This confusion comes about in part because people observed dominant and recessive inheritance patterns before anyone knew anything about DNA and genes, or how genes code for proteins that specify traits. The critical point to understand is that there is no universal mechanism by which dominant and recessive alleles act. Whether an allele is dominant or recessive depends on the particulars of the proteins they code for. The terms can also be subjective, which adds to the confusion.
The same allele can be considered dominant or recessive, depending on how you look at it. The sickle-cell allele, described below, is a great example.
However, these patterns apply to few traits. Sickle-cell disease is an inherited condition that causes pain and damage to organs and muscles. Instead of having flattened, round red blood cells, people with the disease have stiff, sickle-shaped cells. The long, pointy blood cells get caught in capillaries, where they block blood flow.
The disease has a recessive pattern of inheritance: only individuals with two copies of the sickle-cell allele have the disease. People with just one copy are healthy. In addition to causing disease, the sickle-cell allele makes people who carry it resistant to malaria, a serious illness carried by mosquitos. Malaria resistance has a dominant inheritance pattern: just one copy of the sickle cell allele is enough to protect against infection. This is the very same allele that, in a recessive inheritance pattern, causes sickle-cell disease!
People with two copies of the sickle-cell allele have many sickled red blood cells. People with one sickle-cell allele and one normal allele have a small number of sickled cells, and their cells sickle more easily under certain conditions.
So we could say that red blood cell shape has a co-dominant inheritance pattern. Just like it is possible to get three heads in a row when you flip a coin, it is also possible to pass the same gene version three times in a row. In other words, it is possible for these parents to have all blue eyed kids. In fact, the chances of this happening are around 1 in 8. If the dad were to pass the blue gene version three times in a row, then all three kids in the second generation could have blue eyes.
There are other situations where we need to be careful too. For example, if we had a family of all blue-eyed people, then it might look like blue is dominant because parents always pass it on to their kids.
It is for these reasons we usually need more than one generation and lots of different families to figure these things out. Each observation is evidence that a trait is either dominant or recessive. OK, so from this we can figure out that brown is dominant over blue. Now we know that if someone has a brown allele and a blue allele, they'll have brown eyes. We can also figure out which allele each person carries. To make this easier, we need to use a little genetics shorthand.
When we show something as dominant, we use capital letters. For the recessive allele we use lowercase. As I mentioned above, people have two copies of each gene, which means that you can have BB or Bb and have brown eyes. However, you can only have blue eyes if you are bb.
In other words, a dominant allele will always allow a specific trait to show up no matter if we have two dominant copies BB or just one Bb. A trait from a recessive allele will only appear if it is paired with another recessive allele bb. As a final point, genetics is rarely this simple. There are always exceptions to genetics rules. For example, sometimes a dominant trait won't be seen because of something called incomplete penetrance. Or sometimes a new mutation will change the eye color gene between generations confusing the interpretation.
Again, we can get around these things by looking at lots of family trees. Here are a few samples of family trees so you can practice.
Can you figure out what traits are dominant or recessive? The black circles and squares represent people with freckles. Are freckles dominant or recessive? Click on the image to see if you guessed right. Click here to learn more about the genetics of freckles. The black circles and squares represent people with red hair. Is red hair dominant or recessive? Click here to learn more about the genetics of red hair. Erika Bustamant e, Stanford University.
Why some alleles are dominant and others recessive. Some common dominant and recessive traits in more detail Fun and easy way to create a family's health history. Knowing your family history is one of the most useful tools in understanding genetics. In that case, we call it recessive.
A dominant gene, or a dominant version of a gene, is a particular variant of a gene, which for a variety of reasons, expresses itself more strongly all by itself than any other version of the gene which the person is carrying, and, in this case, the recessive.
Now, it usually refers to inheritance patterns frequently used in conjunction with a Punnett square where, if an individual has two versions of a gene, and one is observed to frequently be transferred from one generation to another, then it is called dominant.
Biochemically, what is going on in this case is that the genetic variation, for a variety of reasons, can either induce a function in a cell, which is either very advantageous or very detrimental, which the other version of the gene can't cover up or compensate for. In that case, you're going to have a dominant mutation, and that dominant mutation can be benign. In some instances, offspring can demonstrate a phenotype that is outside the range defined by both parents.
In particular, the phenomenon known as overdominance occurs when a heterozygote has a more extreme phenotype than that of either of its parents. A well-known example of overdominance occurs in the alleles that code for sickle-cell anemia. Sickle-cell anemia is a debilitating disease of the red blood cells, wherein a single amino acid deletion causes a change in the conformation of a person's hemoglobin such that the person's red blood cells are elongated and somewhat curved, taking on a sickle shape.
This change in shape makes the sickle red blood cells less efficient at transporting oxygen through the bloodstream. The altered form of hemoglobin that causes sickle-cell anemia is inherited as a codominant trait. Specifically, heterozygous Ss individuals express both normal and sickle hemoglobin, so they have a mixture of normal and sickle red blood cells. In most situations, individuals who are heterozygous for sickle-cell anemia are phenotypically normal.
Under these circumstances, sickle-cell disease is a recessive trait. Individuals who are homozygous for the sickle-cell allele ss , however, may have sickling crises that require hospitalization. In severe cases, this condition can be lethal. Producing altered hemoglobin can be beneficial for inhabitants of countries afflicted with falciparum malaria, an extremely deadly parasitic disease.
Sickle blood cells "collapse" around the parasites and filter them out of the blood. Thus, people who carry the sickle-cell allele are more likely to recover from malarial infection.
In terms of combating malaria, the Ss genotype has an advantage over both the SS genotype, because it results in malarial resistance, and the ss genotype, because it does not cause sickling crises. Allelic dominance always depends on the relative influence of each allele for a specific phenotype under certain environmental conditions.
For example, in the pea plant Pisum sativum , the timing of flowering follows a monohybrid single-gene inheritance pattern in certain genetic backgrounds. While there is some variation in the exact time of flowering within plants that have the same genotype, specific alleles at this locus Lf can exert temporal control of flowering in different backgrounds Murfet, Investigators have found evidence for four different alleles at this locus: Lf d , Lf , lf , and lf a.
Plants homozygous for the lf a allele flower the earliest, while Lf d plants flower the latest. A single allele causes the delayed flowering. Thus, the multiple alleles at the Lf locus represent an allelic series, with each allele being dominant over the next allele in the series.
Mendel's early work with pea plants provided the foundational knowledge for genetics, but Mendel's simple example of two alleles, one dominant and one recessive, for a given gene is a rarity. In fact, dominance and recessiveness are not actually allelic properties. Rather, they are effects that can only be measured in relation to the effects of other alleles at the same locus. Furthermore, dominance may change according to the level of organization of the phenotype. Variations of dominance highlight the complexity of understanding genetic influences on phenotypes.
Murfet, I. Flowering in Pisum : Multiple alleles at the Lf locus. Heredity 35 , 85—98 Parsons, P. The evolution of overdominance: Natural selection and heterozygote advantage.
Nature , 7—12 link to article. Stratton, F. The human blood groups. Nature , link to article.
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