Biology 10th Class Biology

Chapter Number 15 Inheritance – 10th Class Biology Notes

Chapter Number 15 Inheritance – 10th Class Biology Notes

Q#1) Define the following.

Inheritance.  2) Heridity 3)Variation 4) Genetics 5) Genes 6) Chromosome 7) Chromation 8) Genotype 9) Evolution 10) Allele.


Answer: 1) Inheritance: Characters that are transmitting from parents to offsprings are called inheritance. These characters are of two types’ (a) Heridity (b) Variation.

Heridity : Characters in offsprings that resemble to parents are called heridity.

Example: Egg of frog produces frog.

Variation: Characteristics in individuals that is different from other individuals of the same species is called variaton or characteristics of individuals different from their parents are called variation.

Example: Eye colours (blue and black).

4)Genetics: Branch of biology that deal with the study of inheritance is called genetics.

5) Gene: Gene is the basic unit of inheritance is called genetics.

6) Chromosomes: Chroma means colour and Soma means body. (These absorbed the dye so named chromosome).Dark staining structure in which the organism genitic material (DNA) is organized.In eukaryotes chromosome is composed of DNA, protein and RNA while in prokaryotes this composed of only small DNA or RNA.

7) Chromation: Collection of DNA, Protein fibres which during prophase form chromosomes is called chromation.

8) genotype: genotype is the genetic make up of an organism.

9) Phenotype: it is the physical appearance of an organism as a result of genetic make up. Example: ( tallness,colour,etc.)

organic evolution: organic evolution is the change in the characteristics of a population over the couse of generations.

Allele or allelomorph: the alternate form of the same gene at a particular site or locus is called allele.


                                  Figure : Location of alleles on chromosomes.


Q#2) : Discuss the relationship between chromosome and gene ? Describe the structure of chromatin? Also write a short note on nucleosome (unit of chromatin)?

Answer: Relationship between chromosome and gene:

Gene present on contain a specific part of DNA . Each gene  has a specific information.each organism has a constant number of chromosomes.human body cells have 23 pairs of homologous chromosomes making a total of 46 chromosomes.

The two chromosomes of a pair are called homologous chromosome homologous chromosomes are same in size, shape and characteristics. Chromosome is composed of chromatin material. This chromatin is composed of DNA and histone protein. DNA wrap around the histone protein and form a rounded structure called nnucleosome..

Nucleosome: nucleosome is the unit of chromatin material. Nucleosome is the combination of DNA and proteins. Nucleosomes are arranged in linear and condensed form that it gives chromatin and form chromosomes. Each nucleosome having 200 nucleatides of duplex (double helix) DNA and 8 histone proteins.

The protein forming an octamere. Histone has positively charge (due to arginine and lysine). So they attract negative charge phosphate of DNA so form supercoiled structure.



Q#3) how does the DNA of chromosomes work? “OR” write a note on the role of DNA ( gene) in protein synthesis? “OR” how characteristics (traits) are controlled by DNA (protein)?

Answer: role of DNA in protein synthesis: DNA is the genetic material. It contains all the instructions and controls all functions of the cells. It gives information for the synthesis of proteins. These proteins then perform structural role or funtional role.

Some proteins functions as enzymes. Gene are present on DNA which in turn are located on chromosomes. Genes contain nitrogenous bases. Each triplet of nitrogenous ( nucleotides ) code for an aminoacid. Many peptides ( aminoaced ) combined to form proteins. These proteins form in two steps, called transcription and translation.

. Transcription: The formation of mRNA form DNA is called transcription. At this stage DNA unzip, and replication of DNA takes place. A single strand of mRNA formed.

. Translation: The formation of protein from RNA is called translation. In cytoplasm the mRNA carried its sequence of nucleotide to ribosome . The ribosome read this sequence and joins specific aminoacids brought by transper RNA (tRNA).



Q#4: What are genotype and phenotypes ? What are the types of genotype?

Answer: Genotype: (from gene) The specific combination of genes is known as genotype. It is the genetic make up of an orginism. Genotypes are of two types.

