The major axes of development in early fruit-fly embryos are established as a result of initial differences in the distribution of specific mRNAs and proteins encoded by genes in the female parent (genetic maternal effect). These differences in distribution establish concentration gradients of morphogens, which cause different genes to be activated in different parts of the embryo.

21.7 The anterior-posterior axis in a Drosophila embryo is determined by concentrations of Bicoid and Nanos proteins.

Table 21.4 Segmentation genes and the effects of mutations in them

Class of Gene

Effect of Mutations

Examples of Genes

Gap genes

Delete adjacent

hunchback, Kr├╝ppel, knirps,

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