DISEASES TRANSMITTED ACCORDING TO THE MENDELIAN TANCE Before considering these diseases it may be well to briefly review the more salient principles of mendelian inheritance.
Its principles have been found to hold true for all forms of life and we can perhaps illustrate it most conveniently by reference to the vegetable kingdom.
If the pollen of red flowered peas (male) is employed in fertilizing the ovaries (female) of a white flowered pea, the mature plants which develop from the seeds of this fertiliza tion (hybrids) are all red flowering like their male parent. The red flowering characteristic is stronger in leaving its influence upon its offspring than the white flowering charac teristic of the mother. The red characteristic is therefore said to be dominant and the white character recessive. The recessive character is not lost in the first generation of hybrids, but is only latent, as we shall find when those hybrids are crossed with each other. The offspring of those hybrids when brought to flower will not all be found similar to their parents, but will be arranged in groups according to very definite rules. In the first place three fourths of their number will be red flow ered, i.e., present the dominant characteristic, while one fourth will be white flowered, that is, present the recessive character. The two groups arc in a ratio of 3 : 1, sometimes called the Mendelian ratio. If now then like individuals of the third generation be crossed with each other, it will be found that among the progeny of the white flowers, only white flowered offsprings will occur in the fourth generation. On the other hand, of the progeny of the red flowers, one fourth will be as red as the original male parent, while one half will be less clearly red and show clearly that they are hybrids, and one-fourth will be white. Thus of the fourth generation, one-fourth will present only the dominant characteristic, one fourth the recessive character and the remaining half, though appearing like the dominant, possess the properties of hybrids. (Fig. 113).
One may designate by the term determiner the unknown substance in the germ cells which determines in some way the different characteristics of an individual, such as the color of the skin, the kind of hair, mental traits, etc.
If each of two parents possess a given character and each one transmits the determiner for that character to their offspring, then the offspring will possess two determiners for a given character, one from each parent. Then, unless there is some thing to prevent it, this offspring will in turn transmit two determiners to its progeny. If the other parent of this genera tion also contributes two determiners the third generation will possess four determiners for each character and so the num bers of determiners would multiply. This however is prevented by the division of the chromosomes in the germ cell before maturity is reached, in which it is assumed the determiners reside. Thus only one half of the determiners pass into each mature germ cell and there is actually no increase. Therefore in the union of hybrid germ cells each containing one deter miner for a given character, the determiners unite and depend ing upon whether they are similar or dissimilar, dominant or recessive, the offspring will according to our example, be either like the dominant male plant, the recessive female plant, or the hybrids, which possess both the dominant and recessive determiners alone. The former we can designate as a duplex, the latter (the hybrids) as simplex combinations of deter miners. Now when an individual who appears normal but has the dominant determiner in duplex is united with a hybrid, two kinds of offspring may result. One half will have the dominant character in simplex and one half in duplex but all will appear as normal. On the other hand if an individual with a recessive determiner in duplex, is united with a hybrid with one dominant and one recessive determiner, two kinds of offspring are likewise possible. One half will have one domi nant and one recessive character each (simplex) but appear like the normal who has the dominant character in duplex and one half will have the recessive character in duplex and will not appear true to the normal type. (Fig. 13).