A dominant trait refers to a genetic feature that hides the recessive trait in the phenotype of an individual. A dominant trait is a phenotype that is seen in both the homozygous AA and heterozygous Aa genotypes. Many traits are determined by pairs of complementary genes, each inherited from a single parent. Often when these are paired and compared, one allele (the dominant) will be found to effectively shut out the instructions from the other, recessive allele. For example, if a person has one allele for blood type A and one for blood type O, that person will always have blood type A. For a person to have blood type O, both their alleles must be O (recessive).
When an individual has two dominant alleles (AA), the condition is referred to as homozygous dominant; an individual with two recessive alleles (aa) is called homozygous recessive. An individual carrying one dominant and one recessive allele is referred to as heterozygous.
A dominant trait when written in a genotype is always written before the recessive gene in a heterozygous pair. A heterozygous genotype is written Aa, not aA.
Recessive trait
The term "recessive allele" refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in homozygous genotypes (organisms that have two copies of the same allele) and never in heterozygous genotypes. Every diploid organism, including humans, has two copies of every gene on autosomal chromosomes, one from the mother and one from the father. The dominant allele of a gene will always be expressed while the recessive allele of a gene will be expressed only if the organism has two recessive forms.[2] Thus, if both parents are carriers of a recessive trait, there is a 25% chance with each child to show the recessive trait.
The term "recessive allele" is part of the laws of Mendelian inheritance formulated by Gregor Mendel. Examples of recessive traits in Mendel's famous pea plant experiments include the color and shape of seed pods and plant height.
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