HEREDITY 



575 



will again contain two chromosomes of 

 each kind, as did the primary oocyte and 

 spermatocyte from which the gamete (egg 

 or sperm) arose. Further, with future mitotic 

 divisions, all of the resulting somatic cells 

 will contain the diploid (double) number of 

 chromosomes. Only when maturation recurs 

 will you find the haploid (single) number 

 present in cells. Though chromosome pairs 

 are present in somatic cells, they do not 

 usually synapse in somatic cells. 



Returning to the first maturation division, 

 it should be remembered that the two mem- 

 bers of a pair of homologous chromosomes 

 in the primary spermatocyte or oocyte came 

 originally from two sources, one from the 

 father and one from the mother of the or- 

 ganism undergoing maturation. When the 

 chromosomes orient on the first maturation 

 division spindle, the paternal chromosome 

 of a particular pair may go to the same pole 

 as the paternal one from another pair, or 

 it may go to the opposite pole. Each pair 

 then is oriented independently of every 

 other pair, and hence any combination of 

 maternal and paternal chromosomes is pos- 

 sible in an egg or sperm, a fact which per- 

 mits a wide variety of recombinations of 

 genes, and hence traits, to occur. On the 

 average, half of the chromosomes of an egg 

 or sperm will be of paternal and half of 

 maternal origin, but one can predict that on 

 rare occasions, a sperm or egg may contain 

 by chance exclusively maternal or paternal 

 chromosomes. This independent behavior 

 of chromosome pairs in maturation is the 

 physical basis for the independent assort- 

 ment of genes, Mendel's Second Law. 



Distribution of genes during 

 fertilization and maturation 



Since the homologous chromosomes con- 

 sist of genes that affect the same traits, Fig. 

 405 may be used to understand the distribu- 

 tion of genes during fertilization and ma- 

 turation. Instead of writing the complete 

 descriptive name of each trait, the geneticist 



assigns symbols to represent the character. 

 For example (a) may be used to designate 

 a gene for albinism and (A) a gene for 

 pigment. Here, beginning at the left, the 

 sperm containing genes represented by (A) 

 and (B) fertilizes the egg containing genes 

 (a) and (b) to form a zygote {AaBb); 

 next shown, without including the steps 

 that lead up to it, is reduction division in 

 an individual with the genotype (AaBb). 

 The possible arrangements of the chromo- 

 somes on the spindle at the reduction divi- 

 sion is a matter of chance. Those shown not 

 only can occur but do so with equal fre- 

 quency. It depends on how the chromo- 

 somes are arranged in the equatorial plane at 

 reduction division as to what combination 

 of chromosomes and genes will result in the 

 gametes. With 2 pairs of chromosomes, as 

 used in this illustration, there are 4 differ- 

 ent possible combinations of chromosomes 

 to form 4 different kinds of gametes as in- 

 dicated: (AB), [ab], ((2B),and (Ab). 



Alleles 



Two genes at the same position (locus) 

 in the homologous chromosomes, but pro- 

 ducing somewhat different effects on the in- 

 dividual are called alleles. For example, the 

 gene responsible for red-green color blind- 

 ness is an allele of the alternative normal 

 gene. An individual is homozygous for a 

 trait when both the genes at corresponding 

 loci in homologous chromosomes are iden- 

 tical. An individual is heterozygous for a 

 trait if the two genes at any one locus in 

 homologous chromosomes are different for 

 a given trait. An organism may be homozy- 

 gous for some pairs of genes and heterozy- 

 gous for other pairs. 



Dominants and recessives 



In a heterozygote the genes are of two 

 types, dominants and recessives. A gene is 

 said to be dominant when the trait it repre- 

 sents appears in the heterozygote; its allele 



