One example of recessive inheritance can be seen in the appearance of earlobes; some people have the lobe hanging free (detached ear lobes) while others have the lobe attached to their head. This can be seen in the British royal family, judging from photographs.
This tends to show a pattern whereby the gene allele for detached earlobes (E) is dominant over attached (e). Therefore, heterozygotes containing both an allele for detached and one for attached earlobes have detached ears. And if two such heterozygous parents produce children, then there is one in four chance of any of their offspring inheriting both attached alleles and so showing attached earlobes; the classic 3:1 ratio.
Prince Andrew and Sarah Ferguson both have attached ear lobes (suggesting that they are e/e), and their two daughters seem to have inherited the same trait. This would imply that both Queen Elizabeth (her sister had attached ear lobes) and the Duke of Edinburgh are heterozygotes (E/e), as one of their four children (Andrew) shows attached ear lobes.
Recessively inherited disorders occur in the same way, only resulting if two copies of the defective gene are inherited. Inheriting only one defective copy will not lead to any ill-effects as a second functional copy of the gene will compensate. So while a dominant disease requires only one parent to carry the gene, a recessive disease can only result if both parents have at least one copy of the defective gene.
Furthermore, only 25% of the offspring of two such heterozygous (i.e. they only carry one mutant copy of the gene) parents will show the disease. We can see this if we assume the dominant non-disease causing gene as the ‘D’ and the recessive, disease-causing, gene as ‘d’.
In recessively inherited disorders the parents are often unwitting heterozygous carriers of a gene in which children inheriting two copies suffer from the disease. The family tree of a recessive gene is very different from a dominant one, as only one or two generations will suddenly show the disease when individuals with the same gene marry
Inbreeding is how we get championship horses.
Carl Gunter, (Louisiana state representative, explaining why he was fighting a proposed antiabortion bill that allowed abortion in cases of incest)
The ancient taboo of incest in most cultures probably stems from the realisation that inbreeding, particularly within the first generation, leads to higher occurrences of children born with defects. The reason inbreeding is so deadly to a population is that, while normally only a small minority of the general population will carry a copy of a particular gene causing a specific recessive disease (i.e the chance of marrying a carrier of the same recessive gene is low), this chance will increase if the individuals are related and hence share some genetic material. Therefore, some recessively inherited conditions are found in higher frequencies in some close-knit communities such as Amish or various Jewish communities. Another example of inbreeding leading to higher incidents of genetic diseases can be seen in the Negev Bedouin people, a population of 140,000 people who roam the Negev desert in Southern Israel. Their tradition is to marry within the family so as to strengthen bonds among extended families; around 65% marry their 1st or 2nd cousins. Although they do not carry any more mutations than the general population, because so many marry relatives they have a higher chance of marrying someone carrying the same mutation increasing the odds of producing a child with a recessive genetic disease. In fact, many of the diseases seen in the Bedouin are extremely rare, resulting from mutations not previously seen in other populations of the world, due to their cultural isolation.
The French painter Henri de Toulose-Lautrec was born from a first-cousin marriage with the recessively inherited bone disorder Pycnodysostosis. His parents must have both had a copy of the mutated gene for the disease and at least three of his cousins, the offspring of his paternal uncle married to his maternal aunt, also suffered from the same disorder, supporting the mode of inheritance being autosomal recessive and exemplifying again the risks of inbreeding.
Charles Darwin, along with his cousin Francis Galton who coined the term eugenics, was one of the first to realise the ill effects of inbreeding and the advantages of cross-breeding. His theories may have actually stemmed, to some degree, from his own experiences as he himself married a cousin Emma Wedgwood in 1839. They had ten children with seven either dying prematurely or remaining childless; although dozens of his descendants nevertheless became eminent, particularly in the fields of science.