Class 10 Biology Chapter 1 Notes Kerala Syllabus Genetics of Life Questions and Answers

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SSLC Biology Chapter 1 Notes Questions and Answers Pdf Genetics of Life

SCERT Class 10 Biology Chapter 1 Genetics of Life Notes Pdf

SSLC Biology Chapter 1 Questions and Answers – Let Us Assess

Question 1.
Are basic building blocks of DNA and RNA the same? Explain.
Answer:
The basic building block of DNA is nucleotide. Each nucleotide is composed of a deoxyribose sugar, a phosphate group, and a nitrogen base. In DNA the nitrogen bases namley Adenine, Thymine, Guanine and Cytosine are present.In DNA the nitrogen bases namley Adenine pairs with Thymine and Guanine pairs with Cytosine.

The nucleic acid RNA also plays a crucial role in the synthesis of proteins. They are also made up of nucleotides. Each of the nucleotide contains a ribose sugar, a phosphate group, and a nitrogenous base. The nitrogen bases in RNA are Adenine, Guanine, Uracil, and Cytosine. Most of the RNAs have a single strand

Question 2.
Analyse the statements and choose the appropriate one.
i. F₁ has similarity with both the parents.
ii F₁ has no similarity with any of the parents’ character intermediate to them,
iii F₁ has similarity with one of the parents
a) i – Dominance, ii – Incomplete dominance, iii – Co – dominance
b) i – Incomplete dominance, ii – Dominance, iii – Co – dominance
c) i – Co-dominance, ii – Incomplete dominance, iii – Dominance
d) i – Dominance, ii – Co-dominance, iii – Incomplete dominance
Answer:
c) i – Co-dominance,
ii – Incomplete dominance,
iii – Dominance

Question 3.
Which of the following is contributed by organisms that reproduce sexually, to their offspring?
a) All genes
b) Half of their genes
c) One fourth of their genes
d) Double the number of genes
Answer:
b) Half of their genes

Question 4.
A tall pea plant with purple flowers (dominant character) is crossed with a dwarf plant with white flowers.
a) Illustrate the dihybrid cross of these and write the F₂ ratio.
b) Did characters that differ from the parents appear in the F₂ generation? Why?
c) If both the genes are not assorting independently, how does it affect the F₂? ratio?
Answer:

F₂ generation – 9 Tall plant with purple flowers : 3 Tall plant with white flowers : 3 Dwarf plant with purple flowers : 1 Dwarf plant with white flowers generatio.

b) Yes. The Tall plant with white flowers and Dwarf plant with purple flowers are differ from the parents that appear in the F₂ generation. When two or more different traits are combined, each trait is inherited independently to the next generation without mixing each other. (A pair of alleles in an organism does not influence the separation of another pair of alleles.)

c) If both the genes do not assort independently, it means they are linked and present close together on the same chromosome. So, they tend to be inherited together.As a result, the expected F₂ ratio of 9 : 3 : 3 : 1 will not appear. Instead, we will get more offspring with the parental combinations and fewer with new combinations.

Question 5.
How does dominance, co-dominance and incomplete dominance differ from one another?
Answer:
In co – dominance both alleles exhibit their traits at the same time. F₁ has similarity with both the parents. In incomplete dominance a dominant allele cannot fully hide the allele of the recessive trait. F₁ has no similarity with any of the parents’ character intermediate to them.
In dominance the F₁ has similarity with one of the parents

Question 6.
Different phenotypic ratios are obtained in monohybrid and dihybrid cross. Why? What does it indicate about the inheritance of characters?
Answer:
The phenotypic ratios are obtained in monohybrid is 3 : 1 and dihybrid cross is 9 : 3 : 3 : 1. When two or more different traits are combined, each trait is inherited independently to the next generation without mixing each other. If more characters are considerd they leads to the formation of more different charecters in offsprings.

Question 7.
Even though a gene responsible for certain characters has more than two alleles, why does that particular gene have only two alleles in an individual?
Answer:
Each gene in our body is present in pairs of chromosomes – one from the mother and one from the father. So, for any gene, an individual can inherit only two alleles: One from each parent.Even if the gene has more than two alleles in the whole population (like in human blood groups: I^A, I^B, and i), a single person can have only two of these at a time (like I^A, I^B or I^Ai or I^Bi).

Question 8.
Although the DNA possesses all genetic in-formation for protein synthesis, RNA is also required for protein synthesis. Why?
Answer:
DNA stays safely inside the nucleus and cannot leave it.“But proteins are made in the cytoplasm, at the ribosomes. So, DNA needs a messenger to carry the genetic code from the nucleus to the ribosomes. That messenger is RNA – specifically mRNA (messenger RNA).

mRNA copies the code from DNA (this is called transcription). mRNA carries this code out of the nucleus to the ribosome. At the ribosome, tRNA and rRNA help read the code and build the protein.

