Mono Hybrid Cross
The principle of dominance and segregation were deduced from monohybrid cross.
MONOHYBRID CROSS - DEFINITION
A cross made to study the inheritance of one character or two contrasting forms or two different alleles is known as monohybrid cross.
Crossing of pure tall (TT) and dwarf (tt) to observe the inheritance of height character or two contrasting forms (Tall, Dwarf) or two different alleles (T t). Such crossing is called mono hybridisation and resultant ‘F1’ progeny is called monohybrids.
PHENOTYPIC EXPLANATION OF MONO HYBRID CROSS
Mendel crossed pure tall plant with pure dwarf plant. To do this, he transferred pollen grains of tall plant (male parent) to the stigma of dwarf plant (female parent or emasculated parent). It is called normal cross.
In another cross, Mendel transferred the pollen grains of dwarf plant (male parent) to the stigma of tall plant (female parent or emasculated parent). It is called reciprocal cross.
In both the above crosses, Mendel got all ‘F1’ monohybrid tall plants. Why ‘F1’ progeny is tall? This resulted him to propose the principle of dominance.
B. Law of Dominance - Definition:
Mendet proposed this principle by observing ‘F1’ progeny.
Law of dominance states that when a pair of contrasting forms (Tall, Dwarf) were crossed, the contrasting form that expressed (Tall) in ‘F1’ generation is called dominant contrasting form. The contrasting form that did not express in ‘F1’ generation is called recessive contrasting form (Dwarf).
In the above experiment tall is dominant over dwarf. What happened to the recessive allele,dwarf in ‘F1’?
To find out the answer for above Question, Mendel continued the experiment.
C. Self pollination of ‘F1’ individuals:
Mendel allowed self pollination in the ‘F1’ individuals. In ‘F2’ he got both tall and dwarf in the ratio of 3 : 1. So, the recessive contrasting form is expressed in 114 ‘F2’ generation. The appearance of recessive progeny resulted the proposal of law of segregation.
D.Observation of ‘F3’ generation:
Mendel allowed self pollination in ‘F2’ individuals to observe ‘F3’ generation. ‘F2’ dwarf plants produced all dwarf plants. It indicates that the dwarf plant is a true breeding plant. 1/3 ‘F2’ tall plant produced only tall plants. It indicates that 1/3 among ‘F2’ tall plants is a true breeding plants. 2/3 ‘F2’ tall plants behaved like ‘F1’ by producing tall and dwarf in the ratio of 3:1.
E. Law of segregation:
It was explained by Mendel by observing recessive progeny of ‘F2’ generation.
Law of segregation states that the two alleles of heterozygous or monohybrid or ‘F1’ are seperated during gamete formation. Hence gametes are always pure. The law of segregation is also called law of purity of gametes. The gamete receives either dominant or recessive allele but never both. In ‘F1’ hybrid tall, the alleles of tall and dwarf seperated and enter into two different gametes.