Certain diseases appear to be transmitted from parent to offspring by either dominant or recessive characteristics of the germ plasm. Their characteristics are briefly presented.
(a) Huntington's chorea is a condition in which there are marked involuntary, irregular contractions of the muscles leading to a more or less constant movement of the limbs. There is also dementia. It begins in middle life, affects both sexes about equally and is usually transmitted whenever present in either parent. It is therefore a dominant trait. Many of the cases in this country are traceable to several families belonging to an old New England colony.
(b) Albinism refers to a condition in which there is an absence of pigment from the skin, hair and eyes, a condition seen in white mice or rabbits. It is rare in man. It is undesirable since the lack of pigment makes the skin and eyes very sensitive to light. It is a typically recessive condition.
(c) Deaf-Mutism refers to a kind of deafness which begins early in life before the power of speech has been acquired, hence these individuals can neither hear nor speak. Fay has exten sively studied this inheritance. Of 335 matings of congenitally deaf parents, 25 per cent. yielded deaf offspring. A total of 779 children were born of whom 26 per cent. were deaf. That all children of such marriages are not deaf is doubtless due to the fact that the parents are not deaf in the same way and one parent brings into the combination something which the other lacks.
(d) Insanity may result from various causes. Some cases are the result of organic brain disease. Such are not inheritable.
On the other hand with functional insanity without well defined lesions, some evidence of heredity is available. It is the tendency to insanity or the neuropathic trait which is inherited. It follows the laws of heredity governing recessive characters.
(e) Feeble Mindedness like insanity in many instances is a recessive trait. It is not always easy to determine who is and who is not a feeble minded individual. At one end of the scale we have those who are exceptionally able. In a general way we may say that a child is feeble minded who for no obvious reason at the age of ten is no further advanced in his school work than are other children at the age of six or seven. Within normal variations we may have children one or two years behind or in advance of the average.
Mendel's law is also exemplified by what are known as sex limited diseases, examples of which are color blindness and hemophilia. These are diseases that appear almost only in the male line, but are transmitted only through the female line. Thus if a given man be affected, his sons will not have the disease nor will they transmit it to their offspring, nor will his daughters have the disease, but they will transmit it to their sons. The explanation of this is associated with the deter miner from the male chromosome. Striking examples of this type of heredity are observed with multiple sclerosis, atrophy of the optic nerve, color blindness, myopia, ichthyosis, muscular atrophy and hemophilia. •