A basic tenet of Mendelian genetics is that the parental origin of a gene does not affect its expression, and therefore reciprocal crosses give identical results. However, the expression of some genes is significantly affected by their parental origin. This differential expression of genetic material depending on whether it is inherited from the male or the female parent is called genomic imprinting.

A gene that exhibits genomic imprinting in both mice and humans is Igf2, which encodes a protein called insulin-like growth factor 2 (Igf2). Offspring inherit one Igf2 allele from their mother and one from their father. The paternal copy of Igf2 is actively expressed in the fetus and placenta, but the maternal copy is completely silent (Figure 4.22). Both male and female offspring possess Igf2 genes; the key to whether the gene is expressed is the sex of the parent transmitting the gene. In the present example, the gene is expressed only when it is transmitted by a male parent. In other genomically imprinted traits, only the copy of the gene transmitted by the female parent is expressed. In some way that is not completely understood, the paternal Igf2 allele (but not the maternal allele) promotes placental and fetal growth; when the paternal copy of Igf2 is deleted in mice, a small placenta and low-birth-weight offspring result. Some of the different ways in which sex interacts with heredity are summarized in Table 4.7.

EPIGENETICS Genomic imprinting is just one form of a phenomenon known as epigenetics. As we have seen, most traits are encoded by genetic information that resides in the sequence of nucleotide bases of the DNA—the so-called genetic code, which will be considered in more detail in Chapter 11. Some traits, however, may be caused by alterations to the DNA that do not affect the DNA base sequence, but affect the way in which the DNA sequences are expressed. An example of this type of alteration is the addition of methyl groups to some of the DNA bases (DNA methylation). These changes are often stable and heritable in the sense that they are passed from one cell to another.

103

In genomic imprinting, whether the gene passes through the egg or the sperm determines how much methylation of the DNA takes place. The pattern of methylation on a gene is copied when the DNA is replicated and therefore remains on the gene as it is passed from cell to cell through mitosis. However, the methylation may be modified or removed when the DNA passes through a gamete, so a gene that is methylated in sperm may be unmethylated when it is eventually passed down to a daughter’s egg. Ultimately, the amount of methylation determines whether the gene is expressed in the offspring.

These types of reversible changes to DNA that influence the expression of traits are termed epigenetic marks. The inactivation of one of the X chromosomes in female mammals (discussed earlier in this chapter) is another type of epigenetic change. We will consider epigenetic changes in more detail in Chapter 12.