Jenny Read

Abstract

How come 50% of Triturus embryos never hatch? In a balanced lethal system, two chromosome forms carry distinct lethal alleles that are reciprocally compensated for by functional genes on the alternative chromosome form. Therefore, both chromosome forms, and in effect their linked lethal alleles, are required for survival. All adult Triturus newts invariably possess two forms of chromosome 1 that can be distinguished under the microscope, known as 1A and 1B. Yet, according to the rules of Mendelian inheritance, half of the offspring produced are homozygous (possessing two copies of either 1A or 1B). These homozygotes die at the late tail-bud stage, approximately halfway through embryological development (Figure 1). Which genes cause this lethality is still unknown.