Homozygous genotype and heterozygous genotype.

Homozygous genotype: The genotype in which gene pair consists of two identical alleles is called homozygous genotype. Example: AA , aa , TT, tt, etc.

Heterozygous genotype: The genotype in which the gene pair consists of different alleles is called heterozygous genotype. Example: Aa, Tt, Rr, etc.


Allele or allelomorph: the alternate form of the same gene at a particular site or locus is called allele. OR gene occur in pair, a single gene from a particular gene pair is called an allele. For Example in Aa gene pair two alleles  A and a are present.

When in the heterozygous condition one allele masks or prevents the expression of the other, it is called the dominant allele. The allele which is not expressed is called recessive. The dominant alleles are represented by capital letters and recessive alleles by lower case letters.

Albinism is a recessive trait i.e. it is produced when both alleles are recessive. In humans, allele ‘A’ produces normal body pigments while allele ‘a’ does not produce pigments. If genotype is AA or Aa, the individual will produce pigments. On the other hand if genotype is aa, no pigments will be produced and the individual will be albino.

Phenotype: (from greek phainein meaning to show ) The observable physical appearance of an organism is called phenotype. Example: Round seeds, white color, etc.


Question 5: Write a note on Watson and Crick model OR double helical structure of DNA?

Answer: Watson-Crick Model of DNA:  Watson and Crick model was proposed by James Watson and Francis Crick in 1953. This model shows the structure of DNA. According to this model DNA is a double stranded helical molecule. It consists of two sugar phosphate back bones on the outside, held together  by hydrogen bonds between pairs of nitrogenous bases from the inside.

The inside steps make paired with each other by hydrogen bonds.  Adenine (A) form double bond with thymine (T), While Guanine form Triple bond with Cytosine (C). Adenine and Guanine are larger bases and are called purines. Purines are double ring structures. Thymine and Cytosine are small and single ring structures and are called pyrimidine. Watson and Crick was awarded noble prize in 1962.

Chapter Number 15 Inheritance




Question 6 : Describe the process of replication of DNA?

Answer: DNA Replication: It is the process of producing two identical daughter DNA,s from one original DNA molecule.

Before cell division the DNA replicates to make the copies of chromatids of chromosome. DNA is capable of self replication. This process completes in following steps.

 Separation of Strands: In this step hydrogen bond between DNA strands breaks and the DNA becomes unzip, like an open zipper. The splitting first of all occurs at double bond. (A=T). Helicase enzymes breaks the hydrogen bonds.

Binding of Primer: RNA primase binds with the initiation point of the 3′ -5′. It attract the RNA nucleotides and make a primer (a small stretch of RNA). Primer starts the process of replication.

Duplication: Nitrogenous bases present in nucleoplasm are paired with the single strand by polymerase enzymes, so that a full strand fully prepared. To each parental strand a new strand forms. The new formed DNA contain one old parental strand and one new strand and that is why this replication is called semiconservative replication.

Termination: The last step is termination. This occurs when primer is removed. From the two new strands errors are removed by nucleases and the two strands are joined by ligase enzymes.


Figure : How does DNA replicate?


For your information:

A dominant allele only suppresses the expression of recessive allele.It does not affect its nature.


Mendel used 28,000 pea plants in his experiments.

Question 7: Who was Gregor Mendel ? write a note on his laws of inheritance?

Answer: Gregor John Mendel: Gregor Mendel is called the father of genetics due to his valuable work in this field. He was an Austrian monk who lived in small monastery in Brno. He performed his famous experiments on pea plant (Pisum sativum) in 1856-1864 and provide a base to modern genetics.

Mendel laws of inheritance: Mendel select pea plant for his experiments and put forward two important laws of inheritance that marks the real beginning of the study of genetics. Mendel used the pure breeding strains.

Question 8 : Why Mendel was successful in his experiments? OR Why Mendel select pea plant for his experiments? OR What are the characteristics properties of a pea plant?

Answer: Mendel was a good mathemician. This gives help in finding ratio. Secondly Mendel selected pea plant for his experiments that has many advantages for his experiments.

