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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 Mendelís 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.

Table of Contents

7.0 Introduction
7.1 Gregor Mendel
7.2 Mendel's Experiment on Sweet Pea
7.3 Terminology Used
7.4 Law of Dominance
7.5 Monohybrid Ratio
7.6 Law of Segregation
7.7 Dihybrid Ratio
7.8 Law of Independent Assortment
7.9 Test Cross or Back Cross
7.10 The Concept of "Factor"

Chapter 8





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