Q15-14TYU - Campbell Biology

Question

Butterflies have an X-Y sex determination system that is different from that of flies or humans. Female butterflies may be either XY or X0, while butterflies with two or more X chromosomes are males. This photograph shows a tiger swallowtail gynandromorphy, which is half male (left side) and half female (right side). Given that the first division of the zygote divides the embryo into the future right and left halves of the butterfly, propose a hypothesis that explains how nondisjunction during the first mitosis might have produced this unusual looking butterfly.

Step-by-Step Solution

Answer

The formation of aneuploids occurs due to non-disjunction during meiosis. This leads to an unusual pattern in the butterfly.

Step 1: Sex determination in butterfly

Sex determination happens in a slightly different pattern in butterflies compared to flies and humans. The female butterflies carry the XY or X0 chromosome. 

Two or more X chromosomes are present in the male butterflies.

Step 2: Non-disjunction

Non-disjunction is the failure of the separation of homologous chromosomes during the anaphase stage of mitosis and meiosis. This leads to the formation of daughter cells with an unequal number of chromosomes. 

This abnormal number of chromosomes in a cell is referred to as aneuploidy.

Step 3: Unusual pattern of the butterfly

During embryogenesis, the zygote cleaves to form the right and left portions of the butterflies. However, non-disjunction occurs in the chromosomes of the butterflies, which does not cause proper segregation of them during meiosis. 

The improper segregation of the chromosomes results in an abnormal number of chromosomes in the gametes. The fusion of such gametes leads to the condition of aneuploidy. 

Thus, the color pattern in the butterfly tiger swallowtail gynandromorphy appears due to the inappropriate segregation of the chromosomes.