Advantages of using pea plant in his experiments: Mendel argued that an organism for genetic experiments should have the some basic characteristics. All these properties are shown by pea plants, so Mendel select pea plants for his experiments. Important characteristics of a pea plant are as follows.

  • There should be a number of different traits that can be studied (see figure below).
  • The organism should have contrasting traits e.g. for the trait of height there should be only two very different phenotypes i.e. tallness and dwarfness.
  • The organism (if it is a plant) should be self-fertilizing (self pollination) but cross fertilization (cross pollination) should also be possible.
  • The organism should have a short but fast life cycle.
  • The organism must produce large number of flowers and seeds, so that it can be helpful in making ratio.
  • The organism must need less area for cultivation.
  • The organism should produce different color of flowers.

Question 9: What is Monohybrid cross, Monohybrid ratio and Monohybrids?

Answer: 1) Monohybrid Cross: A cross in which only one contrasting trait is studied at a time is called Monohybrid cross.

Monohybrids: The offsprings of monohybrid cross are called Monohybrids.

Monohybrid ratio: The phenotypic ratio of different types of individuals occurring in the F2 generation is called Monohybrid ratio.


Question10: Describe Mendel’s law of segregation OR Law of monohybrid cross?

Answer: Mendel law of segregation Or Law of Monohybrid cross: This law states that when a pair of contrasting traits are brought together in the same cross, members of allelic forms (pairs) remains together in the first generation (F1) but separates in the next generation. OR This law states that whenever two alleles of a traits are brought in a same individual, they stay together but at the time of gametes formation they separates from each other.

Explanation: Mendel crossed a true breeding round seeded plant with a true breeding wrinkled seeded plant. All resulting seeds of the next generation were round. Mendel called the parents as “P1” while the resulting generation as first filial generation “F1”. He says that round is dominant over wrinkled. Next year Mendel planted the round seeds of F1 and allowed the new to self fertilize. The Offsprings in the next generation were as follows.

Rounded seeds = 5474

Wrinkled seeds = 1850

Total seeds = 7324.

In this generation the parents are called P2 and the offsprings were named F2 generation (second filial generation).


Question 11: Define Dihybrid cross, Dihybrids and Dihybrid ratio?

Answer: Dihybrid Cross: A cross in which two contrasting traits are studied at a time is called dihybrid cross.

Dihybrid Ratio: The phenotypic ratio of different types of individuals occurring in the F2 generation is called Dihybrid Ratio.

Dihybrids: The offsprings of a dihybrid cross are called dihybrids.


Question 12: Explain how Mendel proved the law of independent assortment?


Answer: Mendel’s Law of Independent Assortment: According to this law “ When two contrasting traits are brought together in the same cross, alleles in each pair are inherited or assorted independent of each other.  “OR”  When two traits are brought together in the same cross, the distribution of alleles for one traits in the gametes doesnot effect the distribution of allele for the other trait.

“OR” “The alleles of a gene pair segregate (get separated and distributed to gametes) independently from the alleles of other gene pairs”.  “OR”  “The alleles of a gene pair segregate (get separated and distributed to gametes) independently from the alleles of other gene pairs”.


Explanation: In this law two characters are under consideration at a time. In this cross Mendel noticed that one type of character had no effect on the other. In other words we can say that member of allelic forms assorted independent of each other.

Example: Mendel crossed true breeding plants having round yellow seeds (RRYY) seeds with plants having wrinkled green seeds (rryy). Seeds of F1 generation show only dominant characteristics that is seed with round and yellow phenotype (RrYy).

When F1 generation are self crossed, the F2 generation shows four (4) types of seeds.


Round Yellow seeds = 9/16

Wrinkled yellow seeds = 3/16

Round green seeds = 3/16

Wrinkled green seeds = 1/16


The F2 generation appears in ratio of 9:3:3:1

It shows that out of 556 seeds, 315 are round yellow, 108 are round green, 101 are wrinkled yellow and 32 wrinkled green seeds.


