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Factsheet: What's hereditary and what's not?

This factsheet forms part of the Progress Educational Trust (PET) project 'When It Takes More Than Two'. The factsheet provides a brief guide to heredity as it relates to donor conception.

Advertising around donor eggs and sperm
Sperm banks and fertility clinics often try to attract people to use their services by describing the types of donors they have. Advertising can refer to how well-educated the donors are, citing donors who have 'degrees, MBAs and PhDs'. Other criteria used to market donors include religion, occupation and personal interests. This information helps to build a picture of the donor. However, none of these inclinations or experiences can be inherited via the use of the donor's sperm or eggs. These characteristics do not necessarily reflect any genetic predisposition for intelligence, morality or fitness. They are shaped significantly by the circumstances and environment in which someone is raised, and cannot be used to determine whether a donor has 'intelligent' genes, 'athletic' genes, or genes for other desirable traits such as morality.

What traits can be inherited?
The simplistic view of inheritance is that a single pair of genes (one from each parent) controls one characteristic. These genes (and therefore this characteristic) can be passed down to future generations following classic genetic rules (which are referred to as Mendelian inheritance). However, this is usually not the case - even a 'simple' characteristic, such as skin, hair or eye colour, is influenced by several different genes. Knowing the hair colour of a sperm donor may give one an idea of what colour the resulting child's hair could be, but the resulting child's hair colour will also depend on which particular versions of several genes the child inherits. For example, a 'blonde hair' gene can be masked by a 'brown hair' gene.
Other qualities, such as height and intelligence, are even less clear-cut. Not only do they involve many genes, but they are also affected greatly by other factors. For example, height not only involves the complex interaction of hundreds of genes controlling biological processes (such as bone growth and hormones), but is also affected by nutrition.
Roughly speaking, the average full-height of children with the same biological parents will be between the height of those parents. However, if a woman who is of very short stature selects a very tall sperm donor, she will not in fact boost her chances of having a very tall child. The relevant genetics are not that straightforward.
Intelligence is far more contentious. First, it is a highly subjective quality. Second, there is a lack of conclusive evidence about the genes involved in intelligence, while factors such as upbringing and diet are known to play a significant role.
When it comes to sperm and egg donors, some of the traits advertised (such as hair, eye and skin colour) will be inherited genetically, and some of the traits advertised (such as height) will be partly inherited genetically but will also be affected significantly by non-genetic factors. Other traits (such as morality) will not be inherited genetically at all, and depend entirely upon the circumstances and environment in which a person is raised and in which they live as an adult.