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Principle of Genetics is a science that deals with inheritance i.e., how characters pass from parents to progeny. It states what causes variation among them. Genetics is the study of rules and standards that govern the biological phenomenon of heredity. Heredity is the passing on similar or different traits in closely related species.
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Key Terms: Genetics, Mendel’s law of heredity, Variation, Parental traits, Inheritance, Genes, DNA, Heredity, Species
What is heredity?
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A cat always gives birth to kittens and not to puppies. An apple tree grows from the apple seed. This mechanism is true for all other living organisms; humans, animals, plants and microorganisms. This is because of a phenomenon called heredity. It is the passing on of characters from parents to their offspring.
Hereditary
These characters can be similar or different depending upon the DNA encoded in units called genes. For example, siblings may look similar or different from each other. It depends upon the information processed during the transmission of genes.
Genetics
Genetics is the scientific study of heredity and genes. It studies the similarities and differences in the transmission of physical and physiological traits. These similarities and differences are called variations. Genetics understand the rules related to transmission as a result of changes in DNA sequence. These sequences provide the instructions for building molecules.
Structure of DNA
There are two ways to form the biological basis for heredity.
Transmission genetics: It is the study of how genes pass from one generation to another. The organisms are crossed to study this mechanism.
Cytogenetics: It studies the structure of DNA within the cell nucleus.
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Variation
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Any difference caused by genetic differences or environmental factors in the offspring is called variation. The degree to which an offspring differs from its parent depends on the variation in genes inherited from them. Genetic variation results in different forms or alleles of genes.
It is visible in physical appearance, metabolism, behaviour, mental abilities and other characteristics. It makes us unique.
For example, the colour of the eye is determined by the different alleles of the gene. People with blue and brown eyes will have different alleles of genes. Our weight is partially influenced by our genes. It depends on our environment, i.e., how much we eat and exercise.
Types of Variation
The differences between the types of variation is tabulated below.
| Genotypic variations | Somatic variations |
|---|---|
| These are hereditary. | These are not hereditary |
| The differences in characteristics of progeny which are the consequences of different DNA sequences in genes are genotypic. | These are the result of different environments and are not caused by differences in DNA sequences. |
| The structures or number of chromosomes present in the cells will be different. | Several factors that affect them are climate, food supply and actions of the organisms. |
| Physical features like the colour of the eye or skin, face structure and height will fall under this. | Characteristics like weight, accidental injuries, sleeping abilities etc. fall under this. |
| They are passed down to future generations. Evolution relies on it. | They are not passed down to future generations. Evolution here is not possible. |
| Favourable characteristics are selected and transmitted. This is known as natural selection. | Natural selection is not possible. |
Read More: Difference between Prokaryotic and Eukaryotic Transcription
Mendel’s Law of Heredity
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Gregor Mendel(1822-1844), the father of genetics demonstrated the mechanism of transmission of characters from one generation to another. He conducted hybridisation experiments (pollination/cross-pollination) on garden peas (Pisum sativam) for seven years. He selected fourteen true-breeding pea plant varieties. He made them into seven pairs of contrasting traits such as tall or dwarf plants, round and wrinkled, red and yellow seeds etc.
Below is the contrasting traits studied by Mendel in pea:
| Characters | Contrasting traits |
|---|---|
| Stem height | Tall/dwarf |
| Flower colour | Violet/white |
| Pod shape | Axial/terminal |
| Flower position | Inflated/constricted |
| Pod colour | Green/yellow |
| Seed shape | Round/wrinkled |
| Seed colour | Yellow/green |
Laws of Inheritance
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Gregor Mendel gave three laws of inheritance:
Law of Dominance: The traits manifested in the offspring are the dominant alleles and the traits not expressed are recessive alleles. Both alleles occur in pairs in the heterozygous condition of the genotype.
Law of Segregation: The alleles although stay together, never gets mixed. They separate only at the time of gametogenesis so that each gamete receives the dominant trait. Also note that in a monohybrid cross, the alleles of a personality are separated in subsequent generations.
Law of Independent Assortment: Alleles of a character can undergo any sort of combination when more than a pair of characters are taken into consideration. This results in a phenotype different from both parents.
