Hygiene, dental and general . e male; none of the sexcells have the characteristic for blackness. Animals C (female) and D (male)are hybrids. In each case half of the sex cells carry the characteristic forblackness and half of them lack this characteristic. When C and D are crossedthe offspring of the second generation are in the ratio to three black to onewhite. The diagram illustrates each possible chance-combination of sperm andegg and it will be seen that, of the second generation animals, E is homozygous,F and G are heterozygous like C and D, and H is a homozygous white. the characteristi
Hygiene, dental and general . e male; none of the sexcells have the characteristic for blackness. Animals C (female) and D (male)are hybrids. In each case half of the sex cells carry the characteristic forblackness and half of them lack this characteristic. When C and D are crossedthe offspring of the second generation are in the ratio to three black to onewhite. The diagram illustrates each possible chance-combination of sperm andegg and it will be seen that, of the second generation animals, E is homozygous,F and G are heterozygous like C and D, and H is a homozygous white. the characteristic for blackness. Such a union results in awhite pig of pure stock so far as color is concerned. Two other possibilities involve the mating of a female cellcontaining the characteristic with a male cell lacking the char- 84 HYGIENE: DENTAL AND GENERAL acteristic or the exactly opposite condition. In either casewe have a hybrid individual like the parents, that is, one andonly one of the pair of chromosomes with which the color. Fig. 17.—Eminent Twins. Edwin P. Grosvenor (left), prominent New York-lawyer, and his twin brother Gilbert Grosvenor (right), President of the NationalGeographic Society. (Photograph from Gilbert Grosvenor.) factor is connected contains the characteristic. Such animalsare black in color because a pigment factor is present, al-though only in half quantity, and if again bred among them- HYGIENE OF REPRODUCTION 85 selves we once more have a 3-1 ratio, that is, three black off-spring to one white. Naturally since this ratio depends uponprobability or chance it is not exact. However, the greaterthe number of offspring the more exact is the ratio. Thisthree to one ratio is in reality a ratio of one pure black: 2hybrid blacks: 1 pure white. Such an hereditary trait as theone just described is called a unit character. The study of genetics has taught us that many character-istics are inherited like the unit factors mentioned being the case, it is obv
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