From Atoms to Traits

 1.  Gregor Mendel swept away the confusion about blending inheritance of Darwin’s theory by implementing his famous breeding experiments with peas, conducted in the 1850s and 1860s. His experiment showed when true-breeding pea plants of contrasting types were crossed, the off – spring usually resembled one of the two parents. With further crosses, both forms of a trait could reappear in undiluted form in future generation; yet, the genetic information for alternative forms had not blended away.

     Mendel’s experiment changed the general perception of heritable variants from ephemeral and blendable to discreet entities passed from parents to offspring, present even though they are not always visible. Furthermore, Mendel’s experiment answered the question how new traits could spread in subsequent generations that Darwin was unable to solve.

2.   James D. Watson and Francis Crick proposed a structure for the DNA (deoxyribonucleic acid) molecule in 1953, with stunning implications for our physical understanding of heredity and variation. DNA is a long; two- stranded helix, with a backbone made of repetitive chains of sugar and phosphate. The complementary pairing between four possible chemical bases: adenine, cytosine, guanine, and thymine holds the two strands of the polymer together. On the other hand, these four possible chemical bases also form the foundation of a simple genetic language (A, C, G, T)

     The four chemical letters in the DNA alphabet can occur in any sequence along one strand of the helix, spelling out different instructions that are passed down from parents to offspring. The double-stranded helix provides a clear mechanism for copying genetic information as well. Cs always pair with Gs, and As pair with Ts across the middle of the DNA molecule

3.  1) Point mutation: substitution of a single letter for another at a particular position in the polymer. For example, in whippet dogs, a single base pair change makes the difference between a slender silhouette and the hulking animal. The mutation inactivates the gene for a signaling molecule that regulates muscle growth. In animals with both copies of the gen mutated, muscle growth is uncontrolled for lack of “stop “ signal. When only one copy of the gene is disabled, the dogs are moderately more muscular and prized as racers.

2) Duplication of new letters. Sequences containing the same base pair repeated eight or more times, known as homopolymers, are highly prone to copying errors. For example, in pigs, the gain of two additional C-G pairs in such a sequence inactivates a gene for a signal receptor in pigment cells producing light-colored coats.  On the other hand, copying mistakes within individual cells may also cause the duplicated sequence to lose bases, restoring the gene’s function and producing dark patches on the body.  

3) Gene copy number. Entire gens can be duplicated by copying errors during cell division, leading to differences between species and to variation among members of the same species. The genome of chimpanzees, which eat green plants, normally contains just a single gene of the starch-digesting enzyme salivary amylase, whereas humans can carry up to 10 copies of the gene.

4) Insertion of new letters. For example, in pea plants, an 800-base-pair sequence inserted into a gene produces peas that are wrinkled rather then smooth. The intruding DNA element disables a gene necessary for starch synthesis, altering the peas’ sugar and water content. Such mobile elements are seen in the genomes of most multicellular organisms, including humans.

5) Regulatory changes. Mutations in the DNA that controls when and where genes are activates can produce profound trait changes by altering the formation of entire body parts during the organism’s development. Changes in the regulatory regions of a single gene that controls patterns of cell division during stem development account for much of the shape difference between the bushy teostinte plant and its descendent, the tall modern cornstalk.  

4.    It’s a subspecialty within evolutionary biology that has come to be known as evo-devo, concentrating on studying the effects of changes in important developmental genes and the role they play in evolution.

5.    An enzyme called lactase, produced in the intestines, allows infants and children to digest the complex milk sugar lactose. Only a minority of people continues to produce lactase as adults. In 2002, this ability was traced in Europeans to mutation in the regulatory DNA that controls the lactase gene. More recently, different mutations affecting the same gene were found to predominate in East African and Saudi Arabian populations who traditionally herd milk-producing animals. The differing DNA changes indicate that the trait of lactase tolerance has arisen independently many times in the past 9,000 years. 

            As we have learnt from the TV documentary, A Journey of Man, people from Africa started to migrate 50,000 years ago. As years passed by, this extraordinary group of people had spread out all over the world. They carried Y-chromosome and passed on genes that allow them to digest the complex milk sugar lactase to the next generations. This is why the ability of digesting was traced in Europe, Saudi Arabia and East Africa.