Genes determine traits, or characteristics, such as eye, skin, or hair color, of all organisms. Each gene in an individual consists of two alleles: one comes from the mother and one from the father. Some alleles are dominant, meaning they ultimately determine the expression of a trait. Other alleles are recessive and are much less likely to be expressed. When a dominant allele is paired with a recessive allele, the dominant allele determines the characteristic. When these traits or characteristics are visibly expressed, they are known as phenotypes. The genetic code behind a trait is known as the genotype.
|About||When an allele is dominant, the characteristic it is connected to will be expressed in an individual.||When an allele is recessive, the characteristic it is connected to is less likely to be expressed. Recessive traits only manifest when both alleles are recessive in an individual.|
|Documented||Uppercase Letter (T)||Lower Case Letter (t)|
|Example||Brown eyes trait, A and B blood type||Blue eyes trait, O blood type|
With respect to eye color, the allele for brown eyes (B) is dominant, and the allele for blue eyes (b) is recessive. If a person receives dominant alleles from both parents (BB) she will have brown eyes. If she receives a dominant allele from one parent and a recessive gene from the other (Bb) she will also have brown eyes. But if she receives recessive alleles from both parents (bb), she will have blue eyes.
Thus, in the case of Bb (dominant and recessive), brown (B) dominates and determines the eye color. This genetic material, which determines traits (the phenotype) is called the genotype. The genotype is considered homozygous when an individual has either two dominant alleles or two recessive alleles. The genotype is considered heterozygous when an individual has one dominant allele and one recessive allele. Note that genetic inheritance is complex and cannot always be explained in this simple manner — some people have green eyes, for example, and or one blue eye and one brown eye (heterochromia iridum).
Immediately below is a Punnett square, a table that demonstrates the probability of inheriting a certain trait, which in this case is eye color. The allele for brown eyes is upper case B and for blue eyes is lower case b.
|Brown (B)||Blue (b)|
|FATHER (Bb)||Brown (B)||BB||Bb|
If both parents contribute the dominant (B) allele, the child will be BB and have brown eyes. The child will also have brown eyes if she inherits the dominant allele (B) from one parent and the recessive allele (b) from the other parent. This is because the dominant allele (B) will override the recessive one (b). If both parents contribute the recessive allele (b), the child will be bb and have blue eyes, even though both parents may have brown eyes themselves.
Notice from the table above that both parents have brown eyes, but they also both have recessive alleles that they might pass on to a child. In such a scenario where both parents carry a dominant and recessive allele, there is a 75% chance the child will have brown eyes (BB or Bb) and a 25% chance he or she will have blue eyes (bb). 3 of the 4 scenarios modeled in the Punnett square show at least one B allele.
The last scenario (bb) shows how it is possible for the offspring to inherit recessive alleles from both parents, and thereby display a recessive phenotype even though neither of its parents does.
If one of the parents is BB, it is impossible for the child to have blue eyes, as the table below shows.
|Brown (B)||Blue (B)|
|FATHER (Bb)||Brown (B)||BB||BB|
If one parent is BB and one is Bb, there is a 50% chance of having a BB child and a 50% chance of having a Bb child, but all children this couple produces will have brown eyes.
Other Types of Genetic Dominance
When a parent has a homozygous trait (RR) that cannot completely dominate the other parent's different homozygous trait (WW), the genotype of both parents is said to be incompletely, or partially, dominant. Neither parent's dominant trait can overtake the other parent's dominant trait, and characteristics from both parents merge in the offspring. This results in a new, blended trait (phenotype) with a heterozygous genotype that can then be passed on to future offsprings. An example of incomplete dominance is found in the snapdragon plant. When a red flower snapdragon (RR) is crossed with a white flower snapdragon (WW), the result is a pink flower (RW). Note that in the case of incomplete dominance, recessive alleles are never present in either parent.
With codominant genes, both characteristics from both parents are seen. For example, in the camellia shrub, flowers can be red or white, but if a plant receives its genes from two parent plants, one with white flowers and one with red, its flowers will have splotches of both red and white. As with incomplete dominance, recessive alleles are never present in either parent when codominance occurs.
Some characteristics can be mixtures of the types of dominance described above. Human blood types are an example. A and B blood types are codominant. If a child receives the A blood type from one parent and the B blood type from the other, he will be type AB. This blood type has characteristics that are a mixture of type A and type B. However, both A and B are dominant over type O, another blood type. So if this child were instead to receive A from one parent and O from the other, he will be type A; likewise, if he receives B from one parent and O from the other, he will be type B.
Disorders and Diseases
Some human diseases are hereditary. If one or both parents have a heritable disease, it may be passed down to a child. Genetic abnormalities may be passed down on dominant alleles (autosomal dominant inheritance) or recessive alleles (autosomal recessive inheritance).
It is possible for a person to be a carrier of a disease but not have symptoms of the disease personally. This occurs when the disease is carried on a recessive allele. In other words, the trait cannot manifest in any person with a more dominant, healthy allele.
If one parent is a carrier of a disease, while the other has two healthy alleles, the disease will not be manifested in any of their offspring. However, if both parents are carriers, they have a 25% chance of having a child who is completely unaffected by the disease they both carry, a 50% chance of having a child who is also a carrier of the disease, and another 25% chance of having a child who suffers from the disease. Another way of looking at it is that any child they have has a 75% of being personally unaffected by the disease.