Читать книгу Health Psychology - Michael Murray - Страница 47

The science of genetics began with Hippocrates. His theory of ‘pangenesis’ suggested that heredity material in the form of ‘pangenes’, collected from around the body, enters the sperm and ovaries. Information from specific parts of the parents’ bodies were communicated to the offspring to create the corresponding body part. For example, information from the parents’ hearts, lungs and limbs was believed to transmit directly from these body parts to create the offspring’s heart, lungs and limbs. The theory that inheritance was based on the ‘blending’ of parental traits was also popular and could not be dismissed until the research by Gregor Mendel (1822–1884), an Austrian monk. Mendel cultivated and tested the physical characteristics of 28,000 pea plants, which made excellent study items as they have easily recognizable and constant features, such as seed texture, colour and height. Mendel observed seven traits that existed in one out of two possible forms, for example the flower colour is white or purple, the seed shape is either round or wrinkled, and the seed colour and pod colour are both green or yellow. With only one of two possible results from cross-pollination, Mendel could determine which traits were passed down to the offspring with what frequency. The modern-day science of genetics was born.

A world-wide survey of human mitochondrial DNA (mtDNA)¹ led to the claim that ‘all mitochondrial DNAs stem from one woman’ and that she probably lived around 200,000 years ago in Africa (Cairn et al., 1987). This prolific woman has been called ‘Mitochondrial Eve’ or ‘African Eve’. Allowing 25 years per generation, that is just 8,000 generations to create the entire human population of more than 7.5 billion people alive today. How does the science of genetics explain the huge diversity of descendants from Mitochondrial Eve?

¹ Mitochondria are structures within cells that convert the energy from food into a usable form.

First, the blending theory of the ancients was found to be inadequate. In his study of peas, Mendel proposed that there can be no blending because the gene alternate for yellow is ‘dominant’ over the gene alternate for green. The dominant trait is observed whenever a single copy of its gene is inherited. When Mendel crossed the hybrid offspring, green seeds would reappear in one-quarter of plants in the next generation. Mendel concluded that the ‘recessive’ green trait appears only when a copy of the recessive gene form is inherited from each parent. Although Mendel published his discoveries in 1866, his ideas were not appreciated until the early twentieth century (Figure 3.2).

In 1905, the study of meiosis revealed that gender is based on chromosomes, thread-like structures inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of DNA. Chromosome keeps DNA tightly wrapped around spool-like proteins called histones. Without this tight packaging, DNA molecules would be too long to fit inside cells. For example, if all of the DNA molecules in a single human cell were unwound from their histones and placed end-to-end, they would stretch six feet. Bearing in mind that the body contains 37.2 trillion cells, one can appreciate the need for histones.

In humans, each cell normally contains 23 pairs of chromosomes, a total of 46. Of these pairs, called autosomes, 22 look the same in both males and females. The twenty-third pair, the sex chromosomes, differ between males and females. One sex chromosome (X) is much bigger than the other (Y). A mismatched pair of one X and one Y chromosome occurs in male cells, while a matched pair of X chromosomes occurs in female cells. Females produce eggs with only X chromosomes, while males produce sperm with an X or a Y chromosome.

Thomas Morgan studied inheritance in the common fruit fly by crossing white-eyed male flies with red-eyed female flies which produced only red-eyed offspring. However, white-eyed mutants reappeared in the following generation, indicating a recessive trait, but only in males of the second generation. Morgan correctly concluded that being white-eyed must be a sex-linked recessive trait, with the gene for eye colour being physically located on the X chromosome.

Recessive inheritance has explained genetic disorders such as alkaptonuria and albinism, while other disorders are based on dominant genes such brachydactyly (short fingers), congenital cataracts and Huntington’s chorea. Duchenne muscular dystrophy, red-green colour blindness and haemophilia are also sex-linked disorders.

Mendel’s ideas were exploited and taken into a scientific cul-de-sac by the eugenics movement, which proposed that the human species could be improved by breeding from ‘superior’ white stock, while reproduction of the ‘genetically unfit’ was to be stopped. Eugenicists misused ideas of dominant and recessive genes to explain in simplistic terms complex human behaviours and mental illnesses, and failed to take account of environmental effects in human development. Eugenics reached its lowest point in the ‘Final Solution’ of Nazism and the Holocaust of Jewish and Romani people in the Second World War.

As early as 1881 Albrecht Kossel had isolated five nucleotide bases – adenine, cytosine, guanine, thymine and uracil – which were all later shown to be basic building blocks of DNA and RNA in all living things. Deoxyribonucleic acid (DNA) carries the instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses. DNA is composed of nucleotide deoxyribose sugar, a phosphate group and one of four nitrogen bases — adenine (A), thymine (T), guanine (G) and cytosine (C). Phosphates and sugars of adjacent nucleotides link to form a long polymer. The ratios of A to T and G to C are found to be constant in all living things. Uracil is only present in RNA, replacing thymine.

Figure 3.2 Inheritance of traits

In Mendel’s research with round and wrinkled peas, he observed that a quarter of the peas were wrinkled in the second generation, suggesting that the characteristic is produced by two ‘factors’ (genes)

Source: Public domain, Mariana Ruiz Villarreal, 12 September 2008

In 1953, an American, James Watson, and an Englishman, Francis Crick, described the structure of DNA as a double helix, shaped like a twisted ladder. This discovery was in part based on an image produced by Raymond Gosling and Rosalind Franklin using X-ray crystallography, ‘Photo 51’. On 28 February 1953 in a pub in Cambridge, Crick allegedly announced that he and Watson had ‘discovered the secret of life’. Two months later the discovery was reported in Nature (Watson and Crick, 1953). Crick and Watson demonstrated that alternating deoxyribose and phosphate molecules formed the twisted uprights of the DNA ladder, while the rungs of the ladder are formed by complementary pairs of nitrogen base, with A always paired with T and G always paired with C, an elegant and beautiful structure (Figure 3.3).

An important molecule related to DNA is ribonucleic acid (RNA), which carries out coding, decoding, regulation and expression of genes. As noted above, RNA and DNA are both nucleic acids, and, along with proteins and carbohydrates, constitute the four macromolecules that are necessary for all known forms of life. The type of RNA that transcribes information from DNA as a sequence of bases and transfers it to a ribosome is called messenger RNA. Messenger RNA translates instructions from DNA to make proteins, without which we would not have evolved from the slime that we apparently evolved from.

Figure 3.3 The DNA double helix

Source: Reproduced from National Institutes of Health/National Human Genome Research Institute (2017). Public domain