This posting has been replaced by Identity of the Product of Mendel's Green Cotyledon Gene (Update).
2. To the difference in the color of the seed albumen (endosperm). The albumen of the ripe seeds is either pale yellow, bright yellow and orange colored, or it possesses a more or less intense green tint. This difference of color is easily seen in the seeds as their coats are transparent.Mendel's reference to the color of albumin, or endosperm, is inaccurate. He was actually observing the color of the cotyledons—the "seed leaves" that surround the embryo in the pea seed. These tiny leaves are covered by a seed coat that is partially transparent.
In wild-type peas the seeds turn yellow as they mature (i) but certain mutants exhibit a "stay-green" phenotype where the peas retain their green color (I). The figure shows seeds from a plant with the II genotype (top) and the ii genotype (bottom). The seed coat has been removed from the lower pair of each group of four peas.
In a paper just published in the Proceedings of the National Academy of Sciences (USA) a group in Japan has identified the "stay green" gene that Mendel worked with (Sato et al., 2007). It turns out that the gene, called SGR (stay-green), encodes an enzyme that is localized to chloroplasts and plays a role in the degradation of chlorophyll during senescence and maturation of seeds. When the enzyme is defective chlorophyll isn't broken down and the tissue stays green.
This brings to three the number of Mendel's genes that have a known function. The wrinkled pea phenotype is caused by a defect in the gene for starch branching enzyme (Bhattacharya et al., 1990) [Biochemist Gregor Mendel Studied Starch Synthesis]. The tall/short phenotypes are caused by defects in the gene for gibberellin 3β-hydroxylase (Martin et al., 1997). Gibberellins are plant growth hormones.
[Photo Credit: The photograph of mutant and wild-type pea seeds is taken from Figure 1 of Sato et al. (2007)]
Bhattacharyya, M. K., Smith, A. M., Ellis, T. H., Hedley, C., and Martin, C. (1990) The wrinkled-seed character of a pea described by Mendel is caused by a transposon-like insertion in a gene encoding starch-branching enzyme. Cell 60:115-122.
Martin D.N., Proebsting W.M., Hedden P. (1997) Mendel's dwarfing gene: cDNAs from the Le alleles and function of the expressed proteins. Proc. Natl. Acad. Sci. (USA) 94:8907–8911.
Sato Y., Morita R., Nishimura M., Yamaguchi H., and Kusaba M. (2007) Mendel’s green cotyledon gene encodes a positive regulator of the chlorophyll-degrading pathway. Proc. Natl. Acad. Sci. (USA) (early publication, [August 20, 2007]).