Read more: Genotype and phenotype
Notations Used in Breeding Experiments
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The dominant trait is represented in capital letters and the recessive trait in small letters. For example, Tallness is represented as T and dwarfness as t.
A homozygous pair (where both the alleles are of equivalent nature) is written as TT or tt.
A heterozygous pair(two alleles of different nature) is written as Tt.
Breeding Experiment
Punnet Square
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Punnet square is the square diagram that is used to predict the genotypes of a particular cross or breeding experiment. The square is named after Reginald C. He invented this approach in 1905. All possible combinations which can happen between maternal alleles and paternal alleles are presented in a tabular summary.
Read More: Difference between Cytogenesis and Molecular Genesis
Things to Remember
- Any offspring resembles their parents because of the phenomena called heredity. The characteristics or traits that are passed from one generation to another result in this phenomenon.
- Genetics scientifically studies this phenomenon. It focuses on how the transmission happens i.e., how the similarities are carried on from the parents to their offspring and what results in the differences in traits.
- Variation or differences in the inheritance occurs due to the specific arrangement of genes. These can be due to hereditary reasons or environmental factors. These are called genotypic or somatic variations respectively.
- Gregor Mandel discovered the mechanism of transmission of traits through the process of pollination in pea plants. Based on his experiment, he gave three laws of inheritance.
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Sample Questions
Ques: Compare in any three ways the chromosomal theory of inheritance as proposed by Sutton and Bovery with that of experimental results on pea plant presented by Mendel. ( CBSE 2019,3 marks)
Ans:
| Sutton and Bovery | Mendel |
|---|---|
| Chromosomes occur in pairs | Factors occur in pairs |
| It is the chromosomes that separate in gamete formation such that only one pair is transmitted. | Here factors segregate at the time of gamete formation. |
| Independent pairs of chromosomes segregate independently from each other | One pair of factors segregate independently of other pairs. |
Ques: How would you find out whether a given tall pea plant is homozygous or heterozygous? Substantiate your answer with the help of Punnett squares. (CBSE 2019, 3 marks)
Ans: By test cross/crossing the plant with unknown genotype with the recessive plant
Crossing the plant
Ques: Name the type of cross that would help to find the genotype of a pea plant bearing violet flowers. (CBSE 2017,1 mark)
Ans: Test cross would help to find the genotype of a pea plant bearing violet flowers. It is a genetic cross between a homozygous recessive individual and a corresponding suspected heterozygote and determines whether the organism carries the dominant trait of former or latter. It was first used by Gregor Mandel in plant hybridisation.
Ques: Which law of Mendel is universally accepted? State the law. (2 marks)
Ans: Mendel’s law of segregation is universally accepted.
It states that “the two alleles of a gene remain separate & do not contaminate each other in F1 or the hybrid. At the time of gamete formation two alleles separate & parts into different gametes.”
Ques: Give any two similarities between the behaviour of genes (Mendel’s factor) during inheritance and chromosomes during cell division. (2 marks)
Ans: Two similarities between the behaviour of genes during inheritance and chromosomes during cell division are:
- Both genes and chromosomes occur in pairs in somatic and diploid cells
- 2. Genes and chromosomes retain their numbers, structure and individuality throughout the life of the organism and are passed down to another generation in the same manner.
Ques: With the help of one example, explain the phenomena of co-dominance and multiple allelism in the human population. (CBSE 2014, 3 marks)
Ans: When two alleles of a trait are expressed equally, co-dominance occurs. ABO blood groups are controlled by gene I. Gene I have three alleles lA, lB and lO. These alleles determine the type of sugar on the RBC surface. Alleles lA and lB are co-dominant and express the AB blood group. Multiple allelism happens when there are three different alleles and express themselves based on dominance and recessiveness.
Ques: Write any three advantages of selecting a pea plant for the experiment by Mendel. (3 marks)
Ans: Any three advantages of Mendel selecting pea plants for the experiment are:
- Peas have contrasting traits. These traits are seed colour, pod colour, pod shape, flower shape, the position of flower, seed shape and plant height.
- Peas self pollinate as their corolla completely encloses the reproductive organs until pollination is complete.
- It is easily available and can be grown easily.
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