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What happens in a cross

Orchids (and humans) have many thousands of distinct genes, each of which contains the information for creating a specific molecule that does a specific job.


Recall that orchids have two copies of each gene.  These copies may be different alleles, or identical alleles.  During a mating, each parent contributes one copy of each gene to the progeny. 


Consider a parent that has for its two copies of gene G the alleles Q and q.  (Dominant alleles are usually written with a capital letter, while recessive alleles are designated with a lower case letter, but you can designate alleles with whatever symbols you like.)  So, we would write the genotype of gene G in this parent as Qq.


Now let’s assume that the other parent has the genotype qq.


What would a cross between these two parents with respective genotypes Qq and qq produce?


The Punnett square (which is simply a diagram that helps you see how the alleles might mix in a cross) will help us see the result:




What this says is that half the progeny will have the Qq genotype, while the other half will end up with the qq genotype.  It does not mean that if you have exactly two progeny, one will be Qq and the other qq.  It does mean that if you had, say, 1000 progeny, you’d get approximately 50% of one type, and 50% of the other. 


Now, let’s try a real world example.  Assume that Q represents the dominant color allele for flower color, while q represents the recessive album (often incorrectly referred to as albino) allele.  In Paphiopedilum plants, alba/album forms of the plant do not produce certain pigments in the flower, resulting in beautiful green, white, or green/white flower forms.


Here’s a picture of both color and album forms of Paphiopedilum fairrieanum:





The color form carries QQ alleles for its two copies of the flower color gene, and the album form carries qq for its two copies.  (For you perceptive types, the color form could also be Qq.)


What would happen if we crossed these two plants?

QQ (color) x qq (album) 





All of the progeny from this cross would flower out as the color form, and they would all have the same genotype, Qq.  Now, what would happen if we picked one of these Qq plants and crossed it with an album qq plant?


Qq (color) x qq (album)



Approximately half the crop would flower as the color form (Qq), while the other half would show the album form (qq). And, here's the proof:



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