Grade 9 Chapter 16: Heredity and Variation

Introduction

This chapter delves into the fascinating world of heredity and variation. We will explore how characteristics are passed down through generations (inheritance), the principles discovered by Gregor Mendel, and the role of chromosomes and DNA. We will also look at how variations arise and discuss some genetic disorders caused by chromosomal abnormalities.

Key topics covered:

• Inheritance: Heredity, Characteristics and their appearance
• Mendel's laws of inheritance
• Chromosomes, DNA, and RNA
• Genetic Disorders: Diseases due to chromosomal aberrations and gene mutations

Inheritance

The branch of biology that studies the transfer of characteristics of organisms from one generation to the next, and genes in particular, is called 'genetics'. New progeny is formed through the process of reproduction. Except for a few minor differences, the offspring shows great similarities with parents. Organisms produced by asexual reproduction show minor variations. However, offspring produced through sexual reproduction show comparatively greater variations.

Heredity

Heredity is the transfer of characteristics from parents to offspring. It is due to heredity that puppies are similar to dogs, squabs are similar to pigeons, and infants are similar to humans.

Inherited Traits and Expression of Traits

How do specific traits or characteristics appear in organisms? Though there are many similarities between parents and their offspring, there are some differences too. These similarities and differences are all the effect of heredity. Let us study the mechanism of heredity. Information necessary for protein synthesis in the cell is stored in DNA. The segment of DNA which contains all the information for synthesis of a particular protein is called a 'gene' for that protein. It is necessary to know the relationship of these proteins with the characteristics of organisms.

To understand the concept of heredity, let us consider the characteristic 'plant height'. We know that there are growth hormones in plants. Increase in height of plants depends upon the quantity of growth hormones. The quantity of growth hormones produced by a plant depends upon the efficiency of the concerned enzyme. Efficient enzymes produce a greater quantity of the hormone due to which the height of the plant increases. However, if the enzymes are less efficient, a smaller quantity of hormone is produced leading to a stunting of the plant.

Chromosomes

The structure in the nucleus of cells that carries the hereditary characteristics is called the chromosome. It is made up mainly of nucleic acids and proteins. During cell division, chromosomes can be clearly seen under the compound microscope. 'Genes' which contain the information about hereditary characteristics in coded form are located on chromosomes. Each species has a specific number of chromosomes.

Structure of Chromosome

Each chromosome is made up of DNA and it appears dumbbell-shaped midway during cell division. There is a constricted region on each chromosome. It is called the 'Primary constriction' or 'Centromere'. This divides the chromosome into two parts. Each part is called an 'arm'. The centromere has a specific position in each chromosome. Depending upon this, there are four types of chromosomes:

• Metacentric: The centromere is exactly at the mid-point in this chromosome, and therefore the chromosome looks like the English letter 'V'. The arms of this chromosome are equal in length.
• Sub-metacentric: The centromere is somewhere near the mid-point in this chromosome which therefore looks like English letter 'L'. One arm is slightly shorter than the other.
• Acrocentric: The centromere is near one end of this chromosome which therefore looks like the English letter 'J'. One arm is much smaller than the other.
• Telocentric: The centromere is right at the end of this chromosome making the chromosome look like the English letter 'i'. This chromosome consists of only one arm.

Generally, in somatic cells, chromosomes are in pairs. If the pair consists of similar chromosomes by shape and organization, they are called 'homologous chromosomes' and if they are not similar they are called 'heterologous chromosomes'. In case of organisms that reproduce sexually, one of the chromosomal pairs is different from all others. Chromosomes of this different pair are called 'sex chromosomes' or allosomes, and all other chromosomes are called 'autosomes'.

(Note: Refer to textbook diagrams 16.2 Organization of chromosome and 16.3 Types of chromosomes for visual representation)

Number of chromosomes in different organisms: Crab (200), Maize (20), Frog (26), Roundworm (4), Potato (48), Human (46).

