Determination of the radiation-induced mutation rate for sex-linked lethals and detrimentals in the mouse

The costs of human inbreeding and their implications for variations at the DNA level

An analysis of the world literature on the children of first cousin marriages reveals that the depression of survival in offspring followed from birth (including late miscarriages; at about six months gestation or later) to a median age of 10 years is constant (4.4% +/- 4.6) across a wide range of values for population prereproductive mortality. There is thus no evidence for the action of conditional lethals. On the basis of these data, it is calculated that the average human is heterozygous for only 1.4 lethal equivalents capable of acting over this portion of the life cycle. The implications of these results are discussed in the context of genetic counseling, and the biomedical significance of variation in DNA.

Use of an inversion to test for induced X-linked lethals in mice

A long X-chromosomal inversion in the mouse was used to suppress crossing-over and thereby to scan 85% of the X-chromosome, or 5% of the genome, for recessive lethal mutations induced by radiation. After a fractionated absorbed dose of 500 + 500 rad X-rays 24 h apart to spermatogonia, 2/536 irradiated and 0/529 control X-chromosomes carried a confirmed lethal. This corresponds to a rate for recessive lethals of 1.9 x 10(-6)/rad/X-chromosome for single exposures (allowing for the enhancing effect of fractionation). This is believed to be the first demonstration of the induction of transmissible X-linked lethals in mammals. The results are consistent with previous findings by other methods and indicate the relatively low rate of induction of lethals and the value of inversions in detecting them.

A sex-linked coat-colour mutation in the mouse non-transmissible through the female

A sex-linked mutation with a phenotype similar to that of an allele of the Mottled series has been discovered in a mosaic male. This mutation can be normally transmitted through the sperm of the original male but not through the oocytes. Several interpretations of this abnormal transmission are discussed.