Mendelian Genetics

Mendelian Genetics Overview

Before Mendelian Inheritance was commonly recognized, many people believed that heredity was akin to mixing two paint buckets, creating an intermediate color. For example, a black-haired parent and a blond-haired parent would give birth to a child with brown hair.

Mendel demonstrated that inheritance is not based on this blending concept. Instead, individuals have discreet units of heredity, which we now know as genes, and these genes are passed down to offspring. The characteristics the offspring display are based on the alleles they inherit and the dominance of those alleles.

Mendel began his experiments using peas that were pure-bred for specific traits. For example, he knew which of his plants were pure-bred for purple flowers because he self-pollinated them for years, over and over, and the flowers they produced were always purple. He eventually cross-pollinated these purple pure-breeds with white pure-breeds, creating a hybrid. The pure breeds were called the parent generation (P), and the hybrids were called the first filial generation (F1). He saw that the F1 flowers were all purple!

Now Mendel took two F1s and crossed them together to produce the second filial generation (F2). This F2 generation appeared different: most of its flowers were purple, yes, but some were white again! In fact, after performing this F1 x F1 cross time and time again, Mendel noticed a consistent ratio of purple to white flowers in the F2 generation. Purple flowers were consistently 3/4 of the crop, while white flowers were 1/4 (Fig. 1). These findings helped consolidate Mendel's Theory of Inheritance.

Mendelian Genetics Definitions

Before we go on, it's important to define some terms in Mendelian genetics.

• What is a gene? A gene is the basic unit of heredity.
  • For each trait, organisms get one gene from each parent, so there are two genes per trait.

• What is an allele? An allele is a variant of a gene.
  • In Mendel's pea plants, some peas were wrinkly, and others were round. These are two variants, or two different alleles, of the gene deciding pea shape. If an organism's two alleles are the same, it is homozygous (AA or aa) for that trait. If the two alleles are different, it is heterozygous (Aa). (Homo - the same, Hetero - different).

• What is a phenotype? Phenotype refers to how an organism looks, regardless of its alleles.

• What is a genotype? Genotype refers to the exact allelic makeup of an organism, regardless of how the organism looks.

• What is a dominant allele? A dominant allele is an allele that shows up in the phenotype of a heterozygote.
  • The Round (R) allele is dominant in peas over the Wrinkled (r) allele. So in a plant heterozygous for pea shape, with one copy of the round allele and one copy of the wrinkled allele, the plant would have the Rr genotype, and its peas would appear completely round, just as if it were a RR homozygote with two copies of the round allele (Fig. 2).

• What is a recessive allele? A recessive allele is an allele that does not show up in the phenotype of a heterozygote.
  • An organism must be homozygous for a recessive allele for it to be observed in its phenotype. Because wrinkled peas are recessive, we need an rr genotype to observe a wrinkled pea.

Basic Principles of Mendelian Genetics

Three principles make up the Mendelian Theory of Inheritance. These principles are the cornerstone of the entire field of genetics. To understand the exceptions to these laws and the more complex concepts that build on them, we must first understand each of the three in detail.

1) The Law of Dominance

2) The Law of Segregation (read more about this in the article "Mendel's Law of Segregation")

3) The Law of Independent Assortment

Mendelian Theory of Inheritance

The Law of Dominance

The Law of Dominance states that, in a heterozygote, the dominant allele is expressed exclusively.

We can observe this when we cross two homozygous parent organisms for different alleles, and see that their offspring is heterozygous for both alleles but has the same phenotype as the parent with the dominant allele.

Let's use the wrinkly and round peas again to examine this. Also, we will use a Punnett Square, a tool used in genetics to determine the possible genotypes of future offspring made by crossing two parent organisms (Fig. 3).

The Law of Segregation

The Law of Segregation states that when an organism is making gametes, it separates its gene pair, or alleles so that each one is individually packaged. Then, during reproduction, one maternal and one paternal gamete will fuse so that their offspring will get one random allele from each parent for two alleles.

The Law of Independent Assortment

The Law of Independent Assortment states that alleles of different genes are inherited independently of one another. Thus, an allele inherited for one gene doesn't influence or affect the ability to inherit an allele of a different gene.

For example, a parent plant with purple flowers and wrinkly peas passes down their wrinkled shape and purple flower alleles independently and equally.

Exceptions to Mendelian genetics

It's important to note that while Mendelian genetics is foundational, not every trait fits neatly into these three laws, and we do see exceptions.

Exception 1: Multiple Genes

Multiple genes control some characteristics. These are called polygenic traits. An example of this is your height, which is influenced by over 50 genes!

Exception 2: Multiple Alleles

Even if a trait is controlled by just one gene, there may be more than two alleles for that gene. In Mendel's pea plants, every trait he studied had only two possible alleles (wrinkled or round, green or yellow, normal-sized or dwarf, purple or white flowers, etc.) But the gene determining human blood types, for example, has three possible alleles A, B, and O.

Exception 3: Codominance

When Mendel crossed purple flowers and white flowers, he didn't get light-purple flowers, so he postulated that all traits have an all-or-nothing, dominant or recessive phenotype. However, we have discovered some traits in some animals where both alleles can be expressed together, called codominance. An example of this is speckled chickens, which have both white and black feathers from their pure white and pure black parents (Fig. 4).

Exception 4: Incomplete Dominance

Sometimes, an offspring's phenotype is the intermediate of its two parents; thus, neither allele is completely dominant. This blending form of inheritance is reminiscent of the popularly held concepts in Mendel's era. We can see this form of inheritance in Palomino-colored horses, whose tan coat color is in between their brown and white parent's coats (Fig. 5).

Exception 5: Pleiotropy

If a gene is pleiotropic, it has multiple effects on the phenotype. Unlike the allele for wrinkled peas, which didn't affect height or flower color, or anything other than pea shape, some genes in higher organisms have multiple effects. For example, PKU, a disease in humans due to an altered gene, causes features like slow growth, reduced skin pigment, and intellectual disability. One gene alteration has multiple effects.

Exception 6: Gene Linkage

Gene linkage means that a gene at a particular spot on a chromosome influences the ability to inherit a different gene on the same or different chromosomes. Two linked genes tend to assort together, and inheriting one would increase the likelihood that you inherit another. In humans, genes for hair color and eye color exhibit some gene linkage, which you may have noticed if you've thought of how often blonde hair and blue eyes occur together.