Choreography:
Following the footwork of
Classical (Mendelian) Genetics
Foundational Ideas
Tomatoes have genes for different traits (e.g. fruit color) and different versions of these genes are called alleles (e.g ‘R’ for red, ‘r’ for yellow). These represent functional changes in the pathways of molecule production, for ‘r’ (yellow) it’s a dysfunction in the creation of an enzyme called phytoene synthase, the precursor to all carotenoids.
A basic red cherry tomato that evolved from wild relatives is the most evolutionarily simple and it’s genetic composition is like a reference point, referred to as wild-type and when a gene is different from that it is called a mutant allele. In this case ‘R’ is the wild-type allele and ‘r’ is a mutant allele.
Tomatoes have 12 chromosomes and two copies of each chromosome, so there are two alleles of each gene within the tomato DNA.
Mechanically, a Dominant allele (indicated by capital letter e.g. ‘R’) masks the effect of a recessive allele (indicated by lowercase e.g. ‘r’). In other words, one is enough for the Dominant allele to be expressed, but the recessive trait is only expressed if both alleles are the recessive ones (loss of function).
When the alleles are the same on both sets of chromosomes it is called homozygous, when they are different it is called heterozygous.
Genotypes are the genetic make up and phenotypes are the observable expression of that genotype.
For each recessive gene that is different, you can expect the double recessive in ¼ of the next generation. You multiply, like calculating the odds of flipping a coin, to get how many seeds you should expect to grow to get that many genes lined up. So if there are 3 different recessive genes involved, like dwarf, potato-leaf and tangerine orange color, then 1 in 64 of the F2s should have all three, and so you should grow an according number. You can do it in steps, say you get a potato-leaf, tangerine orange but not dwarf, you can backcross with a dwarf tangerine orange and the F2s from that will be all tangerine orange, 1/16 potato-leaf tangerine orange dwarf, and then if you select only seedlings with the potato leaf trait, 1/4 of those will be potato-leaf tangerine orange and dwarf.
R (Red fruit) and r (yellow fruit); RR homozygous will be red, Rr and rR heterozygous will be red, rr will be yellow. About 1 in 4 will be yellow.
C (regular leaf) and c (compound leaf aka potato leaf). CC homozygous will be regular leaf, Cc and cC heterozygous will be regular leaf, cc homozygous will be potato leaf. About 1 in 4 will be potato leaf.
When crossing RR/cc (Red fruit regular leaf with and rr/CC (yellow fruit) potato leaf; About 1 in 4 will be yellow. About 1 in 4 will be potato leaf. About 1 in 16 will be both yellow and potato leaf. This helps demonstrate the difference between selecting for a Dominant or recessive trait. If you wanted for example a red fruit with a potato leaf, you can select at the seedling stage just the potato leaf seedlings to continue (screening out the regular leaf), and all subsequent generations will have potato leaves; it is locked in. But for the red fruit, you can weed out the yellow flower phenotypes(or yellow fruits) and save the red fruit ones, but because of the way that the Dominant trait masks the recessive one, you can't be sure that it is RR until you grow the next generation and find that out of them there are no yellow ones appearing any more (sampling/tracking).