Mendel explained that the two traits i.e. seed shape and seed colour are not tied with each other. The segregation of ‘R’ and ‘r’ alleles happens independently of the segregation of ‘Y’ and ‘y’ alleles. 

 Question 13: Write a note on punnet square?

Answer: Punnet square: The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after R. C. Punnett (an English mathematician).

The gametes of both parents having all possible genetic set-ups are determined. A checker board is used to cross all the possible gametes of one parent with all the gametes of other parent. In this way, a biologist can find all the possible genotypes of offsprings.


Question 14: Explain the phenomenon of incomplete dominance with the help of example.What do you mean by co-dominance. Give an example.  “OR” Write a note on deviation from the Mendel laws?


Answer: CO-DOMINANCE AND INCOMPLETE DOMINANCE: After the discovery of Mendel’s work, scientists began experiments on the genetics of various organisms. These experiments proved that all the traits in organisms do not follow Mendel’s laws. For example, it was found that there are many traits which are controlled by more than one pair of genes.

Similarly for many traits there are more than two alleles in a gene pair. Co-dominance and incomplete dominance are two examples of such deviations from Mendel’s laws.

Co-dominance : Codominance is a relation in which both parent characters in heterozygous expresses in equal (same). Both the alleles expresses completely and donot show any dominant recessive relationship. As a result, the heterozygous organism shows a phenotype that is different from both homozygous parents.

An example of co-dominance is the expression of human blood group AB. The ABO blood group system is controlled by the gene ‘I’. This gene has three alleles i.e. IA, IB and i. The allele IA produces antigen A in blood and the phenotype is blood group A. The allele IB produces antigen B in blood and the phenotype is blood group B.

The allele i does not produce any antigen and the phenotype is blood group O. The alleles IA and IB are dominant over i. When there is a heterozygous genotype of IA IB, each of the two alleles produces the respective antigen and neither of them dominates over the other.





Between Alleles




Antigen ABlood Group AAllele IA is dominant over i



Antigen BBlood Group BAllele IB is dominant over i
iiNo AntigenBlood Group OAllele i is recessive
IAIBAntigen A & Antigen BBlood Group ABAlleles Iand IB are co-dominant


In-complete dominance: When the phenotype of heterozygote is intermediate between the phenotype of the two homozygotes, it is called incomplete dominance or partial dominance. The hybrid individual shows the characters of both parents and results a new character (intermediate character).

So the heterozygote individual is a mixing or blend of both parents and thus shows a new character different from both the parents. In this type of dominance relationship in F1 generation one character expresses more than the other, as in Four O Clock plant in red expreses 60% and white expresses 40% in F1 generation.

Example: In Four O Clock plants, the 3 flower colours are red, pink and white. There is no specific gene responsible for producing pink flowers. In Four O clock plant, the trait of flower colour is controlled by two alleles (let us say them R and r). The true breeding plants RR and rr have red and white flowers, respectively.

When a homozygous red flowered plant (RR) is crossed with homozygous white flowered plant (rr), the heterozygous (Rr) plants of F1 generation produce pink flowers (pink is a blend of red and white colours). This result clearly indicates that neither of the red flower allele (R) and white flower allele (r) is dominant. However, when two heterozygous plants with pink flowers (Rr) are crossed, F2 generation shows phenotypes of red, pink and white flowers in the ratio 1:2:1.


Question15: What is variation? Discuss sources of variations? What are the different types of variation?

Answer: Variation: The differences shown by the individuals of one kind of organisms are called variations.  “OR”  Characters in offsprings that are different from their parents are called variations.

Sources of Variations: The main sources of variations in sexually producing organisms are as follows.

Genetic Recombination: The process of forming new alleleic combination in offsprings by exchanges between genetic material is called genetic recombination. The genetic recombination occurs by crossing over during prophase-1 of meiosis-1 process.

Random fertilization:Random fertilization means that one of a million different kinds of sperms could fertilize an egg.  Each gamete has a unique genetic composition. During fertilization one of the millions of sperms combines with a single egg cell. The chances involved in this combination also act as a source of variation.

