GENETICS 693 



The upper limit of a person's mental ability is determined geneti- 

 cally, but training, experience and other environmental influences play 

 a role in determining how fully the inherited abilities are developed. 

 Since the coordinated action of many pairs of genes is involved in the 

 inheritance of intelligence, the fortuitous combination of genes which 

 produced high intelligence in one or both parents may be separated so 

 that the offspring are less intelligent than either parent. Conversely, the 

 chance combination of favorable genes may produce a brilliant child 

 from parents of average intelligence; however, geniuses are never pro- 

 duced by two feeble-minded parents, 



294. Medical Genetics 



Within the past two decades rapid progress has been made in the 

 analysis of human genetics, and there are now several good texts of medi- 

 cal genetics and a number of medical schools have established depart- 

 ments or courses of instruction in the subject. It is proving possible to 

 detect genetic carriers of disease, i.e., individuals heterozygous for a 

 recessive trait such as sickle cell anemia, and thus to provide more ac- 

 curate estimates of the probability that the potential offspring of a 

 particular couple will have some particular inherited trait. The proper 

 use of our knowledge of medical genetics permits the physician to iden- 

 tify certain diseases more accurately and at an earlier stage in their 

 development, and thus to begin treatment or preventive measures. As 

 the infectious diseases are gradually conquered, the chronic diseases, 

 many of which are inherited, become more important in medical prac- 

 tice. Inherited conditions cause about half of all cases of blindness and 

 deafness, and they play a role in diabetes, epilepsy, certain heart dis- 

 eases, mental disease, cerebral palsy, arthritis and many metabolic dis- 

 eases. A knowledge of medical genetics is useful in certain medico-legal 

 cases, such as in disputed parentage, which was discussed previously. 



Questions 



1. Distinguish between complementary genes and supplementary genes; multiple factors 

 and multiple alleles; penetrance and expressivity; an inherited trait and a congenital 

 trait. 



2. A mating of an albino guinea pig and a black one gave 6 white (albino), 3 black and 

 3 brown offspring. What are the genotypes of the parents? What kinds of offspring, 

 and in what proportions, would result from the mating of the black parent with 

 another animal that has exactly the same genotype as it has? 



3. Mating a red Duroc-Jersey hog to sow A (white) gave pigs in the ratio of 1 red : 2 

 sandy! 1 white. Mating this same hog to sow B (sandy) gave 3 red : 4 sandy : 1 white 

 pigs. When this hog was mated to sow C (sandy) the litter had equal numbers of red 

 and sandy piglets. Give the genotypes of the hog and the three sows. 



4. A walnut-combed rooster is mated to three hens. Hen A (walniU-combed) has off- 

 spring in the ratio of 3 walnut : 1 rose. Hen B (pea-combed) has offspring in the ratio 

 of 3 vvalnut : 3 pea : 1 rose : 1 single. Hen C (walnut-combed) has only walnut-combed 

 offspring. What are the genotypes of the rooster and the three hens? 



5 The size of egg laid by one variety of hens is determined by three pairs of genes; hens 



■ with the genotype AABBCC lay eggs weighing 90 grams and hens with the genotype 



Tabbcc lay eggJ weighing 30 grams. Each dominant gene adds 10 grams to the weight 



