An Introduction to the Invertebrates, Second Edition - tiera.ru

4 THE PROCESS OF EVOLUTION: NATURAL SELECTIONFig.1.2 Cell division: (a) mitosis,(b) meiosis, (c) crossing over.Squares represent genes,circles represent points of junction.same process of mitosis but then divide by ‘meiosis’, a process inwhich the number of chromosomes is halved and (usually) the twogenes in each allelomorphic pair are separated as Mendel postulated(Figure 1.2b). The fusion of gametes combines half the genes of eachparent to make a new individual. Gametes are described as ‘haploid’since they contain half the number of chromosomes of the ‘diploid’zygote and adult individual.Only the gametes carry genes to the next generation. Changesoften occur in the somatic cells, caused by use or disuse or by directeffects of the environment, but such changes cannot be transmitted