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