Heredity

• 1. Introduction

  ›

  Heredity is the reason a child can inherit a brown-eye trait from one parent and carry it for life. The source calls it inheritance or biological inheritance, the passing of traits from parents to their offspring. It works through asexual reproduction or sexual reproduction, and in both cases the offspring acquire the genetic information of their parents. Yet some of what we observe in a person is never passed down at all. A suntan, for instance, comes from the meeting of a person's genes and sunlight, and it dies with the body that grew it. So what exactly travels from one generation to the next, and what gets left behind? Why did Charles Darwin's grand theory of evolution stall for decades on this single question? And how did a monk counting pea plants quietly hold the answer the whole time?

• 2. Genes And The Genome

  ›

  Eye color sits under the control of genes, and the complete set of genes inside an organism is called its genotype. The full set of observable traits, the structure and behavior we can see, is the phenotype. These traits arise from the interaction of the genotype with the environment, which is why many aspects of a phenotype are never inherited. People with the inherited trait of albinism do not tan at all and are very sensitive to sunburn, a striking case of genes shaping how the body meets the world. DNA carries the heritable information from one generation to the next. The source describes it as a long polymer built from four types of bases, which are interchangeable. The sequence of those bases spells out genetic information much like letters spelling out a passage of text. Before a cell divides through mitosis, the DNA is copied so each of the two resulting cells inherits the sequence. A portion of a DNA molecule that specifies a single functional unit is a gene, and within cells these long strands condense into structures called chromosomes. The specific location of a sequence on a chromosome is a locus, and when that sequence varies between individuals, the different forms are called alleles. Mutations can change DNA sequences and produce new alleles, and a mutation inside a gene may alter the trait that gene controls.

• 3. Beyond The Gene

  ›

  Most traits are not the neat work of a single allele. The source notes they are controlled by multiple interacting genes within and among organisms, and developmental biologists point to complex genetic networks and cell-to-cell communication. These interactions may underlie developmental plasticity and canalization. Heritable changes have been confirmed that the DNA molecule cannot directly explain, and these fall under epigenetic inheritance systems. DNA methylation marking chromatin, self-sustaining metabolic loops, gene silencing by RNA interference, and the three dimensional conformation of proteins such as prions are all places where such systems have been found at the organismic level. Niche construction shows heredity reaching an even larger scale. The regular, repeated activities of organisms in their environment generate a legacy of effect that feeds back into the selection regime of later generations. Descendants inherit genes plus the environmental characteristics their ancestors shaped, a process the source calls ecological inheritance. Cultural traits, group heritability, and symbiogenesis are further forms of heredity that operate above the gene, gathered under the title of multilevel or hierarchical selection, a subject of intense debate in the history of evolutionary science.

• 4. Darwin's Missing Mechanism

  ›

  When Charles Darwin proposed his theory of evolution in 1859, one of its major problems was the lack of an underlying mechanism for heredity. He believed in a mix of blending inheritance and the inheritance of acquired traits, a model called pangenesis. Blending inheritance carried a fatal flaw. It would push a population toward uniformity in only a few generations, stripping away the very variation that natural selection needs to act upon. That pressure led Darwin to adopt some Lamarckian ideas in later editions of On the Origin of Species and in his later biological works. His own approach was mostly to describe how heredity appeared to work rather than to name its machinery. He noticed that traits not expressed in a parent at the time of reproduction could still be inherited, and that certain traits could be sex-linked. Darwin's cousin Francis Galton took up this model, then heavily modified it, laying the framework for the biometric school of heredity. Galton found no evidence to support the parts of pangenesis that relied on acquired traits. The decisive blow came in the 1880s, when August Weismann cut the tails off many generations of mice and found their offspring kept developing tails.

• 5. From Animalcules To Alleles

  ›

  Theophrastus once proposed that male flowers caused female flowers to ripen, one of many ancient guesses about heredity. Hippocrates speculated that seeds were produced by various body parts and passed to offspring at conception, while Aristotle thought male and female fluids mixed at that moment. In 458 BC, Aeschylus cast the male as the parent and the female as a nurse for the young life sown within her. The Doctrine of Epigenesis, originated by Aristotle, held that a parent's modified traits passed to an embryo as it continually developed, resting on the inheritance of acquired traits. The opposing Doctrine of Preformation claimed that like generates like, treating procreation as the revealing of what had been created long before. Cell theory in the 19th century disputed preformation by proving all cells arise from preexisting cells. The Dutch microscopist Antonie van Leeuwenhoek, who lived from 1632 to 1723, discovered animalcules in the sperm of humans and other animals. Some scientists claimed to see a little man, a homunculus, inside each sperm, and these spermists believed the female contributed only the womb and its prenatal influences. The rival ovists placed the future human in the egg and held that the gender of a child was determined well before conception. In 1878, Alpheus Hyatt led an investigation into the laws of heredity by compiling data on family phenotypes such as nose size and ear shape, and on pathological conditions, paying attention to the age at which traits appeared.

• 6. The Monk And The Synthesis

  ›

  Gregor Mendel, a Moravian monk, published his work on pea plants in 1865 and gave us the idea of particulate inheritance of genes. His findings went largely unknown and were not rediscovered until 1900. At first people assumed Mendelian inheritance only explained large qualitative differences, like those Mendel saw in his peas. The additive effect of quantitative genes was not realised until R.A. Fisher's 1918 paper, The Correlation Between Relatives on the Supposition of Mendelian Inheritance. Traits that can be traced to a single locus became known as Mendelian Traits. In the 1930s, Fisher and others merged the Mendelian and biometric schools into the modern evolutionary synthesis. This synthesis bridged experimental geneticists, naturalists, and palaeontologists, holding that all evolutionary phenomena fit known genetic mechanisms, that evolution is gradual, and that selection acting on the phenotype is the main mechanism of change. The role of genetic drift was equivocal. Strongly supported at first by Dobzhansky, it was downgraded later as results came in from ecological genetics. Trofim Lysenko broke from this consensus in the Soviet Union, emphasising Lamarckian ideas on the inheritance of acquired traits. The movement that bears his name, Lysenkoism, distorted agricultural research and led to food shortages in the 1960s that seriously affected the USSR.

• 7. Dominant And Recessive

  ›

  In peas, the allele for green pods, written G, is dominant to the allele for yellow pods, written g. An allele is dominant when it always shows in the organism's appearance as long as at least one copy is present. A pea plant with the pair GG, a homozygote, or the pair Gg, a heterozygote, will have green pods either way. The allele for yellow pods is recessive, and its effect appears only when both chromosomes carry it, the homozygous pairing gg. This points to zygosity, the degree to which both copies of a chromosome or gene share the same genetic sequence. The source frames a full mode of inheritance through ordered categories: the number of involved loci, from monogenetic at one locus to polygenetic across many; the chromosomes involved, whether autosomal or situated on a sex chromosome; and the genotype to phenotype correlation, which runs from dominant and recessive to overdominant and underdominant. Further layers include penetrance and expressivity, imprinting phenomena, sex-linked and sex-limited expression, and locus to locus interactions such as epistasis. Determining a mode of inheritance is achieved primarily through statistical analysis of pedigree data, and when the loci are known, the methods of molecular genetics can be brought in. Among the common genetic disorders the source lists are Fragile X syndrome, sickle cell disease, phenylketonuria, and haemophilia, each a reminder that these rules of inheritance carry real consequences for human health.