Mutation: Any sudden (accidental) change in the nucleotide sequence of the genetic material (DNA,RNA, Or Chromosome) is called mutation. Different alleles arise during mutation that lead to different products (Proteins). Original allele is called wild type and the new formed is called mutant allele.

Gene flow: Movement of genes from one population to the other is called gene flow. It is also a source of genetic variation. Migration into and out of population is responsible for a marked change in allele. For example: gene flow from white color human to black color human leads to embrown color.

Chromosomal Combination: Variations are also caused by different combinations of chromosomes in gametes and then in zygote. In the case of humans, the possible number of chromosomal combinations at fertilization is 70,368,744,177,664. In other words, a couple can produce more than 70 trillion genetically different children!

Types of Inheritable Variation: Following are the two main types of variations.

 Discontinuous variations.

 Continuous variations.

Discontinuous variations: A characteristic of any species with only a limited number of possible values is called Discontinuous variations. The inheritable variations are of two types i.e. discontinuous and continuous variations. Discontinuous variations show distinct phenotypes. The phenotypes of such variations cannot be measured. The individuals of a population either have distinct phenotypes, which can be easily distinguished from each other.

Blood groups are a good example of such variations. In a human population, an individual has one of the four distinct phenotypes (blood groups) and cannot have in between. Discontinuous variations are controlled by the alleles of a single gene pair. The environment has little effect on this type of variations.

Continuous Variations: Continuous variations are those which continuously transfer from parents to offsprings through ages. It deals with many phenotypes. It means that it starts from one extreme and ends on another extreme. Between these two extremes one can obsereved many intermediate characters. Continuous variations are controlled by many genes and are often affected by environmental factors.

Example: Human heights. In human populations the individuals have a range of heights ( from very small to tall ). No population can show only two or three distinct heights.

Question 16: How would you prove that variations lead to evolution?

Answer: Variations lead to Evolution (Organic evolution or Biological evolution) :

Organic evolution (biological evolution) is the change in the characteristics of a population or species of organisms over the course of generations (time). The evolutionary changes are always inheritable. The changes in an individual are not considered as evolution, because evolution refers to populations and not to individuals.

On earth various species are present from unicellular and multicellular. All these organisms are originated from simple and imperfect forms.  We called it modification or evolution. These variations occurs very slow during evolution and so one cannot able to check them. Organic evolution includes two major processes.

Alternation in genetic characteristics (traits) of a type of organism over time.

Creation of new types of organisms from a single type.

Example: In 15th century some rabbits were released to “porto santo” islands in Europe. After 400 years they becomes quite different from one another not only in color and size but also in other characters. Most important is this they cannot produce offspring’s when breed with European . although they were first taken from this European group of rabbits.


Question 17: What are different Concepts of evolution? Explain.

Answer: Concepts of evolution: There are two important theories about evolution.

Theory of Special creation: According to this theory all the current organisms are created from a single ancestory about thousand years ago. According to this theory, all the different forms of life that occur today on planet earth, have been created by God, the almighty. This idea is found in the ancient scriptures of almost every religion. According to Hindu mythology, Lord Brahma, the God of Creation, created the living world in accordance to his wish.

According to the Christian belief, God created this universe, plants, animals and human beings in about six natural days. The Sikh mythology says that all forms of life including human beings came into being with a single word of God. Special creation theory believes that the things have not undergone any significant change since their creation. The theory of Special Creation was purely a religious concept, acceptable only on the basis of faith. It has no scientific basis.

Theory of natural selection: Darwin (1809-1882) proposed this theory after his 5-year voyage on the HMS (His Majesty’s Ship) Beagle. He also published a book “On the Origin of Species by means of Natural Selection” in 1859. According to Darwin as time and generation continue, adaptations (variations) are passed on generation after generation. With the passage of time new species may evolve from a common ancestor.


Question 18 : What is role of natural selection in evolution?   

Answer: Mechanism of Evolution – Natural Selection

Almost every population contains several variations for the characteristics of its members. In other words, there are morphological and physiological variations in all populations. Natural selection is the process by which the better genetic variations become more common in successive generations of a population.