Question 9.
How do co-dominance and multiple allelism function in the determination of blood group in the ABO blood grouping in human beings? Explain.
Answer:
The ABO blood group system in humans is an excellent example of co-dominance and multiple allelism, both of which play a crucial role in determining blood type.

Role of Multiple Allelism in ABO Blood Group – Multiple allelism occurs when a gene has more than two alleles in the population. The ABO blood group gene (I gene) has three alleles:

I^A – Produces A-type antigen on red blood cells.
I^B – Produces B-type antigen on red blood cells.
i (I^O) – Does not produce any antigen.

An individual can inherit only two alleles (one from each parent), but the presence of three alleles in the population creates different combinations, leading to four possible blood groups: A, B, AB, and O.

Genotype	Blood group
IAIA or IA IO	A
IBIB or IB IO	B
IAIB	AB
IOIO	O

Role of Co-Dominance in ABO Blood Group – Co-dominance occurs when both alleles are expressed equally. Co-dominance ensures that both IA and IB alleles are equally expressed in individuals with blood group AB.

Question 10.
All ova formed in females contain one type of sex determining chromosome. Why?
Answer:
Humans have 46 chromosomes (23 pairs), and one pair is the sex chromosomes. Females have two X chromosomes (XX). During the formation of ova (egg cells), only one X chromosome goes into each egg. Since both sex chromosomes in females are X, every ovum gets only one X chromosome.

Biology Class 10 Chapter 1 Notes Kerala Syllabus Genetics of Life

Question 1.
The completed worksheet 1.1 of page 11

Answer:

1. Number of strands in DNA	Two
2. Molecules used to make strands	Sugar and phosphate
3. Molecules used to make rungs	Nitrogen bases
4. Different types of nitrogen bases	Adenine, Thymine, Guanine and Cytosine
5. Formation of rungs	Formed by the pairing of nitrogen bases.
6. Mode of nitrogen base pairing	Adenine pairs with Thymine and Guanine pairs with Cytosine.
7. Molecules in a nucleotide	A deoxyribose sugar (5 carbon sugar), a phosphate group, and a nitrogen base.

Question 2.
How does the normal sugar differ from a sugar molecule in DNA?
Answer:
The sugar in DNA is deoxyribose, which differs from normal sugar, ribose, found in RNA. The key difference is Deoxyribose (DNA sugar): Lacks an oxygen atom at the 2′ carbon (hence “deoxy”), making it C₅H₁₀O₄. Ribose (Normal sugar in RNA): Has an -OH (hydroxyl) group at the 2′ carbon, making it C₅H₁₀O₅. Deoxyribose makes DNA more stable, suitable for long-term genetic storage. DNA is less prone to degradation than RNA.

Indicators in the text book page 12
Question 3.
Building blocks of chromosomes:
Answer:
DNA and histone proteins are the primary components of a chromosome.

Question 4.
Histone and nucleosome:
Answer:
Eight histone proteins join together to form a histone octamer. DNA strands wind around this octamer to form a structure called nucleosome.

Question 5.
Formation of chromosome:
Answer:
DNA and histone proteins arc the primary components of a chromosome. Eight histone proteins join together to form a histone octamer. DNA strands wind around this octamer to form a structure called nucleosome. The chromosomes are formed by packing and coiling numerous nucleosomes and recoiling the chains of nucleosomes.

Question 6.
Chromatid, centromere
Answer:
Chromatids are the parts of a chromosome which are connected by means of centromere.

Question 7.
What is the relationship between chromatin reticulum and chromosomes?
Answer:
Chromatin reticulum and chromosomes are two forms of the same genetic material, DNA. In a resting (non-dividing) cell, the DNA is long, thin, and spread out in the nucleus. This loose network is called chromatin reticulum. When a cell gets ready to divide, the chromatin condenses and coils up tightly to form chromosomes. So, chromosomes are condensed forms of chromatin reticulum. Both carry the genetic information needed for the functioning and inheritance of traits.

Question 8.
Why Are Chromosomes Seen in Pairs?
Answer:
In most organisms, including humans, chromosomes are seen in pairs because body cells are diploid (2n), meaning they contain two sets of chromosomes – one inherited from each parent.

Question 9.
Reasons for Chromosomes Being in Pairs
Answer:
Each human cell has chromosomes in pairs. One chromosome in each pair comes from the mother (called the maternal chromosome) and the other comes from the father (called the paternal chromosome). These pairs are called homologous chromosomes. They have the same genes in the same positions, but they may carry different forms (called alleles) of those genes. During meiosis (the special cell division that forms sperm and egg cells), these homologous chromosomes separate. This ensures that each gamete (sperm or egg) gets only one chromosome from each pair. When fertilization happens, the number of chromosomes remains stable in the next generation.

