Question

The discovery of patterns of inheritance enabled many fields of biology to be unified. One process that is better understood by genetics is meiosis. Describe the process of meiosis and describe Mendel's two experiments (monohybrid and dihybrid) and Morgan's experiment on recombination. Describe how these threOKe experiments illustrate the process of meiosis.

Answer 1

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

Monohybrid: Definition

In biology, a monohybrid cross is defined as the breeding experiment conducted between parents, which differ in one specific trait ('mono' meaning one). Or in other words, the parents are heterozygous (having dissimilar alleles) at only one locus. Over here, one parent has a dominant gene for a specific phenotypic character (e.g. tall trait), while the other has recessive gene for the 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 more precise, the parental organisms are heterozygous for two different characters. In this case, one parent has dominant genes for two characters (e.g. tall plant bearing red flowers) and the other parent has recessive genes for the two characters (dwarf plant bearing 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 cross is again made between the two tall members of the F1 generation (Tt x Tt). In this F2 generation, the expected offspring along with their genetic component are three tall plants (TT, Tt, Tt) and one dwarf plant (tt). Thus, the ratio of dominant to recessive phenotype is 3:1.

Dihybrid

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

Thus, with monohybrid cross, inheritance of a single trait can be traced easily, while dihybrid type is useful for studying the pattern of genetic inheritance in organisms for two different traits.

Morgan had discovered that eye color in Drosophila expressed a sex-linked trait. All first-generation offspring of a mutant white-eyed male and a normal red-eyed female would have red eyes because every chromosome pair would contain at least one copy of the X chromosome with the dominant trait. But half the females from this union would now possess a copy of the white-eyed male's recessive X chromosome. This chromosome would be transmitted, on average, to one-half of second-generation offspring—one-half of which would be male. Thus, second-generation offspring would include one-quarter with white eyes—and all of these would be male.

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

• Certain characteristics are sex-linked—that is, occur together because 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, genes proximate to one another tend to remain together. But sometimes, as a mechanistic consequence of reproduction, this linkage between genes is broken, allowing for new combinations of traits.