Patterns of Inheritance: Not Always Easily Visible
We always talk about our outward characteristics such as hair color, eye color, height, weight, and skin tone, as things that we have inherited from our parents or that we take after from some member of our family. However, very few of us actually know the process of how those unique characteristics that make us who we are and make us recognizable are passed on from previous generations—or how they are inherited.
In fact, this information was not known until Gregor Mendel, an Augustinian monk and largely known as the father of modern genetics, published his findings on the inheritance patterns of nearly 30,000 plants, most of which were pea plants. Though we use diagrams like Punnett square to predict genotypes of a particular experiment on dihybrid crosses, it has not always been that way. Mendel began his studies of variations among plants in 1856 and a few years later published what is now known as Mendel’s Laws of Inheritance. In other words, he spent years studying the inheritance patterns and condition of plants and how they contributed to the variation and helped us understand and know what we know of genetics, and the patterns of inheritance and each dominant trait today.
We Inherit Our Traits
So, what does this mean for us? Before we get into the specifics as well as the “not always easily visible” aspect of things, we need to define a few terms. We inherit our characteristics, also called traits in genetics, as a result of gene combinations that we receive from the mother and the father. Half of the genetic material is inherited from each parent, so we essentially have two copies of each gene (or alleles). Each pair of alleles come in two forms: dominant and recessive and make one trait.
Dominant alleles are always visible, hence the term “dominant.” This is regardless of whether there is one or two of them. Recessive genes, on the other hand, are only visible on the outside of both copies of the allele are present. Let us look at an example from Mendel’s work. Mendel realized that when he crossed a green pea plant with a yellow pea plant, all of the offspring were yellow. In the next generation, the offspring color ratio was 1:3 green to yellow, and this gave rise to the terms “dominant” and “recessive.” In this example, the color yellow is dominant and the color green is recessive. In other words, if the color yellow is YY and the color green is yy, the first generation of offspring were all mixed, namely of Yy genotype. Letters are typically used to denote allele combinations, with capital letters referring to dominant traits and lower caps letters denoting recessive traits. The term “genotype” refers to the genetic constitution of an organism.
A mixed genotype or Yy is also called a heterozygous genotype, and will always have the color yellow. This, as we mentioned earlier, is because one copy of the dominant gene is enough for the outward traits to be visible. A YY genotype is called homozygous or having two copies of the same gene. Specific to our example, dominant genes will be expressed regardless of whether they are heterozygous or homozygous, while a recessive allele will only show on the outside if each copy is present, or if they are homozygous. Specific to Mendel’s example, the first round of offspring were all heterozygous Yy and, therefore, yellow. The second generation was actually yy (green), yY or Yy (which is the same outward trait of yellow), and YY (also yellow). Mendel did not know this right away. Understanding this required cross-breeding of thousands of plants in order to realize the rules of what and how a genotype produces a specific phenotype. He coined the term “test-cross” referring to crossing two parents to find out what offspring they will yield in order to understand the parental genotypes. More specifically, the parent whose genotype is in question is crossed with a homozygous recessive parent. The offspring trait distribution will help to determine the genotype in question.
The aforementioned example of yellow and green peas holds when the parents are so-called true-breeding, meaning that each parent is a homozygote for their trait and they produce offspring that all have the same outward characteristics.
Patterns of Inheritance
Juxtaposing this onto humans, understanding patterns of inheritance is particularly important when it comes to specific diseases, such as cystic fibrosis, muscular dystrophy, breast cancer, Huntington’s disease, Marfan syndrome, hemophilia, diseases connected to blood, and other x-linked disorders. Here, it is also important to differentiate between autosomal and sex chromosomes, as that affects disease inheritance. In addition to understanding the terms dominant and recessive, it is also important to know that traits, by virtue of being passed down via genes that are located on chromosomes, are also passed down on either autosomal (non-sex chromosomes) and sex chromosomes. This is important because it affects disease inheritance patterns.
For example, an autosomal disorder is caused by a single genetic mutation on any of the 22 autosomal chromosomes. If the mutation happens to be on a dominant gene, then the offspring will inherit it from a parent who also has the disease even if the other parent is otherwise healthy. An example of an autosomal dominant disease is Huntington’s disease that is caused by a mutation in the Huntington gene (HTT). One defective copy is enough to cause the disease that is marked by poor coordination and irregular or abnormal movements. This, in turn, is a result of the HTT mutation that produces a defective protein that is harmful to nerve tissue.
On the other hand, sex-linked diseases are marked by mutations on the genes of the X or Y chromosome (one of the two sex chromosomes, X and Y). Women carry two X chromosomes and men carry one X and one Y chromosome. This means that while women can be safe from some recessive X-linked diseases, men, by virtue of carrying just one X chromosome are not and will almost always express an X-linked recessive mutation.
This is the very basic and abridged version of patterns of inheritance, designed to guide you through a little bit of what you see and what lies beneath the surface, so to say. Rest assured that there is much more to the patterns of inheritance, but that is largely beyond the scopes of this post for now.