Deoxyribonucleic Acid (DNA)

Chromosomes are mainly made up of DNA. This acid was discovered by the Swiss biochemist, Frederick Miescher in 1869 while studying white blood cells. Initially this acid was reported to be only in the nucleus of cells. Hence, it was named nucleic acid. However, it was later realized that it is present in other parts of the cell too. Molecules of DNA are present in all organisms from viruses and bacteria to human beings. These molecules control the functioning, growth and division (reproduction) of the cell and are therefore called 'Master Molecules'.

The structure of the DNA molecule is the same in all organisms. In 1953, Watson and Crick produced a model of the DNA molecule. As per this model, two parallel threads of nucleotides are coiled around each other. This arrangement is called a 'double helix'. This structure can be compared with a coiled and flexible ladder.

Structure of DNA Molecule

Each strand in the molecule of DNA is made up of many small molecules known as 'nucleotide'. There are four types of nitrogenous bases: adenine, guanine, cytosine and thymine. Adenine and guanine are called as 'purines' while cytosine and thymine are called as 'pyrimidines'.

In the structure of the nucleotide, a molecule of a nitrogenous base and phosphoric acid are each joined to a molecule of sugar (deoxyribose). As there are four types of nitrogenous bases, nucleotides also are of four types.

Nucleotides are arranged like a chain in a molecule of DNA. The two threads of the DNA molecule are comparable to the two rails of a ladder and each rail is made up of alternately joined molecules of sugar and phosphoric acid. Each rung of the ladder is a pair of nitrogenous bases joined by hydrogen bonds. Adenine always pairs with thymine (A=T), and cytosine always pairs with guanine (C≡G). This is known as complementary base pairing.

(Note: Refer to textbook diagrams 16.4 DNA (Watson and Crick's Model) and 16.5 Structure of DNA for visual representation)

Genes

Each chromosome is made up of a single DNA molecule. Segments of the DNA molecule are called genes. Due to variety in the sequence of nucleotides, different kinds of genes are formed. These genes are arranged in a line. Genes control the structure and function of the cells and of the body. Also, they transmit the hereditary characteristics from parents to offspring. Hence, they are said to be the functional units of heredity. That is why, many similarities are seen between parents and their offspring. Information about protein synthesis is stored in the genes.

DNA Fingerprinting

The sequence of the genes in the DNA of a person i.e. the genome of the person is identified. It is useful to identify the lineage and to identify criminals because it is unique to every person.

Ribonucleic Acid (RNA)

RNA is the second important nucleic acid of the cell. This nucleic acid is made up of ribose sugar, phosphate molecules and four types of nitrogenous bases: adenine, guanine, cytosine and uracil (Uracil replaces Thymine found in DNA).

The nucleotide (smallest unit of the chain of the RNA molecule) is formed by combination of a ribose sugar, phosphate molecule and one of the nitrogenous bases. Large numbers of nucleotides are bonded together to form the macromolecule of RNA. According to function, there are three types of RNA:

1. Ribosomal RNA (rRNA): The molecule of RNA which is a component of the ribosome organelle is called a ribosomal RNA. Ribosomes perform the function of protein synthesis.
2. Messenger RNA (mRNA): The RNA molecule that carries the information of protein synthesis from genes (i.e. DNA chain in the cell nucleus) to ribosomes in the cytoplasm which produce the proteins, is called messenger RNA.
3. Transfer RNA (tRNA): The RNA molecule which, according to the message of the mRNA, carries the amino acid up to the ribosomes is called transfer RNA.

(Note: Refer to textbook diagram 16.6 Types of RNA for visual representation)

Mendel's Principles of Heredity

Genetic material is transferred in equal quantity from parents to progeny. Principles of heredity are based upon this fact. If both the parents make equal contribution to inheritance of characteristics, which characteristics will appear in the progeny? Mendel carried out research in this direction and put forth the principles of heredity responsible for such inheritance. The experiments performed by Mendel, almost a century ago are quite astonishing. All of Mendel's experiments were based upon the visible characteristics of the pea plant (Pisum sativum).

