Difference between Linkage and Crossing Over

Linkage: T.H. Morgan performed a series of experiments with Drosophila (1910-1914). He showed that two genes were found in coupling phase (cis) or repulsion phase (trans), because these were present on the same chromosome pair. Such genes are called linked genes and the phenomenon was called linkage.

The genes on a given chromosome cannot assort independently and are called linked genes. The state of many genes being present on the same chromosome is called Linkage.

The number of linkage groups for any organism is equal to the haploid chromosome number. In man the number of linkage groups is 23, corresponding to the chromosome number n=23. Drosophila has 4 linkage groups.

Crossing Over: Recombination results from crossing over. The process of crossing over occurs by a mechanism called breakage and reunion theory (Darlington; 1937) and comprises four steps: synapsis, tetrad formation, crossing over and disjunction.

During prophase of meiosis I chromosomes physically exchange their segments. At zygotene homologous chromosomes are brought together and they are synapsed (side to side pairing). Each homologue has two chromatids at this stage. At pachytene chromatids exchange segments, resulting crossover types.

Linkage vs Crossing Over

Linkage

1. It is tendency of genes on a chromosome to remain together and passed as such in next generation.

2. It brings more parental types.

3. Strength of linkage between two genes increases if they are closely placed on a chromosome.

4. With increase in age, link age increases

5. It helps to maintain a newly improved variety.

Crossing Over

1. It is exchange of genes or chromosomal parts to break established linkages and formation of new linkages.

2. It produces recombinations.

3. Frequency of crossing over between two genes decreases if they are closely placed.

4. Crossing over decreases.

5. It is the source of variations for producing new varieties.

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Difference Between Incomplete Dominance and Co-Dominance

Incomplete Dominance: Correns discovered incomplete dominance. It is the phenomenon where dominant alleles do not completely express itself. This phenomenon was first studied in flower colour of Mirabilis jalapa or Four O’clock plant. In this plant, red flowers are incompletely dominant over white flowers, the heterozygotes being pink.

Co-Dominance: When the heterozygote exhibits a mixture of phenotypic characters of both homozygotes.

In shorthorn cattle, alleles for red and white coat colour occur. Crosses between red (r1r1) and white (r1r2) coat. A close examination of hairs of roan animals reveals that the coat is made up of a mixture of red hairs and white hairs. Here co-dominance occurs, rather than intermediate dominance.

Incomplete Dominance vs Co-Dominance

Incomplete Dominance

1. Effect of one of the two alleles is more conspicuous.

2. It produces a fine mixture of the expression of two alleles.

3. The effect in hybrid is intermediate of the two alleles.

4. The expressed phenomenon is new. It has no allele of its own.

5. The incompletely dominant allele has quantitative effect.

Co-Dominance

1. Effect of both the alleles are equally conspicuous.

2. There is no mixing of the effect of the two alleles.

3. Both the alleles produce their effect independently. Ex I^A and I^B, Hb^S and Hb^A .

4. The expressed phenotype is combination of two phenotypes and their alleles.

5. A quantitative effect is absent.

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Difference between Darwinism and Lamarckism

The evolutionary idea contributed  proposed by Charles Darwin called ‘Darwinism’ or Natural selection theory, explaining the mechanism of evolution  is clearly stated in his book ‘ Origin of species’.

The important postulates of the theory are: Over production, Struggle for existence, Variations, Survival of the fittest, and Origin of species.

Lamarckism is popularly known as theory of inheritance of acquired characters or theory of use and disuse organs.

Darwinism vs Lamarckism

Darwinism

1. It does not believe in the internal vital force.

2. They do not form part of Darwin’s natural selection theory.

3. An organ can develop further or degenerate only due to continuous variations.

4. Struggle for existence is very important in this theory.

5. Only useful variations are transferred to the next generation.

6. Darwin’s natural selection theory is based on survival of the fittest.

Lamarckism:

1. This theory states that there is an internal vital force in all organisms.

2. It considers new needs or desire produce new structures and change habits of the organism.

3. According to this theory if an organ is constantly used it would be better developed whereas disuse of organ results in its degeneration.

4. It does not consider struggle for existence.

5. All the acquired characters are inherited to the next generation.

6. Lamarckism does not believe in survival of the fittest.

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