Mendelian genetics definitions allele one alternative

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Mendelian Genetics Definitions Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual Allelic pair - the combination of two alleles which comprise the gene pair Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote Genotype - the specific allelic combination for a certain gene or set of genes Using symbols we can depict the cross of tall and short pea plants in the following manner: The F 2 generation was created by selfing the F 1 plants. This can be depicted graphically in a Punnett square. From these results Mendel coined several other terms and formulated his first law. First the Punnett Square is shown. Union of Gametes At Random D d Punn ett Squar e D DD (Tall ) Dd (Tall) d Dd dd
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(Tall ) (Shor t) The Punnett Square allows us to determine specific genetic ratios. Genotypic ratio of F 2 : 1 DD : 2 Dd : 1 dd (or 3 D_ : 1 dd) Phenotypic ratio of F 2 : 3 tall : 1 dwarf Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete Confirmation of Mendel's First Law Hypothesis With these observations, Mendel could form a hypothesis about segregation. To test this hypothesis, Mendel selfed the F 2 plants. If his law was correct he could predict what the results would be. And indeed, the results occurred has he expected. From these results we can now confirm the genotype of the F 2 individuals. Phenotype s Genoty pes Genetic Description F 2 Tall Plants 1/3 DD 2/3 Dd Pure line homozygote dominant Heterozygotes F 2 Dwarf Plants all dd Pure line homozygote recessive Thus the F 2 is genotypically 1/4 Dd : 1/2 Dd : 1/4 dd
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This data was also available from the Punnett Square using the gametes from the F 1 individual. So although the phenotypic ratio is 3:1 the genotypic ratio is 1:2:1 Mendel performed one other cross to confirm the hypothesis of segregation --- the backcross. Remember, the first cross is between two pure line parents to produce an F 1 heterozygote. At this point instead of selfing the F 1 , Mendel crossed it to a pure line, homozygote dwarf plant. Backcross : Dd x dd Male Game tes d Femal e Game tes D DD (Tall) d dd (Short) Backcross One or (BC 1 ) Phenotypes : 1 Tall : 1 Dwarf BC 1 Genotypes : 1 Dd : 1 dd Backcross - the cross of an F 1 hybrid to one of the homozygous parents; for
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