Simple Genetic Basis Research Articles

Further breeding experiments with New Zealand Romney N-type sheep to 1956

Another NS-grade sheep, a horned ewe from an ordinary Romney flock, is judged on progeny to have been N/+. Further breeding results are consistent with the simple genetic basis of horns in N/+ ewes previously postulated, namely, a fortifier gene F. Additional evidence is given for a genetic basis of the comparative scarcity of halo-hairs on the main area of the body of low-N N/+ lambs. It is a reasonable guess that selection would fairly quickly build up a stock in which the gene N could be called recessive, in that its expression in heterozygotes fell short of VI. In work with low-N sheep two horned rams, twin sons of a low-N ram with very large horns, have proved not to possess N. Horns in these rams clearly have a simple genetic basis. One N-grade ram has been shown to be +/nr, and it is likely that his twin brother, which closely resembled him, was also +/nr. The N-type characterization of these twin rams was feeble in several respects. There is evidence that the nr gene is helped to N-grade expression by multiple factors which are thus dominigenes. This is a third possible way, in addition to picking up N from a low-N or nr from a carrier, in which an nr/nr sheep mated with non-N may have an N-grade lamb. The original hypothesis of a main dominigene remains unproved. Better evidence is given that the nr gene helps horns to grow in N/+ . +/nr ewes. After repeating the back-cross, N/ +. +/nr x + / + .nr/nr, the conclusion is adopted that N and nr are independent, residing on different chromosomes. Results from that back-cross have strengthened the belief that at least the great majority of sheep of both sexes of the high-dosage genotype N/+ .nrlnr have the complete halo-coverage and hornedness of N/N. +/ +

ON SEX DETERMINATION IN THE DIASPINE SCALE PSEUDAULACASPIS PENTAGONA (TARG.) (COCCOÏDEA)

SUMMARY In the diaspine scale, Pseudaulacaspis pentagona, the males are haploid, with eight chromosomes, and the females are diploid, with 16. Meiosis is normal in the female and replaced by a simple mitosis in the male. Mating is required for the production of any offspring, either male or female. Egg production affords an example of combined sexual dimorphism and dichronism; the mothers first produce a series of eggs containing coral-colored female embryos and then, without interruption, lay a series containing pinkish-white male embryos. Cytological study of early embryogeny shows that a chromosome set is eliminated to produce the haploid condition of the male embryos. Chromosomes broken by X-rays were used as genetic markers to demonstrate that the eliminated set is of paternal origin. Symbionts, yeast-like in appearance, are present at the distal end of both male and female embryos. The polyploid sector of the embryo stems from the fusion of a cleavage nucleus with both polar bodies and is pentaploid with 40 chromosomes. As would be expected on a simple genetic basis, irradiation of fathers preferentially reduces the number of daughters.

GENETICS AND HUMAN CONGENITAL MALFORMATIONS

A BRIEF SURVEY of the vast subject of "Genetics and Human Congenital Malformations" can scarcely be expected to be systematic. Accordingly, discussion of the subject will be limited to a series of more or less unrelated observations concerning genetics and the study of human teratology. The first portion of this discussion will be devoted to three general points concerning which there seems to be room for misunderstanding between the genetically and nongenetically oriented student of teratology. The latter portion of the discussion will be concerned with drawing attention to some general genetic approaches which may be profitable in specific areas. In approaching the material in this fashion, familiarity with the current status of work on human teratology is assumed. It is further assumed that the principal reason for this Conference on Teratology is to consider useful lines of attack on outstanding problems in this area. It is important at the outset to establish the fact that, from the genetic point of view, human malformations are a very "mixed bag" indeed. At the one extreme there are malformations which have a clear-cut and relatively simple genetic basis. Many of the skeletal malformations, such as the relatively benign brachydactyly or polydactyly, or the more serious ectrodactyly (split hand) or acheiropodia, fall into this category. At the other extreme there are a few malformations, such as tracheo-esophageal fistula1 which thus far have not been demonstrated to show a noteworthy tendency to concentrate in particular families. The majority of congenital malformations fall into neither of these extremes, showing clearly statistical tendencies to occur in certain families, but in these tendencies not following any simple pattern.

The causal sequence of fruit development

In plums different pollinations can result in differences in the time of maturity and in the size of the fruit. The pollen influences the development of the fruit inasmuch as defective embryos promote earlier ripening and smaller fruits. Further, the wider the difference between the two parents, the more defective the embryos. The developmental differences in the fruit are an expression of different degrees of seed growth. In some varieties of pears frost injury to the styles induces parthenocarpy, i.e. the complete absence of embryo development. Difference in the fruit due to the action of the pollen (formerly known as xenia) fall into three classes: (1) where the developing zygote is affected, (2) where the endosperm is affected, (3) where the effect is on the maternal tissues. In classes (1) and (2) the effects or differences are due to the action of paternal genes in heredity, and since they are readily explicable on a simple genetic basis there is no reason why they should continue to be referred to as xenia. In class (3) they are due to differences in the constitution and development of the embryo, endosperm and seed which affect the development of the maternal tissue. At one extreme the fruits are seedless (parthenocarpy), and at the other they have seeds larger than normal.