How can a dominant gene be recessive

Examples of inheritance patterns include:. An allele of a gene is said to be dominant when it effectively overrules the other recessive allele. The allele for brown eyes B is dominant over the allele for blue eyes b. So, if you have one allele for brown eyes and one allele for blue eyes Bb , your eyes will be brown. This is also the case if you have two alleles for brown eyes, BB. However, if both alleles are for the recessive trait in this case, blue eyes, bb you will inherit blue eyes.

For blood groups, the alleles are A, B and O. The A allele is dominant over the O allele. Blood group A is said to have a dominant inheritance pattern over blood group O. If the father has two O alleles OO , he has the blood group O. For each child that couple has, each parent will pass on one or the other of those two alleles.

This is shown in figure 1. This means that each one of their children has a 50 per cent chance of having blood group A AO and a 50 per cent chance of having blood group O OO , depending on which alleles they inherit. The combination of alleles that you have is called your genotype e. The observable trait that you have — in this case blood group A — is your phenotype. If a person has one changed q and one unchanged Q copy of a gene, and they do not have the condition associated with that gene change, they are said to be a carrier of that condition.

The condition is said to have a recessive inheritance pattern — it is not expressed if there is a functioning copy of the gene present. If two people are carriers Qq of the same recessive genetic condition, there is a 25 per cent or one in four chance that they may both pass the changed copy of the gene on to their child qq, see figure 2. As the child then does not have an unchanged, fully functioning copy of the gene, they will develop the condition.

There is also a 25 per cent chance that each child of the same parents may be unaffected, and a 50 per cent chance that they may be carriers of the condition. Recessive genetic conditions are more likely to arise if two parents are related, although they are still quite rare.

Examples of autosomal recessive genetic conditions include cystic fibrosis and phenylketonuria PKU. Not all genes are either dominant or recessive. Sometimes, each allele in the gene pair carries equal weight and will show up as a combined physical characteristic. The A and B alleles are said to be co-dominant.

Someone with one copy of A and one copy of B has the blood group AB. A cell reproduces by copying its genetic information then splitting in half, forming two individual cells. Occasionally, an alteration occurs in this process, causing a genetic change. When this happens, chemical messages sent to the cell may also change. For example, skin cancer can be caused by a build-up of spontaneous changes in genes in the skin cells caused by damage from UV radiation.

Other causes of spontaneous gene changes in somatic cells include exposure to chemicals and cigarette smoke. About half of the Australian population will be affected at some point in their life by a condition that is at least partly genetic in origin. Scientists estimate that more than 10, conditions are caused by changes in single genes. Having a genetic susceptibility to a condition does not mean that you will develop the condition.

It means that you are at increased risk of developing it if certain environmental factors, such as diet or exposure to chemicals, trigger its onset. If these triggering conditions do not occur, you may never develop the condition. Some types of cancer are triggered by environmental factors such as diet and lifestyle.

For example, prolonged exposure to the sun is linked to melanoma. Avoiding such triggers means significantly reducing the risks. Related Content:. What is a gene? What is inheritance? What is genetic variation? What are single gene disorders? What is a genetic disorder? How helpful was this page? What's the main reason for your rating? Which of these best describes your occupation? What is the first part of your school's postcode? How has the site influenced you or others? Thankyou, we value your feedback!

We use cookies to improve this site. 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. That is, individuals with one copy of each allele have an in-between phenotype. So is the sickle cell allele dominant, recessive, or co-dominant?

It depends on how you look at it. If we look at the proteins the two alleles code for, the picture becomes a little more clear. The affected protein is hemoglobin, the oxygen-carrying molecule that fills red blood cells. The sickle-cell allele codes for a slightly modified version of the hemoglobin protein. The modified hemoglobin protein still carries oxygen, but under low-oxygen conditions the proteins stick together. When a person has two sickle cell alleles, all of their hemoglobin is the sticky form, and the proteins form very long, stiff fibers that distort red blood cells.

When someone has one sickle-cell allele and one normal allele, only some of the hemoglobin is sticky. Non-sticky hemoglobin is made from the normal allele, and sticky hemoglobin is made from the sickle-cell allele every cell has a copy of both alleles.

The protist that causes malaria grows and reproduces in red blood cells. Just exactly how the sickle-cell allele leads to malaria resistance is complex and not completely understood.

However, it appears that the parasite reproduces more slowly in blood cells that have some modified hemoglobin. And infected cells, because they easily become misshapen, are more quickly removed from circulation and destroyed. To see more examples of how variations in genes influence traits, visit The Outcome of Mutation. Dominant and recessive are important concepts, but they are so often over-emphasized. After all, most traits have complex, unpredictable inheritance patterns.

However, at the risk of adding even more over-emphasis, here are some more things you may want to know:.