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| Chromosome Theory of Inheritance |
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| In the year 1902, Boveri and Sutton proposed the idea of chromosome theory of inheritance which was subsequently confirmed by Morgan in the year 1933. According to this theory, the Mendelian factors which are now called as genes, are located on the chromosomes and it is the chromosomes which segregate and assort independently during meiosis and recombine during the process of fertilization. |
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| Let us think of a pair of contrasting character represented by the alleles R and r. A pair of homologous chromosomes carrying these alleles undergo pairing in prophase I and subsequently separate (or segregate) during anaphase I of first meiosis. In anaphase II of second meiosis, the chromatids separate and enter into the gamete along with the allele present in each. |
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| The idea of independent assortment can be explained as follows: |
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| 1. Let us think of two pairs of contrasting characters represented by the alleles R and r, Y and y. |
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| There are two ways by which the two homologous chromosomes segregate during metaphase I of first meiosis. This in turn leads to an independent assortment of the chromosomes along with the genes located on them during meiosis. As a result, four types of gametes are formed, in equal proportions (25% each). |
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| 2. Take the situation of a test cross for a dihybrid inheritance. Even here, four different types of gametes are formed by the heterozygous plant. The homozygous recessive plant produces only one type of gamete. The progeny shows four types of combinations two parental types and two recombinations. This is the outcome of segregation and independent assortment of the chromosomes which contain the alleles of the two different genes. Thus, it confirms the idea of independent assortment. |
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