A, a young ear of corn. B, formation of lily zygote the union of male and female gametes: pe, Primary endosperm (triple fusion) nucleus; sn, sperm nucleus; e, egg. C, the single-celled zygote and endosperm of lily: pe, primary endosperm nucleus; sn, sperm nucleus; e, egg; z, zygote; sy, synergid. D, the corn embryo 5 days after pollination: en, endosperm; pl, posterior lobe; su, suspensor; p, pericarp. E, the corn embryo lo days after pollination: co, coleoptile; st, future stem tip; en, endosperm; Pl, posterior lobe; r, root apex; su, suspensor. F, position and relative size of the corn embryo 10 days after pollination: em, embryo; en, endosperm; nu, nucellus remnant; p, pericarp; sk, silk remnant. G, the corn embryo 12 days after pollination: co, coleoptile; 11, first lea st, stem tip; se, scutellum; en, endosperm; r, root tip; p, pericarp; su, suspensor. H, the corn embryo 15 days after pollination: sc, scutellum; co, coleoptile; l¹, first foliage leaf; 1², second foliage leaf; 1³, third foliage leaf; st, stem tip; r, radicle; cz, coleorhiza. I, the corn kernel at 30-35 days after pollination: en, endosperm; sk, silk scar; p, pericarp; sc, scutellum; co, coleoptile; 1, five folded leaves; st, stem tip; r, radicle; sc, scutellum; cz, coleorhiza.

A plant grown from a resulting kernel carries on one of its chromosome No. 1 homologs a gene that gives colorless pericarp. On the other homolog it carries a gene that gives red pericarp. The kernels of this plant will be red. The reason is that when the no-color and color-producing genes are in the same plant, the color gene has a dominant effect and suppresses the other factor.

During meiosis of the new plant, following the pairing of the two No. 1 chromosomes and their subsequent migration to opposite poles, two of the resulting megaspores carry color genes, and two carry no-color genes.

When the three megaspores disintegrate, there is an equal chance that the surviving megaspore will carry the color- or no-color-producing gene. When we consider all the kernels on an ear of corn, we can expect that half of the egg cells will carry a gene for red pigmentation and the other half will carry the no-color gene.

As we pointed out, meiosis also precedes the formation of pollen in the tassel of the plant. Half of the pollen therefore bears a no-color gene. The other half carries the gene for red grains.

Let us assume that we self-pollinate this plant: We collect pollen of the tassel and apply it to the silk in a way to keep away the pollen from nearby plants. There is an even chance that the egg cells will be fertilized with a sperm nucleus that carries a color gene. There also is an even chance that they will be fertilized by a sperm with the no-color gene. Because half of the egg cells carry the no-color gene and half of the pollen grains also carry this gene,one-quarter of the new zygotes on an ear of corn will carry the no-color gene in each of the two No. 1 homologous chromosomes.

It follows that one-quarter of the corn kernels will give rise to plants bearing yellow or white ears of corn.

Similar reasoning establishes that one-quarter of the new zygotes will carry the color gene in both of the homologous chromosomes, and the resulting plants will have red kernels.

Finally, one-half of the new zygotes will be heterozygous that is, they will be carrying the colorless factor on one chromosome and the color factor on the other. Because of dominance of the color gene over the colorless gene, this class of zygotes will give rise to plants with kernels that have colored pericarps.

If the pigments of underlying kernel parts were yellow in both parents, we would therefore expect three-quarters of the progeny of the self-pollinated ear of corn to give rise to red ears of corn and one-quarter of the progeny to bear yellow ears. Here we have the familiar 3:1 ratio that occurs with the segregation of gene pairs when one of the genes is completely dominant over the other. Such ratios, called Mendelian ratios, were discovered by Gregor J. Mendel in Austria about 100 years ago.

When a plant is heterozygous for two pairs of genes located on two different chromosome pairs, both of which may affect the same character or two different characters, their assortment during meiosis is completely independent and at random. When the plant is self-pollinated, the proportion of segregates of the various types in the resulting offspring can be predicted by simple mathematical calculations.

Many characters are determined by a large number of genes. The precise influence of any one gene cannot be measured. Studies on the inheritance of such characters are restricted to quantitative measurements that reflect the aggregate effect of a number of genes. Inheritance of characters controlled by many genes is known as quantitative inheritance.

Some genetic factors carried by the generative nuclei of pollen grains have an immediate effect on characteristics of the developing endosperm. It will be recalled the endosperm develops from a triple-fusion nucleus, two components of which are provided by the ovule parent and one component by the pollen parent. When the pollen parent carries a dominant endosperm character and the ovule parent carries the recessive counterparts, the effect of the pollen parent is observable in the developing seed. This immediate effect upon the endosperm is known as xenia.

If the ovule parent of the corn, for instance, carries the genetic factor resulting in white endosperm and the pollen parent carries the corresponding gene for yellow endosperm, the developing endosperm will have a light yellow appearance. If the pericarp is colorless so that the endosperm color can be seen, the end result will be that a yellow ear of corn will be borne on a genetically white plant.

Other factors that produce xenia have been found in corn. Several brown and blue colors exhibit xenia when crossed onto white corn. Similarly, the genetic factors that condition starchy field corn will exhibit xenia when crossed onto plants of sweet corn.

EARLIER we mentioned the linear arrangement of genes on the chromosomes. The genes conditioning two characters may be located on the same chromosomes. If the two genes are located close to each other on the chromosome, the two characters they control in most instances will be inherited together. This phenomenon is called linkage.