Deoxyribonucleic acid, or DNA, is the hereditary material found in humans and almost all other organisms. DNA contains the instructions needed for an organism to develop, survive and reproduce. DNA is made up of molecules called nucleotides which are arranged into sequences called genes. Genes carry the information that determines traits.
Genes
A gene is a distinct sequence of nucleotides within a DNA molecule that contains the instructions for making a specific protein or RNA molecule. Genes vary in size, with the largest human gene being dystrophin at 2.4 million nucleotides. Most genes are much smaller, containing anywhere from a few hundred to tens of thousands of nucleotides.
Genomes can contain anywhere from a few hundred genes to over 40,000 genes. The human genome is estimated to contain around 20,000 protein-coding genes. Genes make up only a small percentage of the human genome – less than 2% – with the rest being non-coding DNA.
Alleles
Alleles are variants of a gene. For example, there are three common alleles of the gene that codes for blood type: IA, IB, and i. The IA and IB alleles determine blood types A and B, while the i allele determines blood type O.
Most genes have two alleles, or forms, that code for slightly different versions of a trait. Each parent passes down one of their two alleles to their offspring. If the two alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous.
Dominant and Recessive Alleles
Some alleles are dominant while others are recessive:
- Dominant allele – An allele that is fully expressed when only one copy is present. For example, the IA allele that produces blood type A is dominant.
- Recessive allele – An allele that is only expressed when two copies are present (the individual is homozygous). For example, the i allele that produces blood type O is recessive.
If an individual inherits two different alleles for a specific trait, the dominant allele will be expressed while the recessive allele will be masked. For example, a person with an IA (blood type A) allele and an i (blood type O) allele will have blood type A.
How Genes Influence Traits
There are estimated to be around 20,000 genes in the human genome that influence both the physical and behavioral traits that make us unique individuals. Here are some examples of how different genes influence human traits:
- Eye color – The main genes involved in eye color are HERC2 and OCA2 on chromosome 15. Different variations of these genes lead to different levels of melanin pigment production in the iris, resulting in eye colors like brown, hazel, green, blue and gray.
- Hair color – Similar to eye color, genes like MC1R, ASIP, TYRP1 influence the type and amount of melanin produced, leading to variation in hair color including black, brown, blond, redhead and gray.
- Height – Many genes influence height, including SHOX, FGFR3, HHIP, and NAT2. Each contributes a small amount to overall height potential.
- Disease risk – Variants of genes like BRCA1/BRCA2, APOE, and LRRK2 can increase risk for diseases like breast cancer, Alzheimer’s, and Parkinson’s disease.
- Behavioral traits – Genes influence personality and behavioral traits like risk-taking, introversion/extroversion and empathy. However, life experiences and environment also play a key role.
While genes do influence traits, it is not always a direct cause and effect. The environment also plays a key role in how traits are expressed. Additionally, most traits are polygenic, meaning they are influenced by many genes working together.
Genotype vs Phenotype
Genotype refers to the genetic makeup of an individual organism. It is the specific set of alleles that an individual inherits from their parents. The phenotype is the physical manifestation, or expressions, of those alleles.
For example, two individuals may have the same genotype for a gene that codes for height. However, their adult heights could be different due to environmental factors like nutrition.
The relationship between genotype and phenotype is not always direct. Some alleles may never be expressed if they are recessive and masked by dominant alleles. Additionally, the environment plays a key role in how traits are expressed.
How Sequences of DNA Determine Genetic Traits
The order of nucleotides in a DNA sequence determines the genetic code for making proteins. Even small changes in the DNA sequence can result in new traits:
- Changes in a single nucleotide are called point mutations. These can have minor or major effects on the resulting protein product.
- Insertions or deletions of nucleotides can shift the genetic code completely, resulting in a nonfunctional protein.
- Copying errors during cell division can lead to extra or deleted pieces of DNA called copy number variants. These can duplicate or delete entire genes.
- Repetitive sequences called microsatellites are prone to expansion or contraction. Changes in their length can impact gene function.
Mutations occur randomly but natural selection determines which are passed on based on their impact on fitness and reproductive success. Over many generations, useful mutations are propagated through populations.
Mendelian Inheritance
In the 19th century, Gregor Mendel outlined the basic principles of inheritance by studying the passing on of traits in pea plants:
- Segregation – Organisms carry two copies of each gene called alleles. Each parent randomly passes one allele to their offspring.
- Independent assortment – Genes for different traits are inherited independently of each other.
- Dominance – Some alleles are dominant and mask the effects of recessive alleles.
These form the basis of Mendelian inheritance which explains simple monogenic traits controlled by one gene with two alleles. However, modern genetics has shown that most traits are far more complex involving multiple genes, gene-environment interactions, and epigenetics. Still, Mendelian inheritance gives a good basic framework.
| Generation | Genotype | Phenotype |
|---|---|---|
| Parental | IAIA x iaia | Blood type A x Blood type O |
| F1 | IAia | Blood type A |
| F2 | IAIA, IAia, iaia | Blood type A or O |
This table shows an example of Mendelian inheritance of blood type as a monogenic trait. The IA and ia alleles show dominant/recessive inheritance. Based on random segregation and assortment, the offspring genotypes and phenotypes can be predicted.
Polygenic Inheritance
While Mendelian inheritance applies to single-gene traits, polygenic inheritance involves the actions and interactions of multiple genes to determine a trait. This is the case for most human traits like height, skin color, and susceptibility to diseases. Polygenic inheritance follows these patterns:
- Multiple genes each make minor contributions to the trait.
- Individuals fall along a spectrum for the trait rather than discrete phenotypes.
- Environmental factors also play a role in how the trait is expressed.
- Inheritance patterns are more difficult to predict mathematically.
Polygenic traits do not follow simple dominant/recessive patterns. Prediction of offspring phenotypes requires knowing the alleles each parent contributes at many loci across their genome and accounting for environmental effects.
Conclusion
In summary, genes are distinct sequences of DNA that provide the instructions for specific traits. Different versions of genes called alleles lead to variation in traits between individuals. Some alleles are dominant and mask the effects of recessive alleles. The sequencing of nucleotides in DNA determines the genetic code. Changes in DNA sequences can lead to new traits.
While Mendelian inheritance applies to single gene traits, polygenic inheritance involves multiple genes interacting to influence a trait. The environment also plays a key role. Understanding genetics helps explain human diversity and variations in traits like eye color, height, disease risk and many other aspects of health and biology.
