The nucleus of a cell contains DNA molecules, which code for proteins.
In humans, DNA is organized into 46 linear double-stranded molecules called chromosomes . Chromosomes of similar length pair up to form 23 pairs.
On a pair of chromosomes, each chromosome contains the same genes in (roughly) the same places.
A gene is a DNA sequence that codes for a protein, which controls a particular trait of a person.
If two chromosomes (A and B) are together in a pair; Half of chromosome A has genes for eye color, and the other half of chromosome B also has genes for eye color.
However, genes can vary slightly, one gene can code for blue eye color or brown eye color.
A different version of the same gene is called an allele. On a pair of chromosomes, a person can have two different alleles (heterozygous) or two of the same (homozygous).
Some alleles can be expressed if the person only has one copy. These are called “dominant” alleles.
If a person has one allele for brown eyes (dominant) and one allele for blue eyes, they will have brown eyes (if they have two alleles for brown eyes, they will also have brown eyes).
Some alleles can only be expressed if the person has two copies.
These are called “recessive” alleles. For example, if a person has two alleles for blue eyes, they will have blue eyes.
- Dominant allele + dominant allele = dominant trait (homozygous dominant).
- Dominant allele + recessive allele = dominant trait (heterozygous dominant).
- Recessive allele + recessive allele = recessive trait (recessive heterozygous).
Traits or characteristics, such as hair and eye color, are determined by genes, sections of information about DNA.
DNA is like a blueprint for the individual, and genes are the individual instructions for building each piece.
For each trait, two copies of a gene are inherited: one from the mother and one from the father.
Each of these copies may be different. Different copies of the same gene are called alleles.
Organisms can be homozygous or heterozygous for a gene.
The term homozygous means that the organism has two copies of the same allele for a gene.
An organism can be homozygous dominant, if it carries two copies of the same dominant allele, or homozygous recessive, if it carries two copies of the same recessive allele.
The term heterozygous means that an organism has two different alleles of a gene.
Carriers are always heterozygous.
People with cystic fibrosis are homozygous recessive, unlike Huntington’s disease which is autosomal dominant, people with the disease can be homozygous dominant or heterozygous.
Dominant and recessive genes
Some copies of a gene are stronger than others. The instructions for these stronger alleles are followed to make traits, even when there is a copy of a weaker allele.
These stronger alleles that are expressed over others are called dominant alleles. Alleles that are not expressed if there is a dominant allele around are called recessive.
Genotypes and phenotypes
Taking into account the alleles of a gene present in an organism and the physical results, it brings us to the terms genotype, phenotype, and trait.
The genotype of an organism is its specific combination of alleles for a given gene.
The phenotype is the physical manifestation of an organism’s allelic combination (genotype). A trait is the general aspect of physiology that is shown in the phenotype.
The genotype and phenotype can be demonstrated using the human examples such as cystic fibrosis, people with normal-normal or mutant-normal genotypes have the normal phenotype, while people with the mutant-mutant genotype have the disease phenotype.
Dominant and recessive alleles
An allele is simply another copy of the same gene.
Each person receives two copies of each gene, one from the mother and the other from the father.
Each copy is called an allele of the other.
For example, the brown color of the eyes in a person is determined by two genes (BB).
One copy of the gene is contributed by the male father, and the other copy is contributed by the female father.
If the person receives dominant versions of the gene (dominant alleles) from both parents, they will have brown eyes.
If both of your alleles for eye color are recessive (bb), then you will have blue eyes.
Dominant alleles are those that express their trait even when a person has only one copy of the allele.
In other words, it is not necessary for a person to have two dominant alleles in order to express the trait.
For example, a person with BB or Bb will have both brown eyes.
On the other hand, recessive alleles are those that need to be present in two copies in order to express the trait encoded by them.
For example, only a person with bb will have blue eyes. If a person has only one recessive allele, then they will only express the trait of the other allele, which is dominant.
Often times, organisms will carry both a dominant and a recessive allele of a gene. These organisms can be called carriers of the recessive allele.