We investigated the parent and cell division of origin of the additional sex chromosome in 142 males with a 47,XXY constitution and 50 females with a 47,XXX constitution. In 66 of the 47,XXY males the additional chromosome was paternal in origin and in 76 it was maternal in origin, while among the 47,XXX females only 5 had an additional paternal X chromosome, the remaining 45 having an additional maternal chromosome. Among the 107 maternally derived aneuploids for whom it was possible to determine the cell division of origin, 73 were the result of a mat MI error, 24 the result of a mat MII error and 10 the result of a post zygotic mitotic (PZM) error involving the maternal X chromosome. Among those in which the non-disjunction was attributable to an error at the first meiotic division (MI) we observed three different mechanisms of origin. Approximately 30% were associated with complete absence of recombination (nullichiasmate); approximately 24% were associated with a normal number of recombinant events but an abnormal distribution of exchanges (perturbed recombination), while approximately 45% were associated with a normal number and distribution of recombinant events (normochiasmate). Nondisjunction due to an error at the second meiotic division (MII) was associated with a slight reduction in the total number of recombinant events and an abnormal distribution of exchanges. Thus of the four different meiotic mechanisms of origin, three were associated with an abnormal number and/or distribution of exchange events. There was no evidence of an increased paternal age in the aneuploids of paternal origin.(ABSTRACT TRUNCATED AT 250 WORDS)

Three thousand seven hundred and eighry shoot ^apices of Fevuca scabrella Torr. were examined or-er a three-yea"r period. stages of dcvelopment and tr.ieht oi ,pii., ,no". the root-stcm-traniition were iecorded. lnitiation and

I h e s of a variety of common \\heat, S-615, rno~~osornic for chrornoson~es 111 and S \ ' I of homoeologous group 3 had culn~s less solid in the top internode than normal S-615. Monosomics of homoeologous group 5, namely, V, IX, and XVIII, were less solid in the bottom four internodes than S-615. These live chromosomes carry genes for solid stem in this variety. Monosomics X I X , X X , and X X I , the D-genome chromosomes of homoeologous groups 6, 2, and 7 respecti\-ely, were more solid than the normal check in both top and lower internodes, indicating that the missing chromosomes carry genes for hollow stem. Chromosome X I I I , a I~ornoeologue of X X , which in Chinese Spring has a gene for hollow stem, does not affect the amount of pith in the culm of S-615.The concept of the culm phenotype in a given environment resulting from an interaction of genes promoting pith development and those opposing it makes it possible to reconcile results of genetic experiments on solid stem in wheat that previously appeared contradictory.'Manuscript

Current hard red spring wheat (Triticum aestivum L. em Thell.) cultivars have only a moderate level of resistance to common root rot caused by Cochliobolus sativus (Ito and Knrib.) Dreschl. ex Dastar. The objective of this study was to determine if wheat‐alien amphiploid and chromosome substitution lines derived from Aegilops squarrosa L. and Agropyron species could be used as new sources of root rot resistance. A series of greenhouse studies established that Agrotana, a wheat‐A. trichophorum amphiploid, carried resistance to common root rot. Tests of the chromosome substitution series for the D genome in the winter wheat cultivar Winalta by chromosomes 1 to 7 of Ae. squarrosa demonstrated that the substitutions had no consistent effect on disease severity. Similarly, substitutions of chromosomes 4D, 5D, and 6D of Winalta by homoeologons chromosomes from A. elongatum and chromosome 4B for chromosome 4 from A. intermedium had no effect on root rot severity. Isolations from the subcrown internodeshowed that Agrotana had a significantly lower percentage of plants infected with C. sativus than did other exotic lines and common wheat cultivars, suggesting that it might have a different resistance mechanism against common root rot.