Dominance in genetics may be understood by comparing heterozygotes to homozygotes. In a heterozygous individual, only one trait is expressed phenotypically (physical expression of the trait).
For example, say we have two alleles for flower color at a locus- Red flowers are dominant over white flowers: A- produces red flowers a- produces white flowers.
Individuals with the genotypes:
A_ - red flowers
aa - white flowers
So, individuals with the genotype AA at that locus (position of alleles on the homologous chromosomes-homozygous dominant) are true breeding plants for red flowers.
Individuals with the genotype aa at that locus are homozygous recessive for white flowers, and are therefore true breeding for white flowers.
The plants with the genotype Aa (heterozygous). However, dominance is determined by this genotype because although this plant possesses both alleles for white and red flowers, it only phenotypically expresses the allele for red flowers. Therefore, the allele, A, for red flowers is considered to be dominant.
If we cross two heterozygous plants: Aa x Aa
Resulting Progeny in F1 generation: 1/4 AA 1/2 Aa 1/4 aa - Genotypic ratio
3/4 A_ (red) 1/4 aa (white) - Phenotypic ratio
Since most of the resulting progeny express red flower petals, the red allele is the dominant allele because it will be expressed even when the white allele is present.
For example, say we have two alleles for flower color at a locus- Red flowers are dominant over white flowers: A- produces red flowers a- produces white flowers.
Individuals with the genotypes:
A_ - red flowers
aa - white flowers
So, individuals with the genotype AA at that locus (position of alleles on the homologous chromosomes-homozygous dominant) are true breeding plants for red flowers.
Individuals with the genotype aa at that locus are homozygous recessive for white flowers, and are therefore true breeding for white flowers.
The plants with the genotype Aa (heterozygous). However, dominance is determined by this genotype because although this plant possesses both alleles for white and red flowers, it only phenotypically expresses the allele for red flowers. Therefore, the allele, A, for red flowers is considered to be dominant.
If we cross two heterozygous plants: Aa x Aa
Resulting Progeny in F1 generation: 1/4 AA 1/2 Aa 1/4 aa - Genotypic ratio
3/4 A_ (red) 1/4 aa (white) - Phenotypic ratio
Since most of the resulting progeny express red flower petals, the red allele is the dominant allele because it will be expressed even when the white allele is present.
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