Question

Given: bicoid genes are maternal genes; kruppel genes are gap genes, even-skipped genes are pair rule genes, gooseberry genes are gooseberry. Predict the effect of scenarios below on expression levels of each of these gene classes in the embryo described (will expression increase, decrease, or be unaffected?) and explain the reason for your prediction.
a. a fly embryo has a mutation that blocks expression of the kruppel gene

b. a fly embryo has a mutation that blocks expression of the bicoid gene.

c. a fly embryo has a mutation that increases expression of the even skipped gene.

Answer 1

In case of drosophila maternal effect genes, segmentation genes, homeotic genes these classes of genes act one after another in sequence and regulate fine details of segmentation pattern and devlopment of drosophila. Firstly maternal effect genes (bicoid, nanos) are expressed followed by Gap genes(kruppel,knirp),followed by pair rule gene(even skip,odd skip) followed by segment polarity genes(gooseberry genes) and at last homeotic genes.

a. fly embryo having mutation that blocks expression of kruppel gene-

Kruppel gene is Gap gene which function to divide embryo in 3 broad segments namely head, thorax, abdomen. When it is blocked embryo will not be divide in 3 broad segments and gap will be present in embryo.

Due to gap present and lack of segment will result into decrease expression of pair rule genes and gooseberry genes.

Pair rule genes divide 3 broad segments into parasegments and even skipped gene when its expression is decreased every even segment (2,4,6,8) will be defective.

Gooseberry genes are the segment polarity genes which decides anterior and posterior poles of segment. No segments will result in its decreased expression.

Q.b fly embryo has a mutation that blocks expression of the bicoid gene - Bicoid gene is maternal effect gene. These genes present in egg prior to fertilization and expressed even before fertilization.These genes maintain egg polarity. When its expression is blocked development from first stage itself will be defective. As bicoid expression is blocked there will be absence of head and thorax segment in embryo. When maternal effect genes are expressed properly then only further expression of genes will occur but when these are blocked, there will be decreased expression of gap genes, pair rule genes, segment polarity genes further.

Q.c fly embryo has a mutation that increases expression of even skipped gene-

When expression of even skipped gene is increase more no of even segment will be formed and there will be increase in expression of gooseberry gene which may result in segments in which posterior half of segment is replaced by anterior half of adjacent segment.