HEREDITY AND VARIATION 





gametes, i.e. the closer the lit ige be. 



loci of two genes are sufficiently far apart foi ... -. it. or i 



over to take place between them, they will i .th, u 



A 

 B 



A a 



Bb 



Co 

 a 



B 



rr\ 



A 



B 



«- 1 



a- 

 b 



3 



rb 



Ctjc 



C*k 



> 



c 



f S 



Fig. 441. 



The cytological basis of linkage (diagrammatic) a, two somatic chromosomes 

 carrying the genes A, B, C and a, b, c, respectively ; each chromosome 

 two chromatids, b, During the close pairing of the chromosomes in mekxts, bn 

 occur in the chromatids, which join up with different partners, i.e. segments oi the 

 chromatids become interchanged, c, At anaphase of the first division the ' 

 chromosomes with their interchanged segments are distributed to different nuclei, 

 and at anaphase of the second division each of the four daughter cbrooa 

 (d, e, f, g) is distributed to a different tetraspore. d and g are non-crc> md 



e and / cross -over types. The zygotes from gametes of those four I 

 independent inheritance for gene C and linkage between A and B. From Crane .mil 

 Lawrence.) 



of the fact that they lie on the same chromosome. By means of intensive 

 gene-analysis leading to the establishment of linkage groups, it is poasibl 

 construct " chromosome-maps " indicating the relative positions of the known 

 genes in the several chromosomes. 



Polyploidy. 



While gene-mutation — a change in the constitution of a gene — is the most 

 usual source of heritable variation, there an ral Other ways in which the 



hereditary material may undergo alteration. A portion of a chromosome D 

 break loose and (a) remain separate, or {b) rejoin in the inverted positj 

 (c) become attached to another chromosome or i combine with other 

 detached chromosome-segments, sometimes to form .1 ring. Such " chi 

 some-rearrangements " are shown diagrammatically in Fig. 442. The peculiar 

 behaviour of many Evening Primroses [Oenothera spp.), which, although 

 complex hybrids, breed practically true, is connected \\ith the formation of 

 chromosome-rings. Chromosome-rearrangements, though <>f frequent occur- 

 rence, will not be considered further here. An important .. 

 polyploidy. 



