Y-Linked Characteristics

Y-linked traits—also called holandric traits—exhibit a distinct pattern of inheritance. These traits are present only in males, because only males possess a Y chromosome, and are always inherited from the father. Furthermore, all male offspring of a male with a Y-linked trait will display the trait because every male inherits his Y chromosome from his father.

THE USE OF Y-LINKED GENETIC MARKERS DNA sequences in the Y chromosome undergo mutation with the passage of time and thus vary among individual males. These mutations create variations in DNA sequence—called genetic markers—that, like Y-linked traits, are passed from father to son and can be used to study male ancestry. Genetic markers can also occur on other chromosomes and are frequently used in mapping genes. Although the markers themselves do not encode any physical traits, they can be detected with the use of molecular methods. Much of the Y chromosome is nonfunctional, and so mutations readily accumulate. Many of these mutations are unique; they arise only once and are passed down through the generations without undergoing recombination. Individual males possessing the same set of mutations are therefore assumed to be related, and the distribution of these genetic markers on Y chromosomes provides clues about the genetic relationships of present-day people. Y-chromosome sequences have also been used extensively to examine past patterns of human migration and the genetic relationships among different human populations.

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Y-linked genetic markers have been used to study the offspring of Thomas Jefferson, principal author of the Declaration of Independence and third president of the United States. In 1802, a political enemy accused Jefferson of fathering a child by his slave Sally Hemings, but the evidence was circumstantial. Hemings, who worked in the Jefferson household and accompanied Jefferson on a trip to Paris, had five children. Jefferson was accused of fathering the first child, but rumors about the paternity of the other children circulated as well. Descendants of Hemings’s children maintained that they were related to the Jefferson line, but some Jefferson descendants refused to recognize their claim.

To resolve this long-standing controversy, geneticists examined markers from the Y chromosomes of male-line descendants of Hemings’s first son (Thomas Woodson), her last son (Eston Hemings), and a paternal uncle of Thomas Jefferson with whom Jefferson had Y chromosomes in common (descendants of Jefferson’s uncle were used because Jefferson himself had no verified male descendants). Geneticists determined that Jefferson possessed a rare and distinctive set of genetic markers on his Y chromosome. The same markers were also found on the Y chromosomes of the male-line descendants of Eston Hemings. The probability of such a match arising by chance is less than 1%. The markers were not found on the Y chromosomes of the descendants of Thomas Woodson. Together with the circumstantial historical evidence, these matching markers suggest that Jefferson was the father of Eston Hemings, but not Thomas Woodson.

CONCEPTS

Y-linked characteristics exhibit a distinct pattern of inheritance: they are present only in males, and all male offspring of a male with a Y-linked trait inherit the trait.

CONNECTING CONCEPTS

Recognizing Sex-Linked Inheritance

What features should we look for to identify a trait as sex linked? A common misconception is that any genetic characteristic in which the phenotypes of males and females differ must be sex linked. In fact, the expression of many autosomal characteristics differs between males and females. The genes that encode these characteristics are the same in both sexes, but their expression is influenced by sex hormones. The different sex hormones of males and females cause the same genes to generate different phenotypes in males and females.

Another misconception is that any characteristic that is found more frequently in one sex than in the other is sex linked. A number of autosomal traits are expressed more commonly in one sex. These traits are said to be sex influenced. Some autosomal traits are expressed in only one sex; these traits are said to be sex limited. Both sex-influenced and sex-limited characteristics will be considered in more detail later in the chapter.

Several features of sex-linked characteristics make them easy to recognize. Y-linked traits are found only in males, but this fact does not guarantee that a trait is Y linked because some autosomal characteristics are expressed only in males. Y-linked traits are unique, however, in that all the male offspring of an affected male express the father’s phenotype, and Y-linked traits can be inherited only from the father’s side of the family. Thus, a Y-linked trait can be inherited only from the paternal grandfather (the father’s father), never from the maternal grandfather (the mother’s father).

X-linked characteristics also exhibit a distinctive pattern of inheritance. X linkage is a possible explanation when the results of reciprocal crosses differ. If a characteristic is X linked, a cross between an affected male and an unaffected female will not give the same results as a cross between an affected female and an unaffected male. For almost all autosomal characteristics, the results of reciprocal crosses are the same. We should not conclude, however, that when the reciprocal crosses give different results, the characteristic is X linked. Other sex-associated forms of inheritance, described later in the chapter, also produce different results in reciprocal crosses. The key to recognizing X-linked inheritance is to remember that a male always inherits his X chromosome from his mother, not from his father. Thus, an X-linked characteristic is not passed directly from father to son; if a male clearly inherits a trait from his father—and his mother is not heterozygous—it cannot be X linked. image TRY PROBLEM 14