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Name: Mary B.
Status: Educator
Age: 30s
Location: N/A
Country: N/A
Date: 2002

For my 3rd grader's science fair project, we want to use a family tree to prove that 1 out of 4 will have the recessive gene. We read this a few years ago in an older son's science book, but we have not been able to locate the book or find the same information. Is this a valid theory? (We have looked in the library, encyclopedias and other on-line places)

The wording of your question is confusing. Are you asking whether 1 in 4 people have a trait or condition in their bodies (their phenotype) or whether they carry one recessive gene for a trait along with a dominant gene that masks it (their genotype)? Our genes are present in our cells in pairs. We say a person is a heterozygote if he or she has two different alleles, or "flavors," or versions, of a particular gene. If so, one of the alleles may be recessive to the other. In such a case the recessive gene will not show its presence in the person, unless a carrier test is available to detect its presence.

You often cannot prove if the members of a family have a recessive gene, unless it is a particular condition for which carrier testing exists.

If two parents are carriers of the same recessive gene, then the chances of the recessive condition showing up in their children is 1 in 4 for each pregnancy. If they have a large enough family (many children) or if we combine the data for many similar sets of parents then, on average, about one quarter of the children may be expected to show the trait.

It is not typical to find a family tree in which one quarter of the members, from various generations and branches of the family, have a recessive condition. On the contrary, since the parents are unaffected carriers, it is more likely that the condition will show up in one or more of their children as a sudden unexpected occurrence. Consanguinity, or marrying among relatives, increases the chances that a child could have a recessively inherited condition. This is because cousins carry a greater chance or carrying the same recessive gene as each other, inherited from a common ancestor.

Please note: There is a big difference between gene frequencies in a family and in the population. A rare gene may exist in a population for a condition that is either dominantly or recessively inherited. For example, the gene achondroplasia is dominantly inherited, so that a person who has the gene does have the condition, and has a 50% chance of passing it on with each pregnancy. Yet the incidence of this condition is very low, certainly way less than 1 in 2 or 1 in 4. A recessive condition may occur in 1 out of 4 of the children in a family, but only 1 out of 3,600 babies born to the population. In such a case, about 1 in 30 people in this population will be carriers of the recessive gene.

Sarina M Kopinsky, MSc, CGC, HED

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