Earlier, we have seen that there is great variation within every species in nature. In this chapter, we shall study the factors that give rise to these variations.
The branch of biology which 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.
Transfer of characteristics from parents to offspring is called heredity. 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 :
Though there are many similarities between parents and their offsprings 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 characteritic ‘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.
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. Each chromosome is made up of DNA and it appears dumbell-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.
Types of chromosomes :
Types of chromosomes can be easily identified during cell division.
- 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 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 than others. Chromosomes of this different pair are called ‘sex chromosomes’ or allosomes and all other chromosomes are called ‘autosomes’
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 sturcture can be compared with a coiled and flexible ladder
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 ‘pyrimidines’. In the structure of the nucleotide, a molecule of a nitrogenous base and phosphoric acid are each joined to a molecule of sugar.
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 and cytosine always pairs with guanine.
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.
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. The nucleotide i.e. 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.
- 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.
- 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.
- 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.
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). These characteristics are as follows –
We shall study the following crosses to clearly understand the conclusions of Mendel’s experiments.
In this experiment, Mendel brought about the cross between two pea plants with only one pair of contrasting characters. This type of cross is called a monohybrid cross. So as to study the monohybrid ratio, let us consider the characteristic ‘plant height’ with a pair of contrasting characteristics tall plant and dwarf plant.
Parental generation (P1 ) : Tall pea plants and dwarf pea plant were used in this cross. Hence, this is parent generation (P1 ). Mendel referred to the tall and dwarf plants as dominant and recessive respectively. The tall plant was referred to as dominant because all the plants in the next generation were tall. The dwarf plant was referred to as recessive because this characteristic did not appear in next generation at all. This experiment has been presented by the ‘Punnet Square’ method as shown below
Depending upon these observations, Mendel proposed that the factors responsible for inheritance of characteristics are present in pairs. Today, we refer to these factors as genes. Dominant genes are denoted by capital letters whereas recessive genes are denoted by small letters. As genes are present in pairs, tall plants are denoted by TT and dwarf plants are denoted by tt. During gametogenesis, these genes separate from each other. Due to this, two types of gametes one containing factor T and other containing factor t are formed.
Mendel’s experiment on dihybrid cross:
In the dihybrid cross, two pairs of contrasting characteristics are under consideration. Mendel performed more experiments on hybridization in which he considered more than one pair of contrasting characteristics. He brought about a cross between a pea plant producing rounded and yellow coloured seeds and a pea plant with wrinkled and green coloured seeds. In this cross, two pairs of contrasting characteristics were considered colour of seeds and shape of seeds. Hence, it is called a dihybrid cross.
Parental generation (P1 ) : Mendel selected the pea plants producing rounded yellow seeds and wrinkled green seeds as parent plants, as shown in the chart –
During gamete formation in P1 generation, the pair of gametes separate independently i.e. in RRYY plants, only RY type gametes are formed and not RR and YY. Similarly, in rryy plants, only ry gametes are formed. Thus we can say that each pair of genes is represented in the gamete by only one gene from that pair
Based on the conclusions from the monohybrid cross, Mendel expected that in the F2 generation of dihybrid cross, plants would produce rounded-yellow seeds. He was proved right. Though the genotype of these plants was RrYy, their phenotype was like the parents producing rounded-yellow seeds, because yellow colour is dominant over green and round shape is dominant over wrinkled. Due to the combination of two different characteristics in the F1 generation of the dihybrid cross, these plants are called dihybrid plants.
Plants of the F1 generation of dihybrid cross produce four types of gametes RY, Ry, rY, ry. Of these gametes, RY and ry are similar to those of the P1 generation.
F2 generation is formed through the selfing of F1 plants. The pattern of inheritance of charactistics from F1 to F2 is shown in brief in the table given on page 187 and an activity about its presentation in the form of a ratio has been given in a box, beside it. The 16 different possible combinations through the union of 4 types of male gametes and 4 types of female gametes are shown in a chess-board like table (Punnet Square / Checker board) on page 187. Male gametes are shown at the top of table and female gametes are shown in left column. Observations based on a study of the F2 generation will be according to the table on page 187.
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.
Mendel has shown in his experiments that there exist two type of genes, dominant and recessive.
If we take into account the number of chromosomes in human cells, their sex-related types, the types of genes on the chromosomes – dominant and recessive – we can see where genetic disorders originate and how they are inherited.
- 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 is less. Instead, if there is an increase in number of any autosomal pair, physical or mental abnormalities arise and the lifespan is shortened with a shortened life span. Following are some disorders
- Down syndrome (46+1, Trisomy of 21st Chromosome) :
Down syndrome is a disorder arising due to chromosomal abnormality. This is the first discovered and described chromosomal disorder in human beings. This disorder is characterised 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 expentancy of about 16–20 years.
- 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.
- Klinefelter syndrome (44+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.
- Diseases occuring 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.
This disease is spread in only one way i.e. reproduction. Hence, husband and wife should get their blood exmined either before marriage or after it.
- A carrier or sufferer should avoid marriage with another carrier or sufferer.
- A person suffering from sickle cell anaemia should take a tablet of folic acid daily
- 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.