MENDEL'S LAWS OF INHERITANCE 283 



allelomorph or allele. One of a pair of genes or characters that are contrasted in 

 inheritance. Tall and dwarf are examples of allelomorphic characters; D and d, 

 of allelomorphic genes. The term is applied to both characters and genes, but 

 characters that are not inherited as alternatives are not allelomorphic. 



gamete. A germ cell, either male or female. 



gametic formula. The formula of a gamete in terms of the genes it is known to 

 carry. Because of the principle of gametic purity, a gamete can have but a 

 single gene from any allelomorphic pair. Examples: d, D, Ab, AbC, abC, etc. 



zygote. The product of the union of two gametes. By extension, it is also applied 

 to the individual that is produced by the development of a zygote and is used 

 in opposition to the haploid gamete. 



zygotic formula or zygotic constitution. The formula for a zygote or individual 

 that it, he, or she is known to carry. Examples: dd, DD, Dd, Aa BB cc, etc. 



homozygote. An individual with like genes for the pair or pairs under considera- 

 tion. Examples: DD, dd, AA bb, etc. 



heterozygote. A zygote or individual with unlike genes for the pair or pairs 

 under consideration. Examples: Dd, Aa Bb, etc. Note: an individual may be 

 homozygous for some genes and heterozygous for others. Example: A A Bb. 



hybrid. The result of a cross of unlike parents, a heterozygote. 



monohybrid. A cross that involves but a single pair of contrasting genes. Some- 

 times applied to the F\ product of such a cross. 



dihybrid. A cross that involves two pairs of contrasting genes. 



trihybrid. A cross that involves three pairs of contrasting genes. 



genotype. The description of an individual in terms of the genes that he, she, 

 or it is known to possess. 



phenotype. The description of an individual in terms of its visible characters. 

 Tall is a phenotypic description, in the pea, and may be due to either of two 

 genotypes, DD or Dd. 



Pi. The original parent generation; typically consists of a male and female that 

 are each homozygous and differ in regard to one or more pairs of contrasting 

 genes. 



Fi. The first filial generation, the immediate offspring of the Pi; always all alike 

 and all hybrid. 



F 2 . The second filial generation, the offspring of Fi individuals, and the genera- 

 tion that shows various diagnostic phenotypic ratios. 



THE DIHYBRID CROSS 



Mendel's studies did not stop with the making and analyzing of mono- 

 hybrid crosses. His experiments also included several dihybrid and 

 trihybrid crosses that resulted in the discovery of a second great principle 

 of inheritance. We shall examine one of these crosses in detail. 



Among the varieties at Mendel's disposal was one in which the ripened 

 seeds were round in shape and had a yellow color; another variety had 

 seeds that, when ripe, were shriveled or wrinkled and retained their green 

 color. The cross between these varieties was effected precisely as was the 

 one between tall and dwarf — the pollen from an individual of one variety 



