Concepts 9

Dizygotic twins develop from two eggs fertilized by two separate sperm; they have, on average, 50% of their genes in common. Monozygotic twins develop from a single egg, fertilized by a single sperm, that splits into two embryos; they have 100% percent of their genes in common.

mon and dizygotic twins have on average only 50% in common, genetically influenced traits should exhibit higher concordance in monozygotic twins. For instance, when one member of a monozygotic twin pair has asthma, the other twin of the pair has asthma about 48% of the time, so the monozygotic concordance for asthma is 48%. However, when a dizygotic twin has asthma, the other twin has asthma only 19% of the time (19% dizygotic concordance). The higher concordance in the monozygotic twins suggests that genes influence asthma, a finding supported by other family studies of this disease. Concordance values for several human traits and diseases are listed in Table 6.2.

The hallmark of a genetic influence on a particular characteristic is higher concordance in monozygotic twins compared with concordance in dizygotic twins. High concordance in monozygotic twins by itself does not signal a genetic influence. Twins normally share the same environ-ment—they are raised in the same home, have the same friends, attend the same school — so high concordance may be due to common genes or to common environment. If the high concordance is due to environmental factors, then dizygotic twins, who also share the same environment, should have just as high a concordance as that of monozy-gotic twins. When genes influence the characteristic, however, monozygotic twin pairs should exhibit higher concordance than dizygotic twin pairs, because monozy-Comparisons of dizygotic and monozygotic twins can be gotic twins have a greater percentage of genes in common. used to estimate the importance of genetic and environ- It is important to note that any discordance among mental factors in producing differences in a characteristic. monozygotic twins must be due to environmental factors, This is often done by calculating the concordance for a trait. because monozygotic twins are genetically identical. If both members of a twin pair have a trait, the twins are The use of twins in genetic research rests on the impor-

said to be concordant; if only one member of the pair has tant assumption that, when there is greater concordance in the trait, the twins are said to be discordant. Concordance is monozygotic twins than in dizygotic twins, it is because the percentage of twin pairs that are concordant for a trait. monozygotic twins are more similar in their genes and not Because identical twins have 100% of their genes in com- because they have experienced a more similar environment.

[Table 6.2 Concordance of monozygotic and dizygotic twins for several traits 1

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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