A monohybrid cross between a red-flowered snapdragon (Antirrhinum majus) and a white flowered variety does not produce red or white flowered plants in F1 as expected from mendelism. Instead the flowers are pink, i.e. intermediate between the two parents. This is because neither red flower colour nor white is dominant, but each allele has its influence in color development and the hybrid appears pink. If the F1 pink flowers are self-pollinated, the F2 progeny shows red, pink and white flowered plants in the proportion 1 : 2 : 1. It may be recalled that this is the same genotypic ratio that Mendel obtained in garden peas. The difference is that in the present case the heterozygous progeny is distinct in appearance from the homozygotes. The name intermediate inheritance is also given to crosses where F1 hybrids show incomplete or partial dominance with no phenotypic resemblance to either parent. This type of inheritance has been found in crosses involving many other plants.
In 1900 Bateson and his colleagues studied inheritance of comb shape in fowls. There are four types of combs in fowls: rose, pea, walnut and single. Bateson first performed a cross between rose and single. The F1 hens all had a rose comb, and on inbreeding gave rise to an F2 progeny of rose and single in the ratio 3 : 1. The cross indicates that rose and single comb are controlled by a single gene and that rose is dominant over single. In the second cross when chickens with pea comb were mated with single comb, the F1 progeny had pea comb, and F2 had pea and single in the proportion 3 : 1. Obviously, the gene for pea comb is dominant over single. Bateson then crossed rose and pea. Surprisingly, the F1 birds had an altogether different comb of the walnut type! An F2 progeny raised by inbreeding the walnut type consisted of four types ofchickens—walnut, rose, pea and single in the ratio 9 : 3 : 3 : 1. As this ratio is typical for dihybrid inheritance it became clear that rose and pea combs were controlled by two pairs
of genes.
In 1900 Bateson and his colleagues studied inheritance of comb shape in fowls. There are four types of combs in fowls: rose, pea, walnut and single. Bateson first performed a cross between rose and single. The F1 hens all had a rose comb, and on inbreeding gave rise to an F2 progeny of rose and single in the ratio 3 : 1. The cross indicates that rose and single comb are controlled by a single gene and that rose is dominant over single. In the second cross when chickens with pea comb were mated with single comb, the F1 progeny had pea comb, and F2 had pea and single in the proportion 3 : 1. Obviously, the gene for pea comb is dominant over single. Bateson then crossed rose and pea. Surprisingly, the F1 birds had an altogether different comb of the walnut type! An F2 progeny raised by inbreeding the walnut type consisted of four types ofchickens—walnut, rose, pea and single in the ratio 9 : 3 : 3 : 1. As this ratio is typical for dihybrid inheritance it became clear that rose and pea combs were controlled by two pairs
of genes.
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