Gregor Johann Mendel (1822-1884) was an Austrian scientist. He studied the inheritance of some characteristics of the pea plant. He showed that inheritance of these characteristics follows certain principles. Later, these principles became popular by his name. Mendel's work was recognized only in the 20th century. After a reconfirmation of these principles, the same principles now form the basis of modern genetics.

Seven contrasting characteristics studied by Mendel in pea plants:

• Seed Shape: Round (Dominant) / Wrinkled (Recessive)
• Seed Colour: Yellow (Dominant) / Green (Recessive)
• Flower Colour: Purple (Dominant) / White (Recessive)
• Pod Shape: Inflated (Dominant) / Constricted (Recessive)
• Pod Colour: Green (Dominant) / Yellow (Recessive)
• Flower Position: Axial (Dominant) / Terminal (Recessive)
• Plant Height: Tall (Dominant) / Dwarf (Recessive)

(Note: Refer to textbook diagram 16.7 Seven mutually contrasting visible characteristics)

Monohybrid Cross

This experiment involves a cross between two pea plants with only one pair of contrasting characters. Let's consider the characteristic 'plant height'.

• Parental Generation (P₁): Tall pea plant (TT) crossed with Dwarf pea plant (tt). (Genotypes: TT and tt; Phenotypes: Tall and Dwarf)
• Gametes from P₁: T from Tall plant, t from Dwarf plant.
• First Filial Generation (F₁): All plants were Tall (Genotype: Tt). Mendel observed that only one trait (Tallness) appeared, concluding it was the dominant trait, while Dwarfness was recessive.
• Parental Generation (P₂): Selfing (self-pollination) of F₁ plants (Tt x Tt).
• Gametes from F₁: T and t from each parent.
• Second Filial Generation (F₂): Produced using a Punnett Square.
• F₂ Phenotypic Ratio: Tall : Dwarf = 3 : 1
• F₂ Genotypic Ratio: TT (Pure Tall) : Tt (Hybrid Tall) : tt (Pure Dwarf) = 1 : 2 : 1

Phenotype: External appearance or visible characteristics (e.g., Tall, Dwarf).

Genotype: The pair of genes (factors) responsible for the visible characteristics (e.g., TT, Tt, tt).

(Note: Refer to textbook diagram/table for Mendel's experiment of the Monohybrid Cross)

Dihybrid Cross

This experiment involves a cross between two pea plants with two pairs of contrasting characters. Let's consider seed shape (Round/Wrinkled) and seed colour (Yellow/Green).

• Parental Generation (P₁): Round-Yellow seeds (RRYY) crossed with Wrinkled-Green seeds (rryy). (R=Round, r=Wrinkled, Y=Yellow, y=Green)
• Gametes from P₁: RY from RRYY, ry from rryy.
• First Filial Generation (F₁): All plants had Round-Yellow seeds (Genotype: RrYy). Round and Yellow are dominant traits.
• Parental Generation (P₂): Selfing of F₁ plants (RrYy x RrYy).
• Gametes from F₁: Each F₁ plant produces four types of gametes: RY, Ry, rY, ry.
• Second Filial Generation (F₂): Produced using a Punnett Square (16 possible combinations).
• F₂ Phenotypic Ratio: Round-Yellow : Round-Green : Wrinkled-Yellow : Wrinkled-Green = 9 : 3 : 3 : 1
• F₂ Genotypes: RRYY, RRYy, RrYY, RrYy, RRyy, Rryy, rrYY, rrYy, rryy (in specific ratios).

Mendel's Law of Independent Assortment: Based on the dihybrid cross, Mendel concluded that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters during gamete formation.

(Note: Refer to textbook diagram/table for Mendel's experiment of the Dihybrid Cross and Punnett Square)

Genetic Disorders

Diseases or disorders occurring due to abnormalities in chromosomes and mutations in genes are called genetic disorders. Chromosomal abnormalities include either increase or decrease in numbers and deletion or translocation of any part of the chromosome. Examples are physical disorders like cleft lip, albinism and physiological disorders like sickle cell anaemia, haemophilia, etc.

