Gene Interactions

Abstract

In the study of some traits, there are considerable deviations from the expected Mendelian ratios (3:1 when studying one trait and 9:3:3:1 when two traits are studied at the same time). There are several reasons for this behavior, with the changes in phenotypic frequencies as a consequence of changes in genotypic frequencies being the first thing to study. When the genotypic frequencies change, the probable causes are sex-linked inheritance, gene linkage, or extrachromosomal inheritance. If the change in the expected Mendelian proportions is not the consequence of changes in genotypic frequencies, the explanation for this situation is given by gene interactions, both intra-allelic (between alleles of the same locus) when only one trait is being studied as well as interallelic (between alleles of different loci when two traits are being studied at the same time). The intra-allelic interactions are that of dominance-recessiveness (which was the one that was presented to Mendel in the cases he studied), partial dominance, and codominance. The best-known interallelic interactions are complementation, inhibition, supplementation, dominant epistasis, and duplicated genes. Another cause of the deviation of the expected Mendelian proportions is due to pleiotropism, which is the effect that only one gene has on several traits at the same time. The biological reason for this occurring is based on the fact that a polypeptide (made from a gene) can have a central function in several metabolic pathways so that when this gene is affected, many of the traits will also be affected.

Solved Problems

• 1. By crossing a sesame plant whose capsules possess 4 locules with another 6-loculus capsule, an offspring was obtained consisting of 4-locule plants. By self-pollinating these plants, 152 plants with capsules of 4 locules were obtained, and 49 with 6-locule capsules. Provide a genetic explanation of these results.

The genetic explanation must be made on the basis of observed phenotypic proportions. In F1, 100% of the offspring repeat the phenotype of one of the parents. In F2, 75% of the individuals have the 4-loculus capsule phenotype and 25% of the individuals have the 6-loculus capsule phenotype. The observed proportions correspond to the proportions expected in a dominance-recessiveness interaction. If the genotypes are represented according to this type of interaction as AA or Aa = 4-loculus capsules and aa = 6-loculus capsules, 100% of the gametes of an AA individual will be A, and 100% of the gametes of an aa individual will be a. Therefore, the AA x aa cross will result in an offspring with 100% of the Aa individuals. This offspring is the F1. If an F1 plant is self-pollinated, or outcrossed with another F1 plant, the probabilities of the cross will be given by the probability of gamete formation, i.e., pollen grains will have a probability of 50% of being A and 50% of being a; likewise the ovules. The possible crosses, with their respective offspring will be:

25% of the individuals will have the AA genotype, 50% of the individuals will have the Aa genotype, 25% of the individuals will have the aa genotype. Due to dominance, AA and Aa genotypes show the same phenotype, in this case 4-locule capsules. This is the explanation for why 75% of the individuals in the crossbreed have 4-locule capsules, and 25% 6-locule capsules. Finally, the crossover can be represented as follows:

• 2. By studying the expression of the aconitase isoenzyme in two progenitor lines (P1 and P2), F1 and 8 individuals F2, the following zymogram was obtained:

Provide a genetic explanation for these results

The F1 individual possesses the phenotype of both parents at the same time (both bands). 25% of the F2 individuals repeat the phenotype of P1, 50% have the phenotype of both parents and 25% have the phenotype of P1. If allele names are assigned to the present bands such as A1 to the band at the top of the zymogram and A2 to the one at the bottom of the zymogram, the cross can be represented as follows:

The explanation for obtaining these phenotypic proportions is that within the locus that determines the presence or absence of the band there is a codominance interaction.

• 3. In an ornamental species, when crossing red-flowered plants with a white-flowered one, 12 pink-flowered plants are obtained. From its self-pollination, 49 red-flowered plants, 52 white-flowered plants, and 101 pink-flowered plants were obtained. Provide a genetic explanation for these results

The phenotypic proportions observed in the F2 generation are 25% of individuals with red flowers, 50% of individuals with pink flowers, and 25% of individuals with red flowers. If RR is denoted as red flowers, Rr is denoted as pink flowers, and rr is denoted as white flowers, the results can be explained by the following crossing.

In this case, the interaction that occurs is that of partial or incomplete dominance since the R allele does not completely dominate over r, giving rise to a phenotype that is not present in either parent, is intermediate between the two parents.

• 4. In an ornamental plant, when crossing plants with green leaves with plants with purple leaves, plants with green leaves were obtained. When these plants were crossed among themselves, 180 plants with green leaves, 45 with white leaves, and 15 with purple leaves were obtained. Provide a genetic explanation for these results

In this situation, the proportions observed in F2 do not correspond to any of the following of the expected phenotypic frequencies for traits determined by only 1 gene (3:1 ó 1:2:1). For this reason, it could be thought that for the expression of this trait two enzymes act, i.e., two genes. For this purpose, the proportions must be calculated, which results in 12:3:1, which is a modification of the 9:3:3:1 ratio for traits determined by 2 genes (9 + 3: 3: 1), i.e., the individuals with green leaves have the genotypes A_B_ and A_bb (i.e., for this phenotype, the genotype in gene B is irrelevant), the individuals with white leaves are aaB_ and the individuals with purple leaves are aabb. To confirm this hypothesis, we can schematize the crossing, which would be:

The genetic explanation for this segregation is given by the presence of dominant epistasis of A over B. Individuals that present at least one dominant allele of the gene A will be phenotypically equal, regardless of the genotype in gene B. Because of this the genotypic frequencies A_B_ and A_bb are summed. The phenotype white leaves appears when a dominant allele of A is absent, but at least one dominant allele of B is present. The purple leaves appear when there are no dominant alleles at neither of the two genes.

• 5. Suppose that by crossing sesame plants with black seed with sesame plants of white seed, we obtain an offspring of 19 plants with black seeds. When self-pollinated, 360 plants with black seeds and 278 plants with white seeds are obtained. Provide a genetic explanation for these results

The ratios observed are 9:7, which is a modification of the ratio 9:3:3:1 (9: 3 + 3 + 1). The crossover would be as follows:

These results are explained by an interallelic complementation interaction.

• 6. Crossbreeding of several black-coated bulls with several red-coated cows resulted in offspring of 12 black-coated animals. Crossbreeding between F1 individuals resulted in offspring of 21 black-coated animals and 7 red-coated animals. Provide a genetic explanation for these results.

The genetic explanation must be made on the basis of phenotypic proportions observed. In all F1s of all crosses, 100% of the offspring repeat the phenotype of one of the parents. In the F2, 75% of the individuals have the black coat phenotype and 25% of the individuals have the red coat phenotype. The observed proportions correspond to the expected proportions in a dominance:recessiveness interaction. If the genotypes are represented according to this type of interaction as AA or Aa = black coat and aa = red coat, 100% of the gametes of an AA individual will be A, and 100% of the gametes of an aa individual will be a. Therefore, the AA x aa cross will result in an offspring with 100% of the Aa individuals. This offspring is the F1, the proposed model corresponds to the observed situation, 100% of the individuals in F1 have the same phenotype, correspondent to the same genotype. If individuals of F1 are crossed, the probabilities of the cross will be given by the probability of gamete formation, i.e., the spermatozoa will have a probability of 50% of being A and 50% of being a; likewise the ovule. The A allele is shown to be dominant over the a allele, i.e., for coat color in this species, there is a dominance-recessiveness interaction.