poglavlje 3a nastavak
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 Mendelian ratios for the inheritance of one character controlled by one pair of genes. The astute breeder uses these ratios to determine the genotype of the parental plants and the relevance of genotype to further breeding.
This simple example may be extended to include the inheritance of two or more unrelated pairs of genes at a time. For instance we might consider the simultaneous inheritance of the gene pairs T (tall)/t (short) and M (early
maturation)/m (late maturation). This is termed a polyhybrid instead of monohybrid cross. Mendel's second law allows us to predict the outcome of polyhybrid crosses also:
If complete dominance is assumed for both pairs of genes, then the 16 possible F2 genotype combinations will form 4 F2 phenotypes in a 9:3:3:1 ratio, the most frequent of which is the double-dominant tall/early condition. Incomplete dominance for both gene pairs would result in 9 F2 phenotypes in a 1:2:1:2:4:2:1:2:1 ratio, directly reflecting the genotype ratio. A mixed dominance condition would result in 6 F2 phenotypes in a 6:3:3:2:1:1 ratio. Thus, we see that a cross involving two independently assorting pairs of genes results in a 9:3:3:1 Mendelian phenotype ratio only if dominance is complete. This ratio may differ, depending on the dominance conditions present in the original gene pairs. Also, two new phenotypes, tall/late and short/early, have been created in the F2 generation; these phenotypes differ from both parents and grandparents. This phenomenon is termed recombination and explains the frequent observation that like begets like, but not exactly like.
A polyhybrid back-cross with two unrelated gene pairs exhibits a 1:1 ratio of phenotypes as in the monohybrid back-cross. It should be noted that despite dominance influence, an F1 back-cross with the P1 homozygous-recessive yields the homozygous-recessive phenotype short/late 25% of the time, and by the same logic, a back-cross with the homozygous-dominant parent will yield the homozygous dominant phenotype tall/early 25% of the time. Again, the back-cross proves invaluable in determining the F1 and P1 genotypes. Since all four phenotypes of the back-cross progeny contain at least one each of both recessive genes or one each of both dominant genes, the back-cross phenotype is a direct representation of the four
possible gametes produced by the F1 hybrid.
So far we have discussed inheritance of traits controlled by discrete pairs of unrelated genes. Gene interaction is the control of a trait by two or more gene pairs. In this case genotype ratios will remain the same but phenotype ratios may be altered. Consider a hypothetical example where 2 dominant gene pairs Pp and Cc control late-season anthocyanin pigmentation (purple color) in Cannabis. If P is present alone, only the leaves of the plant (under the proper environmental stimulus) will exhibit accumulated anthocyanin pigment and turn a purple color. If C is present alone, the plant will remain green throughout its life cycle despite environmental conditions. If both are present, however, the calyxes of the plant will also exhibit accumulated anthocyanin and turn purple as the leaves do. Let us assume for now that this may be a desirable trait in Cannabis flowers. What breeding techniques can be used to produce this trait?
First, two homozygous true-breeding P1 types are crossed and the phenotype ratio of the F1 offspring is observed.
The phenotypes of the F2 progeny show a slightly altered phenotype ratio of 9:3:4 instead of the expected 9:3:3:1 for independently assorting traits. If P and C must both be present for any anthocyanin pigmentation in leaves or calyxes, then an even more distorted phenotype ratio of
9:7 will appear.
Two gene pairs may interact in varying ways to produce varying phenotype ratios. Suddenly, the simple laws of inheritance have become more complex, but the data may still be interpreted.
Summary of Essential Points of Breeding
1 - The genotypes of plants are controlled by genes which are passed on unchanged from generation to generation.
2 - Genes occur in pairs, one from the gamete of the staminate parent and one from the gamete of the pistillate parent.
3 - When the members of a gene pair differ in their effect upon phenotype, the plant is termed hybrid or heterozygous.
4 - When the members of a pair of genes are equal in their effect upon phenotype, then they are termed truebreeding or homozygous.
5 - Pairs of genes controlling different phenotypic traits are (usually) inherited independently.
6 - Dominance relations and gene interaction can alter the phenotypic ratios of the F1, F2, and subsequent generations.
Polyploidy
Polyploidy is the condition of multiple sets of chromosomes within one cell. Cannabis has 20 chromosomes in the vegetative diploid (2n) condition. Triploid (3n) and tetraploid (4n) individuals have three or four sets of chromosomes and are termed polyploids. It is believed that the
haploid condition of 10 chromosomes was likely derived by reduction from a higher (polyploid) ancestral number (Lewis, W. H. 1980). Polyploidy has not been shown to occur naturally in Cannabis; however, it may be induced artificially with colchicine treatments. Colchicine is a poisonous compound extracted from the roots of certain Colchicum species; it inhibits chromosome segregation to daughter cells and cell wall formation, resulting in larger than average daughter cells with multiple chromosome sets. The studies of H. E. Warmke et al. (1942-1944) seem to indicate that colchicine raised drug levels in Cannabis. It is unfortunate that Warmke was unaware of the actual psychoactive ingredients of Cannabis and was therefore unable to extract THC. His crude acetone extract and archaic techniques of bioassay using killifish and small freshwater crustaceans are far from conclusive. He was, however, able to produce both triploid and tetraploid strains of Cannabis with up to twice the potency of dipbid strains (in their ability to kill small aquatic organisms). The aim of his research was to "produce a strain of hemp with materially reduced marijuana content" and his results indicated that polyploidy raised the potency of Cannabis without any apparent increase in fiber quality or yield.
