172 FUNDAMENTALS OF CYTOLOGY 



Two contrasting characters such as tallness and dwarfness are said 

 to be allelomo7'phs or alleles. The same terms are also used for the pair 

 of differential genes influencing them. The corresponding adjectives 

 are allelomorphic and allelic. 



An individual is said to be homozygous for a given allelic factor pair 

 if it has received the same type of factor from the two parents — a pea, 

 for example, with the constitution T T or t t. If it has unlike members 

 in the pair, such as T t, it is said to be heterozygous. It may be homozy- 

 gous for some pairs and heterozygous for others, or it may conceivably 

 be either homozygous or heterozygous for all its factors. Thus an 

 organism with the genotypic constitution A A B b c c is homozygous for 

 the factors A A and cc and heterozygous for Bb. It is a pure dominant 

 with respect to A and a, a pure recessive with respect to C and c, and a 

 hybrid with respect to B and b. The phenotypic appearance of the 

 organism is here determined by the dominant factors A and B and the 

 recessive c. It is a common practice to represent dominant factors by 

 capital letters and their recessive alleles by the corresponding small 

 letters. 



Explanation in Terms of Chromosomes. — The basic reason for the 

 manner in which an allelic pair of characters is inherited becomes evident 

 when factor distribution is compared with chromosome distribution. 

 Each parent furnishes the offspring with one chromosome set, or genome. 

 Referring to Fig. 75, it is seen that in meiosis the two members of an}' 

 homologous chromosome pair, e.g., the ones marked A and a, are so 

 distributed that a descendant of A lies in half the spores or gametes, 

 while a descendant of a lies in the rest of the spores or gametes. If 

 such an organism is self-fertilized or crossed to a similar one, both A 

 gametes and a gametes being functional in each sex, random unions will 

 result in combinations of three kinds (A A, A a, a a) in the ratio of 1:2:1. 

 Hence the inference is that a given allelic pair of genes is located in a 

 homologous pair of chromosomes. If the genes T and t are so located 

 in a chromosome pair of the garden pea, Mendel's results with tallness 

 and dwarfness are accounted for. His first law was a result of chromo- 

 some disjunction at meiosis, although at the time of its formulation 

 almost nothing was known about chromosome behavior. 



Mendel^lso studied six other pairs of heritable characters in peas. He 

 observed that all seven pairs (including tallness and dwarfness) were 

 inherited independently, i.e., while each character gave the usual ratio 

 with its allele, there was no tendency on the part of any character to 

 appear more often with a given nonallelic character than with any other 

 after a testcross. This independence of character pairs was stated in 

 Mendel's second law. A case of this kind is illustrated in the left-hand 

 part of Fig. 123, which shows also the cytological basis for it. The two 