Human beings have 46 chromosomes in the form of 23 pairs. There is great variation in the size and shape of these chromosomal pairs. These pairs have been numbered. Out of 23 pairs, 22 pairs are autosomes and one pair is of sex chromosomes (allosomes). Chromosomes in women are represented as 44+XX and in men, as 44+XY.

(Note: Refer to textbook diagram 16.8 Human Karyotype (Chromosome chart))

A. Disorders due to Chromosomal Abnormalities

Following are the disorders that occur due to numerical changes in chromosomes. Offspring are not sterile if there is change in the number of autosomes. Instead, if there is an increase in number of any autosomal pair, physical or mental abnormalities arise and the lifespan is shortened.

1. Down syndrome (Trisomy of 21st Chromosome): This disorder arises due to a chromosomal abnormality. This is the first discovered and described chromosomal disorder in human beings. This disorder is characterized by the presence of 47 chromosomes. It is described as trisomy of the 21st chromosome. Infants with this disorder have one extra chromosome with the 21st pair in every cell of their body. Therefore they have 47 chromosomes instead of 46. Children suffering from Down's syndrome are usually mentally retarded and have a short lifespan. Mental retardation is the most prominent characteristic. Other symptoms include short height, short wide neck, flat nose, short fingers, scanty hair, single horizontal crease on palm, and a life expectancy of about 16-20 years.
2. Turner syndrome (Monosomy of X chromosome): As with autosomes, abnormalities in sex chromosomes also cause some disorders. Turner syndrome (or 44+X) arises due to either inheritance of only one X-chromosome from parents or due to inactivation of the gender related part of X-chromosomes. Instead of the normal 44+XX condition, women suffering from Turner syndrome show a 44+X condition. Such women are sterile i.e. unable to have children due to improper growth of the reproductive organs.
3. Klinefelter syndrome (46+XXY): This disorder arises in men due to abnormalities in sex chromosomes. In this disorder, men have one extra X chromosome; hence their chromosomal condition becomes 44+XXY. Such men are sexually sterile because their reproductive organs are not well developed.

B. Diseases occurring due to mutation in single gene (monogenic disorders)

Disorders or diseases occurring due to mutation in any single gene into a defective one are called monogenic disorders. Approximately 4000 different disorders of this type are now known. Due to abnormal genes, their products are either produced in insufficient quantity or not at all. It causes abnormal metabolism that may lead to death at a tender age. Examples of such disorders are Hutchinson's disease, Tay-Sachs disease, galactosaemia, phenylketonuria, sickle cell anaemia, cystic fibrosis, albinism, haemophilia, night blindness, etc.

