Genetic Legacy: Navigating Mendelian Inheritance Patterns
We have all had the experience when meeting up with family hearing relatives say to you that you look just like one of your parents, or a sibling, or another family member. Some traits, like personality, are shaped by your environment and the way you grew up, while others, like hair color, height, or what hand you write with are all heritable traits that can be linked to your genetics. When you take a science class, you are sure to encounter the concept of genetics again. It is also very likely you will hear about Gregor Mendel, an Austrian monk who became known as the “father of modern genetics” in part due to the experiments he conducted with pea plants. From these experiments, the whole world came to know about the patterns of Mendelian inheritance and understand how traits get passed down over time.
What were Mendel’s Pea Plant Experiments about?
Mendel’s earliest experiment with pea plants examined monohybrid inheritance. Monohybrid inheritance meant he would focus on one distinct trait that differed between two pea plants and cross them to see how the trait would be exhibited in the offspring. Both of these pea plants were homozygous (had 2 forms of the same allele) for each respective trait. The first generation of offspring, known as F1, had only tall pea plants. Their genotype was heterozygous, meaning they had two different alleles for the trait. When he crossed two plants from the F1 generation, he found that the ratio of tall plants to short ones was 3:1. After completing this experiment he conducted a further study to determine if the monohybrid inheritance pattern extended to other traits in peas, like texture and color. It was found to be consistent and helped him determine one of the Mendelian inheritance patterns.
He then conducted a dihybrid cross experiment, looking at two traits that had two different alleles each. He crossed peas which were yellow and round with peas that were green and wrinkled. When he pollinated F1, he found they had 4 different characteristics (round-yellow, round-green, wrinkled-yellow, and wrinkled-green), which were exhibited in a 9:3:3:1 ratio. From these experiments, another two Mendelian inheritance patterns were noted.
What are Laws of Mendelian Inheritance?
The three Mendelian Inheritance laws are the Law of Dominance, the Law of Independent Assortment, and the Law of Segregation.
Law of Dominance
This is the first law of Mendelian Inheritance. This law states that a hybrid plant will only inherit the trait that is in the dominant allele. The dominant allele will be the trait that is exhibited in the majority of the offspring. The recessive allele will then be suppressed in the first generation of offspring, which is present in genotype, but not the phenotype. The phenotypic ratio of 3:1 confirms this Mendelian inheritance pattern.
Law of Independent Assortment
This is the second law of Mendelian Inheritance. This law states that a pair of traits will segregate independently of other traits during the process of meiosis and gametic separation. In the dihybrid cross, traits following this law will manifest in a 9:3:3:1 ratio. The exception to this law of Mendelian inheritance is sex-linked traits, which will not follow this ratio that other traits follow in a dihybrid cross.
Law of Segregation
This is the third law of Mendelian Inheritance. This law states that during gamete production, two copies of every hereditary trait will segregate so that the offspring will receive one allele from each parent. They will then reunite in a random pattern following the fertilization process. This law is also known as the law of purity of gametes since a gamete will hold either a dominant or recessive allele, but not both. This is because a gamete is a haploid cell with only one set of chromosomes. The four basic concepts that relate to this law of Mendelian inheritance are that a gene has more than one form of an allele, gamete production in meiosis separates allelic pairs, every organism has two alleles, and two alleles in the pair are heterozygous.
What are the exceptions to these laws?
There are a few instances in which these laws cannot predict the pattern in which traits are passed down, which will be discussed below.
Sex-linked traits:
Some traits, like color blindness, are linked to the sex chromosomes, which means they do not exhibit in the same pattern of Mendelian Inheritance found in his dihybrid cross pea plant experiments.
Co-dominance:
Codominance defies the first law of Mendelian inheritance, which dictates that the dominant trait will be the only one expressed in the offspring. In codominance, both the different alleles will be expressed in equal measure. One example of codominance is found in our own blood. People with an AB blood type express both alleles for Type A and B, and one type does not dominate over the other. Even offspring with codominant traits will still follow the genotype ratios of 1:2:1, but the phenotype does differ.
Incomplete Dominance:
Incomplete dominance describes how a heterozygous organism can blend traits passed down by both parents. This also defies the first law of Mendelian inheritance because again, both traits are being expressed to a degree. An example of how incomplete dominance would be exhibited is if you have two flowers of the same species, one red (dominant) and one white (recessive) crossed and fertilized. The genotypic pattern is 1:2:1 for homozygous dominant, heterozygous, and then homozygous recessive, but the two heterozygous flowers are pink, which is a blend of both red and white.
These are the primary principles of Mendelian inheritance patterns that can explain how and why certain traits are passed down across generations. Though there are also exceptions to these rules as mentioned above, these principles will help guide you in understanding genetics and heredity.
Author: Maaida Kirmani
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