Crossover occurs when two chromosomes, normally two homologous instances of the same chromosome, break and then reconnect but to the different end piece. If they break at the same place or locus in the sequence of base pairs, the result is an exchange of genes, called genetic recombination.
<span>The correct answer is b.0.5 to 1 micrometre thick.</span>
Usually, the respiratory membrane is about 0.6 micrometres thick. This membrane consists of the alveolar epithelial cells, the capillary endothelial cell, and basement membranes which are formed by the alveolar and capillary cells.