PinkMonkey Online Study Guide-Biology
7.4 Law of Dominance
Unit characters : On the basis of his experiments
and observations, Mendel concluded that the characters from parents
to offsprings are transmitted in the form of some kind of hereditary
units or particles called factors (now called genes). These
factors determine the characters in the individual. For each factor
representing a character there are two alternate forms called alleles.
Mendel represented the allelic factors by appropriate alphabets.
In each of the seven pairs selected by him, the dominant allele
was represented by the capital alphabet and its recessive allele
by the same but smaller alphabet (See Table 1.1) (e.g., Round
seed = R, Wrinkled seed = r).
Table 1.2 Showing dominant and recessive characters in some plants and animals
|
Examples |
Dominant |
Recessive |
Appearance of F1 hybrids |
PLANTS |
Sorghum |
(i) Pearly grain |
Chalky grain |
Pearly |
(ii) Awnless |
Awned |
Awnless |
Maize |
Full Endosperm |
Shrunken endosperm |
Full |
Rice |
Starchy endosperm |
Glutinous endosperm |
Starchy |
ANIMALS |
Rabbit |
Black coat |
white coat |
Black |
Mice |
Normal body size |
dwarf |
Normal |
Man |
(i) Brown eyes |
blue eyes |
Brown |
(ii) Short stature |
tall |
short |
Law of dominance : In a hybrid union, the allele which expresses itself phenotypically is the dominant allele while the other allele which fails to express itself phenotypically is the recessive allele. The hybrid individual shows phenotypically only the dominant character.
The law of dominance is often described as Mendel’s first law of inheritance.
Dominance is seen in various characters in many plants and animals (See Table 1.2). But this is not of universal occurrence. There are many cases where dominance is incomplete or absent.
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