Table 2.

Incidence of Abnormal^* Phenotypes in Patch Heterozygotes Relative to Backcrossing onto a Balb/c Background^†

Heterozygote	Third	Sixth
Phenotype	generation	generation
Normal	19 (86.4%)	58 (93.5%)
Abnormal*	3 (13.5%)	4 (6.5%)

Heterozygote Patch mice originally on a C57Bl/6 genetic background were crossed with wild-type Balb/c mice in order to enrich the background for Balb/c. When third generation heterozygotes were mated, 13.5% of the heterozygote embryos had an abnormal phenotype. As the genetic background became more enriched in Balb/c, the incidence of abnormalities in heterozygotes decreased. Note that by the sixth generation, only 6.5% of the heterozygotes embryos (embryonic day 8.5–17.5) were abnormal.

^*Abnormal phenotype included an enlarged heart, wavy neural tube, severe growth retardation, facial abnormalities, and bubbles along the ectoderm.

^†Due to the low incidence of abnormal phenotype, the relative incidence of abnormal phenotype approaches significance, p = 0.13 using the chi-square test.