The central concept of natural selection is the evolutionary fitness of an organism. Fitness means an organism’s ability to survive and reproduce. Organisms produce more offspring than can survive and these offspring vary in fitness. These conditions produce struggle for survival among the organisms of population .

The organisms with favourable variations are able to reproduce and pass these variations to their next generations. On the other hand, the rate of the transmission of unfavourable to next generations is low. We can say that the favourable variations are “selected for” their transmission to next generations, while the unfavourable variations are “selected against” their transmission to next generations.


Example 1: In this example we can see a mouse population with variations in skin colour. Cat preys upon light and medium coloured mouse. In first generation, light coloured mouse is preyed upon by cat.

Figure: The concept of natural selection

Only medium and dark coloured mouse can make their next generations. In next generation, population again contains light, medium and dark coloured mouse. Cat preys upon the light and medium coloured mouse. Now only the dark coloured mouse make new generation. If this happens in many generations, we will see only the dark coloured (favourable variation) mouse in the population (Fig. 15.7).

As a result of natural selection, the allele that gives more fitness of characteristics (favourable variations) than other alleles becomes more common within population. So, the individuals with favourable variations become a major part of population while the individuals with harmful or unfavourable variations become rarer.

Example 2: In England, the moths had two variations i.e. dark and white coloured moths (Fig.15.8). The moths used to rest on the light coloured tree trunks (on which white lichens had grown). In the 19th century when industries were established in England, the lichens on tree trunks died (due to polluted air) and the naked tree trunks turned dark.

Now the white moth variation became harmful because a white moth resting on a dark tree trunk was easily visible to the predatory birds. The natural selection selected dark moths to reproduce. In this way dark coloured moth became more common and at last the white

moths disappeared from population. In this case, the dark colour variation in moth may be considered an adaptation to environment.

Figure: White and dark coloured moths

Different populations face different environments and they have to adapt to different conditions.


French biologist C. de Buffon (1707–1788) was the first to hint at evolution. His countryman  J. de Lamarck(1744–1829) was the first to propose a mechanism of evolution. Lamarck’s ideas were soon rejected due to the vagueness of the mechanisms he proposed.

Question 19: Write a note on artificial selection and crop improvement?

Answer: Artificial selection OR Selective breeding (Crop improvement by selective breeding) : Artificial selection or Selective breeding means intentional breeding between individuals for certain traits or combination of traits. OR The selection of best varieties from a farm stock to produce best varieties is called artificial selection. The term “artificial selection” was first expressed by the Persian scientist Abu Rayhan Biruni in the 11th century. Charles Darwin also used this term in his work on natural selection.

He noted that many domesticated animals and plants had special properties that were developed by:

  • Intentional breeding among individuals with desirable characteristics; and
  • Discouraging the breeding of individuals with less desirable characteristics. In artificial selection, the bred animals are known as breeds, while bred plants are known as varieties or cultivars.

Uses and importance of Artificial selection: Artificial selection increase the productivity of crops. All present day plants are mainly artificially selected. Many improved plant varieties are produced by this techniques such as cereals, fruits and vegetables. Similarly animals best breeds like that of sheep, goat, cow and hen are also produced using artificial selection.

Question: What are hereditary characters and non-hereditary characters?

Hereditary Characters: – Those characteristics, which are transmitted, form parents to offspring. For example eye colour, hair colour etc.

Non-hereditary Characters: – Those characteristics, which are not, transferred form parents to offspring. For example parent’s habits or any body part lost or weakened due to any disease.


  1. Describe the structure of chromatin.

Answer: See answer of question no. 2.


  1. Describe Mendel’s law of segregation.

Answer: See answer of question no. 10.


  1. Explain how Mendel proved the law of independent assortment.

Answer: See question no. 12.


  1. How would you prove that variations lead to evolution?

Answer: See question no. 16.


  1. Explain the phenomenon of incomplete dominance with the help of example.

Answer: See question no. 14.


  1. What do you mean by co-dominance. Give an example.

Answer: See question no. 14.



