abstracts of papers presented at the 1962 meetings - Genetics

ABSTRACTS 949
some closely linked to scarlet (st) is under study with respect to its action on the eye pigment
phenes in the white series of “alleles.” Its action in relation to wsat is readily detected in the
compounds, wsat, U and wsat;st, but not in wsat alone. When homozygous in compounds of
wSat with the genetic blocks to ommochrome formation indicated, the modifier suppresses the
residual pteridine components normally found in the eyes of these and similar compounds with
other dark members of the white series. However, when appropriately comparable compounds
involving st were prepared with the following members of the white series: apricot (wa), blood
(wal), cherry (wch), coral (wco), colored (wcol), eosin (we), and spotted-2 white (sp2-w) no
readily visible effect was observed. Other white mutants are being tested.-Preliminary work
utilizing paper chromatographic methods of HADORN and MITCHELL (1951) and HADORN and
KUHN (1955) confirm and quantify in a different way earlier results derived by colorimetry.
In wsatu; z males, the drosopterines from the heads are almost completely suppressed, and certain
other components of the pteridine inventory are significantly reduced, especially isoxanthopterin.
(Some of this work was done at the Zoological Institute of the University of Zurich
under Special Research Fellowship No. GF 12,237, USPHS.)
COMSTOCH, R. E., and LAURENCE H. BAKER, Institute of Agriculture, University of Minnesota,
St. Paul, Minn.: Monte Carlo studies of linkage effects in population genetics.-An electronic
computor was employed to simulate the genetic and nongenetic factors involved in
reproduction, phenotype expression, and selection. The net effect of procedure was as if gene
frequencies and effects on total genotypic value and genetic variances could be determined in
full detail for a sample of loci in a natural population. Information relative to 35 “genes” was
obtained.-Variables in the study were number of parents per generation, number of offspring
per parent, linkage relations among the “genes,” degree of dominance and amount of nongenetic
variance.-It had been predicted from mathematical analysis that linkage disequilibrium
generated by finite population size could lead, in the presence of selection, to: (1) reduction in
the rate of “drift loss” of alleles, (2) seeming advantage (at the locus level) of the heterozygote
over homozygotes in the absence of overdominance, (3) decrease in additive genetic variance,
and (4) inflation of dominance variance and total genotypic variance. All of these were
demonstrated.-Studies were designed to simulate artificial selection for a single quantitative
trait. Conditions were vaned to simulate heritability over the range from 15 to 50% when
linkage was absent. Rate of “drift loss” of alleles was reduced as much as 50% by linkage
depending on the combination of variables. Heritability was sometimes reduced to near zero
at the same time that total genetic variance increased because of inflation of dominance
variance.-These results suggest that the full impact of linkage in pulation genetics may be
much greater than generally realized.
DAVIS, R. H., and VAL W. WOODWARD,
University of Michigan, Ann Arbor, Mich., and Rice
University, Houston, Tex.: Euidence for the dual function of the pyr-3 locus of Neurospora.-A
survey of 30 pyr-3 mutants of Neurospora was undertaken in regard to the activity of the en-
zyme asparate transcarbamylase (ATC), and the response of each mutant to a nonallelic sup-
pressor mutation, s. The ATC reaction yields the pyrimidine precursor, ureidosuccinic acid
from carbamyl phosphate (CAP) and aspartate. The suppressor mutation depresses the orni-
thine transcarbamylase reaction, which utilizes CAP and ornithine to yield citrulline. It was
found that 23 pyr-3 mutants displayed no activity for ATC and were unsuppressible by s; the
remaining seven mutants displayed normal ATC activity and were suppressible. Because the s
gene had its effect on one mode of CAP utilization, it is felt that its suppressor action in regard
to pyr-3 mutants containing ATC lies in compensating for a deficiency in a pyrimidine-specific
CAP. This implies that pyr-3 mutants may affect CAP synthesis as well as the ATC reaction.
Complementation among the pyr-3 mutants studied supports the view that two functions are
controlled by the pyr-3 gene. The distribution of the two types of pyr-3 mutations on the genetic
map of the locus suggests that if two reactions are involved, they are catalyzed by a single