Monohybrid and Dihybrid Crosses: Definition, Explanation, and Examples

Monohybrid cross Definition

 It is recombination that is created when two parents vary in one feature at the same time

Mono-hybrid cross explanation

 Johann Gregor Mendel was an Austrian monk. He made a number of investigations with pea plants. In the garden, he found several unique features in pea plants such as Tall and Dwarf plants, Green and Yellow cotyledons, Round and Wrinkled seeds, etc. These characters occurred repeatedly creation after creation since pea plants are self-pollinated. In other words, these features were mating true. Mendel chose Garden pea as his investigational organism as it is an annual plant with well-defined characteristics and is grown and mixed easily. Mendel was fortunate in his choice of a diploid organism through many creations of natural self-generation. Gardens peas had evolved into pure lines. A single alternation in a trait was therefore indicated by a visible difference between varieties furthermore in the seven pairs of distinguishing traits.

 Mendel chose to study one form that was dominant over a well-defined distinct alternative. Mendel created a hybrid by crossing two plants with opposing traits. He mixed a tall species with a Dwarf variant to get this hybrid. He got the cross seed and cultivated it separately. only tall plants came from all of the seeds when these individuals were self-pollinated. The further creation formed tall and dwarf plants in a 3:1 ratio and he achieved a similar outcome in the second creation with other contrasting features in a 3:1 ratio. Each parent had two factors in charge of every feature. They produced so the tall parent had TT and the dwarf parent had tt. There was only one factor in the gametes. The tall parent’s gamete had one T while the dwarf gamete had one t. The zygote possessed one component from each parent i.e T and t. when the gametes merged in fertilization, The first filial generation often known as the F1- generation was generated by crossing despite having one factor for tallness and one factor for dwarfness. All of the F1- generation individuals grew to be tall. In the next mixture between the F1- generation, 50% of the gametes generated by the parent had the tall factor T and 50% had the dwarf factor t. When fertilization took place among the gametes 3 types of combinations were formed i.e. as TT, Tt, and tt that is in the second filial generation 25% of the plants had both tall factors, 50% of the plants had the mixtures of both factors and 25% of the plants had both dwarf factors. In the outward appearance the tall plants formed 75% and dwarf plants 25% this is called the phenotypic ratio 3:1 though genetically there were 3 types namely homozygous tall 25%, heterozygous tall 50%, and homozygous dwarf 25%. this ratio 1:2:1 is called a genotypic ratio. As only one trait was taken into account it is referred to as the monohybrid ratio

Monohybrid cross Examples

Red flower Plants( RR)   x        White flower Plants (rr)

                    (Red)RR           X         rr (white)

                       I                                   I                

       (gametes) R                                  r


   (selfing) Rr          X   Rr  (Hybrid Red) F1  (Generation)

           R              r
R         RR (Red flower)        Rr  (hybrid flower)
r        Rr (Hybrid flower)        rr (Pure white)

Phenotypic Ratio            Red  :  White

                                     3    :   1

Genotypic Ratio       Pure Red :  Hybrid Red :  Pure White

                               1       :     2      :         1

Di-Hybrid Cross: Definition

It is a cross that is created when two parents vary in two features at the same time

In the Dihybrid cross, Mendel crossed Tall plants with Rounded (smooth) seeds and Dwarf plants with Wrinkled (rough) seeds. From his earlier studies of Monohybrid crosses, Mendel knew that smooth seededness was dominant over wrinkled seededness and tallness was dominant over dwarfness. So he predicted that F1-offsprings in this dihybrid cross would be tall and smooth seeded and the expected F1- Progeny tall with smooth seeded was obtained on self-pollination of F1 individuals. He in F2- generation observed that there were 4- types of phenotypes i.e. Tall Smooth, Tall Wrinkled, Dwarf Smooth, and Dwarf Wrinkled. When Mendel counted the F2-progeny, 9/16 were tall smooth, 3/16 were tall wrinkled, 3/16 were dwarf smooth and 1/16 were dwarf wrinkled giving a 9:3:3:1 ratio. Mendel pondered why 9:3:3:1 ratio rather than some other ratio.

