Mendelian Genetics

Mendelian Genetics

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Mendelian Genetics and how are inherited. Gregor Mendel, a scientist from many years ago, came up with three laws that explain how genes are passed down from one generation to the next. These genes are what determine an organism's physical appearance.

Introduction to Mendelian Genetics

Mendelian genetics, also known as classical genetics, were developed in the 19th century by Gregor Mendel, an Austrian monk who is known as The Father of Genetics. Mendel studied the common garden pea and identified three laws of inheritance that are applicable to all living organisms, not just peas.

Mendelian Genetics Overview

Before Mendelian Inheritance was widely accepted, people believed that heredity was like mixing two paint colors to make an intermediate. For instance, black blond would with.

endel's experiments with pea plants showed that inheritance does not work this way. Instead, organisms have distinct units of inheritance, called genes, which are passed down to their offspring. The traits that offspring exhibit depend on the alleles they inherit and the dominance of those alleles.

Mendel began his studies by self-pollinating plants that were pure-bred for specific traits. He then cross-pollinated these pure-breeds with other pure-breeds that had different traits, creating hybrids. The first generation of hybrids always displayed the trait of the pure-bred parent, and Mendel called this the first filial generation (F1).

When Mendel crossed F1 plants with each other, he observed that the second filial generation (F2) displayed a mixture of the traits of the original pure-breeds. For instance, when he crossed F1 plants with purple flowers and F1 plants with white flowers, the resulting F2 generation had mostly purple flowers, but some were white. Mendel noticed a consistent ratio of 3:1 for purple to white flowers in the F2 generation.

These experiments helped Mendel develop his Theory of Inheritance, which showed that traits are determined by discrete units of inheritance (genes) that are passed down from generation to generation.

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).

A Punnett Square of Peas Homozygous for their Shape
A Punnett Square of Peas Homozygous for their Shape

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

In some cases, an offspring's phenotype is not completely dominant or recessive, resulting in a blending form of inheritance. This was a popular concept before Mendelian Inheritance was widely accepted. One example of this is seen in Palomino-colored horses, whose coat color is a blend of their brown and white parents' coats.

In contrast, some genes in higher organisms have multiple effects on the phenotype, a phenomenon known as pleiotropy. For example, the altered gene responsible for PKU, a disease in humans, has multiple effects on the individual's phenotype, including slow growth, reduced skin pigment, and intellectual disability. This demonstrates that some genes can have far-reaching effects beyond a single trait.

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.

Mendelian Genetics - Key Takeaways Mendelian genetics is based on three laws: The Law of Dominance, The Law of Segregation, and The Law of Independent Assortment. The Law of Dominance states that the dominant allele is the only allele on display in the phenotype of a heterozygote. The Law of Segregation states that alleles separate independently into gametes. The Law of Independent Assortment states that alleles of different genes are inherited independently without affecting each other. Pure-bred plants are always homozygous, and they are called the P or parental generation.F1 plants are the offspring produced from crossing two different P plants.F2 plants are the offspring produced from crossing two of the same F1 plants. Mendel's Laws have several exceptions, including gene linkage, polygenic traits, codominance, incomplete dominance, and more.

Mendelian Genetics

What is mendelian genetics?

Mendelian Genetics refers to a pattern of inheritance in traits controlled by a single gene, with dominant and recessive alleles.

Which traits are Mendelian?

Traits that are Mendelian are those determined by a single gene that has only two possible alleles, one dominant, one recessive.

Why is mendelian genetics important?

Mendelian genetics is important because it is the foundation of our modern understanding of genetics, and its pattern of inheritance occurs in all living organisms.

What is the meaning of Mendelian genetics?

Mendelian genetics refers to classical genetics or genetics that follow Mendel's three laws; the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment.

What are the 3 principles of Mendelian genetics? 

The Three Principles of Mendelian Genetics, also known as Mendel's Laws, are the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment.

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