Mendelian inheritance, dominant and recessive characters

Darwin (1859) recorded that some hybrids, instead of standing between their parents, closely resemble one of them.¹ He tied this to fertility. Such hybrids are usually extremely sterile, which showed him that fertility is independent of resemblance to either parent. Rádl (1930) described a separate line of work, followed by Naudin and by Mendel, which Darwin did not take up. In Rádl's account Mendel held that an organism is made of many characters, as substances are made of atoms. He also held that in hybrids the parental characters do not fuse but lie side by side.⁶ Mendel also held, Rádl reports, that as a rule only one of these characters develops while the other is latent.

Loeb (1906) reported Mendel's terms with the pea. The spherical seed was dominant and the angular seed recessive.² First-generation hybrids looked like the dominant parent, yet their sex cells differed. Loeb inferred that half carried the determinant for each character. Crossing such hybrids among themselves, he argued, gives one fourth pure recessive offspring by chance meeting of egg and pollen cells of the recessive type.³ Loeb (1912) stated the law of segregation in its simplest form. A hybrid between forms differing in one character forms two kinds of sex cells in equal numbers.⁴

Loeb (1912) argued that segregation had found its physical basis. He credited Boveri with placing the transmission of paternal characters in the chromosomes of the nucleus, and said the proof came from Mendelian breeding work. From experiments by Correns, Doncaster and especially Morgan, he concluded that the sex chromosomes carry hereditary characters that appear mainly in one sex.⁵ He added that the problem of sex determination is a special case of segregation, as Mendel had already suggested.

Wallace (2013) restates the rule in modern terms. Each parent gives one copy of each gene through the gametes, and the individual later separates the two copies into its own sex cells.⁷ Neuhof, Levin and Rechavi (2016) note that epimutations, which leave the DNA sequence unchanged, can segregate with the chromosomes in accordance with Mendel's rules.⁸ Trewavas (2025) criticises the teaching use of Mendel. He says Mendel's characters are exceptions, since most morphological and physiological characters do not behave simply.⁹