Genes are inherited equally through both parents in sexual species. Although a few examples of maternal inheritance had been reported, the broader sentiment at that time was that genes typically are transmitted to progeny from both parents via Mendel’s recently rediscovered laws of heredity. What remained uncertain were the physical composition of Mendelian particles, and how and precisely where such genes are housed in cells.
Drosophila; sex chromosomes
Genes are inherited equally through both parents in sexual species. Although a few examples of maternal inheritance had been reported (see Chapter 13), the broader sentiment at that time was that genes typically are transmitted to progeny from both parents via Mendel’s recently rediscovered laws of heredity (see Chapter 11). What remained uncertain were the physical composition of Mendelian particles, and how and precisely where such genes are housed in cells (but see Chapter 9).
In 1908, Thomas Hunt Morgan established a Drosophila (fruit fly) laboratory at Columbia University (New York) to which he recruited several soon-to-be famous graduate students and postdocs, including Alfred Sturtevant, Hermann Muller, C.B. Bridges, and Theodosius Dobzhansky. During the 1910s and 1920s, this “fly lab” made several landmark discoveries that eventuated in Morgan’s receipt of the 1933 Nobel Prize in Physiology or Medicine for his body of work on hereditary transmission mechanisms in Drosophila. Morgan’s lab confirmed the suspicions that genes are sequentially housed along chromosomes, that each chromosome contains a set of linked genes with partially non-independent transmission from one generation to the next, and that the closeness of genetic linkage quantitatively predicts departures from Mendel’s law of independent assortment for unlinked loci.
Morgan also discovered sex chromosomes and the phenomenon of sex-limited inheritance. In Drosophila (as in humans, mammals, and many other organisms), females carry two X chromosomes whereas males carry one X and one Y chromosome. Females transmit one copy of the X to each son and daughter, and males pass their copy of the X to daughters and the Y to sons. Genes housed on these sex chromosomes thus have a sex-linked mode of inheritance that differs from the standard pattern of Mendelian inheritance as applied to unlinked loci on the autosomes (chromosomes that are not sex-linked). Many years later other categories of sex linkage were uncovered, such as the ZW system of birds and butterflies in which females have the sex-chromosomal constitution ZW and males are ZZ.
The discovery of sex linkage was important because it (1) revealed taxonomically widespread departures from standard autosomal inheritance, and (2) initiated critical thought about the optimal evolutionary strategies of genes that are transmitted and expressed differently in males and females (see Chapter 48).
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