16. Heredity and Variation

In asexual reproduction a single parent passes on almost identical genetic material, so the offspring differ only slightly. In sexual reproduction, genes from two parents combine in new ways, so the offspring show greater variation.

A cross between two plants that differ in only one pair of contrasting characters is a monohybrid cross. Consider plant height: tall (T, dominant) and dwarf (t, recessive).

F₂ result: phenotypic ratio 3 tall : 1 dwarf; genotypic ratio 1 TT : 2 Tt : 1 tt.

From this Mendel concluded that factors (genes) occur in pairs, that one factor is dominant over the other, and that the two factors of a pair separate during gamete formation, so each gamete carries only one of them. The dwarf character, hidden in F₁, reappears in F₂.

A cross that considers two pairs of contrasting characters is a dihybrid cross. Mendel crossed a pea plant with round, yellow seeds (RRYY) and one with wrinkled, green seeds (rryy). Round (R) and yellow (Y) are dominant.

F₂ phenotypic ratio: round-yellow 9 : wrinkled-yellow 3 : round-green 3 : wrinkled-green 1, i.e. 9 : 3 : 3 : 1.

Genotypic ratio: RRYY 1 : RRYy 2 : RRyy 1 : RrYY 2 : RrYy 4 : Rryy 2 : rrYY 1 : rrYy 2 : rryy 1 — i.e. 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 (9 genotypes in 16 combinations).

Two new combinations, wrinkled-yellow and round-green, appear in F₂. This shows that the two pairs of characters are inherited independently of each other.

1. Characters considered: a monohybrid cross considers one pair of contrasting characters, e.g. tall × dwarf; a dihybrid cross considers two pairs at a time, e.g. round-yellow × wrinkled-green.
2. F₁: monohybrid F₁ plants are monohybrids (Tt); dihybrid F₁ plants are dihybrids (RrYy).
3. Gametes of F₁: two types (T, t) in a monohybrid cross; four types (RY, Ry, rY, ry) in a dihybrid cross.
4. Combinations in F₂: 4 in the Punnett square of a monohybrid cross; 16 in a dihybrid cross.
5. Phenotypic ratio: 3 : 1 in a monohybrid cross; 9 : 3 : 3 : 1 in a dihybrid cross.
6. Genotypic ratio: 1 : 2 : 1 (3 genotypes) in a monohybrid cross; 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 (9 genotypes) in a dihybrid cross.

No, it is not right. Genetic disorders are caused by changes in chromosomes or genes that a person is born with. They are not infectious and do not spread by touch, by sharing food or by living together; they pass only from parents to children through reproduction. So there is no reason to avoid such a person. People with disorders like sickle-cell anaemia, Down syndrome or albinism need understanding, love, medical care and support, not isolation. Avoiding them hurts them emotionally and is a form of discrimination. We should treat them with respect and help them lead a full life.

The thread-like structure in the nucleus of a cell that carries the hereditary characteristics is called a chromosome. It is made up mainly of DNA and proteins, and carries genes in coded form. Chromosomes are clearly seen during cell division. Each has a constricted region, the centromere (primary constriction), which divides it into two arms. Each species has a fixed number of chromosomes; humans have 46.

1. Metacentric: the centromere is exactly at the mid-point; the two arms are equal; the chromosome looks like the letter ‘V’ during cell division.
2. Sub-metacentric: the centromere is near the mid-point; one arm is slightly shorter; it looks like ‘L’.
3. Acrocentric: the centromere is near one end; one arm is much smaller than the other; it looks like ‘j’.
4. Telocentric: the centromere is right at the end; there is only one arm; it looks like ‘i’.

Chromosomes in a pair that are alike in shape and organization are homologous. The pair that decides sex is the sex chromosomes (allosomes); the rest are autosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes — 44+XX in women and 44+XY in men.

• According to the model of Watson and Crick (1953), DNA consists of two long strands coiled around each other; this arrangement is called a double helix. It can be compared to a coiled, flexible ladder.
• Each strand is made of many small units called nucleotides. Each nucleotide has a sugar (deoxyribose), a phosphoric acid (phosphate) molecule and a nitrogenous base.
• There are four nitrogenous bases: adenine and guanine (purines), and cytosine and thymine (pyrimidines). So there are four types of nucleotides.
• The two rails of the ladder are made of alternate sugar and phosphate molecules. Each rung 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).
• The structure of DNA is the same in all organisms; only the sequence of nucleotides differs. Segments of DNA with specific sequences are genes.

DNA fingerprinting is the identification of the sequence of genes in a person’s DNA. Since this is unique to every person (except identical twins), it is a very reliable method of identification.

In my opinion it is a very useful technique:

• It helps identify criminals from tiny samples of blood, hair or skin left at a crime scene, and also proves innocent people not guilty.
• It helps settle disputes about parentage and lineage.
• It helps identify victims of accidents and disasters.
• It is used in research on heredity, evolution and genetic diseases.

