Heredity

The transmission of particular characters from parents to offspring is termed Heredity and Evolution. When a living organism is developed or gradually progressed from a simple life form to a complex organism it is termed Evolution. In all living organisms, the rules of inheritance determine the pattern of inheritance. Evolution is due to the inheritance of these variations in a particular organism. When there is reproduction between two genetically diverse organisms it gives rise to variations in any living organisms. Sexual reproduction causes more successful variations as compared to asexual reproduction.

Human beings can be distinguished from one another based on the inherited characteristics that children get from their parents. They reproduce sexually and hence various heredity traits are passed on, which has resulted in such dynamic changes in the later generations over the years. The human body and animals show a variety of changes that occur due to Heredity and Evolution.

Heredity

The Biological process that transfers the characteristics and traits from one generation to other (i.e. from parents to their children) is termed heredity. It is responsible for variation among the offspring and thus the evolution of a species over a period of time. Genes are the functional units of heredity that transfer character from parents to offspring. Example: the colour of skin or hair, eyes, height, etc.

Inheritance of Traits

The characteristics that are inherited from the parents are termed traits. An Inherited Trait is a particular genetically determined feature that distinguishes a person from another, the traits are inherited from one generation to other and this is the cause of the variations in population. For example, the colour of skin and eyes in humans. The set of genes is responsible for a particular character in a trait.

Allelic Relations

The genes are present as a pair for a specific trait. An Allele is a variant form of a gene. It is one or two more forms of a DNA sequence (a single base or a segment of bases) at a particular genomic location. Generally, an individual inherits two alleles, one from each parent, for given any genomic location where such variation exists. If the two alleles are the same the individual is homozygous for that allele and if the alleles are different the individual is heterozygous. Allelic heterogeneity refers to multiple mutations that occur in the same gene. Genetic variants determine phenotypic variability. Characterization of these allelic variations may open largely uncharted territory in genomics for biomedical research and may eventually lead to the discovery of the causative genes of usual hereditary diseases and their functionality of actions. 

Rules for the Inheritance 

Mendel’s Contribution

Gregor Johann Mendel referred to as the ‘Father of genetics, was an Australian Monk. He framed this law of Inheritance using his scientific and mathematical knowledge. Mendel did this experiment to understand the concept of heredity, His work laid the foundation of modern genetics. He used pea plants for his experiment as he found them easy to grow and they had a greater number of visible characteristics like tall/short, inflated/constricted pod shape, violet/white flower, and round/wrinkled seeds.

During his experiment, Mendel found that genes are the factors that control the expression of traits. Genes are present in pairs for a specific trait and they are termed alleles. Depending on the expression of traits the genes could be either dominant or recessive. 

Pea plant in Mendel Experiment.

Dominant Traits: The traits that express themselves in the offspring in every possible combination and can be seen by the naked eye are called Dominant traits.

In Mendel’s experiment, the tall trait in the pea plants tends to express more than the small trait. Therefore, the tall trait of plants is said to be dominant over the small trait.

Recessive Traits: A trait which is not expressed itself in the presence of a dominant allele is known as a Recessive trait. 

A recessive trait is expressed in an offspring when it is contributed by both parents. So, the recessive trait is present in an organism but cannot be seen due to the presence of the dominant trait.

The morphological expression of a single character which is observable physically is termed a phenotype. Examples – are tall or short, round or wrinkled seeds, the colour of a flower, etc.

The genetic constitution of the allele pair for a specific trait is termed as the genotype. Example – Tt or tt or TT.

Mendel was a mathematician so he used statistics to record the traits in each generation by using a statistical method known as Punnett square for predicting the possible genotype and phenotype in the offspring.

Monohybrid cross

A monohybrid cross is the hybrid of two individuals with homozygous genotypes. Only one character is considered. In this cross, Mendel showed the inheritance of dominant and recessive characters. 

E.g. If a Round seed (RR) is crossed with a wrinkled seed (rr), we get 3 Round seeds and 1 wrinkled seed at the end of the F2 generation. The ratio of characters, arising after the cross at the end of the F2 generation is called the monohybrid ratio. In this case, the monohybrid ratio is 3:1. For the cross below the shape of the seed is considered.

Monohybrid cross

Dihybrid cross

A dihybrid cross is a cross between two individuals with two observed traits that are controlled by two distinct genes. In this cross, two characters are considered. The ratio of characters arising at the end of the F2 generation is known as the dihybrid ratio.

E.g. If a plant with a Round green pea (RRyy) is crossed with a plant having a wrinkled and yellow pea (rrYY), the first generation would have all-around yellow peas (RrYy). While crossing the same for an F2 generation, the result had greater variations and the new combinations were Green Round, Yellow round, yellow wrinkled and green wrinkled. Thus, the dihybrid ratio is 9:3:3:1.

