When a parent with blue eyes and parent with brown eyes pass down their genes for eye color to their offspring, this is an example of heredity.

The children inherit the genes that consist of deoxyribonucleic acid (DNA) from the parents, and they may have blue or brown eyes. However, genetics is complex, and more than one gene is responsible for eye color.

Likewise, many genes determine other traits like hair color or height.

Heredity is the study of how parents pass down their traits to their offspring through genetics. Many theories about heredity have existed, and the general concepts of heredity appeared before people understood cells completely.

However, modern-day heredity and genetics are newer fields.

Although the foundation for studying genes appeared in the 1850s and throughout the 19th century, it was largely ignored until the early 20th century.

Human traits are specific characteristics that identify individuals. The parents pass down these through their genes. Some easy-to-identify human traits are height, eye color, hair color, hair type, earlobe attachment and tongue rolling. When you compare common vs. uncommon traits, you are usually looking at dominant vs. recessive traits.

For example, a dominant trait, such as brown hair, is more common in the population, while a recessive trait, such as red hair, is less common. However, not all dominant traits are common.

If you are going to study genetics, you have to understand the relationship between DNA and heritable traits.

The cells of most living organisms have DNA, which is the substance that makes up your genes. When cells reproduce, they can pass down the DNA molecule or genetic information to the next generation. For instance, your cells have the genetic material that determines if you have blonde hair or black hair.

Your genotype is the genes inside the cells, while your phenotype is the physical traits that are visible and influenced by both the genes and environment.

There are variations among the genes, so DNA sequences differ. Genetic variation makes people unique, and it is an important concept in natural selection because favorable characteristics are more likely to survive and pass on.

Although identical twins have the same DNA, their gene expression may vary. If one twin receives more nutrition than the other does, he or she may be taller despite having the same genes.

Initially, people understood heredity from a reproductive perspective. They figured out basic concepts, such as the pollen and pistils of plants being similar to the egg and sperm of humans.

Despite breeding hybrid crosses in plants and other species, genetics remained a mystery. For many years, they believed blood transmitted heredity. Even Charles Darwin thought blood was responsible for heredity.

In the 1700s, Carolus Linnaeus and Josef Gottlieb Kölreuter wrote about crossing different plant species and discovered that the hybrids had intermediate characteristics.

Gregor Mendel’s work in the 1860s helped improve the understanding of hybrid crosses and inheritance. He disproved established theories, but his work was not fully understood upon publication.

Erich Tschermak von Seysenegg, Hugo de Vries and Carl Erich Correns rediscovered Mendel's work in the early 20th century. Each of these scientists studied plant hybrids and reached similar conclusions.

Genetics is the study of biological inheritance, and Gregor Mendel is considered its father. He established the key concepts of heredity by studying pea plants. Heritable elements are genes, and traits are specific characteristics, such as flower color.

Often called Mendelian inheritance, his findings established the relationship between genes and traits.

Mendel focused on seven characteristics in pea plants: height, flower color, pea color, pea shape, pod shape, pod color and flower position. Peas were good test subjects because they had fast reproductive cycles and were easy to grow. After he established pure-breeding lines of peas, he was able to cross-breed them to make hybrids.

He concluded that traits like pod shape were heritable elements or genes.

Alleles are the different forms of a gene. Genetic variations such as mutations are responsible for creating alleles. Differences in DNA base pairs can also change function or phenotype. Mendel's conclusions about alleles became the basis for two major laws of inheritance: the law of segregation and the law of independent assortment.

The law of segregation states that allele pairs separate when gametes form. The law of independent assortment states the alleles from different genes sort independently.

Alleles exist in either dominant or recessive forms. Dominant alleles are expressed or visible. For example, brown eyes are dominant. On the other hand, recessive alleles are not always expressed or visible. For instance, blue eyes are recessive. In order for a person to have blue eyes, he or she must inherit two alleles for it.

It is important to note that dominant traits are not always common in a population. An example of this is certain genetic diseases, such as Huntington disease, which is caused by a dominant allele but not common in the population.

Since there are different types of alleles, some organisms have two alleles for a single trait. Homozygous means there are two identical alleles for one gene, and heterozygous means there are two different alleles for a gene. When Mendel studied his pea plants, he found that the F₂ generation (grandchildren) always had a 3:1 ratio in their phenotypes.

