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

[Halloamey]

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.

[ischel1]

I was completely fascinated by your post. Easy to see that you are into genetics. Very informative and easy for this novice to understand. Thanks for all the info.

[Brutal_Dreamer]

Thank you, Amey! I have promoted this post to an article in the article library. Excellent.

Cheers,
BD

[Halloamey]

Thanks a lot Mary, I am really glad that you liked and understood it, yesterday night after typing this thing down, I though, hmm its quite some work, it would be really bad if no one understands its, now I feel great atleast three people like it.

I was completely fascinated by your post. Easy to see that you are into genetics. Very informative and easy for this novice to understand. Thanks for all the info.

[Halloamey]

Thanks a lot Bruce

Thank you, Amey! I have promoted this post to an article in the article library. Excellent.

Cheers,
BD

[TwoStems]

Ah brings me back to genetics in my high school Biology class Very informative post, Amey. Thank you!

[Halloamey]

Thanks a lot Ali ! I like teaching, infact I did some lectures for bachelor students this year, that was a nice experience for me.

Ah brings me back to genetics in my high school Biology class Very informative post, Amey. Thank you!

[Germinatorman]

Love this Amey, especially as you have broken this down. I am having problems with genetics the Hosta plant. Three sets of chromosomes in the meristem of a leaf causes interesting issues when doing meristem culture - I always thought that a meristemmed plant would be identical to its one and only 'parent'. Not so.
Love your enthusiasim with the subject!

[pavel]

Well done, Amey!

For those who might be interested, F1 stands for Filial 1 ... or in other words the first generation of offspring (sons and daughters) resulting from the parental (P) generation. The 'grandchildren' if you will, would be denoted as the F2 generation and so forth.

As Amey alluded to, genetics often gets quite complicated as one examines traits involving codominance (both traits are expressed), incomplete dominance (phenotype is a blending of the two traits), traits involving more than one gene, epigenetics, and mutations. With the lip example, above, it would seem that incomplete dominance might be at work as the lip is not as frilly as Rl. digbyana but is definitely much more frilly than C. purpurata. (Any idea if that is indeed the case, Amey?)

For those interested in furthering their genetics understanding, I'm sure Amey could be prevailed upon to do further installments. (Don't you like how subtly I volunteered you for such a task, Amey? )

Don, while the goal is to have identical plants from meristem culture, the reality -- as you have seen -- is that is not always the case. Even in cases wherein polyploidy (having more than two sets of homologous chromosomes) is not a factor, mutations can and do still occur. Plants that are polyploids simply make things even more complicated.

[Germinatorman]

More complicated, yes - but they also give us more combinations to deal with! More surprises in the tray!!!
Don