86 



CELL HEREDITY 



At meiosis a hetorozvgote produces gametes which have either one allele 

 or the other. The sperm and eggs of an M /M' heterozygote are either 

 M^' or M^ . If these two types are produced in a 1:1 ratio, and if sperm 

 combine with eggs at random, then there are in equal frequencies four 

 possible diploid combinations at fertilization: 



Parental (Pj ) 

 generation 



Genotype 



Father 



M »fN 



M'"M 



Mother 



M^M^ 



Genotype 

 of gametes 



Filial (Fi) 



generation 



genotypes 



Average 

 frequency 



Ratio 



(" M^ j ( ^^ ) 



The two heterozygotes are indistinguishable; therefore the proportions 

 among the progeny of an MN x M'N mating should be on the average 

 1:2:1. Table 3.4 shows that this is indeed a fact. A confirmation of the 

 genetic heterogeneity of the gametes of heterozygotes is found in the re- 

 sults of a cross of a heterozygote with a homozygote; there should be 

 only two diploid combinations in equal proportions and this is found 

 to be the case. 



The heterozygotes in the example just given expressed both alleles and 

 formed M and N red blood cell antigens. More often one allele dom- 

 inates. For example, an individual heterozygous for genes determining 

 brown and blue eye color has brown eyes; the gene for brown eyes is 

 dominant and that for blue eyes is recessive. The heterozygote cannot 

 readily be distinguished from a brown-eyed homozygote, showing that 

 individuals may have indistinguishable phenotypes although their 

 genotypes differ. This circumstance determines the ratio of phenotypes 

 observed among the progeny. Consider a mating of a homozygous 

 brown-eyed individual (B/B) with a homozygous blue eyed one (b/b). 

 Their Fj offspring would all be brown-eyed but heterozygous {B/b). If 



