Heredity and Evolution

Why do children resemble their parents? And how did the millions of species on Earth come to be? The answers lie in two linked ideas: heredity, the passing of traits from one generation to the next, and evolution, the slow change of species over vast stretches of time. Together they explain both the unity and the diversity of life. They are also important, frequently tested topics.

Heredity is the transmission of characteristics from parents to offspring. The instructions for these traits are carried by genes, which are segments of DNA found on thread-like structures called chromosomes inside every cell.

Each offspring receives genes from both parents. That is why children share features with their mother and father. Genes are the units of heredity, faithfully copied and passed on. The basic plan of an organism is preserved across generations.

The rules of heredity were first worked out by Gregor Mendel, an Austrian monk, through careful experiments on pea plants in the nineteenth century.

Mendel found that traits are controlled by pairs of factors (now called genes). Some forms are dominant: they show up in the offspring. Others are recessive: they are hidden unless both copies are recessive. For example, crossing tall and short pea plants gave all tall offspring in the first generation. Short plants reappeared in the next generation. Mendel's work was ignored in his lifetime. It later became the foundation of the science of genetics.

How is the sex of a child decided? It depends on special sex chromosomes. In humans:

• females have two X chromosomes (XX), and
• males have one X and one Y chromosome (XY).

A child always receives an X from the mother, and either an X or a Y from the father. So it is the father's chromosome that determines whether the child is a girl (XX) or a boy (XY). This corrects a common misunderstanding.

Over enormously long periods, species change and new species arise. This process is evolution. The great explanation was given by Charles Darwin through the idea of natural selection:

• individuals in a species vary,
• those with traits better suited to their environment are more likely to survive and reproduce, and
• over many generations, these favourable traits become common, gradually changing the species.

Evidence for evolution comes from fossils (the preserved remains of ancient life), from similarities in body structure across species, and from embryos. Evolution links all living things in a single, branching tree of life stretching back billions of years.

Modern biology does not just study heredity. It can rewrite it. Genetic engineering is the deliberate altering of an organism's genes, usually by moving a gene from one organism into another. Its core method is recombinant DNA technology, which cuts a useful gene out of one genome and joins it into another. Genes can be moved freely across the living world: between different plant species, from animals into plants, and from microorganisms into higher organisms. An organism that carries a foreign gene this way is called transgenic, or genetically modified (GM).

Bt crops are the best-known example. The soil bacterium Bacillus thuringiensis carries a gene, the Cry gene, for a protein that is toxic to insect larvae. Inserting this gene lets a crop resist pests on its own.

• Bt cotton: carries the Bt gene to kill the bollworm. Bollgard I and Bollgard II are the two Bt cotton technologies.
• Bt brinjal: carries the Cry1Ac gene to resist the fruit and shoot borer. Its release was opposed over fears for human health and for biodiversity. The gene comes from a bacterium, not a fungus, and the seeds are not terminator seeds.

Genetic engineering aims at more than pest resistance:

• Drought tolerance: crops bred to survive on little water.
• Better nutrition: Golden rice is engineered to make beta-carotene, a pro-vitamin A that the body turns into vitamin A.
• Longer shelf life: produce that stays fresh longer.
• Hybrid vigour: India's GM mustard (DMH-11), developed at Delhi University, uses the barnase-barstar bacterial genes to allow controlled cross-pollination and hybridisation, not pest resistance.

A related case is canola, a rapeseed-mustard oilseed bred for very low erucic acid. Its oil is valued because it is rich in unsaturated fatty acids.

Not every plant-breeding method changes the genome, so it is worth sorting the techniques apart:

• Cytoplasmic male sterility: a trait that makes a plant unable to produce fertile pollen. Breeders use it to force cross-pollination, which is central to making hybrid and transgenic crops.
• Gene silencing (RNAi): switches off a chosen gene, and can be used to give crops a new trait such as virus resistance.
• Budding and grafting: conventional vegetative propagation, where a bud or shoot of one plant is joined onto another. These are old horticultural methods that do not alter the genome, so they do not create transgenic crops.

Newer tools edit and copy genes with great precision:

• CRISPR-Cas9: a targeted gene-editing system. Cas9 is an enzyme that acts as molecular scissors, cutting DNA at a chosen spot.
• RNA interference (RNAi): a gene-silencing technique. It is used in gene-silencing therapies, in cancer treatment, and in making crops resistant to viral pathogens.
• Somatic Cell Nuclear Transfer (SCNT): the cloning method. The nucleus of a body (somatic) cell is placed into an emptied egg to grow a genetic copy. Dolly the sheep, cloned by Ian Wilmut in 1996, was produced this way. This is reproductive cloning, not in-vitro fertilisation (IVF). Dolly was famous as the first mammal cloned from an adult body cell. She was not the first cloned mammal, because earlier mammals had already been cloned from embryo cells.

Gene editing can be aimed at different kinds of cells, and the target matters. Editing a body (somatic) cell changes only that one person. Editing the germline, meaning the egg-producing or sperm-producing cells, changes a gene that will be passed to every later generation. A genome can also be edited at the early embryonic stage, before birth, so that the change is built into the whole organism as it grows. Each of these is now technically possible with tools such as CRISPR-Cas9.

Scientists can also mix cells from two species. A chimera is an organism made of cells from more than one source. For example, human induced pluripotent stem cells, which are ordinary body cells reprogrammed back into a flexible, embryo-like state, have been injected into a pig embryo to create a human-animal chimera.

Several feats of cell and molecular biology are now routine, and one common claim is false:

• Artificial functional DNA can be synthesised in the laboratory from its chemical building blocks.
• Isolated DNA can be copied outside a living cell, for example by the polymerase chain reaction (PCR), a test-tube method that multiplies DNA.
• Plant and animal cells can be made to divide in petri dishes, the technique of cell culture.
• However, a fully functional chromosome cannot be created merely by joining together DNA segments taken from different species. A working chromosome needs more than spliced DNA, so this last claim is the false one.

Other techniques identify organisms or read the genome:

• DNA fingerprinting: compares each person's unique DNA pattern. It establishes the paternity of a child and is a standard forensic tool.
• DNA barcoding: uses a short standard gene sequence to tell apart look-alike species and to detect undesirable materials in processed food. It cannot assess an organism's age.
• Microsatellite DNA: short repeated DNA sequences that vary between individuals, used to study evolutionary relationships among species.
• Genome sequencing: in crops it finds markers for disease resistance and drought tolerance, speeds up the breeding of new varieties, and reveals host-pathogen relationships.
• Transcriptome: the full set of mRNA molecules expressed by an organism.
• Aerial metagenomics: collecting environmental DNA straight from air samples to survey the organisms in a habitat.

In India, the Genetic Engineering Appraisal Committee (GEAC), the apex regulator for genetically modified organisms, is constituted under the Environment (Protection) Act, 1986, the country's main environmental law. The same law bars the import of GM food without the competent authority's approval.