Indicators in the text book page 14
Question 10.
Different types of chromosomes.
Answer:
Somatic chromosomes (22 pairs) and Sex Chromosomes (1 pair)

Question 11.
Somatic chromosomes and their functions.
Answer:
These are chromosomes that control physical characteristics. There are twenty two pairs of somatic chromosomes.

Question 12.
Homologous chromosomes
Answer:
A pair of identical chromosomes together form a homologous chromosome. One of these is inherited from the mother and the other from the father.

Question 13.
Sex chromosomes and their function
Answer:
These are the chromosomes which are involved in sex determination. They are of two types. X chromosome and Y chromosome. The Y chromosome is comparatively smaller than that of the X chromosome. The SRY gene on the Y chromosome is responsible for the development of testis in the embryo.

Question 14.
Are there multiple organinsms with same number of chromosomes?
Answer:
Yes, different organisms can have the same chromosome number, but this does not mean they are related or have similar characteristics. Chromosome number only indicates the total number of chromosomes in a species, not the genetic content or complexity.

Examples of Different Organisms with the Same Chromosome Number

Chromosome Number	Organisms
48	Chimpanzees, Gorillas
78	Dogs, Chickens
48	Potatoes, Chimpanzees
24	Rice, Tomatoes

Question 15.
Prepare a table including the chromosome number of various organisms that are seen in your surroundings
Answer:

Organism	Common Name	Scientific Name	Number of Chromosomes
Human	Human being	Homo sapiens	46 (23 pairs)
Dog	Domestic dog	Can is familiaris	78 (39 pairs)
Cat	Domestic cat	Felis catus	38(19 pairs)
Cow	Domestic cow	Bos taurus	60 (30 pairs)
Rice	Rice plant	Oryza sativa	24 (12 pairs)
Onion	Onion	Allium cepa	16 (8 pairs)
Housefly	Housefly	Musca domestica	12 (6 pairs)

Question 16.
The completed table 1.1 of page 14

Answer:

	Genetic constitution	Total Number of chromosomes	Number of somatic chromosomes	Number and type of sex hormones
Female	44 + XX	46	44	2, XX
Male	44 + XY	46	44	2, XY

Question 17.
The completed table 1.2 of page 16

Answer:

	Number of strands	Type of sugar molecule	Nitrogen bases
DNA	2	Deoxy ribose sugar	Adenine, Thymine, Guanine, Cytosine
RNA	1	Ribose sugar	Adenine, Guanine, Uracil, Cytosine

Indicators in the text book page 18
Question 18.
Stages of Protein Synthesis:
Transcription and Translation
Answer:

Question 19.
Processes take place in the nucleus
Answer:
Transcription – mRNA is formed from a speific nucleotide sequence (gene) in DNA with the help of various enzymes. The mRNA contains messages for protein synthesis.

Question 20.
Processes take place in the cytoplasm
Answer:
Translation – tRNAs (transfer RNA) carry- specific amino acids to the ribosome based on message in the mRNA that has reached the ribosome from the nucleus. The rRNAs (ribosomal RNA), which are part of ribosomes combine amino acids to make protein.

Question 21.
The completed illustration 1.8 of page 18

Answer:

Answers of Indicators in the text book page 23

Question 22.
The characters considered and their traits.
Answer:
Character – Height of plant
Traits – Tall, dwarf

Question 23.
Dominant and recessive traits in the first generation
Answer:
Dominant trait – Tall
recessive trait – Dwarf

Question 24.
Importance of self-pollination
Answer:
Mendel hypothesised that there must be certain factors within the seed that control traits. To find out what might have happened to the factor responsible for dwarfness, he self-pollinated the plants obtained in the first generation to produce a second generation of plants.

Question 25.
Traits in the second generation.
Answer:
Tall, Dwarf in 3 : 1 ratio

Question 26.
The completed illustration 1.10 of page 24

Answer:

Answers of Indicators in the text book page 24
Question 27.
Phenotype and genotype of the parental plants
Answer:
Phenotype of the parental plants – Tall, its genotype – TT
Phenotype of the parental plants – Dwarf, its genotype – tt

Question 28.
Phenotype and genotype of first generation plants
Answer:
Phenotype of the first generation plants – Tall, its genotype – Tt

Question 29.
Genotype of the tall parental plant and the first generation plant
Answer:
Phenotype of the parental plants – Tall, its genotype – TT and Phenotype of the first generation plants – Tall, its genotype – Tt

Question 30.
Ratio of the traits in the second generation
Answer:
3 tall: 1 dwarf

Answers of indicators of page 25
Question 31.
Why was a plant with intermediate height not formed by the combination of tall and dwarf?
Answer:
A trait is controlled by two factors. When gametes are formed, the factors that determine trait gets separated without mixing.

