Abstract
The mammary growth response of immature mice treated with synthetic estrogen was studied in connection with the mechanism of the dairy performance. The study was designed to evaluate the magnitude of the genetic contribution to the phenotypic variance of mammary growth response and to ascertain whether two different mouse strains with regard to the character could be produced by the selection with inbreeding. At the same time, this paper dealt with the effectiveness of selection with inbreeding on the character and other quantitative characters from the point of the variability of inbred mice.
The results obtained are summarized as follows.
1) The value of heritability in the broad sense of this character was estimated as 0.59, indicating that the character was controlled considerably by the genetic factors.
2) The effectiveness of selection with inbreeding appeared after the third generation. Two groups of mice different in mammary growth response were established by selection with inbreeding.
3) The variability of this character showed a gradual decline by the selection with inbreeding. When a selected inbred, an unselected inbred, and a randomly bred groups were compared, the variabilities were the largest in the unselected inbred group, and the smallest in the selected inbred group. It was inferred that the developmental stability would be maintained even by the inbred animals, provided that selection for less variability was applied to them.
4) The same effectiveness as was found in mammary growth response was observed in the mean and the variability of a genetically correlated character (i.e., the body weight on the 35th day).
The author is greatly indebted to Professor M. NAITO and Mr. J. NAGAI for their kind advice and encouragement. This paper is a part of a thesis for doctorate to be submitted to the University of Tokyo. The study was supported by grants from the Scientific Research Fund of the Ministry of Education.
