Animal genetics / Frederick B. Hutt.

  • Hutt, Frederick Bruce, 1897-1991.
Date:
[1964]
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Animal genetics / Frederick B. Hutt. Source: Wellcome Collection.

    39/578 (page 17)
    SIMPLE MENDELIAN INHERITANCE 17 Returning now to our cross of black X red Friesians, the transmis¬ sion of genes for these colors is shown diagrammatically in Fig. 2-1. For that illustration, it is assumed that several animals are produced in the F1 generation, and that these are mated inter se ( among them¬ selves) to produce the second filial (F2) generation. The symbols $ and 9 , used by biologists to designate male and female, respectively, were borrowed from the astronomers, who long ago adopted the spear as their symbol for the planet Mars, and the looking glass for Venus. From the explanations given above it should be easy to follow through the diagram and to see why the F2 generation contains black and red calves in the ratio of 3 : 1. One need only remember that the Pi black parent can produce only gametes carrying the gene B; his mate's gametes must all carry b. The resultant Fi animals must therefore be Bb. Now, when these black Fi cattle are mated together, each animal produces gametes of two kinds and these occur in equal numbers. Half carry B; the other half carry the recessive allele b. To deter¬ mine all the kinds of calves that can be produced from the Fi par¬ ents, we must assume that each type of gamete produced by the cow (with respect to В and b) is fertilized by each kind produced by the bull. That is the purpose of the square at the lower part of Fig. 2-1. Such squares are called Punnett squares, after their originator, R. C. Punnett. This one shows that only four combinations are pos¬ sible and that the resultant F2 generation consists of animals in the proportion of: 1 BB black Ì 3 2 Bb black j 1 bb red 1 As the two kinds of black animals are indistinguishable, the visible ratio in F2 is 3 black : 1 red. It should be clearly understood that if the F2 generation consists of four animals only, these will not invari¬ ably be 3 black and 1 red. All four might be black, or all might be red, although the latter is less likely. With larger numbers, the ex¬ pectation of about 3 black : 1 red is more nearly realized. Mendel's Law of Segregation In the F2 generation, the red cattle are just as red as their grand¬ parent. They do not show any sign that both their Fi parents were black, and they are indistinguishable from cattle produced by two red parents. In cases like this, when the two contrasting parental