. Animal biology. Zoology; Biology. INHERITANCE IN ORGANISMS 559 since the genes are similar all progeny will be alike and will resemble both parents. If two parents, both homozygous but for different characters, are crossed, the offspring will all be hybrid and will exhibit the character which is dominant. In crossing these hybrid or heterozygous individuals it is clear that different genes may be distributed to the different sex cells which the individuals develop and that thus when these unite wdth other cells, varying combinations of genes will occur. For instance, considering only one pai
. Animal biology. Zoology; Biology. INHERITANCE IN ORGANISMS 559 since the genes are similar all progeny will be alike and will resemble both parents. If two parents, both homozygous but for different characters, are crossed, the offspring will all be hybrid and will exhibit the character which is dominant. In crossing these hybrid or heterozygous individuals it is clear that different genes may be distributed to the different sex cells which the individuals develop and that thus when these unite wdth other cells, varying combinations of genes will occur. For instance, considering only one pair of genes, if the dominant gene received from one parent is indicated by A and the recessive gene received from the other parent, by a, some of the sex cells will possess A and others a. In Parenis P Cametes P. Black Bb Black Bb Gametes Of? (D Black BB Black Bb Black Bb r '7 alb/no bb I Fig. 385.—Diagram showing the results of crossing two guinea pigs differing by one character and each homozygous for that character, and the checkerboard showing the results in the F2 generation. {From sketches by D. D. Whitney.) the union of sex cells two A's may be brought together, or two a's, or an A may be united with an a. The individuals with two ^'s and those with two a's are homozygous, while those with A and a are heterozygous or hybrid for this pair of characters and will show the dominant character. These symbols being placed in circles corresponding to the cells, the facts may be fitted into diagrams showing oogenesis, spermatogenesis, and fertilization in successive generations as indicated in the accompanying schemes (Fig. 584). From these schemes it is apparent how segregation, is brought about by the chance union of similar genes and thereby produc- ing the homozygous types from hybrid parents. In the discussion of these successive generations it is usual to refer to the generation which forms the starting point as the P (parental) generation; the first generation pro-. Pleas
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