Question 32.
Hasn’t the character that is not expressed in the first generation appeared in the second generation? How would that be?
Answer:
Yes, traits that are not expressed in F₁ generation can reappear in the 2nd generation (F₂).

Mendel came up with the assumption that during gamete formation the factors that determine a particular character segregate without getting mixed; ie., half of the gametes formed from first generation plant Tt, contain T and the other half contain t.

Question 33.
The hybridisation experiment conducted by considering height of the plant and shape of the seed. The completed illustration 1.11 of page 26

Answer:

Answers of Indicators in the text book page 26
Question 34.
Characters considered and their contrasting traits
Answer:
Characters considered – Height of the plant and shape of the seed
Contrasting traits of character Height – Tall, dwarf Contrasting traits of character Shape of seed – Round seed, wrinkled seed

Question 35.
Phenotype and genotype of the parental plants
Answer:
Phenotype of the parental plants – Tall and round seed, its genotype – TTRR
Phenotype of the parental plants – Dwarf and wrinkled seed, its genotype – ttrr

Question 36.
Dominant and recessive traits of the first generation
Answer:
Dominant trait – Tall and round seed
Recessive trait – Dwarf and wrinkled seed

Question 37.
Alleles of the gametes produced by the first generation
Answer:
TR, Tr, tR,tr

Question 38.
Phenotype of the plants in the second generation
Answer:
Tall and round seed
Tall and wrinkled seed
Dwarf and round seed
Dwarf and wrinkled seed

Question 39.
Phenotypes observed in the second generation that differed in contrast to the parental plants and their genotypes
Answer:
Tall and wrinkled seed – Ttrr, TTrr
Dwarf and round seed – ttRR, ttRr

Question 40.
Phenotypic ratio of the second generation
Answer:
9 Tall and round seed : 3 Tall and wrinkled seed : 3 Dwarf and round seed : 1 Dwarf and wrinkled seed

Indicators in the text book page 30
Question 41.
Phase in which crossing over occurs.
Answer:
Occurs during the first phase of meiosis

Question 42.
Process of crossing over
Answer:
Meiosis is the type of cell divisin that is resposible for the formation of gametes. Pairing of homologous chromosomes (Identical chromosomes inherited from the parents of an organism). The point of contact of the paired chromosomes is called chiasma. The chromatids break at this region and the broken segments are exchanged with each other.

Question 43.
Role of crossing over in the formation of variations
Answer:
During meiosis pairing of homologous chromosomes (Identical chromosomes inherited from the parents of an organism) takes place. The point of contact of the paired chromosomes is called chiasma. The chromatids break at this region and the broken segments are exchanged with each other.This exchange causes a recombination of alleles. This leads to the appearance of new’ traits in the offspring.

Question 44.
Mutation

Answer:
Mutation is the sudden heritable change in the genetic constitution of an organism.

Question 45.
Reasons for mutation
Answer:
Mutations can be caused by errors during DNA replication, exposure to certain chemicals, radiations, etc.

Question 46.
Importance of mutation
Answer:
Mutation causes changes in genes. These genes are transferred through generations which leads to variations in characters. Mutations play a crucial role in the process of evolution.

Std 10 Biology Chapter 1 Notes – Extended Activities

Question 1.
Present the process of transcription and translation in the class, using coloured beads or paper strips to indicate nucleotides.

Question 2.
Prepare and present a time-line animation in class that depicts the steps in the development of genetics.
Answer:
1865 : Gregor Mendel presents his laws of inheritance based on pea plant experiments, laying the foundation for classical genetics.
1869 : Friedrich Miescher discovers DNA (then called “nuclein”) from white blood cells.
1900 : Mendel’s work is rediscovered independently by Hugo de Vries, Carl Correns, and Erich von Tschermak.
1902 : Walter Sutton and Theodor Boveri propose the Chromosome Theory of Inheritance, linking Mendel’s laws to chromosomes.
1910 : Thomas Hunt Morgan demonstrates sex-linked inheritance using fruit flies (Drosophila melanogaster).
1913 : Alfred Sturtevant creates the first genetic linkage map.
1944 : Oswald Avery, Colin MacLeod, and Maclyn McCarty prove that DNA is the genetic material, not proteins.
1950 : Erwin Chargaff formulates Chargaff’s Rules, discovering that adenine pairs with thymine and guanine pairs with cytosine.
1952 : Hershey-Chase experiment confirms that DNA (not protein) carries genetic information.
1953 : James Watson and Francis Crick (with Rosalind Franklin’s X-ray data) propose the double-helix structure of DNA.
1972 : Recombinant DNA technology is developed by Paul Berg.
1973 : Herbert Boyer and Stanley Cohen create the first genetically modified organism (GMO).
1977 : Frederick Sanger develops the DNA sequencing method.
1990 : The Human Genome Project (HGP) begins, aiming to sequence the entire human genome.
1996 : Dolly the Sheep becomes the first cloned mammal using somatic cell nuclear transfer.
2003 : The Human Genome Project is completed, mapping all human genes.
2012 : CRISPR-Cas9 gene-editing technology is developed by Jennifer Doudna and Emmanuelle Charpentier, allowing precise DNA editing.
2018 : First CRISPR-edited human embryos reported in China,
2020 : Nobel Prize in Chemistry awarded to Doudna and Charpentier for CRISPR.

