“Pairs separated with alleles are transferred separately to the next generation. As a result, gene inheritance does not affect gene inheritance elsewhere in the genome. Figure 3. The process of crossing or recombination occurs when two homologous chromosomes align during meiosis and exchange a segment of genetic material. Here, alleles for Generation C were swapped. The result is two recombinant chromosomes and two non-recombinant chromosomes. The exact evidence of this was discovered later when the process of meiosis was understood. In meiosis, the genes of the mother and father are separated, and therefore the alleles of character are separated into two different gametes. Cross-hybridization has led to the development of several new plant and ornamental varieties of plant production and high-yielding disease, which is possible thanks to Mendel`s separation law and the independent assortment law. This law states that the inheritance of one allele has nothing to do with the inheritance of one allele for another characteristic. Parents` alleles are transmitted independently to the offspring.
After fertilization, the resulting zygotes can end with any combination of chromosomes of the parents, and all possible combinations occur with the same frequency. Chromosomal segregation occurs during meiosis in two distinct stages called anaphase I and anaphase II (see diagram of meiosis). In a diploid cell, there are two sets of homologous chromosomes of different parental origin (for example, a paternal set and a maternal set). During the meiosis phase, called “s-interphase” in the meiosis diagram, there is a cycle of DNA replication, so each of the chromosomes originally present now consists of two copies called chromatids. These chromosomes (paired chromatids) then pair with the homologous chromosome (also paired chromatids), which is present in the same nucleus (see Prophase I in the meiosis diagram). The process of aligning paired homologous chromosomes is called synapse (see synapse). During the synapse, genetic recombination usually occurs. Some of the recombination events occur by crossing (with physical exchange between two chromatids), but most recombination events involve an exchange of information but no physical exchange between two chromatids (see synthesis-dependent strand annealing (SDSA)).
After recombination, chromosomal segregation occurs, as indicated by the metaphase I and anaphase I stages in the meiosis diagram. Austrian monk Gregor Mendel conducted groundbreaking experiments with pea plants in the early 1800s, showing the existence of traits (he called them “factors”) that offspring inherit from their parents. His work culminates in the three principles of Mendelian inheritance; The law of segregation, the law of independent assortment and the law of dominance. Mendel observed that true pea plants with opposite traits resulted in F1 generations, all expressing the dominant character, and F2 generations, which expressed dominant and recessive traits in a 3:1 ratio, and proposed the law of segregation. This law states that paired unit factors (genes) must be evenly divided into gametes so that offspring have an equal probability of inheriting either factor. For the F2 generation of a monohybrid cross, the following three genotype combinations could result: homozygous dominant, heterozygous or homozygous recessive. Since heterozygotes can originate from two different signaling pathways (a dominant allele and a recessive allele received from a parent) and because heterozygous and homozygous dominant individuals are phenotypically identical, the law supports Mendel`s observed phenotypic ratio of 3:1. The same segregation of alleles is why we can apply the Punnett square to accurately predict the offspring of parents with known genotypes. The physical basis of Mendel`s law of segregation is the first division of meiosis, in which homologous chromosomes with their different versions of each gene are separated into daughter nuclei.
The role of meiotic chromosome segregation in sexual reproduction has not been understood by the scientific community during Mendel`s lifetime. It is now known that these “laws” are due to key events that occur during meiotic division: there is evidence that CO recombination facilitates the segregation of meiotic chromosomes. [2] However, other studies suggest that although chiasms are favorable, they are not essential for meiotic chromosomal segregation. The budding yeast Saccharomyces cerevisiae is a model organism for the study of meiotic recombination. Mutants of S. It has been found that defective cereviae in CO recombination at Holliday transition resolution effectively undergo correct chromosomal segregation. The pathway that produces the majority of COs in S. cerevisiae and possibly in mammals includes a protein complex, including the heterodimer MLH1-MLH3 (called MutL gamma).
[7] MLH1-MLH3 preferentially binds to Holliday compounds. [8] It is an endonuclease that makes single-stranded breaks in supercoiled double-stranded DNA,[8][9] and promotes the formation of co-recombinant CO-recombinants. [10] The suppressed double mutants for MLH3 (main pathway) and MMS4 (which is required for a smaller Holliday connection dissolution path) showed significantly reduced crossing compared to wild-type (6-17x reduction); However, spore viability was relatively high (62%) and chromosomal disjunction appeared to be largely functional. [10] Although all Mendel pea traits behaved according to the law of independent assortment, we now know that some combinations of alleles are not inherited independently.