140 Dihybrids, Trihyhrids, and Poly hybrids 



of genes could segregate independently as maize has twenty 

 somatic chromosomes. When two plants or animals differ by 

 more than three pairs of genes, the Fq phenotypic and genotypic 

 ratios are correspondingly more complicated than for a trihy- 

 brid. Phenotypic ratios up to a tetrahybrid are: 



Monohybrid 3:1 



Dihybrid 9:3:3:1 



Trihybrid 27:9:9:9:3:3:3:1 



Tetrahybrid 81 : 27 : 27 : 27 : 27 : 9 : 9 : 9 : 9 : 9 : 9 : 3 : 3 : 3 : 3 : 1 



When the Fi is heterozygous for more than three pairs of 

 genes, the F2 ratios become very large. Fortunately, in solving 

 practical genetic problems, it is rarely necessary to deal with 

 such complicated ratios. Polyhybrids are interesting algebra- 

 ically, since they show that phenotypic ratios involving a number 

 of genes in different chromosomes can be obtained by applying 

 the binomial theorem, expanding the binomial (3a + b)", where 

 the exponent of a represents the number of dominant genes in a 

 term, the exponent of b the number of recessives, and n the 

 number of heterozygous genes in the parent. The Fo can thus 

 be determined for any number of heterozygous pairs of genes on 

 separate chromosomes. 



Heterozygous Populations in Man 



From considering the complicated ratios involved in polyhy- 

 brids involving four and more pairs of genes, we learn the im- 

 portant fact of the great diversity resulting from self-fertiliz- 

 ing a plant that is heterozygous for a large number of genes. 

 This great diversity is especially important in studying inherit- 

 ance in human beings. People are frequently encountered who 

 "do not believe in genetics" because children often differ greatly 

 from either or both their parents. When the facts are consid- 

 ered, such divergence from the parents should be expected as 

 well as considerable variation among brothers and sisters, for 

 there are twenty-four pairs of chromosomes in man and a large 

 number of genes in human beings are in a heterozygous condition. 

 As most individuals are heterozygous for a great many genes 

 considerable diversity should be exhibited in their children. 



A plant or animal heterozygous for one pair of alleles, when 

 self- fertilized, or two individuals heterozygous for the same 



