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Genetic Inheritance

Introduction

Genetic inheritance is the process of transferring traits from parents to offsprings during both sexual as well as asexual reproduction. It is done through passing on the genetic material or DNA from parents to offsprings. Although it is seen in all organisms from bacteria to complex mammals, we will mainly focus on the sexually reproducing organisms in this article. 

Genetic inheritance was deeply studied by Mendel who put forward two laws of inheritance after performing thousands of experiments. These experiments proved that all the traits of an organism come from the combination of traits found in his parents. His work on inheritance laid the foundation of modern world genetics. In this article, we will discuss how different traits are passed onto the next generation in sexually reproducing organisms. We will study genetic inheritance in the light of laws put forward by Gregor Mendel. 

Basic Concepts

Before diving into the discussion of genetic inheritance, it is necessary to recall the basic concepts that will help us in understanding the subject much better. 

Homologous and Non-Homologous chromosomes

The genetic material in eukaryotes is present in the form of DNA within the nucleus of the cell. The DNA is highly coiled to form compact structures called chromosomes. The number of chromosomes is constant in all the individuals belonging to one species i.e. all the humans have the same number of chromosomes (46 chromosomes).

These chromosomes are present in the form of pairs. One member of each pair comes from the mother while the other member comes from the father. If the members of a pair are identical in structure and composition, these matching pairs are called homologous chromosomes. Humans have 22 pairs of homologous chromosomes. 

One pair in humans is different in males and females. The chromosomes of this pair are non-homologous chromosomes, also called sex-chromosomes. 

Gene and Alleles

The genetic information in DNA is in the form of genes. A gene is a segment of DNA that codes for the synthesis of one particular protein. These genes are part of chromosomes.

The portion of a chromosome that carries a particular gene is called an allele. Every cell has two copies of one gene on homologous chromosomes together known as alleles. One allele is maternal in origin while the other one is paternal. 

The two alleles may be the same or different i.e. they may code for the same or different proteins. If both the alleles for a gene are similar, they are called homozygous alleles. However, if the two alleles for a gene are different, they are called heterozygous alleles. 

Genotype and Phenotype

The characteristics of an individual are determined by the genes present in it in the forms of allele. The entire genetic makeup or all the alleles present in an individual are collectively called its genotype

The traits shown by an individual are called its phenotype. For example, if an individual has blood group A, this blood group is his phenotype while the alleles responsible for this blood group are called genotype. 

Homozygous genotype includes similar two similar alleles for a characteristic while heterozygous genotype includes two different alleles. 

Transfer of Genes

The genes are transferred from parents to the next generation during the process of sexual reproduction. The male and female gametes fuse to form a single zygote. 

Recall that gametes are formed as a result of meiosis and have half the number of chromosomes as compared to the rest of the cells. When the male and female gametes fuse, a single zygote is formed having a combination of maternal and paternal chromosomes. Thus, half of the chromosomes come from the mother while the rest come from the father.  This results in an individual having traits of both mother and father. 

Mendelian Inheritance

Gregor Mendel was an Austrian monk who conducted several experiments to study the inheritance of many characteristics in pea plants. He is regarded as  “Father of Modern Genetics’ as he was the first person to study and explain the correct process of how different characteristics are transferred from one generation to the next. A number of incorrect hypotheses were prevalent among the scientists before Mendel’s work. He published his work in 1865, but it was approved by the scientific community in 1906. He did his work when the scientists were even unaware of the word ‘gene’. Mendel experimented on thousands of pea plants he grew in his garden. He put forward the two universally accepted laws after rigorous experimentation. 

Experiments

Mendel conducted experiments on pea plants that were grown in his garden. He argues that that the organism selected for studying inheritance should have the following features; 

  • It should have several traits that can be studied
  • The different traits should be contrasting i.e. there should be very different phenotypes for two different traits. For example, for height, the organisms should be tall or short and nothing in between 
  • The organism should allow cross-fertilization
  • The life cycle of the organism should be fast and short

He crossed plants through cross-pollination by transferring the pollen the flower from one plant to the other. He studied seven traits of pea plants;

  1. Seed shape
  2. Seed color
  3. Flower color
  4. Flower position
  5. Pod shape
  6. Pod color
  7. Stem length

Dominant and Recessive Traits

It is important to become familiar with these two terms before studying the laws of inheritance. 

Every trait that is expressed by an individual has two alleles. If a trait is expressed even if only one allele is present, it is called a dominant trait. A dominant trait will express itself in both cases i.e. in the presence of or both alleles. Thus, this trait can have either a homozygous genotype or heterozygous genotype. The alleles of dominant traits are written in capital letters.  

Some traits are expressed only if both the alleles are present. The opposite trait is expressed if one of the alleles is absent. Such traits are called recessive traits. Such traits only appear in the case of homozygous genotypes. The alleles of recessive traits are written in small letters. 

In a heterozygous genotype, if an allele prevents the expression of another allele, it is called the dominant allele. The other allele that is not expressed is called the recessive allele. 

Law of Segregation

Mendel first conducted experiments to study one characteristic at a time. For this purpose, he crossed two plants having only one contrasting trait e.g. seed shape.

Mendel crossed a homozygous round seeded plant with a homozygous wrinkled seeded plant. All the seeds obtained as a result of this cross were heterozygous and had a round shape. Mendel concluded that the trait ‘round seeds’ was dominant while the trait “wrinkled seeds’ was recessive. 

