

Gene linkage and quantitative traits

Independent segregation of two loci is a useful model for genes on different chromosomes and some sufficiently distant loci on the same chromosome. Nearby genes may be transmitted together more often than the independence model predicts. This is linkage. Recombination during meiosis may separate such sets, but its frequency depends on the genomic region. [Clark et al. 2018]

A model of two independent loci with complete dominance gives four phenotypic classes in ratios of nine, three, three and one in a particular cross. Its use requires assumptions about parental genotypes, segregation, expression and survival to hold. It is not a universal distribution of offspring appearance from two cacti. [Clark et al. 2018]

Quantitative traits, such as growth rate or flower size, often vary continuously. Measuring many individuals at comparable ages helps assess them. Choosing the largest seedling may yield an interesting plant, but only evaluating its offspring tests whether the advantage persists in subsequent generations. [Clark et al. 2018]

Inbred lines, depression and heterosis

A plant matching a typical species description is not automatically a “pure line”. Similar appearance does not reveal all hidden variants. A highly homozygous line results from a particular propagation and selection history. In practice, always state which traits and loci were actually assessed. [Clark et al. 2018]

Crossing relatives may increase homozygosity and reveal unfavourable recessive variants. Inbreeding depression is a decrease in selected fitness or vigour components associated with inbreeding. Its magnitude depends on the material and population history. Not every self-pollination produces equally weakened offspring, and self-compatibility must be checked in the particular plant. [Clark et al. 2018]

Heterosis is hybrid superiority assessed for a particular trait against a defined reference. This may be the parental mean or the better parent, so the comparison must be explicit. It does not mean general excellence of every hybrid. Different genetic mechanisms may produce a similar effect, and recombination separates some favourable combinations in the next generation. [Clark et al. 2018]

Mutation, shoot sectors and clonal propagation

A mutation is a change in genetic material. It may arise in a cell giving rise to part of a shoot and be passed to its daughter cells. If the change affects only selected cell lineages, the plant may contain genetically different sectors. Such a mosaic requires different interpretation from a uniform trait present in all tissues examined. [Clark et al. 2018] [Wang et al. 2024]

Offsets and cuttings retain clonal origin but do not guarantee invariance of every cell. The region from which material is taken and the cells regenerating the new shoot matter. A vegetatively maintained trait need not pass through seeds in the same way. This particularly concerns plants with sectoral coloration or irregular growth. [Clark et al. 2018] [Wang et al. 2024]

Grafting primarily connects rootstock and scion physiologically. Performing it alone does not create a seedling whose genome blends both partners. A change in scion growth rate or size may result from water, minerals and physiological signals. A trait observed after grafting therefore needs distinguishing from a heritable change confirmed in offspring. [Clark et al. 2018] [Wang et al. 2024]

Ploidy and chromosome-number notation

Ploidy describes the number of basic chromosome sets. The symbol x denotes chromosome number in one basic set. The symbol n refers to gametic number. In a typical diploid, 2n = 2x indicates two sets in somatic cells. A tetraploid may have 2n = 4x, and its reduced gamete then contains n = 2x. [Clark et al. 2018]

This distinction corrects the simplification that every gamete must have just one basic set. In polyploid plants, chromosome behaviour during meiosis and allele segregation may be more complex than the diploid model presented. Doubling the number of sets is also different from an arbitrary change in the number of individual chromosomes. [Clark et al. 2018]

A larger flower or thicker shoot does not confirm polyploidy. Appropriate tests, such as chromosome analysis or DNA-content measurement with suitable reference material, assess it. Polyploidy may alter development and fertility but does not guarantee greater resistance or better cultivation traits. [Clark et al. 2018]

Organelle genomes and limits of new data

Plant genetic information is not confined to the nucleus. Mitochondria and plastids also have their own genomes. In 2024, Liu and co-authors described organelle genomes of Opuntia cochenillifera. They used short and long sequence reads and bioinformatic analysis. The assembled mitochondrial sequence contained about 1.16 million base pairs. [Liu et al. 2024]

The authors analysed repeats, possible alternative sequence arrangements and similar fragments between mitochondrial and chloroplast genomes. These data help design further studies and markers. They do not provide ready-made instructions for obtaining a particular flower colour. Genome reconstruction and experimental demonstration of variant function are different stages of knowledge. [Liu et al. 2024]

Apomixis, embryo formation without typical gamete union, is also described in opuntias. Obtaining seeds after pollen application therefore does not always confirm hybrid offspring origin. The biology of the material and the method of parentage verification must be considered. [Liu et al. 2024]

Documenting crosses and selection

Assign each parent a permanent number. A pollination record separately lists mother plant and pollen donor, date, measures to limit foreign pollination and the number of flowers pollinated. Then link fruits, seed batches and particular seedlings to this record. Two abbreviated species names quickly become insufficient when more than one cross is made. [Clark et al. 2018]

Reciprocal crosses, in which parents exchange roles, should be conducted and labelled separately. Seeds develop in maternal tissues, and cross direction may affect the outcome for various reasons. A difference does not automatically demonstrate extranuclear inheritance. It may also involve crossing compatibility, seed provisioning or embryo survival. [Clark et al. 2018]

Documentation should also retain plants that do not match expectations and record losses. Assessing only a few selected specimens obscures segregation. Selection seeks desired traits, while inheritance analysis requires knowledge of all observed offspring. Reliable notes allow both objectives without mixing their results. [Clark et al. 2018]

Crested and monstrous growth forms

Cristata and monstruosa describe unusual growth organization, but appearance alone does not determine whether a mutation, injury, developmental regulator or infectious agent caused it. Persistence in a cutting and transmission through seeds are separate questions. Detailed distinctions and a critical review of studies are provided in the account of crested and monstrous forms. [Mauseth 2025] [Maliarenko and Mudrak 2013] [Omar et al. 2014]

Chromosome-set number and pollen compatibility

Dragon fruit lines produced by Cohen and Tel-Zur illustrate the relationship between ploidy and reproduction. Doubling the chromosomes of diploid Selenicereus monacanthus enabled fruit formation after self-pollination. Increasing tetraploid S. megalanthus to eight sets was associated with fruit abortion. The response therefore depends on the material and ploidy level. The yellow dragon fruit reference genome supplies another tool for comparing these traits, retaining the distinction between the reference sequence and the plant’s chromosome complement. [Cohen and Tel-Zur 2012, Cohen and Tel-Zur 2012 p. 345–353, Zaman et al. 2025 p. 4–5, 20][Zaman et al. 2025, Cohen and Tel-Zur 2012 p. 345–353, Zaman et al. 2025 p. 4–5, 20]