Abstract
Provided that there is no chromatid interference, no movement of chiasmata, and no discrepancies between meiotic and mitotic chromosome lengths, then genetic maps and recombination fractions may be directly derived from our meiotic chiasma distribution data. This is illustrated by male chiasma derived genetic lengths and recombination fractions along chromosome 13. The recombination fraction between 13p fluorescent markers and the proposed retinoblastoma locus at 13q14 is estimated at 0.27 to 0.37 and preliminary female chiasma studies suggest a recombination fraction of 0.5 between these two sites. Therefore, it seems unlikely that 13p fluorescent markers may be of any practical help in identifying retinoblastoma gene carriers. This is also borne out by the discordant segregation which has been found in six out of seven retinoblastoma families, which gives a calculated recombination fraction of 0.39 (SE 0.15), not significantly different from 0.5.