Question 3.
Conduct Mendel’s hybridisation experiment on pea plants using dices or beads to represent alleles.

Question 4.
Collect data and draw conclusion on how sex determination in various species takes place and the influence of environmental factors on it.
Answer:
In many organisms, sex determination is influenced by environmental factors rather than just genetic inheritance. These environmental factors can include temperature, social interactions, pH levels, and population density.

1. Temperature-Dependent Sex Determination
In some reptiles, the temperature at which eggs develop determines the sex of the offspring.
Turtles – Warmer temperatures produce females, while cooler temperatures produce males.
Crocodiles and some lizards – Warmer temperatures produce males, and cooler temperatures produce females.

2. Social or Behavioral Sex Determination
In some species, an individual’s sex changes based on social hierarchy or population dynamics. Clownfish – All start as males; the dominant male changes into a female if the leading female dies.

3. pH and Environmental Chemical Influence
Environmental chemicals and pH changes can affect sex determination.

4. Population Density and Sex Determination“ In some species, sex is determined by population density. Daphnia (Water Fleas) – At low population densities, most offspring develop as females; at high densities, more males are produced to promote genetic diversity through sexual reproduction.

5. Discuss the scientific, social and cultural dimensions of skin colour variation.
Melanin, a pigment protein imparts colour to the skin. The rise or fall in the production of melanin is due to difference in the function of alleles of genes responsible for skin colour. This is the reason for the colour difference of human skin.

Question 5.
Discuss the scientific, social and cultural dimensions of skin colour variation.

Genetics of Life Class 10 Notes

Genetics of Life Notes Pdf

• Gene editing process which can bring desirable changes in the genes in DNA.
• DNA is located in the chromosomes found inside the nucleus.
• In 1953, James Watson along with Francis Crick had presented the double helical model of DNA.
• As per double helix model of DNA, DNA has two strands. The strands are composed of sugar and phosphate.
• DNA contain four nitrogen bases – Adenine, Thymine, Guanine, Cytosine
• In DNA the nitrogen base Adenine pairs with Thymine, and Guanine pairs with Cytosine.
• Nucleotide is the basic building block of DNA.
• Each nucleotide is composed of a deoxyribose sugar, a phosphate group, and a nitrogen base.
• Nitrogen base is nitrogen containing alkaline compound.
• DNA and histone proteins are the primary components of a chromosome.
• Eight histone proteins join together to form a histone octamer.
• DNA strands wind around this octamer to form a structure called nucleosome.
• The chromosomes are formed by packing and coiling numerous nucleosomes and recoiling the chains of nucleosomes.
• Chromatids are the parts of a chromosome which are connected by means of centromere.
• Each species possess a specific number of chromosomes.
• Somatic chromosomes are chromosomes that control physical characteristics.
• There are twenty two pairs of somatic chromosomes.
• These are the chromosomes which are involved in sex determination. They are of two types. X chromosome and Y chromosome.
• Gene is a specific sequence of nucleotides in DNA.
• Proteins, which are synthesised according to the instructions of genes, are responsible for the formation of characteristic features and for controlling metabolic activities.
• The nucleic acid RNA also plays a crucial role in the synthesis of proteins.
• Each of the nucleotide of RNA contains a ribose sugar, a phosphate group, and a nitrogenous base.
• The nitrogen bases in RNA are Adenine,Guanine, Uracil, and Cytosine.
• Most of the RNAs have a single strand.
• The proteins are synthesised as a result of the action of genes.
• Stages of protein synthesis are Transcription and Translation.
• Heredity refers to the transmission of characteristics from parents to their offspring. Variations are characters expressed in offspring, that differ from their parents.
• Genetics is the branch of science that deals with genes, heredity, and variation.
• Gregor Johann Mendel is considered as the father of genetics.
• Gregor Mendel’s conclusions are known as the Laws of Inheritance. These laws provide the fundamental genetic framework to understand heredity and variation.
• Mendel initially conducted hybridisation experiments by considering a single pair of contrasting traits. This is known as a monohybrid cross.
• A gene that determines a character can have different forms. These different forms of genes are called alleles. A gene usually has two alleles.
• The observable characteristics of an organism are called phenotype, and the genetic constitution responsible for these characteristics are called genotype.
• Mendel observed the inheritance of two pairs of contrasting traits of the same plant. This is known as
  dihybrid cross.
• Later studies about the complex interaction among genes, environment and other factors revealed some of the limitations of Mendel’s laws. This gave rise to the concept of Non-Mendelian Inheritance.
• Incomplete Dominance – A dominant allele cannot fully hide the allele of the recessive trait.
• Co-dominance – Both alleles exhibit their traits at the same time.
• Multiple allelism – The gene that determines blood group in human beings has more than two alleles.
• Polygenic inheritance – More than one gene controls the colour of the skin.
• Recombination of alleles during fertilisation, crossing over, mutation, etc. causes variations in organisms
• The point of contact of the paired chromosomes is called chiasma. The chromatids break at this region and the broken segments are exchanged with each other.This exchange causes a recombination of alleles. This is crossing over.
• Mutation is the sudden heritable change in the genetic constitution of an organism.

