Dominant and Recessive Genes In Humans

As briefly referred to in the previous Genetics blog, for each of our genes we posess two ‘alleles’. One of these alleles in inherited from our father and one from our mother. There can be many different alleles for one gene and it can be completely up to chance, or perhaps luck, what we inherit from our parents. When speaking in general terms about dominant and recessive alleles, we tend to speak about genes as if for each of them there are two different alleles. This is not always, or often, the case, but it sometimes is and makes it much easier to explain this way.

For example, for a particular gene, say the ability to roll your tongue, there is a dominant and a recessive gene. We can call the dominant allele ‘R’ for being able to roll our tongue and the recessive allele ‘r’ for being unable to roll our tongue. Our parents could posess any combination of these alleles: AA, aa or Aa. Then, it is completely down to chance what we inherit from them.

One unexpected example is that the allele for dwarfism in humans is the dominant allele and the allele for normal growth is recessive. This means that if we inherited both of the different alleles for this gene we would show the dwarfism trait.

Below is a table of dominant and recessive traits shown in humans.

Dominant Trait in Humans

Recessive Trait in Humans

A blood type

O blood type

Abundant body hair

Little body hair

Astigmatism

Normal vision

B blood type

O blood type

Baldness (in male)

Not bald

Broad lips

Thin lips

Broad nose

Narrow nose

Dwarfism

Normal growth

Hazel or green eyes

Blue or gray eyes

High blood pressure

Normal blood pressure

Large eyes

Small eyes

Migraine

Normal

Mongolian Fold

No fold in eyes

Nearsightedness

Normal vision

Rh factor (+)

No factor (Rh -)

Second toe longest

First or big toe longest

Short stature

Tall stature

Six fingers

Five fingers normal

Webbed fingers

Normal fingers

Tone deafness

Normal tone hearing

White hair streak

Normal hair coloring

 

When we are speaking about the inheritance of alleles and the genetic make-up of a person with respect to one gene, we use one of two phrases. The first is homozygous, meaning that the two alleles an individual posesses for one gene are the same i.e. AA or aa. The second is heterozygous, meaning that the two alleles an individual posesses for one gene are different i.e. Aa.

By Robyn Bradbury

26 thoughts on “Dominant and Recessive Genes In Humans

  1. Could you put a note somewhere on here that points out that genetics is not THE determining factor? There are many other factors that come into play. The way this article is written makes it seem like allelic determination is the end all, be all. Thanks.

    • Yeah exactly it also kinda seems like you are claiming ONE gene determines the expression of these traits. Ex eye color is several genes that play together. Ontop of that SNPs and other factors can also change the expression of genes… This would be true if humans had the same gene interactions as a pea plant but in complex humans much much more is happening.

  2. Hi Anne and Tom,

    I am not either of the authors, nor do I speak directly on their behalf. However, I thought I should point out that this blog was written as a basic introduction to some aspects of genetics. This blog is only about the existence of dominant and recessive traits. It is, of course, simplified, as the authors make clear in the blog:

    “When speaking in general terms about dominant and recessive alleles, we tend to speak about genes as if for each of them there are two different alleles. This is not always, or often, the case, but it sometimes is and makes it much easier to explain this way.”

    Tom, I don’t think the authors wanted to claim that one gene is always responsible for every trait, and I certainly didn’t read it that way. It is just a simplification in order to explain some basic genetics. In science, you often have to make simplifications to explain things at a basic level – and then as we learn more we start to understand that things are never that simple (although for some traits, one gene is responsible, and it is that simple)

    Anne, genetics can often be the major determining factor in certain phenotypic traits (blood type, tongue rolling, polydactyly) and the types of alleles found at a given locus may well govern that trait. Obviously, in the broader picture of human development (or of any organism), we know that genes and environment always interact (and environment must be taken to mean a whole host of factors, including the interplay of other genes within the genome in question). In this case, the authors were not trying to espouse any kind of pure genetic determinism, but just write a simple introduction to dominant and recessive traits for a non-scientific audience.

    I hope this helps clear that up. All the best and thanks for reading and for your comments!

  3. I have 0 rh negative blood. My husband has A+ blood. I have two sons. One has A- blood and one has 0+ blood. Is it possible that my A- son got his A from Dad and the Negative part from me, his mother?

