What is heredity?

We are all aware of family resemblances.  The Hapsburg lip & jaw and the presence a sixth finger or toe, polydactyly, red hair and diseases such as hemophilia and sickle cell anemia, are among many traits that occur in families.

These traits are inherited.   People have had a rough understanding of the "rules" heredity for thousands of years, and have used this knowledge to generate the amazing diversity of domesticated plants and animals.

They selected specific traits – bred organisms with those traits, and selected offspring with the desired traits for future breeding. 

Over tens to hundreds of generations, diverse types of organisms were derived.

For example, all breeds of dog (Canis familiaris) appear to be derived from an ancestral species through the process of artificial selection. 

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In the case of heredity we need to define what, exactly, is inherited.  For the plant or animal breeder, what matters are observable (phenotypic) traits, that is the specific features that they want to enhance.  These can range from fruit/vegetable size, to disease resistance, to obedience and passivity. 

A cousin of Charles Darwin, Francis Galton (1822-1911) believed that it was possible to measure a wide range of characteristic, including moral characteristics.  Others believed, that traits such as a tendency to commit murder, are reflected in the shape of the skull (phrenology) and other physical characteristics.


A prominent jaw was thought to be a sign of a murderous personality. Pick-pockets were thought to have slender hands and sparse facial hair.

All of the organisms of a specific type or species have the same set of characters.  The forms these characters take, however, can vary.  It is these differences that are referred to as traits.

For example, all carnations have flowers (the character), but different plants can have flowers of different colors, size, shape and position on the plant (the trait).


Some traits, like height or weight, vary continuously.  Others, like flower color, are often "discontinuous", i.e. only a discrete subset of all possible colors are found to occur. 

Occasionally, it is possible to determine the exact origin of a new trait. Consider the short-legged Ancon sheep.

 

 "It appears that one Seth Wright, the proprietor of a farm on the banks of the Charles River, in Massachusetts, possessed a flock of fifteen ewes and a ram of the ordinary kind. In the year 1791, one of the ewes presented her owner with a male lamb, differing, for no assignable reason, from its parents by a proportionally long body and short bandy legs, whence it was unable to emulate its relatives in those sportive leaps over the neighbours' fences, in which they were in the habit of indulging, much to the good farmer's vexation." - from The Origin of Species by Charles Darwin.
  • Where does the variation between individuals come from?  
  • You release a population of dachshunds into to wild; assuming that there are no other dogs around, which traits might change over time and why?
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