Monohybrid crossover - an overview (2023)

5.9.6. DEADLY GENES AND GENETIC INTERACTION

Genes don't always separate as if separate entities have no effect on other genes. Genes interact in the most complex way. A good demonstration of this situation can be seen in the example shown in FIG.Abb. 69.

The Law of Genetic Segregation

These findings were not compatible with the idea of ​​mixing. Every parental character has been restored intact in F₂, instead of getting "lost" in the F₁. Mendel argued that yellow peas in P₁were not identical to the yellow peas in the F₁why the p₁Yellows were true breeds and the F₁It wasn't yellow. He suggested that the feature found in the F₁was dominant and that the disappearing feature in F₁but reappeared in F₂it was recessive. But what accounted for the reproducible 3:1 ratio?

Mendel proposed - surprisingly presciently - that each plant carried two copies of an inheritance unit for each trait, one male, one female. He further suggested that each unit occurs in alternative shapes that lead to the distinguishing characteristics he studied (yellow-green, round-wrinkled, etc.). Today we call it "units” “Gene" and be "alternative forms” “allelesHe further suggested that the two alleles found in the cells of a mature plant separate (separate) during germ cell formation and come together at fertilization, one from each parent. Mendel went in search of inheritance rights. This was the first: the law ofGen-Segregation.

The law explains the 3:1 ratio in F₂as follows (Figure 5.4), with the visually accessiblePunnett Square(a graph used to predict the outcome of a given cross or breeding experiment). Purebred yellow pea plants (P₁) have two copies of the dominant allele, denotedY; plants that produce only green peas have two copies of the recessive allele, denotedj. (Capital letters usually represent the dominant allele, lowercase letters the recessive.) Gametes of these P₁Plants (YY and yy, denoted as "homozygous") are bothYorj. At fertilization, all are zygotesE(heterozygous). goodYdominates, all plants are yellow. When these plants self-fertilize, males and females produce gametes that are bothYorj. . . . no f₂, so they are 1/4 of the offspringJJ, 1/4 areE, 1/4 aree,and 1/4 arejj. Given theYis dominant, and thatEEeare equivalent, the ratio between green and yellow peas is 3:1.

Monohybrid cross and test cross

Mendel's cross-hybridization studies included purebred plants that differed in a single contrasting trait. Purebred homozygous parental lines have been crossed and the offspring of that cross are designated F₁hybrids or monohybrids. in the f₁By generation, all hybrids resembled the parent with the dominant trait. The genotype of these monohybrid or heterozygous plants can be represented as a genotypeaa, where the uppercase letter represents the dominant allele and the lowercase letter represents the recessive allele. the f₁The hybrid plants were then self-pollinated (aa×aa) and this cross is known asmonohybrid cross. In the offspring of monohybrid crosses or F₂Generation observed Mendel in the F₂Generation. Mendel predicted that plants with an F-dominant phenotype₂generation were of mixed genotypes, with some having a homozygous dominant genotypeAAand others who have a heterozygous genotypeaa. Determine the genotypes of plants with dominant phenotypes in F₂Mendel generation invented the test cross.

The testcross takes an organism with a dominant phenotype but unknown genotype and crosses it with a homozygous recessive individual with a known genotype.ah. In a test cross with a plant of the genotypeAAAll offspring have the dominant phenotype and the heterozygous genotypeaa. However, if a plant with genotypeaais used in a test cross, then the genotypes of 50% of the offspring will have the genotypeaaand show the dominant trait. The other 50% have the recessive phenotype because they have the homozygous recessive genotypeah. Mendel's test-cross method is still used today in plant and animal breeding processes to determine the genotype of plants with dominant phenotypes.

The first generation of hybrids and beyond

Mendel tracked crosses beyond the third generation and found that individuals who appeared to be dominant among the hybrids' offspring were "double important," meaning that they belonged to two species. He wrote: "...of the forms that possess the dominant character in the first generation, two-thirds have the hybrid character, while a third remain constant with the dominant character". produced the dominant phenotype. The second type, when crossed with hybrids, produced both dominant and recessive phenotypes. Plants that didn't grow properly outnumbered other plants two to one.

Today we refer to dominant-looking plants that are homozygous "constantly" as dominant. You have two copies of the dominant allele. Hybrids, called heterozygotes, have a dominant and a recessive allele. Individuals who express the recessive trait constitute the homozygous recessive class and are also true breeders. That is, when crossed with each other, they produce only homozygous recessive individuals. A monohybrid cross results in a 3:1 phenotypic ratio (dominant to recessive) and a 1:2:1 genotypic ratio (dominant homozygote to heterozygote to recessive homozygote).

Mendel performed crosses for each of the seven traits over four to six generations, each time crossing the individuals that were "genuine bred" (the homozygote dominant and the homozygote recessive) and the hybrids themselves. When he did this repeatedly, the proportion of hybrids decreased by 50% with each generation. In the 10th generation, only two hybrids would remain for every 1023 individuals of each homozygous class.