A little excerpt from the mentioned book. If we make a cross with one parent from each of the true-breeding strains, we will find that 100% of the off- apring are of
the compound-pinnate leaf phenotype. (The expression of a trait in a plant or strain is known as the phenotype.) What
happened to the genes for webbed leaves contained in the webbed leaf parent? Since we know that there were just as many
w genes as W genes combined in the offspring, the W gene must mask the expression of the w gene. We term the W gene
the dominant gene and say that the trait of compound-pinnate leaves is dominant over the recessive trait of webbed leaves.
This seems logical since the normal phenotype in Cannabis has compound- pinnate leaves. It must be remembered,
however, that many useful traits that breed true are recessive. The true-breeding dominant or recessive condition, WW or
ww, is termed the homozygous condition; the segregating hybrid condition wW or Ww is called heterozygous. When we
cross two of the F1 (first filial generation) offspring resulting from the initial cross of the ~1 (parental generation) we
observe two types of offspring. The F2 generation shows a ratio of approximately 3:1, three compound pinnate type-to-one
webbed type. It should be remembered that phenotype ratios are theoretical. The real results may vary from the expected
ratios, especially in small samples.
In this case, compound-pinnate leaf is dominant over webbed leaf, so whenever the genes w and W are combined, the
dominant trait W will be expressed in the phenotype. In the F2 generation only 25% of the offspring are homo- zygous for
W so only 25% are fixed for W. The w trait is only expressed in the F2 generation and only when two w genes are
combined to form a double-recessive, fixing the recessive trait in 25% of the offspring. If compound-pinnate showed
incomplete dominance over webbed, the geno- types in this example would remain the same, but the phenotypes in the F1
generation would all be intermediate types resembling both parents and the F2 phenotype ratio would be 1 compound-
pinnate :2 intermediate :1 webbed.
The explanation for the predictable ratios of offspring is simple and brings us to Mendel's first law, the first of the basic
rules of heredity:
I. Each of the genes in a related pair segregate from each other during gamete formation.
A common technique used to deduce the genotype of the parents is the back-cross. This is done by crossing one of the F1
progeny back to one of the true-breeding P1 parents. If the resulting ratio of phenotypes is 1:1 (one heterozygous to one
homozygous) it proves that the parents were indeed homozygous dominant WW and homozygous-recessive ww.
The 1:1 ratio observed when back-crossing F1 to P1 and the 1:2:1 ratio observed in F1 to F1 crosses are the two basic
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