• Albinism: This is a genetic disorder. Our eyes, skin and hair have colour due to the brown pigment, melanin. In this disease, the body cannot produce melanin. The skin becomes pale, hairs are white and eyes are usually pink due to absence of melanin pigment in the retina and sclera.
• Sickle-cell anaemia: Even minor changes in molecular structure of proteins and DNA may lead to diseases or disorders. Normal haemoglobin has glutamic acid as the 6th amino acid in its molecular structure. However, if it is replaced by valine, the shape/structure of the haemoglobin molecule changes. Due to this, the erythrocytes or red blood corpuscles (RBC), which are normally biconcave become sickle-shaped. This condition is called 'sickle-cell anaemia'. The oxygen carrying capacity of haemoglobin in such individuals is very low.
  In this condition, clumping and thereby destruction of erythrocytes occurs most often. As a result blood vessels are obstructed and the circulatory system, brain, lungs, kidneys, etc. are damaged. Sickle-cell anaemia is a hereditary disease. It occurs due to changes in genes during conception. If the father and mother are both affected by sickle-cell anaemia or if they are carriers of this disorder, their offspring are likely to suffer from this disease. Hence, marriages between the persons who are carriers of or suffering from sickle-cell anaemia should be avoided.
  Types of persons affected by sickle-cell anaemia:
  1. Sickle-cell anaemia carrier (AS)
  2. Sufferer from sickle-cell anaemia (SS)
  Symptoms of sickle-cell anaemia: Swelling of hands and legs, pain in joints, severe general body aches, frequent colds and cough, constant low grade fever, exhaustion, pale face, low haemoglobin content.
  Sickle cell anaemia occurs as follows (Inheritance Pattern):
  • Normal (AA) x Normal (AA) -> All Progeny Normal (AA)
  • Normal (AA) x Carrier (AS) -> 50% Progeny Normal (AA), 50% Carrier (AS)
  • Normal (AA) x Sufferer (SS) -> All Progeny Carrier (AS)
  • Carrier (AS) x Carrier (AS) -> 25% Progeny Normal (AA), 50% Carrier (AS), 25% Sufferer (SS)
  • Carrier (AS) x Sufferer (SS) -> 50% Progeny Carrier (AS), 50% Sufferer (SS)
  • Sufferer (SS) x Sufferer (SS) -> All Progeny Sufferer (SS)
  Diagnosis: Under the National Health Mission scheme, the 'Solubility Test' for diagnosis of sickle-cell anaemia is available at all district hospitals. Similarly, the confirmatory diagnostic test, 'Electrophoresis' is performed at rural and sub-district hospitals.
  Remedies: This disease is spread in only one way i.e. reproduction. Hence, husband and wife should get their blood examined either before marriage or after it.
  1. A carrier or sufferer should avoid marriage with another carrier or sufferer.
  2. A person suffering from sickle cell anaemia should take a tablet of folic acid daily.

C. Mitochondrial disorder

Mitochondrial DNA may also become defective due to mutation. During fertilization, mitochondria are contributed by the egg cell (ovum) alone. Hence, mitochondrial disorders are inherited from the mother only. Leber hereditary optic neuropathy is an example of a mitochondrial disorder.

D. Disorders due to mutations in multiple genes (Polygenic disorders)

Sometimes, disorders arise due to mutations in more than one gene. In most such disorders, their severity increases due to effects of environmental factors on the foetus. Common examples of such disorders are cleft lip, cleft palate, constricted stomach, spina bifida (a defect of the spinal cord), etc. Besides, diabetes, blood pressure, heart disorders, asthma, obesity are also polygenic disorders. Polygenic disorders do not strictly follow Mendel's principles of heredity. These disorders arise from a complex interaction between environment, life style and defects in several genes.

Related Topics/Awareness

National Health Mission

Under the National Health Mission, the National Rural Health Mission has been started since April 2005 and the National Urban Health Mission since 2013. The main objectives of this mission are strengthening of the rural and urban health facilities, controlling various diseases and illnesses, increasing public awareness about health, and offering financial assistance to patients through various schemes.

Inter-relationship between tobacco addiction and cancer (uncontrolled growth of cells)

Many people consume tobacco, either by smoking or by chewing. Consumption of tobacco in any form can cause cancer. Smoking of cigarettes and bidi adversely affects the process of digestion. It causes a burning sensation in the throat and a cough. Excessive smoking causes instability and trembling of fingers. A dry cough causes sleeplessness. Tobacco consumption can also lead to shortening of life span, chronic bronchitis, pericarditis, cancer of the lungs, mouth, larynx (voice box), pharynx, pancreas, urinary bladder, etc.

Harmful effects of smoking are due to the nicotine present in tobacco. It affects the central and peripheral nervous system. Arteries become hard i.e. it causes arteriosclerosis and hypertension.

Tobacco smoke contains harmful chemicals like pyridine, ammonia, aldehyde, furfural, carbon monoxide, nicotine, sulphur dioxide, etc. They cause uncontrolled cell division. Tobacco smoke is full of minute carbon particles which cause normal tissue of the lung to transform into thickened black tissue. This causes cancer. While chewing tobacco or tobacco products much of the extract is absorbed into the body. Excessive tobacco consumption may cause cancer of lips or tongue, visual disorders or tremors.

To protect one's body from cancer one must avoid smoking and consumption of tobacco and tobacco products in any form.