82 



Monohybrids 



such gene, for meiosis operates in the same manner in a domi- 

 nant as in a recessive. In the heterozygote, one chromosome bears 

 C and the homologous chromosome bears c. Since these chromo- 

 somes separate during meiosis and enter different gametes, every 

 gamete must have C or c, but never both. Theoretically, exactly 

 half the gametes of the heterozygote would have C and exactly 

 half would have c. The two genes at a given locus segregate jrom 



Fig. 27. A single gene difference in Shepherd's-piirse : left, a rosette of 

 Capsella (Bursa) grandiflora and, right, a rosette if C. Viguieri of the same 

 age. The latter type possesses a dominant gene which produces a heavy 

 fasciation of the stem. 



one another at meiosis; as a result, only one member of the pair 

 of alleles is present in each gamete. This separation of the two 

 genes at any locus is the principle involved in MendeVs first law, 

 often called the law of segregation. The behavior of the genes 

 in the formation of gametes and plant spores is merely a func- 

 tion of the behavior of the chromosomes. 



The genetic constitution of an organism is the result of the 

 particular gametes which happen to unite when that individual is 

 formed. For example, if an egg bearing C is fertilized by a sperm 

 which has the C gene, a homozygous normal-winged fly is pro- 

 duced. If both the egg and sperm happen to contain the gene c, 

 the resulting individual will have curved wings. If the egg is C 

 and the sperm c, or if the egg is c and the sperm C, the new fly 

 will be genotypically heterozygous and phenotypically wild type. 



