When the genotype consists of a dominant and recessive allele in the phenotype will be like?

• Since human cells carry two copies of each chromosome they have two versions of each gene. These different versions of a gene are called alleles.
• Alleles can be either dominant or recessive.
• Dominant alleles show their effect even if the individual only has one copy of the allele (also known as being heterozygous). For example, the allele for brown eyes is dominant, therefore you only need one copy of the ‘brown eye’ allele to have brown eyes (although, with two copies you will still have brown eyes).
• If both alleles are dominant, it is called codominance. The resulting characteristic is due to both alleles being expressed equally. An example of this is the blood group AB which is the result of codominance of the A and B dominant alleles.
• Recessive alleles only show their effect if the individual has two copies of the allele (also known as being homozygous). For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the ‘blue eye’ allele.

Illustration to show the inheritance of dominant and recessive alleles for eye colour.
Image credit: Genome Research Limited

What are sex-linked genes?

• Some genes are found on the sex chromosome, X.
• These genes are inherited with the X chromosome (from the mother if it is a boy or from either mother or father if it is a girl).
• Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY).
• This means females have two alleles for X-linked genes while males only have one.
• Some genetic diseases, are caused by sex linked genes, for example haemophilia.
• The allele for haemophilia is recessive so two copies are needed for a female to have the disease
• However, because males only have one X chromosome, they only need one copy of the haemophilia allele to have the disease.
• This means haemophilia is much more common in males than in females.

For example:

Functioning allele = H

Haemophilia allele = h

XH XH = healthy female

XH Xh = carrier female

Xh Xh = haemophilia female

XH Y = healthy male

Xh Y = haemophilia male

This page was last updated on 2021-07-21

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Inheritance patterns

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. Muscle and organ cells don’t get enough oxygen and nutrients, and they begin to die.

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!

Now let’s look again at the shape of the blood cells. People with two copies of the sickle-cell allele have many sickled red blood cells. People with two copies of the “normal” allele have disc-shaped 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.

Protein function

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 sticking-together effect is diluted, and in most cells, the proteins don’t form fibers.

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.

When the genotype consists of a dominant and a recessive allele?

How are dominant and recessive genes expressed in the phenotype?

In which genotype will the recessive allele be expressed in the phenotype?

When both alleles are dominant and are expressed in the phenotype?