Dihybrid cross Examples

        Tall Smooth seeded plants  X  Dwarf Wrinkled seeded plants  

                TT SS  (Homozygous)     X     tt  ss (Homozygous)

                               I                                     I

(gametes)           TS                                     ts


     (selfing)   TtSs      x          TtSs  F1—-generation

TSTTSS Tall smoothTTSs Tall smoothTtSS Tall smoothTtSs Tall smooth
TsTTSs Tall smoothTTss Tall wrinkledTtSs Tall smoothTtss Tall wrinkled
tSTtSS Tall smoothTtSs Tall smoothttSS Dwarf smoothttSs Dwarf smooth
tsTtSs Tall smoothTtss Tall wrinkledttSs Dwarf smoothttss Dwarf wrinkled

Phenotypic Ratio  Tall Smooth: Tall Wrinkled:Dwarf Smooth: Dwarf Wrinkled

                  9    :      3     :     3      :      1

Genotypic Ratio  Tall Smooth: Hybrid tall Smooth: Tall Wrinkled: Dwarf Smooth: Dwarf Wrinkled

                 1     :       8     :     3     :    3     :    1

Dihybrid Cross: Explanation

Mendel Explained this behavior along the same lines as he did for monohybrid crosses assuming that genes for characters are present in pairs. In parents, Tallness and Smooth Seeded characters are dominant over Dwarfness and Wrinkled Seeded characters. Tall with smooth seeded plants are homozygous having genomes as TTSS and the dwarf with wrinkled seeded plants are homozygous having genomes as ttss. Thus F1-generation from these parents would be double heterozygous having the genetic constitution as TtSs. Mendel predicted that both the pairs of genes would act independently of each other. The allelic pair Tt will segregate to form two types of gametes S and s in equivalent frequency. Thus in all four types of gametes TS, Ts, tS, and ts will be formed in equal proportions. Mendel explained this by closely analyzing F2-phenotypes following one character at a time. A close examination of the checker-board square indicates that tall individuals are 9/16, 3/16 i.e. 12/16 and dwarf 3/16, 1/16 i.e. 4/16 giving a ratio of 3:1 between Tall and Dwarf plants similarly smooth seeded plants are 12/16 and wrinkled seeded 4/16 showing a 3:1 ratio between smooth seeded and wrinkled seeded plants thus the allelic pairs maintain their individuality and independent segregation even when they are together with the same individual.

Back Cross: It refers to the combination of F1-individuals with any of their parents 

 It may be of two types

A.Out Cross:

This is a combination between homozygous dominant parents and F1-individuals.

                           Tall plants (TT)                X            Dwarf plants (tt)

                                            TT (Tall)           X         tt (Dwarf)

                                                I                                   I                

                  (Gametes)          T                                  t


                  (selfing)              Tt    X      Tt  (Hybrid Tall) F1  (Generation)

           T              t
T         TT (Pure Tall)        Tt  (Hybrid tall)

In Outcross, no recessive individuals are obtained in the F2-generation of Monohybrid cross.

In the case of Dihybrid cross:

            Tall Smooth seeded plants  X  Dwarf Wrinkled seeded plants  

                TT SS  (Homozygous)     X     tt  ss (Homozygous)

                               I                                          I

        (gametes)      TS                                     ts


       (Selfing)  TtSs      x          TtSs F1——generation

TSTTSS TallsmoothTTSs Tall smoothTtSS Tall smoothTtSs Tall smooth

In this outcross only one type of phenotype i.e. Tall Smooth individuals are produced.

B.Test Cross: The combination between the F1-individuals and the recessive parents is called a test cross. A monohybrid test cross gives a genotypic and phenotypic ratio of 1:1 while a dihybrid test cross gives a phenotypic and genotypic ratio as 1:1:1:1

In the case of the Monohybrid test cross

                        Tall plants (TT)    X    Dwarf plants (tt)

                                        TT (Tall)           X         tt (Dwarf)

                                 I                                   I                

 (gametes)              T                                  t


                                tt           x        Tt  (Hybrid Tall) F1  (Generation)

           T              t
t         Tt (Tall)     tt (Dwarf)

In case Dihybrid cross

           Tall Smooth seeded plants  X  Dwarf Wrinkled seeded plants  

                TT SS  (Homozygous)     X     tt  ss (Homozygous)

                               I                                     I

   (gametes)       TS                                     ts


                       ttss      x          TtSs F1— generation

tsTtSs Tall smoothTtss Tall WrinkledttSs Dwarf smoothttss Dwarf Wrinkled

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