Dihybrid cross

Sex Determination

The process of determining the sex of an individual, based on the composition of the genetic material is called sex determination. There is various mechanism that determines the sex of newborn organisms. In humans, the sex of a newborn child is determined by the genes inherited from the parents. There are generally 22 pairs of chromosomes in humans and one chromosome is the sex chromosome, which determines all the traits. In humans, the presence or absence of the Y chromosome determines the sex of an individual. Females have a perfect pair (XX) and males have a normal and the other short (XY) chromosome. All gametes formed by females and similar i.e. have X chromosomes. Males have two types of sperm i.e. half with an X chromosome and the other half with a Y chromosome. The sex of the baby will depend on fertilization. There are two possibilities. 

Sex Determination.

Relation to the Theory of Evolution

The most fundamental and important aspects of biology are heredity and evolution, which are connected by inheritable features. These two terminologies assist us in learning and comprehending how the life cycle on Earth works. Both concepts are interrelated to one another, and there can be no evolution without heredity.

The transmission of properties from parent organisms to offspring is known as heredity. The evolution of a species is the outcome of changes in these specific inherited features over many generations that improve survival and reproduction chances. In other words, inheritable traits connects evolution to heredity.

Over several generations, the frequency of an inherited trait changed. Since genes determine characteristics, we can assume that over generations, the frequency of particular genes in a population changed. This is how evolution takes place in an organism

Significance of Heredity

• Heredity is considered an important factor in influencing the development of personality. It focuses on the transfer of half of his or her genes to the offspring. 
• Heredity is the essential factor by which the offspring acquire the personalities, behaviours and characteristics of their parents and grandparents. These characteristics are passed through sexual or asexual reproduction. 
• Heredity is the study of how parents pass down traits to their children through genes. The research conducted on heredity has helped in generating information in terms of various aspects of the theories of heredity.
• It indulges the interest of an individual towards acquiring positive traits as it will be an important contribution in bringing about improvements in their overall quality of life. 
• Both parents contribute half of their genes with a different set of genes passed to different offspring. This creates diversity and variations among the offspring. 

Basic Features of Heredity

• The basic concepts of heredity, derived by Gregor Mendel during his studies in the mid-19th-century became the foundation for the modern science of genetics. 
• The transmission of traits from parents to their children is carried through genes, the functional units responsible for heredity in all living organisms. 
• Many characteristics are influenced by more than one gene, they are referred to as polygenic. Many genes exist in multiple alleles throughout a population. The polygenic and multiple allelic nature of many traits gives a large potential for variability among hereditary characteristics. 
• Heredity is the sum of all biological processes by which particular characteristics are transmitted between different generations.
• The sex cells or the gametes form the bridge across which the heredity must pass between the generations, and are usually invisible to the naked eye.  

FAQs on Heredity

Question 1: Define Heredity.

Answer:

Heredity refers to the transmission of characters or traits from parents to their offspring. It is the continuity of features from one generation to other.

Question 2: What is the genotype of Human males and females?

Answer:

Genotype is the genetic constitution of an organism. In human beings, there are 23 pairs of chromosomes. Out of these 22 chromosome pairs are called autosomes and the sex chromosome is the last pair of chromosomes that help in deciding the gender of that individual. The genotype of the human male is 44 + XY and the Genotype of the human female is 44 + XX.

Question 3: What is an Allele?

Answer:

Allele is an alternative form of a gene occupying the same position on a chromosome and affecting the same characters but in two alternative ways. It is a variation of the same sequence of nucleotides at the same place on a long DNA molecule. The genomic location of any gene or any genetic material is called a locus and the alternative DNA sequences at a locus are called alleles. 

Question 4: State the difference between the dominant trait and the recessive trait.

Answer:

A dominant trait is an inherited character that appears in an offspring if it is contributed by a parent having a dominant allele. A dominant trait overrides the effect of a different variant of the same gene on the other copy of the chromosome. 

A trait that must be contributed by both parents in order to appear in the offspring is termed a recessive trait. This trait is expressed only when the genotype is homozygous. When an organism has two recessive alleles it tends to be expressed in the offspring.

Question 5: State the Monohybrid ratio and Dihybrid ratio.

Answer:

Monohybrid Ratio: 

• In the F1 generation, the ratio is 100% hybrid type.
• In the F2 generation: the phenotypic ratio is 3:1 and the genotypic ratio is 1:2:1.

Dihybrid Ratio: 

• In the F1 generation, the ratio is 100% hybrid type.
• In the F2 generation: the phenotypic ratio is 9:3:3:1 and the genotypic ratio is very complex. 

Question 6: Why sexual reproduction produces more variation?

Answer: 

In sexual reproduction, two different sets of chromosomes from both parents are mixed resulting in a completely new set of chromosomes. They also undergo crossing over during gamete formation and thus each gamete formed is unique. Hence the progeny has more variation. A large set of characteristics is inherited from both parents and is together passed on to the next generation causing diversity among the individual in the next generation.