  1. Define genotype and phenotype.

Answer: See question no. 4.


  1. What do you mean by dominant and recessive alleles?

Answer: See question no. 4


  1. What are the homozygous and heterozygous genotypes?

Answer: See question no. 4


  1. Differentiate between natural and artificial selection.

Answer: Difference between natural and artificial selection;


   Natural selection  Artificial selection
1) Natural selection is the result of natural factors which favour certain variations.1) Artificial selection is when human beings choose certain traits and breeds organisms for that traits.
2) It is nature made selection.2) It is man made selection.
3) It produces great biological diversity.3) It produces varieties of organisms very different from native generation.
4) It occurs in natural population.4) It occurs in domestic population.
5) It is a very slow process.5) It is very faster process.




Allele: Allele or allelomorph: the alternate form of the same gene at a particular site or locus is called allele. OR gene occur in pair. A single gene from a particular gene pair is called an allele. For Example in Aa gene pair two alleles A and a are present.

Artificial selection:

Breeds: Animals which are bred through artificial selection are called breeds.

Chromatin: Collection of DNA, Protein fibres which during prophase form chromosomes is called chromatin. It is the chemical material that make the structure of the chromosome

Chromosomes: a thread-like structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.

Co-dominance: Codominance is a relation in which both parent characters in heterozygous expresses in equal (same). OR  The situation where two allele of a gene pair express their traits independently instead of showing a dominant-recessive relationship.

Cultivar: The plants which are bred through artificial selection are called cultivars.

Dihybrid cross: A genetic cross in which two pairs of contrasting traits are studied at a same time is called Dihybrid cross.

Dominant: In heterozygous condition if one allele masks or prevents the expression of the other is called the dominant allele. OR The allele which in heterozygous condition expresses itself is called dominant.

Gene: Unit of inheritance. A part of DNA that is responsible for the formation of a specific protein is called gene.

Genotype: The specific combination of genes in an individual is called genotype. OR The genetic make up of an organism is called genotype.

Heridity: A characteristic in an individual that is similar to the typical characteristics of other individuals of the same species are called Heridity.

Heterozygous: The genotype that has two different alleles of a trait is called heterozygous. Example: Aa, Tt, Rr, etc.

Histone: The protein present in the structure of chromosome.

Homologous Chromosome: A pair of chromosomes having the same size and shape and carrying alleles for the same traits

Homozygous: The genotype in which gene pair consists of two identical alleles is called homozygous genotype. Example: AA , aa , TT, tt, etc.

Incomplete dominance: When the phenotype of heterozygote is intermediate between the phenotype of the two homozygotes, it is called incomplete dominance or partial dominance.   OR   A type of inheritance in which neither of the pair of contrasting alleles is dominant over the other and the heterozygous individual is intermediate in phenotype.

Inheritance: The transmission of characteristics from parents to offspring is called inheritance.

Locus: Plural Loci; The locations or positions of genes on chromosomes is called locus.

Monohybrid cross: A cross in which only one contrasting trait is studied at a time is called Monohybrid cross.

Mutation: Any accidental change in the nucleotide sequence of the gene is called mutation.

Natural selection: The process in which organisms with favourable variations survive and produce more offspring than less well-adapted organisms is called natural selection.

Nucleosome: The structure formed by the wrapping of DNA around histone proteins is called histone protein.

Organic evolution OR Biological evolution: The modification of characteristics in the species or populations of organisms during their descent, generation by generation is called Organic evolution or Biological evolution.

Phenotype: The expression of the genotype in the form of trait is called phenotype  OR  It is the physical appearance of an organism as a result of genetic make up. Example: ( tallness,colour,etc.)

Recessive trait: The trait that not expresses in heterozygous condition is called recessive.

Trait: The characteristics which are controlled and transmitted to next generations through genes are called traits.

True-breeding: The homozygous individuals are called true breeding. The genotype in which gene pair consists of two identical alleles is called homozygous genotype. Example: AA , aa , TT, tt, etc.

Variations: A characteristic in an individual that differs from the typical characteristics of other individuals of the same species are called variation

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