Recessive traits require two copies of the gene to manifest, while dominant traits only require one. Gregor Mendel first proposed the concept of dominant and recessive traits in genetics. Inheritance of genetic material from both parents creates homozygous or heterozygous alleles, with one allele remaining inactive in heterozygous individuals, representing a recessive trait. Mendel’s work with peas showed that purple was dominant and white was recessive for flower color. The presence of recessive traits explains why two dark-haired parents can have a light-haired child.
A recessive trait is a trait caused by a recessive gene, meaning someone must inherit two copies of the gene for the trait to manifest. In contrast, a dominant trait only requires one copy of the gene. The concept of dominant and recessive traits in genetics was first proposed by Gregor Mendel, a 19th century scientist who is often considered the father of genetics, thanks to his extensive work in the field.
When organisms such as humans reproduce, they produce young with a genetic inheritance from both parents, caused by the fusion of haploid sperm with haploid eggs to create a diploid organism. In the case of humans, the genetic material is encoded in 46 chromosomes, 23 of which come from each parent. Each chromosome contains a set of alleles or genes that encode various information, from the color of a child’s eyes to the way its feet develop.
When both parents’ alleles are the same, someone is said to be homozygous for that allele. If a child inherits two different alleles, such as a gene for red hair and a gene for brown hair, he or she is said to be heterozygous for that allele. When someone inherits the same allele twice, that trait will manifest whether it is dominant or not. When someone is heterozygous, however, one of the alleles will remain inactive and this allele represents a recessive trait.
Gregor Mendel did much of his work with peas. In the course of his research, he learned that purple was a dominant trait for flower color, meaning a flower needed to inherit only one purple gene to produce purple flowers. White, on the other hand, was a recessive trait, so only peas that were homozygous with white at the flower color allele would develop white flowers. For shorthand, he has adopted the convention of using uppercase letters to indicate a dominant trait and lowercase versions to indicate a recessive, in this case P and p to indicate the color of the flower.
The presence of recessives explains why two dark-haired parents can have a light-haired child, because the light-haired gene is a recessive trait, and therefore would not occur in parents who had a dark-haired gene. If both parents of a child carry the dominant and recessive genes, there is a 25% chance that the child will develop light hair, a 50% chance that the child will have dark hair while carrying the hair allele and a 25% chance that the child is homozygous for dark hair.
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