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  • Basics of orchid genetics OR why the hell doesn't my orchid look like its ancestors

    Hello there, recently there have a lot of discussions regarding complex pedigrees of certain orchids. In most hybrids the hereditary influences of the parent species is very obvious, but some times according to pedigree and mathematical calculations a certain species accounts for 8 of the 12 ancestors and yet its effects are not seen in the offspring, why does this happen. The answer lie within the laws of genetics and human selection for certain characteristics. Using a simple example I will try to explain this phenomenon.

    Consider a cross of two Cattleya alliance orchids Rhyncholaelia digbyana and Cattleya purpurata
    for the simplicity of this eg. let us narrow our scope to three characteristics
    1) Lip shape, 2) Flower colour and 3) Growth habit
    Some basic terminology:
    a) alleles are variations in the same gene giving rise to different morphology or phenotypes (like hair colour, same gene with little variation)
    b) genotype: genetic make up of an individual, a phenotype is the manfestation of the genotype.
    c) Dominant allel, the allel whose phenotype is expressed in a hybrid is the dominant allel, whereas the phenotype of the recessive allel is suppressed by the dominant allel in a hybrid. The dominant allel is represented by a capital letter and the recessive allel by the same small letter.
    d) Chromosomes are the super coiled DNA which comprises of various genes joined at a stretch.

    So with these basics lets start
    Diagram1 shows the 3 gene pairs we have considered for the cross, the squares represent the individual genes which will be later shown on the chromosomes.


    Picture two shows the the two parents with their respective phenotype and genotype


    Here now we see the chromosomes and the individual genes on the chromosomes of the parents. Now most species are diploid, i.e they have two identical sets of chromosomes (in case some thing goes wrong with genes on one chromosome, the other can take over). During gamete formation, the cells undergo a process called meiosis, where in the two sets are segregated so that each gamete gets one set hence called haploid. When the gametes from two parents unite the offspring again is diploid with one set from each parent.


    On fertilization a diploid F1hybrid is formed which is shown in the photo.


    Now when this F1 hybrid forms its gametes, we have to consider a process called crossing over of homologous chromosomes. This is the process which generates variation in the parents and the offspring, we did not consider it in the earlier gamete formation, because in that case both the copies of genes were identical and any crossing over would generate only slight variation. During this process the similar chromosomes cross over that is some genes on one of the copy are exchanged with genes on the other chromosome.



    As is evident from the next diagram the gene alleles have exchanged their position on the two chromosomes, still genetically they are the same which can be seen in the genotype. The exchange has been marked by black arrows in the before and after pictures of the recombination event.



    Now the gametes undergo independent assortment and segregate such that 4 different types of gametes are formed which are shown here with their haploid genotypes and the phenotypic characteristics they will encode.



    Now consider you cross this F1 hybrid back to one of its parents C. purpurata, this is called a backcross.



    The offspring from this cross will be a mixture of 4 different genotypes and phenotypes, these have been shown here in this diagram



    What is evident from the picture is that the fourth offspring has a phenotype that is very similar to the F1 parent than C. purpurata inspite arithmetically the plant is 75% C.purpurata and the F1 parent is 50% C.purpurata. Now even if you back cross this offspring again with C.purpurata and select for a frilly lipped clone, then that plant will be 87.5% C. purpurata but it will still not resemble C. purpurata in the flower form it will be just 12.5 % Rl. digbyana, but for that particular gene locus it will still be 50% Rl digbyana with dominant characteristics. This was a very simple example involving just two species and their back crossings. Also we considered 1 gene 1 phenotype hypothesis, in orchids it is hardly so. Also there are phenomenon called co-dominance and epigenetics etc etc which are beyond discussion here, so simply put in arithmetic specie influences are very inaccurate and offsprings with complex ancestries can look very different from their parents.
    This article was originally published in forum thread: Basics of orchid genetics OR why the hell doesn't my orchid look like its ancestors started by Halloamey View original post
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