Health Psychology. Michael Murray
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
The Human Genome
A genome is any organism’s complete set of DNA, including all of its genes. An organism’s genome contains all of the information needed to build and maintain the organism. Accompanied by much fanfare and hype as a ‘landmark in science’, the first draft of the human genome appeared on 12 February 2001. In humans, a copy of the genome with all of its 3,234.83 mega-basepairs is contained in each and every one of the body’s cells.
Knowing the complete sequence of the human genome is similar to having a manual on how to construct the human body. However, this manual has more than 3 billion pages and is not easy to read. Great expectations were raised by the scientists involved with the human genome project. However, understanding how the 3.4 billion complex parts work to create human life, health and disease is a challenge. It is currently estimated that there are 19,000–20,000 human protein-coding genes, although this estimate may be reduced over time. Figure 3.4 gives a graphical representation of the idealized human karotype showing the organization of the genome into chromosomes. The drawing shows both the female (XX) and male (XY) versions of the twenty-third chromosome pair.
Figure 3.4 The idealized human karotype divided into 23 chromosome pairs
Source: Public domain
With the human genome project came the formation of new organizations to capitalize on the project. The National Institutes of Health/National Human Genome Research Institute is steering many research programmes on the human genome. One objective is to identify any gene suspected of causing an inherited disease. More than 2,000 genetic tests enable patients and families to be informed about their genetic risks for disease and to help professionals diagnose disease. The cost of sequencing an individual’s genome is being reduced to below US$1,000. When this cost eventually falls, people will be able to carry copies of their karotype on their smart phones.