What Controls Traits And Inheritance?

DNA

When an egg is fertilized with sperm, the offspring that results from the conception takes half of its DNA from its mother and half from its father.

Each parent passes one-half of each of their 52 pairs of chromosomes to the child. The resulting combination of the two sets of DNA determines the traits that are passed to the child. 

Alleles are a variation of a gene, pronounced ‘AL-eel’, and we inherit two alleles in each pair of genes from our parents. These two copies of the gene contained in your chromosomes are what influence the way your cells work.

Alleles interact with each other in different ways, and these are known as “inheritance patterns." The most common interaction between a pair of alleles is the dominant/recessive relationship. This is the case with traits such as eye color and blood type. 

Dominant and Recessive Alleles 

In blood type groups, the alleles are A, B, and O.

If one parent passes on a dominant gene, even if the other passes on a recessive gene, then the gene expressed in the child will always be dominant. For example, a person with one A allele and one O allele (AO) has blood group A, as the A allele is dominant over the O allele.

Brown eyes come from a dominant gene, whereas blue eyes are classed as recessive. The difference between these two genes is that a person with blue eyes has two recessive genes, whereas people with brown eyes can have one dominant and one recessive gene.

This is why a child can have blue eyes even if both parents have brown eyes, and why two parents with blue eyes can only have a child with blue eyes. 

Codominant and Incomplete Dominance 

However, not all genes are dominant or recessive. 

It’s possible for the alleles in a gene pair to carry equal weight and these will therefore show up as a combined physical characteristic. In the case of blood type groups, the A allele is as ‘strong’ or ‘dominant’ as the B allele.

The A and B alleles are therefore known to be codominant. Someone with one copy of A and one copy of B has the resulting blood group of AB.

You can also have incomplete dominance, which is when a dominant allele does not completely overpower the recessive allele, and the result of this is a blending of both alleles.

This is most commonly shown in heterozygous roses, which have alleles for both red and white colors. This means both alleles are expressed at the same time, resulting in a blending of the two colors to make pink roses. 

In humans, you can see incomplete dominance at work in numerous other traits: for example, a child with wavy hair will usually be the result of a parent with straight hair and a parent with curly hair, due to the expression of both curly and straight alleles. 

Incomplete dominance can be seen in many other physical characteristics as well, such as skin color, height, hand size, and vocal pitch. This is different from codominance, however, which is when both alleles are expressed at the same time, as seen in the AB blood type. 

Personality

On the other hand, what about traits such as our personality types, health conditions, and taste preferences? Where do these fit into the genetic picture? 

Experts suggest that our personality is a mixture of both biological and environmental influences. There are five main personality traits: extraversion, agreeableness, openness, conscientiousness, and neuroticism, and it’s thought that a combination of both nature and nurture influence the development of each of these five traits.

According to VeryWellMind.com, “a study of the five traits looked at 123 pairs of identical twins and 127 pairs of fraternal twins. The findings suggested that the heritability of each trait was 53 percent for extraversion, 41 percent for agreeableness, 44 percent for conscientiousness, 41 percent for neuroticism, and 61 for openness.”

While some studies suggest these personality traits change little throughout adulthood, other studies have shown that maturation may have an impact on the five traits.

As people age, many tend to become less extroverted, less neurotic, and less open to new ideas and experiences. Agreeableness and conscientiousness, on the other hand, tend to increase with age. 

Health Traits 

There’s also the question of health traits: to what extent are these a product of our genetic inheritance? 

Most genes we get from our parents are copies and therefore they work in the same way in us as they do in them. However, sometimes, a gene is not a perfect copy.

Changes in genes are known as mutations, and mutations can be found in everyone. Some mutations work better than the original, and some make very little difference at all, however, some mutations cause problems. 

A condition that is caused by mutations in one or more genes is called a genetic disorder, and there is a group of rare diseases caused by mutations in one gene at a time - known as single-gene disorders.

However, most common diseases are caused by a combination of gene changes, lifestyle choices, and your surrounding environment.

Mutations are either inherited from a parent to a child (known as “hereditary” diseases) or they can happen during a person’s lifetime (known as “acquired” changes). An example of this is the mutations that can be acquired by environmental factors such as overexposure to ultraviolet radiation from the sun.

The acquired mutations you develop during your lifetime are in cells called somatic cells, and these makeup most of your body. They may cause problems for you, such as skin cancer, which can be very dangerous, however, this doesn’t mean you pass these genes onto your children, as they are acquired and therefore are a product of your environment and lifestyle, rather than your genetic makeup. 

Conclusion 

While humans share 99% of their DNA, our traits are what make us individuals - from our hair color, to whether we’re extrovert or introvert, to our predisposition to certain illnesses or genetic diseases.

The most common interaction between a pair of alleles is the dominant/recessive relationship, which is seen in traits such as eye color and blood type.

However, it is also possible for alleles to be codominant - when two exist alongside each other, as with the AB blood type - or to express incomplete dominance - as is the case when somebody has wavy hair, rather than curly or straight like either of their parents. 

But while these processes such as eye color or blood type may be quite simple to understand as we simply inherit them from the share of DNA we acquire from either of our parents, things get a little more complicated when it comes to our personality traits. 

Personality is usually assessed in terms of the five main characteristics of extraversion, agreeableness, openness, conscientiousness, and neuroticism, and the degree to which we express these traits are likely to be a combination of both inherited genes but also our environment.

For example, how we are brought up, the socio-economic context of our upbringing, the friendships we make (as seen when somebody is said to be hanging around with “the wrong crowd).

Equally our health traits - such as our metabolism or predisposition to certain cancers or conditions such as diabetes and obesity - are also dependent on a combination of inherited and acquired traits.

For example, if you smoke, you may increase your risk of certain lung conditions or cancers, or if you eat too much sugar, you may develop type 2 diabetes, whereas somebody else may be born with type 1 diabetes, which is not a direct result of lifestyle, but more a result of the genetic lottery. 

So, in conclusion, our traits - and how they develop or shape us as individuals - are dependent on a combination of both inherited genes and acquired mutations.