Darwin's central problem was that, although he had a clear model for the continuity of physical characteristics, he could not explain how it occurred. The prevailing view was that there was some essence in the blood that was passed on.
Mingling from generation to generation was thought to dilute any beneficial characteristics, hence the preoccupation with blood lines, especially among the nobility. Yet clearly there were characteristics, such as certain diseases that appeared in successive generations within families. There were also some known to skip generations then reappear, often referred to as 'throwbacks'.
It was the rediscovery early in the twentieth century of Mendel's work that showed that there was some set of unitary characters that was passed on. Though they were called "genes", they were still some mythical entity whose effect could be surmised, but whose existence had yet to be proved.
It is unkind of those authors who suggest that Mendel was "lucky" in his choice of the pea.* What he did was select a species for which he could distinguish seven clearly marked characteristics that could be defined in a binary fashion, that is, they were independently assorted.
Most other species are not as amenable as this. Antirrhinums, for instance, have a range of flower colours, and crossing red-flowered plants with white produces plants with pink flowers. The genes responsible are said to be incompletely dominant, where protein produced by both allelles affects the colour. Compare this with co-dominance. If a red bull is crossed with a white cow, the calf will be roan. Its hairs, however, are not an intermediate shade, they are either red or white. In other words, either one allelle or the other determines the colour in an apparently random fashion. Moreover not all genes are inherited independently of each other. For instance, in sweet peas, blue flowers and long pollen grains are inherited together. They are said to be linked and turn out to be on the same chromosome pair.
To recap, in the simple case, each parent may supply one of two alleles, so there are four possible combinations. In two of these, there is one of each allele, that is the offspring is heterozygous for that allele. The recessive allele will be passed on by inheritance, but, in everyday life, its effect will only appear if its carrier mates with another, to produce offspring homozygous for the recessive allele.
An exception is so-called X-linked recessive inheritance. There are a number of conditions, such as hemophilia, which are known to appear in successive generations, but only usually affect males. If the mother is carrying the recessive gene, her daughters will have two copies of the gene, the other one usually dominant. Sons, however will have only the recessive gene from the mother. On average half their sons will be affected, since they have a fifty fifty chance of receiving the gene. If they have children, all their daughters will be carriers, receiving the X. All their sons will be normal, receiving a Y. Women rarely have the condition for, to do so, their mother must be carriers themselves and their fathers hemophiliac.
The occurrence of a simple 3:1 ratio, of homozygous individuals under autosomal inheritance, predicted by Mendel's study, does not always occur. Another common result is a ratio of 2:1. In some plants, a gene concerned with the production of chlorophyll has a dysfunctional recessive allele. Any plants with two copies of this so-called lethal allele do not survive.
We need to be clear at this point, what dominance and recessiveness means. Both alleles produce protein. What is important is the contribution each provides to the organism's functioning.
Sickle cell anemia is experienced by individuals homozygous for the recessive allele of a certain gene. Thus, in a given population, one in four may be affected, and, in the past, would probably perish. However, cruel as it sounds, the two in four that are heterozygous have an advantage, since the recessive allele appears to provide protection from malaria.
Discovering such inherited traits is done by constructing family trees, called pedigree analysis. Originally this was done from family memories and family records, but as the generations pass since official records began to be kept the task has been made easier, as described in more detail in Jones' In The Blood(1)
Being able to study more generations helps with recessive traits which may skip generations, as does studying different family trees. The situation is not always clear, even for single gene conditions, especially if the outcome observed is an indirect effect, or whether the environment is important. Often, in the developmental process, the individual compensates for any problems, and there may be different mutations of a given gene in different families. A good example of this, which will be discussed elsewhere on this site, is the intersex condition, five alpha reductase deficiency.
Interactions of two or more genes or alleles have been studied(2) however most of the attributes we are interested in are polygenic and most are influenced, to a greater or lesser extent, by the environment. Height and body weight, for instance, are determined by a number of genes, but also by diet.
Since polygenic effects are not unitary phenomena like, for instance, an illness, rather than building family trees of individuals, studies rely on statistical measurements of large general populations.
* It is also unkind of some authors to suggest that Mendel 'fudged' his results to get the answer he wanted. He would certainly be no more guilty than any other scientists who discard results that don't fit their hypotheses. They refer to it as finding the norm in a statistical distribution. Science, in fact, is not about finding 'great truths' or 'universal laws', but of teasing out patterns of simplicity in a universe of complexity,(3) something that Mendel achieved with stunning success.
Bibliography and good reading.
Bland, J., (2003) About Gender: Inheritance
Book graphics courtesy of Amazon.co.uk
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Derby TV/TS Group. Text copyright Jed Bland.
06.06.02 Last amended 04.06.03, 24,12.03