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
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.
They know this. It’s just an idea or a ‘dip’ into the idea, not and full ‘dunk’ of an gene’s.
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!
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?