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 Mendelian ratios for the inheritance of one character controlled by one pair of genes. The astute breeder uses these ratios to determine the genotype of the parental plants and the relevance of genotype to further breeding.
maturation)/m (late maturation). This is termed a polyhybrid instead of monohybrid cross. Mendel's second law allows us to predict the outcome of polyhybrid crosses also:
II. Unrelated pairs of genes are inherited independently of each other.
If complete dominance is assumed for both pairs of genes, then the 16 possible F2 genotype combinations will form 4 F2 phenotypes in a 9:3:3:1 ratio, the most frequent of which is the double-dominant tall/early condition. Incomplete dominance for both gene pairs would result in 9 F2 phenotypes in a 1:2:1:2:4:2:1:2:1 ratio, directly reflecting the genotype ratio. A mixed dominance condition would result in 6 F2 phenotypes in a 6:3:3:2:1:1 ratio. Thus, we see that a cross involving two independently assorting pairs of genes results in a 9:3:3:1 Mendelian phenotype ratio only if dominance is complete. This ratio may differ, depending on the dominance conditions present in the original gene pairs. Also, two new phenotypes, tall/late and short/early, have been created in the F2 generation; these phenotypes differ from both parents and grandparents. This phenomenon is termed recombination and explains the frequent observation that like begets like, but not exactly like.
A polyhybrid back-cross with two unrelated gene pairs exhibits a 1:1 ratio of phenotypes as in the monohybrid back-cross. It should be noted that despite dominance influence, an F1 back-cross with the P1 homozygous-recessive yields the homozygous-recessive phenotype short/late 25% of the time, and by the same logic, a back-cross with the homozygous-dominant parent will yield the homozygous dominant phenotype tall/early 25% of the time. Again, the back-cross proves invaluable in determining the F1 and P1 genotypes. Since all four phenotypes of the back-cross progeny contain at least one each of both recessive genes or one each of both dominant genes, the back-cross phenotype is a direct representation of the four
possible gametes produced by the F1 hybrid.
So far we have discussed inheritance of traits controlled by discrete pairs of unrelated genes. Gene interaction is the control of a trait by two or more gene pairs. In this case genotype ratios will remain the same but phenotype ratios may be altered. Consider a hypothetical example where 2 dominant gene pairs Pp and Cc control late-season anthocyanin pigmentation (purple color) in Cannabis. If P is present alone, only the leaves of the plant (under the proper environmental stimulus) will exhibit accumulated anthocyanin pigment and turn a purple color. If C is present alone, the plant will remain green throughout its life cycle despite environmental conditions. If both are present, however, the calyxes of the plant will also exhibit accumulated anthocyanin and turn purple as the leaves do. Let us assume for now that this may be a desirable trait in Cannabis flowers. What breeding techniques can be used to produce this trait?
First, two homozygous true-breeding P1 types are crossed and the phenotype ratio of the F1 offspring is observed.
The phenotypes of the F2 progeny show a slightly altered phenotype ratio of 9:3:4 instead of the expected 9:3:3:1 for independently assorting traits. If P and C must both be present for any anthocyanin pigmentation in leaves or calyxes, then an even more distorted phenotype ratio of
9:7 will appear.
Two gene pairs may interact in varying ways to produce varying phenotype ratios. Suddenly, the simple laws of inheritance have become more complex, but the data may still be interpreted.
Summary of Essential Points of Breeding
1 - The genotypes of plants are controlled by genes which are passed on unchanged from generation to generation.
2 - Genes occur in pairs, one from the gamete of the staminate parent and one from the gamete of the pistillate parent.
3 - When the members of a gene pair differ in their effect upon phenotype, the plant is termed hybrid or heterozygous.
4 - When the members of a pair of genes are equal in their effect upon phenotype, then they are termed truebreeding or homozygous.
5 - Pairs of genes controlling different phenotypic traits are (usually) inherited independently.
6 - Dominance relations and gene interaction can alter the phenotypic ratios of the F1, F2, and subsequent generations.
Polyploidy
Polyploidy is the condition of multiple sets of chromosomes within one cell. Cannabis has 20 chromosomes in the vegetative diploid (2n) condition. Triploid (3n) and tetraploid (4n) individuals have three or four sets of chromosomes and are termed polyploids. It is believed that the
haploid condition of 10 chromosomes was likely derived by reduction from a higher (polyploid) ancestral number (Lewis, W. H. 1980). Polyploidy has not been shown to occur naturally in Cannabis; however, it may be induced artificially with colchicine treatments. Colchicine is a poisonous compound extracted from the roots of certain Colchicum species; it inhibits chromosome segregation to daughter cells and cell wall formation, resulting in larger than average daughter cells with multiple chromosome sets. The studies of H. E. Warmke et al. (1942-1944) seem to indicate that colchicine raised drug levels in Cannabis. It is unfortunate that Warmke was unaware of the actual psychoactive ingredients of Cannabis and was therefore unable to extract THC. His crude acetone extract and archaic techniques of bioassay using killifish and small freshwater crustaceans are far from conclusive. He was, however, able to produce both triploid and tetraploid strains of Cannabis with up to twice the potency of dipbid strains (in their ability to kill small aquatic organisms). The aim of his research was to "produce a strain of hemp with materially reduced marijuana content" and his results indicated that polyploidy raised the potency of Cannabis without any apparent increase in fiber quality or yield.