 However, homozygotes for recessive lethal mutations are , per definition , non-viable, which necessitates maintenance of the mutation in heterozygous organisms . Crosses between heterozygotes yield a wild-type organisms , in addition to hetereozygotes and the non-viable homozygotes.
<meaning-changed>  However, homozygotes for recessive lethal mutations are , per definition , non-viable, which necessitates maintenance of the mutation in heterozygous organisms . Crosses between heterozygotes yield a wild-type organisms , in addition to hetereozygotes and the non-viable homozygotes.
 However, homozygotes for recessive lethal mutations are , per definition , non-viable, because the presence of the recessive lethal allele on both chromosomal homologs causes the organism to die early in development; an organism that is homozygous for a recessive mutation that causes sterility yields essentially the same result (i.e. its genetic material cannot be passed on to progeny, even if the sterile individual itself survives to maturity). This problem forces geneticists wanting to study recessive lethal/sterile mutations to maintain the mutation in heterozygous organisms . Crosses between heterozygotes yield a wild-type organisms , in addition to hetereozygotes and the non-viable homozygotes.