However, DNA information is very personal. It must be collected with proper legal permission, kept confidential, and never misused for discrimination. With these safeguards, DNA fingerprinting is a boon to society.

Structure: RNA (ribonucleic acid) is the second important nucleic acid of the cell. It is made of nucleotides, each formed of a ribose sugar, a phosphate molecule and one of four nitrogenous bases — adenine, guanine, cytosine and uracil (uracil in place of thymine). A large number of nucleotides join to form the RNA macromolecule; it is usually single-stranded.

Function: RNA takes part in protein synthesis, carrying out the instructions stored in DNA.

Types:

1. Ribosomal RNA (rRNA): a component of ribosomes, the organelles that perform protein synthesis.
2. Messenger RNA (mRNA): carries the information for protein synthesis from the genes (DNA) in the nucleus to the ribosomes in the cytoplasm.
3. Transfer RNA (tRNA): according to the message of the mRNA, carries the right amino acids to the ribosomes.

• Some genetic disorders, such as sickle-cell anaemia and thalassaemia, pass from parents to children only through reproduction.
• A person may be a carrier (AS) of sickle-cell anaemia and look completely healthy.
• If both partners are carriers, each child has a 25% chance of being a sufferer (SS) and a 50% chance of being a carrier. If both are sufferers, all children will be sufferers.
• A blood test before marriage tells a person whether he or she is normal, a carrier or a sufferer. Carriers and sufferers are advised not to marry another carrier or sufferer, so that the disease is not passed on.
• Such tests (the solubility test and electrophoresis) are available in government hospitals under the National Health Mission. The test also shows the blood group, which helps avoid problems during pregnancy.

Down syndrome is a disorder caused by a chromosomal abnormality. It was the first chromosomal disorder to be discovered and described in human beings. A person with Down syndrome has an extra 21st chromosome in every cell (trisomy of chromosome 21), so there are 47 chromosomes instead of 46 (45+XX or 45+XY). Its main feature is intellectual disability, with a delay in learning and development. Other features include short height, a short broad neck, a flat nose, short fingers, scanty hair and a single horizontal crease on the palm. Many also have heart and other health problems.

Disorders caused by a mutation in a single gene, which makes it defective, are called monogenic disorders. About 4000 such disorders are known. Because of the abnormal gene, its product (usually a protein or enzyme) is produced in insufficient quantity or not at all. This causes abnormal metabolism, which may even lead to death at an early age. Examples: sickle-cell anaemia, albinism, haemophilia, cystic fibrosis, phenylketonuria, galactosaemia, Tay-Sachs disease and Huntington’s disease. In albinism, for example, the body cannot produce the pigment melanin, so the skin is pale, the hair is white and the eyes are pink.

Sickle-cell anaemia is a hereditary disorder. In normal haemoglobin the sixth amino acid is glutamic acid; in this disorder it is replaced by valine. The shape of the haemoglobin molecule changes, and red blood cells, which are normally biconcave, become sickle-shaped. Their oxygen-carrying capacity is very low, and they clump together and are destroyed easily. They block blood vessels, damaging the brain, lungs, kidneys and other organs. In Maharashtra more than 2.5 lakh people suffer from it, especially in Vidarbha.

Symptoms: swelling of hands and feet, pain in joints, severe general body ache, frequent colds and cough, constant low-grade fever, exhaustion, a pale face and low haemoglobin.

Treatment and remedies:

• Diagnosis by the solubility test (at district hospitals) and confirmation by electrophoresis, under the National Health Mission.
• A sufferer should take a folic acid tablet daily, as advised by the doctor, and get regular medical care.
• Since it spreads only through reproduction, both partners should get their blood tested before marriage, and a carrier or sufferer should avoid marrying another carrier or sufferer.

Leber hereditary optic neuropathy is caused by defective mitochondrial DNA. Mitochondria come to the zygote only from the egg cell, so it is inherited from the mother at the time of zygote formation. Diabetes arises from several genes along with lifestyle and environment. Albinism is due to a single defective gene that prevents melanin formation. Turner syndrome women have only one X chromosome (45 in total) and are sterile; Klinefelter syndrome men have an extra X chromosome and are sterile.

• (a) Model of DNA: use two long pipe cleaners, or strips of cardboard, as the sugar–phosphate rails, and short straws or strips in four colours for the bases (A, T, G, C). Join them as rungs, always pairing A with T and G with C, then twist the ladder gently into a double helix. In the presentation, explain nucleotides, base pairing and genes.
• (b) Presentation on tobacco and cancer: include how tobacco is consumed (smoking and chewing); harmful substances in it (nicotine, carbon monoxide, tar and other chemicals); the diseases it causes (cancer of the lungs, mouth, throat and other organs, bronchitis, heart disease, high blood pressure); the effect of passive smoking; and messages and slogans to quit tobacco. End with where to get help to quit, such as a national tobacco quitline.