The discovery of patterns of i

Meiosis is a series of events that arrange and separatechromosomes and chromatids into daughter cells. During theinterphases of meiosis, each chromosome is duplicated. In meiosis,there are two rounds of nuclear division resulting in four nucleiand usually four daughter cells, each with half the number ofchromosomes as the parent cell. The first separates homologs, andthe second—like mitosis—separates chromatids into individualchromosomes. During meiosis, variation in the daughter nuclei isintroduced because of crossover in prophase I and random alignmentof tetrads at metaphase I. The cells that are produced by meiosisare genetically unique.

Monohybrid: Definition

In biology, a monohybrid cross is defined as the breedingexperiment conducted between parents, which differ in one specifictrait (‘mono’ meaning one). Or in other words, the parents areheterozygous (having dissimilar alleles) at only one locus. Overhere, one parent has a dominant gene for a specific phenotypiccharacter (e.g. tall trait), while the other has recessive gene forthe particular phenotype (e.g. dwarf trait).

Dihybrid: Definition

Contrary to monohybrid cross, parents that differ in two traits(‘di’ meaning two) are bred in a dihybrid cross. To be moreprecise, the parental organisms are heterozygous for two differentcharacters. In this case, one parent has dominant genes for twocharacters (e.g. tall plant bearing red flowers) and the otherparent has recessive genes for the two characters (dwarf plantbearing white flowers) in the chromosome.

Monohybrid

When a cross is made between a tall plant (TT) and a dwarf plant(tt), the two resulting F1 offspring are tall (Tt). Then, a crossis again made between the two tall members of the F1 generation (Ttx Tt). In this F2 generation, the expected offspring along withtheir genetic component are three tall plants (TT, Tt, Tt) and onedwarf plant (tt). Thus, the ratio of dominant to recessivephenotype is 3:1.

Dihybrid

In dihybrid cross, a tall plant having red flower (TTRR) iscrossed with another dwarf plant having white flower (ttrr). Afterthe first F1 generation, we get four heterozygous offspring thatexhibit both tall height and red color flower (TtRr). A secondcross is made between offspring of the F1 generation (TtRr x TtRr),resulting in 16 offspring. The ratio of phenotype character is9:3:3:1, with 9 tall plants having red flower, 3 tall plants withwhite flower, 3 dwarf plants with red flower and 1 dwarf plant withwhite flower.

Thus, with monohybrid cross, inheritance of a single trait canbe traced easily, while dihybrid type is useful for studying thepattern of genetic inheritance in organisms for two differenttraits.

Morgan had discovered that eye color in Drosophila expressed asex-linked trait. All first-generation offspring of a mutantwhite-eyed male and a normal red-eyed female would have red eyesbecause every chromosome pair would contain at least one copy ofthe X chromosome with the dominant trait. But half the females fromthis union would now possess a copy of the white-eyed male’srecessive X chromosome. This chromosome would be transmitted, onaverage, to one-half of second-generation offspring—one-half ofwhich would be male. Thus, second-generation offspring wouldinclude one-quarter with white eyes—and all of these would bemale.

• Discrete pairs of factors located on chromosomes like beads ona string bear hereditary information. These factors—Morgan wouldsoon call them genes—segregate in germ cells and combine duringreproduction, essentially as predicted by Mendelian laws.However:

• Certain characteristics are sex-linked—that is, occur togetherbecause they arise on the same chromosome that determines gender.More generally:

• Other characteristics are also sometimes associated because,as paired chromosomes separate during germ cell development, genesproximate to one another tend to remain together. But sometimes, asa mechanistic consequence of reproduction, this linkage betweengenes is broken, allowing for new combinations of traits.