Heredity and Evolution
Each pollen grain and egg has one high gene, so the plant formed by their union has two… So the gene is of two distinct types or alleles… one allele ‘A’ for Tallness and one ‘a’ for Shortness… So depending on what they inherited, a plant can have same or different alleles… the allele ‘A’ is dominant over ‘a’… so a plant with a combination of ‘aA’ would still be tall… the alleles do not blend.
Now, let’s look at a case, ok? When ‘AA’ breeds with ‘AA’ pollen egg will get one copy of each because sex cell have a number of chromosomes… right? Taking you back to what I taught you a few minutes back.. they have ployed and all other cells will be deployed… Now in the case of ‘AA’ and ‘AA’ the sex cells respectively get ‘A’ and ‘A’ as I mentioned… so the offspring will be ‘AA’ or tall… now in the case of ‘aa’ or ‘aa’, that’s small a’s, the sex cells again get ‘a’ and ‘a’ and the varieties yielded will be stable and short (aa)… In Mendel’s first hybrid experiment, the cross was between ‘AA’ and ‘aa’… so the pollen and the egg carry ‘A’ and ‘a’, so cross was ‘Aa’, which was tall… You remember that I told you everything revolves around phenotype and genotype..? So in this case, the tallness or shortness is the phenotype… and the combination of alleles at gene level, ‘AA’, ‘Aa’, or ‘aa’ is the genotype… So even though the first hybrid experiment gave us the tall plant at the gene level, it was ‘Aa’ and this is called ‘heterozygous’… because ‘hetero’ is different and ‘zygous’ is for zygote and ‘homozygous’ is same… so nothing to memorize here… they are all just logical names derived from various languages, which has gave you that extra twist.
Ok… let’s continue with our story… when the hybrid cell fertilizes its alleles ‘A’ and ‘a’ are sorted out randomly among the pollen grain and eggs… with ‘A’ and ‘a’ appear and roughly in equal proportions… so what are the four possibilities..? ‘short pollen short egg’, ‘tall pollen short egg’, ‘short pollen tall egg’ and ‘tall pollen tall egg’… these are the only four possibilities that can happen.
Now this is too much to write… let’s just put it in a square… let’s just simplify it… ok..? And it’s got a fancy name… It’s called the ‘punnett square’… ok? p-u-n-n-e-t-t… Now let’s write down the four alleles and then it’s just a simple combination… you have the egg here… you have the pollen here… you have ‘A’ here… ‘a’ here… big ‘A’ here and small ‘a’ here… bring them together and what you ‘ll get… AA, Aa, aa, and aA… this is every type of offspring possible, and so, when the hybrids were crossed… he got 1/4 true breeding talls… 1/2 talls which may breed shorts… and, 1/4 true breeding shorts.