578 



COLLEGE ZOOLOGY 



Gamete (sperm) 



Gamete (egg) 



Gametes 



Possible 



arrangements of 



chromosome pairs 



on the reduction 



division spindle 



Cell division 



Gametes 



Figure 405. Diagram to show the union of gametes to form a zygote, from which an animal 

 is eventually developed; when this individual forms gametes with 2 pairs of chromosomes, there 

 are 4 possible combinations of the chromosomes to form 4 different kinds of gametes as follows: 

 (AB), (ab), (Ab), and (aB). Details of the maturation process as shown in Fig. 404 are 

 omitted to save space. It is a genetic custom to represent dominant genes in chromosomes with 

 capital letters and recessive genes with small letters. 



in such a case is said to be recessive. For 

 example, in the fruit fly Drosophila, the 

 gene for red eyes is dominant and that for 

 white eyes recessive; in man, it appears that 

 freckles are dominant and the absence of 

 freckles is recessive; and in cattle, hornless- 

 ness is dominant, and horns recessive. Simi- 

 larly among plants, Mendel found tallness 

 in peas dominant, and shortness recessive 

 (Fig. 408). The result of dominance and 

 recessiveness in allelic genes is that heterozy- 

 gous individuals do not exhibit all that they 

 inherit from their parents, since some of 

 the genes are recessive. 



When we say that we inherit certain 

 traits, we mean that heredity potentialities 

 (the genes) are the things that are really 

 inherited, and never the developed trait. We 

 do not inherit our father's nose, but we 

 may inherit from our father a gene for a 

 nose that looks like his. 



Genotype and phenotype 



The term genotype is used to describe 

 the sum total of the genes inherited from 

 both parents; these include both the domi- 

 nant genes, whose corresponding traits ap- 

 pear in the individual, and the recessive 

 genes that may have no detectable effect. 

 The term phenotype is used to describe the 

 individual as shown by his expressed traits. 

 (Frequently phenotype is incorrectly defined 

 as the appearance of the individual; this 

 definition is limited to physical traits such 

 as hair color, but does not include physio- 

 logic traits such as the blood groups.) The 

 following examples will serve to illustrate 

 what actually happens. 



Homozygotes 



When a homozygous individual with 

 black curly hair mates with a homozygous 



