Heredity and Development Second Edition (1972) / Chapter Skim
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3 Mendelism
3 Mendelism
Pages 49-69
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From page 49... ...
During the 1890s there had been renewed interest in plant hybridization. The three scientists who ‘discovered' Mendel -- Hugo de Vries, Carl Correns, and Erik von Tschermak -- were doing breeding experiments of their own.
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From page 50... ...
In all he studied seven different characters of the pea, and for each he had two varieties, as shown in the following lists: CHARACTER AFFECTED VARIETIES Seed shape round or wrinkled Seed color yellow or green Seed coat color colored or white Pod shape inflated or wrinkled
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From page 51... ...
Dominance can be determined only by making a cross and observing the type of offspring obtained; it could not be predicted before the experiment was performed merely by examining the parent plants. The varieties that Mendel used were found to have these relationships: DOMINANT RECESSIVE round seed wrinkled seed yellow seed green seed colored seed coat white seed coat inflated pod wrinkled pod green pod yellow pod axial flowers terminal flowers long stem short stem Over the course of years, geneticists have introduced some terms that make it easier to discuss crosses.
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From page 52... ...
He allowed a number of the wrinkled plants to self-fertilize to give an F3, and found that all bred true, that is, only wrinkled plants were obtained in the F3. The F2 round plants gave two results: 1.
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From page 53... ...
If self-fertilized, it would breed true. The remaining 2/3 of the round plants are Rr.
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From page 54... ...
Since an F1 plant having the Rr constitution will produce 50 per cent gametes of the R type and 50 per cent gametes of the r type, the mathe matical basis of the F2 frequencies will be as follows: Crosses Involving Two Pairs of Genes. Mendel's next step was to see if the conceptual scheme devised for crosses involving one pair of genes could be applied to crosses involving two pairs of genes.
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From page 55... ...
HEREDITY AND DEVELOPMENT: SECOND EDITION 55 In the formation of gametes by the F1 plant, Mendel assumed that a gamete would have only one member of a pair of genes. Thus, a gamete would have either R or r and in addition either Y or y.
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From page 56... ...
F2 FRE- EXPECTED NUMBER ACTUAL QUENCY F3 RATIOS EXPECTED NUMBER RRYY 1/9 all round- 33 38 yellow RRYy 2/9 3 round- 67 65 RRYy yellow; 1 round-green RrYY 2/9 3 round- 67 60 RrYY yellow; 1 wrinkled yellow RrYy 4/9 9 round- 134 138 RrYy yellow; 3 RrYy round-green; RrYy 3 wrinkled yellow; 1 wrinkled green 301 301
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From page 57... ...
The expected and actual values are rarely identical: we should not expect them to be so any more than we should always expect five heads for every ten tosses of a coin. Mendel went one step farther and crossed plants differing in three contrasting characters.
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From page 58... ...
Mendel studied seven pairs of contrast ing characters. One member of each pair of contrasting characters could be thought of as dominant and the other as recessive.
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From page 59... ...
It must be remembered that, without further work, these conclusions could apply only to the seven pairs of pea genes actually studied by Mendel. The fact that Mendel's rules applied to these would indicate that other genes of peas might behave in a similar way.
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From page 60... ...
It was difficult to accept any biological phenomenon for which there seemed to be no mechanism. Carl Correns conducted extensive crosses among varieties of corn and peas and also published in 1900.
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From page 61... ...
The majority of these gave ratios that were nearly identical with Mendel's pea crosses. That is, in a cross of a pure-breeding dominant and a pure-breeding recessive, the F1 population consisted only of plants with the dominant characteristics.
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From page 62... ...
breeding round plant, which we have designated RR,* is homozygous.
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From page 63... ...
This is not the case, as shown by some experiments with chickens which were reported by Bateson. Poultry breeders recognized a number of comb types, usually involving differences in comb shape.
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From page 64... ...
The answer to this question hinges on the fact that the single comb condition results from the interaction of two different genes.
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From page 65... ...
The genotype of a single comb chicken, therefore, is pprr. A pea comb chicken would have the genotype PPrr or Pprr and a rose comb chicken would have the genotype ppRR or ppRr.
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From page 66... ...
Cuénot reported some crosses in mice that neither he nor Bateson could explain in the usual Mendelian manner. He worked with yellow and agouti hair color genes.
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From page 67... ...
The answers lay elsewhere. Suggested Readings Chapter 3 of Readings contains long excerpts from the report that Bateson made to the Royal Society in 1902, in which he gave a detailed account of Mendel's experiments and adds much new data.
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From page 68... ...
c. In a family of nine children two have blue eyes and seven have brown eyes.
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From page 69... ...
15. Did you formulate a reasonable hypothesis to account for inheritance in yellow mice?
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