Monohybrid cross

Mendel’s Experiment – Monohybrid Cross

  • Monohybrid cross is a cross made to study simultaneous inheritance of a single pair of Mendelian factors. In other words, the cross in which only alternate forms of a single character are taken into consideration is called monohybrid cross.
  • Mendel crossed tall and dwarf pea plants to study the inheritance of one gene.
  • He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called the filial, progeny or the F1
    Mendel observed that all the F1 progeny plants were tall, like one of its parents; none were dwarf.
  • He made similar observations for the other pairs of traits – he found that the F1, always resembled either one of the parents, and that the trait of the other parent was not seen in them.
    Mendel then self-pollinated the tall F1 plants and to his surprise found that in the filial2 generation some of the offspring were ‘dwarf; the character that was not seen in the F1 generation was now expressed.
Character Contrasting traits (dominant/recessive) F1results F2 results F2 ratio
Seed shape Round/wrinkled  All round 5474 round

1850 wrinkled

 2.96:1
 Seed color  Yellow/green  All yellow 6022 yellow 2001 green  3.01:1
 Pod shape  Full/constricted  All full 882 full

299 constricted

 2.95:1
 Pod color  Green/yellow  All green 428 green

152 yellow

 2.82:1
 Flower color  Violet / white  All violet 705 violet

224 white

 3.15:1
Flower position  Axial/terminal  All axial

651 axial

207 terminal

3.14:1
Stem height Tall/dwarf All tall 787 tall

277 dwarf

2.84:1
  • The proportion of plants that were dwarf were 1 /4th of the F2 plants while 3/4th of the F2 plants were tall.
  • The tall and dwarf traits were identical to their parental type and did not show any blending, that is all the offspring were either tall or dwarf, none were of in- between height.
  • Similar results were obtained with the other traits that he studied: only one of the parental traits was expressed in the F, generation while at the F2 stage both the traits were expressed in the proportion 3:1. The contrasting traits did not show any blending at either F, or F2 stage.
  • Based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations.
  • Fie called these things as ‘unit factors’. Nowadays, we call them as genes

Mendel’s first three postulates for Monohybrid Cross

Using the consistent pattern of results in the monohybrid cross,Mendel derived the following three postulates:

(i) Unit factor in pairs in Monohybrid cross: Genetic characters are controlled by unit factors that exist in pairs in individual organisms. Each diploid individual receives one factor from each parent. Because the factors occur in pairs, three combinations are possible: two factors for tallness, two factors for dwarfness, or one of each factor. Every individual possesses one of these three combinations, which determines stem height.

(ii) Dominance/recessiveness in Monohybrid cross: When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other, which is said to be recessive. In each monohybrid cross, the trait expressed in the F, generation results from the presence of the dominant unit factor. The trait that is not expressed in the F1, but which reappears in the F2, is under the genetic influence of the recessive unit factor.

(iii) Segregation in Monohybrid cross: During the formation of gametes, the paired unit factors separate, or segregate, randomly so that each gamete receives one or the other with equal likelihood.

Explanation for the results of the monohybrid crosses:

  • Mendel reasoned that P, tall plant contained identical paired unit factor, as did the P, dwarf plants. The gametes of tall plants all received one tall unit factor as a result of segregation. Likewise, the gametes of dwarf plants all received one dwarf unit factor.
  • Following fertilization, all F, plants received one unit factor from each parent, a tall factor from one and a dwarf factor from the other, reestablishing the paired relationship.
  • Because tall is dominant to dwarf, all F, plants were tall.
  • When F, plants form gametes, the postulate of segregation demands that each gamete randomly receive either the tall or dwarf unit factor.
  • Following random fertilization events during F, selfing, four F2 combinations will result in equal frequency: (1) tall/tall (2) tall/dwarf (3) dwarf/tall (4) dwarf/dwarf
  • Combinations (1) and (4) will clearly result in tall and dwarf plants, respectively. According to the postulate of dominance-recessiveness, combinations (2) and (3) will both yeild tall plants. Therefore, the F2 is predicted to consist of three-fourth tall and one-fourth dwarf plants, or a ratio of 3:1.
  • This is approximately what Mendel observed in his cross between tall and dwarf plants. A similar pattern was observed in each of the other monohybrid crosses.

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