  4. It should be pointed out that some traits, like human stature, are polygenic traits – the result of many genes. The genetics of human stature has been an area of research and disagreement from some time and, while the exact number of genetic loci involved is not known, the wide variability in human stature appears to be consistent with the combined effects of many dominant/recessive genes. The additive effect of all of these genes contributes an estimated 80% – 90% of observed human height variability, with the rest associated with environmental factors like nutrition and medical care.

    • Hi Cindy, in this case it may be that your parents carry brown eye genes which are dominant over those contributing to green colour. If the genetics were as simple as this we could estimate that your parents would produce 3/4 of their children with brown eyes and 1/4 of their children with hazel green eyes. This is just an average so it is not that surprising that you and your sister both have different colour eyes to your parents, whilst your brother shares their colour.

      It is important to note that We now know that eye colour is regulated by more than one gene, and there may be very many genes which contribute to overall eye colour. Furthermore, different genes may interact with each other, and in some cases there is co-dominance (neither gene is dominant). Nonetheless, when children have a discrete trait which differs from their parents (eye colour, hair colour etc.) we can make a reasonable assumption that it is due (at least in part) to recessive genes which the parents carry. The chart and post above is necessarily simplified because genetics is a highly complicated subject, but hopefully it gives you an idea of how genes work to determine physical traits – and why children often different in some of the characters that their parents possess.

      I hope this helps, Dominic

  5. My parents and brother have blue eyes but mine, which were blue until I was about 12, are now green. My husband’s are blue but can be greenish depending on the clothes he is wearing, my daughter’s are blue-grey, and my son’s are green, like my own.

  6. Dwarfism, high blood pressure, and having 6 toes are dominant features?! Really? These were typos, right? Other wise, you are full of crap. rH O-.

    • I didn’t write the article, but I know for a fact that some versions of polydactyly and achondroplasia (dwarfism) are autosomal dominant. Look it up if you don’t believe me. I am not sure what form of high blood pressure the authors were referring to, but certainly some forms have a heritable component. I hope this clears that up for you.

    • Dominant doesn’t mean common. It simply means that if that gene is there, it will be expressed. We don’t all carry a gene for six toes, for example – but my udnerstanding is that if we DO have that gene, then it WILL be expressed. In other words, if you have the gene, you will have the 6th toe.

      Put on a simpler level, tongue rolling is dominant. It is apparently controlled by only one pair of alleles. If you can roll your tongue, it means you must have one R allele. But you may also have an r allele (for not rolling). If you and your spouse are both Rr, then you could have an rr child – who cannot roll their tounge. But if you have a child who is Rr or RR either one, the child WILL be able to roll their tongue.

      • Good point, and you are totally correct about dominant not meaning common. I didn’t realise that this was a source of confusion, but I can now see that it might have been. Dominant refers to the genetic effects, rather than how common a trait is.

  7. Is being double jointed an inherited trait? Is squared nails an inherited trait? How about the largeness of your heel?

    • I don’t know the specific genetics underlying those traits, but they must be heritable – at least partly. Any trait is determined by the combination of genes you inherit from your mother and father, so the trait you possess depends on which genes you inherit from each parent.

  8. My wife is pregnant and we were wondering what colour eyes our child will have,
    I have blue eyes and my wife has brown eyes – what are the chances that my child will have blue eyes?

  9. Hi Gary, this will all depend on whether your wife is homozygous or heterozygous with the dominant brown eye gene. Brown eyes are dominant genes over the blue eye genes and so they will be expressed, but the person could still carry the recessive blue eye gene even though they don’t express it in their phenotype, this is called heterozygous.

    Since you have blue eyes, that means you are homozygous of the recessive blue eye gene and so the brown eye genes will automatically be dominant over your blue eyes.

    So since we don’t know whether your wife is homozygous or heterozygous with the brown eye genes, we won’t know for sure but we could still predict the outcome. So, if your wife is homozygous brown eyes, your child won’t have blue eyes but will just carry the blue eye gene. If your wife is heterozygous of the brown eye gene that could mean that your child has a 50% chance of having blue eyes.

  10. Hi, my wife is pregnant and we were wondering what colour our childs eyes would be. I have blue eyes and my wife has brown eyes.

  11. Hi, my wife is pregnant with our first child and we would like to know what colour eyes our child will have. My wife has brown eyes and I have blue eyes.

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