INTRODUCTION

Nucleic acid
DNA is located in the chromosomes found inside the nucleus. In 1953, James Watson, along with Francis Crick, presented the double helical model of DNA. The strands of DNA are composed of sugar and phosphate. The rungs of DNA are fonned by the pairing of nitrogen bases. A nucleotide is the basic building block of DNA. Each nucleotide is composed of a deoxyribose sugar, a phosphate group, and a nitrogen base. In DNA, the nitrogen bases Adenine, Thymine, Guanine, and Cytosine are found. Adenine pairs with Thymine, and Guanine pairs with Cytosine. RNA is another type of nucleic acid, similar to DNA. It is also made up of nucleotides. Each nucleotide contains a ribose sugar, a phosphate group, and a nitrogenous base. The nitrogen bases in RNA are Adenine, Guanine, Uracil, and Cytosine. Most RNAs have a single strand.

DNA and histone proteins are the primary components of a chromosome. Eight histone proteins join together to form a histone octamer. DNA strands wind around this octamer to form a structure called a nucleosome. Chromosomes are formed by the packing and coiling of numerous nucleosomes and recoiling of nucleosome chains. Each species possesses a specific number of chromosomes.

Protein Synthesis
Proteins are synthesized as a result of the action of genes. Protein synthesis has two stages, namely transcription and translation. mRNA is formed from a specific nucleotide sequence (gene) in DNA with the help of various enzymes. The mRNA contains messages for protein synthesis. tRNAs (transfer RNA) carry specific amino acids to the ribosome based on message in the mRNA that has reached the ribosome from the nucleus. The rRNAs (ribosomal RNA), which are part of ribosomes combine amino acids to make protein.

Mendel’s Experiments and inferences
Genetics is the branch of science that deals with genes, heredity, and variation. Gregor Johann Mendel’s experiments on pea plants (Pisum sativum) and the conclusions he drew from hybridisation experiments laid the foundation for the field of genetics. Therefore, he is considered as the father of genetics. Gregor Mendel’s conclusions are known as the Laws of Inheritance. These laws provide the fundamental genetic framework to understand heredity and variation. In 1900, sixteen years after his death, botanists Hugo de Vries, Carl Correns, and Erich von Tschermak recognized the significance of Mendel’s research. With this, Mendel’s findings were accepted as the foundation of the science of genetics. Genetics has grown into one of the most extensive branches of science through the numerous contributions of various scientists.

Gregor Mendel hypothesized that characters from parents are passed on to offspring through certain factors that are transmitted via gametes. It was only after Mendel’s time that these factors were discovered to be genes, which are located in chromosomes in the nucleus. Mendel’s laws formed the foundation of genetics. However, they could not fully explain the diversity of traits observed in organisms. Later studies on the complex interactions among genes, the environment, and other factors revealed some of the limitations of Mendel’s laws. This gave rise to the concept of Non-Mendelian Inheritance.

Genetic processes responsible for variations
The characteristics the offspring receives from parents may not always be the same. The chief genetic processes which are responsible for this diversity among individuals are recombination of alleles during fertilisation, crossing over and mutation.

The Location of DNA

• A gene editing process which can bring desirable changes in the genes in DNA.
• The Nobel Prize of 2020 in Chemistry was shared by Emmanuelle Charpentier and Jennifer A Doudna for their contributions in the field of gene editing. The award is for the discovery of a technology called CRISPR-Cas 9.
• Gene editing is expected to make revolutionary advances in genetic disease therapy and treatment of cancer.
• Gene editing can also be used to develop crops that are resistant
• DNA is located in the chromosomes found inside the nucleus.