Mendel planted these seeds the next year and allowed the new plants to undergo self-pollination. As a result of this cross, he obtained round and wrinkled seeds in a ratio of 3:1. 

He studied other traits via a similar experiment and obtained the same results.  Based on these experiments, Mendel concluded that each trait is controlled by discrete factors that are separable from one another. For every trait, there exist two such factors. These discrete factors get separated during gamete formation and so that every gamete gets only one of the two discrete factors. They join together as the gametes fuse to form a zygote. These conclusions are called the Law of Segregation. The discrete factors were later called by biologists as genes. 

Law of Independent Assortment

In the next series of experiments, Mendel studied two traits at a time. He crossed two plants having two contrasting traits e.g. seed shape and seed color.

His previous experiments proved that the ‘round seed’ trait was dominant over the ‘wrinkled seed’ trait. Similarly, ‘yellow color’ was dominant over ‘green color’. Mendel crossed a homozygous plant having ‘round and yellow seeds’ with another homozygous plant having ‘wrinkled and green seeds’. The resulting seed had a heterozygous genotype and were all ‘round and yellow’. 

Mendel planted these seeds the next year and found astonishing results. The seeds produced as a result of self-pollination were in the following ratio;

Round Yellow  :  Round Green :  Wrinkled Yellow :  Wrinkled Green 

                   9                 :              3             :              3              :            1

The ‘round and green’ as well as ‘wrinkled and yellow’ were not present in the parent plants. 

After these experiments, Mendel concluded that the inheritance of two or more traits is not tied to one another. The seed color is not tied to seed shape. The segregation of genes responsible for these traits takes place separately from one another. This is known as the Law of Independent Assortment. It states that the segregation of alleles of one gene pair and their distribution to the gametes takes place independently of the alleles of another gene pair. 

This law proved that different traits are transferred from parents to the next generation independently of one another. 

Deviations from Mendelian Inheritance

The experiments performed in the subsequent years proved that some traits do not follow Mendelian inheritance laws. Such traits also exist in organisms that are controlled by more than one gene pair or in other cases, a gene pair can have more than two different alleles. 

Co-dominance and Incomplete Dominance are examples of such deviations. 

Co-dominance

It is a condition in which two different alleles of a gene pair express themselves completely. They do not show a dominant recessive relationship as predicted by Mendelian laws. An example of such a trait is human blood groups. 

The ABO blood groups are controlled by I gene that coded for the synthesis of specific antigens. There are three different alleles for this gene; 

  • IA codes for antigen A and the person has blood group A 
  • IB codes for antigen B and the person has blood group B
  • Io codes for no antigen and the person has blood group O

The presence of the AB blood group is an example of co-dominance. Such individual have one IA allele and one IB allele. Both of these alleles express themselves completely, both A and B antigens are produced, and the result is AB blood group. 

Incomplete Dominance

It is a condition in which two alleles of a heterozygous genotype express themselves incompletely so that a mixture of two traits is formed. 

An example of incomplete dominance is the presence of pink flowers. When a homozygous plant having white flowers is crossed with another homozygous plant having red flowers, the plants of the next generation have pink flowers, a blend of white and red. 

This phenomenon of incomplete inheritance is used in the agriculture industry to produce hybrid plants. Such plants have the characteristics of two different plants. These methods have proved to increase the yield of plants, protect them from diseases, shorten their life cycle, and several other benefits. 

Summary

Inheritance means the transfer of traits from parents to the next generation. All the traits in an individual are obtained from his parents. This transfer of traits is seen in all organisms whether they reproduce sexually or asexually. 

Traits are transferred to the next generation in the form of genetic material i.e. DNA. 

Inheritance of sexually reproducing organisms was first studied by Gregor Mendel who published his results in 1865. 

Mendel conducted several experiments on pea plants grown in his garden to study seven different traits. He formulated two universally accepted laws as a result of these experiments.

Law of Segregation states that all the traits in organisms are controlled by discrete spreadable factors that were called as genes in the subsequent years. These genes separate during gamete formation and unite again during fusion of male and female gametes to form the zygote. 

Law of Independent Assortment states that the segregation and distribution of two more genes takes place independently of one another. They are linked to each other in any case.

Most of the traits follow these laws but some deviations also exist. Examples of such deviations are co-dominance and incomplete dominance. 

An example of Co-dominance is the AB blood group of humans where two different alleles completely express themselves. No allele is dominant or recessive. 

Examples of Incomplete dominance are pink flowers that are produced as a result of a blend of two alleles; one allele for white color and one for the red color.  

References

  1. Pearson H (2006). “Genetics: what is a gene?”. Nature. 441 (7092): 398–401. Bibcode:2006Natur.441..398Pdoi:10.1038/441398aPMID 16724031.
  2. Visscher PM; Hill WG; Wray NR (2008). “Heritability in the genomics era – concepts and misconceptions”. Nat. Rev. Genet. 9 (4): 255–266. doi:10.1038/nrg2322PMID 18319743.
  3. lona Miko: Gregor Mendel and the Principles of Inheritance Nature Education 2008
  4. Henig, Robin Marantz (2009). The Monk in the Garden : The Lost and Found Genius of Gregor Mendel, the Father of Modern Genetics. Houghton Mifflin. ISBN 978-0-395-97765-1.
  5. Gregor MendelExperiments in Plant Hybridization 1965
  6. National Center for Biotechnology Information: Mendel’s experiments U.S. National Library of Medicine, 2000
  7. https://commons.wikimedia.org/wiki/File:Genotype_lg.jpg