This means that the dominant trait showed up three times more often than the recessive one.

Punnett squares can help you understand homozygous vs. heterozygous crosses and heterozygous vs. heterozygous crosses. However, not all crosses can be calculated using Punnett squares due to their complexity.

Named after Reginald C. Punnett, the diagrams can help you predict phenotypes and genotypes for offspring. The squares show the probability of certain crosses.

Mendel’s overall findings showed that genes transmit heredity. Each parents transfers half of his or her genes to the offspring. Parents can also give different sets of genes to different offspring. For example, identical twins have the same DNA, but siblings do not.

Mendel’s work was accurate but simplistic, so modern genetics has found more answers. First, traits do not always come from a single gene. Multiple genes control polygenic traits, such as hair color, eye color and skin color. This means that more than one gene is responsible for you having brown or black hair.

One gene can also affect multiple characteristics. This is pleiotropy, and genes may control unrelated traits. In some cases, pleiotropy is linked to genetic diseases and disorders. For example, sickle cell anemia is an inherited genetic disorder that affects the red blood cells by making them crescent-shaped.

In addition to affecting the red blood cells, the disorder affects blood flow and other organs. This means that it has an impact on multiple traits.

Mendel thought that each gene only had two alleles. However, there can be many different alleles of a gene. Multiple alleles can control one gene. An example of this is coat color in rabbits. Another example is the ABO blood-type group system in humans. People have three alleles for blood: A, B and O. A and B are dominant over O, so they are codominant.

Complete dominance is the pattern that Mendel described. He saw one allele was dominant while the other one was recessive. The dominant allele was visible because it was expressed. Seed shape in pea plants is an example of complete dominance; the round seed alleles are dominant over the wrinkled ones.

However, genetics is more complex, and complete dominance does not always happen.

In incomplete dominance, one allele is not completely dominant. Snapdragons are a classic example of incomplete dominance. This means that the phenotype of the offspring appears to be in between the phenotype of the two parents. When a white snapdragon and a red snapdragon breed, they can have pink snapdragons. When you cross these pink snapdragons, the results are red, white and pink.

In codominance, both alleles are expressed equally. For example, some flowers can be a mix of different colors. A red flower and a white flower may produce offspring with a mix of red and white petals. The two phenotypes of the parents are both expressed, so the offspring has a third phenotype that combines them.

Certain crosses can be lethal. A lethal allele can kill an organism. In the 1900s, Lucien Cuenót discovered that when he crossed yellow mice with brown mice, the offspring were brown and yellow.

However, when he crossed two yellow mice, the offspring had a 2:1 ratio instead of the 3:1 ratio that Mendel found. There were two yellow mice for one brown mouse.

Cuenót found out that yellow was the dominant color, so these mice were heterozygotes. However, about a fourth of the mice bred from crossing the heterozygotes died during the embryonic stage. This was why the ratio was 2:1 instead of 3:1.

Mutations can cause lethal genes. Although some organisms may die in the embryonic stages, others may be able to live for years with these genes. Humans can also have lethal alleles, and several genetic disorders are linked to them.

How a living organism turns out depends on both its heredity and environment. For example, phenylketonuria (PKU) is one of the genetic disorders that people can inherit. PKU can cause intellectual disabilities and other problems because the body cannot process the amino acid phenylalanine.

If you only look at the genetics, you would expect a person with PKU would always have an intellectual disability. However, thanks to early detection in newborns, it is possible for people to live with PKU on a low-protein diet and never develop serious health problems.

When you look at both the environmental factors and genetics, it is possible to see how a person lives can affect gene expression.

Hydrangeas are another example of the environmental impact on genes. Two hydrangea plants with the same genes may be different colors because of soil pH. Acidic soils create blue hydrangeas, while alkaline soils make pink ones. Soil nutrients and minerals also influence the color of these plants. For example, blue hydrangeas must have aluminum in the soil to become this color.

Although Gregor Mendel's studies created the foundation for more research, modern genetics has expanded his findings and discovered new inheritance patterns, such as incomplete dominance and codominance.

Understanding how genes are responsible for physical traits that you can see is a crucial aspect of biology. From genetic disorders to plant breeding, heredity can explain many questions that people ask about the world around them.