Changes in the frequencies of these alleles will then bring about changes in the sex ratio as recorded by the investigator. The complexities and obscurities are inherent in the nature of fitness, the characteristic upon which natural selection acts. Instead of conducting a series of independent studies, we are effectively able to combine these studies into one. Factors affecting genetic resistance of mice to mouse typhoid. Vibrissa number in the house mouse is a very stable character with rare deviations from the count of 19 in normal mice. However, other studies indicate multifactorial inheritance.
Quantitative genetics of skeletal variations in the mouse. Gowen and Calhoun found a positive strain correlation between resistance to Salmonella and total leukocyte count. Quantitative characters: i It shows continuous variation. Characteristics are said to show multiple factor inheritance if their genetic variation depends on allelic differences at more than just a few loci. For further introduction to the subject consult Mather , the Cold Spring Harbor Symposium on Quantitative Biology , Li , Kempthorne , Lerner , and Le Roy.
Probably many genes are involved in the genetic variation of lifespan, although no study has been attempted to estimate the number. Nature and origin of heterosis, p. The pooled data of eight generations showed that the high line had a sex ratio of 45. He made a cross between a large Hamburg chicken with a small Sebright Bantam and found that the F 1 were of intermediate size. However, the cross exhibited heterosis in reproductive capacity. The variation was continuous as well.
This finding stimulated Weir and Schlager to select for total leukocyte count and examine the correlated response of disease resistance. Correlation between mean litter size and mean life span among 12 inbred strains of mice. This is considered as a minimum estimate because the equation is based on a number of assumptions. Studies of natural selection in mice have been made principally to observe its effect on a particular mutant introduced into a wild population, as in the study of Anderson et al. Selection for sex ratio in mice and Drosophila. The theory is that the response of a correlated character is fully explained by its genetic correlation r G with the selected trait. The first method makes it possible to mate assortatively, i.
In other words the animals are homozygous and as genetically alike as identical twins except for the necessary dimorphism caused by X and Y chromosomes. Such genes are called the cumulative or additive or polygenes. An Introduction to Genetic Statistics. Calculations of the maximum amount of genetic variation compatible with the observed data yielded heritability estimates of 6. Fuller also used this method for investigating the inheritance of alcohol preference in mice.
For example, 4 pairs of genes are responsible for determining the size of the chicken and in Hamburg variety only 3 are recessive and 1 dominant. Vibrissa number in the house mouse is a very stable character with rare deviations from the count of 19 in normal mice. Weir and Schlager found no perceptible decrease in litter size in 25 generations of fullsib matings in a study of selection for leukocyte counts. A few common examples of polygenic inheritance are described as below: Seed colour in Wheat: Nilsson-Ehle, crossed two varieties of wheat, red and white in colour and found that all the F 1 offsprings were intermediate between red and white i. The marked differences in corolla lengths were heritable pointing out that they are controlled by genes rather than environmental. The degree of genetic determination can be estimated by comparing two inbred strains, their F 1, F 2, and backcross generations.
When two or three genes for color are present together, they seem to interact in the duplicate manner so that white seed color is produced only when, all the genes are present in the recessive state. If too few animals are born, the species risks extinction. The environmental causes of the phenotypic variation of individuals within a single litter have not been ascertained. An interaction effect is said to exist when differences on one factor depend on the level of other factor. Work on tissue transplantation within strains showed that mice within a single strain were as alike as identical twins at the histocompatibility loci.
Maternal effects and cage effects fall in this category. Environmental differences do exist within litters, but these effects are not attributable to differences between uterine horns, or differences of position and presentation within the uterine horns. Peaceful Uses of Atomic Energy, Geneva 22: 394-399. Estimating the minimum number of loci affecting a quantitative character, Falconer found that 35 loci affect 6-week body weight of mice. Two methods can be used, both necessitating the raising of more mice than when choice of parents for the subsequent generation can be made before mating: 1 One can estimate the genetic value of particular individuals by measuring the characteristic on close relatives of the individual, or 2 one can breed the animals first, save all offspring, measure the parents, and then select the offspring on the basis of the results on the parents.