Structure of DNA

• In 1953, James Watson along with Francis Crick had presented the double helical model of DNA.
• James Watson along with Francis Crick proposed the structure of DNA based on the X-ray diffraction studies conducted by Rosalind Franklin and Maurice Wilkins.
• The crucial information that led to this discovery was obtained from the famous ‘Photo 5 P, an X-ray diffraction image of DNA taken by Rosalind Franklin.
• James Watson, Francis Crick and Maurice Wilkins were awarded the Nobel Prize in Medicine in 1962 for their contributions on the discovery of the double helix model of DNA. Rosalind Franklin passed away at the age of 37 in 1958, so she was not considered for Nobel prize.

• DNA has a double helix model.
• As per double helix model of DNA, DNA has two strands. The strands are composed of sugar and phosphate.
• The rungs of DNA are formed by the pairing of nitrogen bases.
• DNA cntain four nitrogen bases – Adenine, Thymine, Guanine, Cytosine
• In DNA the nitrogen base Adenine pairs with Thymine, and Guanine pairs with Cytosine.
• Nucleotide is the basic building block of DNA.
• Each nucleotide is composed of a deoxyribose sugar, a phosphate group, and a nitrogen base.
• Nitrogen base is nitrogen containing alkaline compound.
• The Deoxyribose sugar in DNA is a 5 carbon sugar.
• The phosphate participates in the formation of bonds that link nucleotides together
• The DNA in each chromosome is about 2 inches (5cm) long.
• If DNA from 46 chromosomes of a human cell joins together, it would be around 6 feet in length (2m).
• The human body is made up of trillions (one lakh crore) of cells. If the DNAs of all the cells joins together, it would be about 67 billion (one billion = 100 crore) miles in length. It is capable enough to wrap around the Earth oxer two million times!

Illustration 1.2 Structure of DNA

The structure of chromosome

• DNA and histone proteins are the primary components of a chromosome.
• Eight histone proteins join together to form a histone octamer.
• DNA strands wind around this octamer to form a structure called nucleosome.
• The chromosomes are formed by packing and coiling numerous nucleosomes and recoiling the chains of nucleosomes.
• Chromatids are the parts of a chromosome which are connected by means of centromere.
• Each species possess a specific number of chromosomes.

Human Chromosomes

• Somatic chromosomes are chromosomes that control physical characteristics.
• There are twenty two pairs of somatic chromosomes.
• A pair of identical chromosomes together form a homologous chromosome.
• One of these is inherited from the mother and the other from the father.
• Sex Chromosomes are the chromosomes which are involved in sex determination.
• Sex Chromosomes are of two types. X chromosome and Y chromosome.
• The Y chromosome is comparatively smaller than that of the X chromosome.
• The SRY gene on the Y chromosome is responsible for the development of testis in the embryo.

Gene

• Genes provide instructions as to how our body should function.
• Gene is a specific sequence of nucleotides in DNA.
• Proteins, which are synthesised according to the instructions of genes, are responsible for the formation of characteristic features and for controlling metabolic activities.

RNA

• The nucleic acid RNA also plays a crucial role in the synthesis of proteins.
• RNA is another type of nucleic acid, similar to DNA are also made up of nucleotides.
• Each of the nucleotide contains a ribose sugar, a phosphate group, and a nitrogenous base.
• The nitrogen bases in RNA are Adenine, Guanine, Uracil, and Cytosine.
• Most of the RNAs have a single strand.

PROTEIN SYNTHESIS
Protein Synthesis

• The proteins are synthesised as a result of the action of genes.
• Stages of protein synthesis are Transcription and Translation.
• Transcription – mRNA is formed from a speific nucleotide sequence (gene) in DNA with the help of various enzymes. The mRNA contains messages for protein synthesis.
• Translation – tRNAs (transfer RNA) carry specific amino acids to the ribosome based on message in the mRNA that has reached the ribosome from the nucleus. The rRNAs (ribosomal RNA), which are part of ribosomes combine amino acids to make protein.
• The RNAs involved in protein synthesis – mRNa, tRNA, rRNA

MENDEL’S EXPERIMENTS AND INFERENCES
Similarities and Differences in Characters

• Some characteristics of parents are also found in their children.
• It is also common to see certain characters in children which differ from their parents.
• Heredity refers to the transmission of characteristics from parents to their offspring. Variations are characters expressed in offspring, that differ from their parents.
• Genes inherited from parents are responsible for both heredity and variations.

