Ap Bio Chapter

10 October 2016

In Arabidopsis, three classes of organ identity genes interact to produce the spatial pattern of floral organs. • The ABC model of flower formation identifies how these genes direct the formation of four types of floral organs. • The ABC model proposes that each class of organ identity genes is switched “on” in two specific whorls of the floral meristem. o A genes are switched on in the two outer whorls (sepals and petals), B genes are switched on in the two middle whorls (petals and stamens), and C genes are switched on in the two inner whorls (stamens and carpels). Sepals arise in those parts of the floral meristems in which only.

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A genes are active. o Petals arise in those parts of the floral meristems in which A and B genes are active. o Stamens arise in those parts of the floral meristems in which B and C genes are active. o Carpels arise in those parts of the floral meristems in which only C genes are active. • The ABC model can account for the phenotypes of mutants lacking A, B, or C gene activity. o When gene A is missing, it inhibits C, and vice versa. o If either A or C is missing, the other takes its place.

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