Genetics in the Garden

• Genetics is the branch of science that deals with genes, heredity, and variation.
• Gregor Johann Mendel’s experiments on pea plants (Pisum sativum) and the conclusions he drew out of hybridisation experiments laid the foundation for the field of genetics.
• Gregor Johann Mendel is considered as the father of genetics.
• Gregor Johann Mendel was bom on 20 July, 1822, at Hyncice a small village of Northern Moravia, which is now known as Czech Republic.
• After joining the Augustinian monastery at Bmo, he became a priest in 1847.
• Between 1851 and 1853, he attended the University of Vienna where he studied Physics, Mathematics, and Natural sciences, and learned statistical methods to analyse data scientifically.
• In 1856, Mendel began to conduct hybridisation experiments on pea plants (Pisum sativum) in the garden of his monastery that focused on seven specific characters such as the colour of flower, shape of the seed etc.
• Based on the analysis of the experimental result, he explained that a pair of factors controls each character and represented those factors using symbols.
• These factors are now known to be genes.
• Gregor Mendel’s conclusions are known as the Laws of Inheritance. These laws provide the fundamental genetic framework to understand heredity and variation.
• In 1865, he presented his findings in the Natural History Society at Bmo. The following year, he published a thesis titled ‘Experiments on Plant Hybridisation.’ However, the scientific community of that time largely ignored Mendel’s discoveries.
• Gregor Mendel passed away in 1884.
• In 1900, sixteen years after his death, botanists Hugo de Vries, Carl Correns, and Erich von Tschermak recognised the significance of Mendel’s research. With this, Mendel’s findings were accepted as the foundation of the science of genetics.
• Genetics has grown into the most extensive branch of science through numerous contributions of various scientists.

Mendel’s Experiments

• Mendel initially conducted hybridisation experiments by considering a single pair of contrasting traits. This is known as a monohybrid cross.
• Mendel hypothesised that there must be certain factors within the seed that control traits.
• To find out what might have happened to the factor responsible for dwarfness, he self pollinated the plants obtained in the first generation to produce a second generation of plants.
• While conducting hybridisation experiments based on the contrasting traits of six other characteristics in pea plants, Mendel obtained results similar to the first experiment.
• Gregor Mendel hypothesised that characters from parents are passed on to offsprings through certain factors that are transmitted through gametes.
• It was only after Mendel’s period that, these factors were discovered to be genes that are located in chromosomes in the nucleus.
• A gene that determines a character can have different forms. These different forms of genes are called alleles.
• A gene usually has two alleles.
• The observable characteristics of an organism are called phenotype, and the genetic constitution responsible for these characteristics are called genotype.

Mendel’s Postulates from Monohybrid cross

• A trait is controlled by two factors.
• When a pair of contrasting traits is subjected to hybridisation, only one of the contrasting traits is expressed in the offspring of the first generation and the other remains hidden. The trait that appears in the first generation is called dominant trait and the hidden trait is called recessive trait.
• The trait hidden in the first generation reappears in the second generation.
• When gametes are formed, the factors that determine trait gets separated without mixing.
• The ratio of dominant to recessive traits in the offspring of the second generation is 3 : 1.

Dihybrid cross
• Mendel observed the inheritance of two pairs of contrasting traits of the same plant. This is known as dihybrid cross.
Mendel’s Postulates: When two or more different traits are combined, each trait is inherited independently to the next generation without mixing each other. (A pair of alleles in an organism does not influence the separation of another pair of alleles.)

Non Mendelian Inheritance

• Mendel’s laws were the foundation of genetics. However, it could not fully explain the diversity of traits observed in organisms.
• Later studies about the complex interaction among genes, environment and other factors revealed some of the limitations of Mendel’s laws. This gave rise to the concept of Non-Mendelian Inheritance.
• Incomplete Dominance – If a red flowered four o’clock plant is hybridised with a white flowered plant, the resulting offspring will have pink flowers. A dominant allele cannot fully hide the allele of the recessive trait.
• Co-dominance – Roan coat pattern, found on some cattle and horses. Both alleles exhibit their traits at the same time.
• Multiple allelism – ABO blood group in humans. The gene that determines blood group in human beings has more than two alleles. Three alleles IA, IB and i determine the blood group.
• Polygenic inheritance – Difference in skin colour. More than one gene controls the colour of the skin. The action of these genes cause variation in the production of melanin that causes difference in skin colour.

Crossing over

• Meiosis is the type of cell division that is responsible for the formation of gametes.
• The process of crossing over, which occurs during the first phase of meiosis.
• Pairing of homologous chromosomes (Identical chromosomes inherited from the parents of an organism).
• The point of contact of the paired chromosomes is called chiasma.
• The chromatids break at this region ( chiasma) and the broken segments are exchanged with each other. This exchange causes a recombination of alleles. This leads to the appearance of new traits in the offspring.

Mutation

• Mutation is the sudden heritable change in the genetic constitution of an organism.
• Mutations can be caused by errors during DNA replication, exposure to certain chemicals, radiations, etc.
• Mutation causes changes in genes. These genes are transferred through generations which leads to variations in characters.
• Mutations play a crucial role in the process of evolution.