This document is a required reading assignment covering chapter 4 in your textbook. Chromosomal basis of genes and linkage The majority of chapter 4 deals with the details of mitosis and meiosis. This information should be reviewed, since it is an important background for our next topics. Presumably everyone has studied these reactions before. However, the exact stages of these cell division processes will not be part of this class. What is important for this class are the differences between them as summarized in Table 4.3 on page 99 of your textbook. The two biggest differences between them are: Mitosis Meiosis 1) Homologues do not pair Homologues pair at the beginning of meiosis 2) Product is two cells Product is 4 cells with ½ the chromosomes of the original identical to original Figures 4.17 and 4.18 show the stages of meiosis as they relate to humans. This material will not be covered on the exam, but it is once again useful background information. The details for which students are responsible begin on page 101. The fundamental finding of interest in this chapter is the demonstration of the physical nature of genetic information. During the early part of the 20 th century, Mendel s ideas continually received new support and were applied to more examples. During this same period, there was a great deal of activity in cytology, which is the microscopic study of cell structures. Animal and plant cells were seen to possess a nucleus and within this central body were chromosomes, which literally means colored bodies. The name reflects the fact that it was possible to stain them with specific dyes. There was no notion at first that chromosomes had anything to do with genetics. However, it was noted that the chromosomes occurred in pairs (Figure 4.4 shows the presence of pairs in humans) and that during reproduction the sex cells (sperm and eggs) each had one copy of each chromosome. When the egg was fertilized, the chromosomes from the two parents formed the pairs in the new offspring. Of course, this is exactly the mathematical pattern presented by Mendel and his factors, which we now call alleles. It was a reasonable extrapolation to combine these two observations and conclude that chromosomes contained genetic information. Or, one could say the alleles are on the chromosomes. In fact, we might more accurately say the chromosomes are composed of alleles. A major finding was the sex chromosomes, which we call X and Y. It was seen that these were present differently in males and females. In humans and other mammals, females are XX and males are XY. Your book summarizes the system of sex determination for some other animals in Tables 4.1 and 4.2 and the related text. There are in fact several different patterns. In birds, it is the male who has two identical sex chromosomes and the female who has two different ones. Of course, understanding these
patterns does not really explain why the different genders arise as a result and that is still not very clear. A major advance came in 1910 when Thomas Hunt Morgan, working with the fruit fly, Drosophila, provided the first direct evidence that chromosomes carried genetic information. Specifically, he showed that the X chromosome, in addition to having information to determine the gender of the fly, also had other non-gender related information, such as certain variants of eye color. Morgan s detailed experiments began with the discovery of a white-eyed male fly in his laboratory stocks (the wild type flies have red eyes). This white mutation arose spontaneously and it would be many decades before it was understood what was different in the alleles so that white eyes were produced. Nonetheless, he proceeded with the general idea of a wild type (red) allele and a mutant (white) allele). The symbolism is important because those who work with Drosophila follow a system that is different from those used with other species. The white allele is represented by a lower case w and the wild type by the lower case w modified by a superscript + sign (w + ). Figure 4.19 summarizes a series of crosses that Morgan did, beginning with this rare white male fly. Note that this figure represents the crosses with phenotypic drawings, genotypic notation and chromosome details. In fact, it could be said that Morgan s main contribution was to deduce the genotypes that gave rise to these phenotypes. Finally, note that their genotype descriptions have one additional level of complication, namely that the alleles are shown as superscripts of an upper case X, which represents the X chromosome. The males have a capital Y to show that chromosome. Study this figure to be sure understand in careful detail. The results in Figure 4.19 have some unexpected features. The most striking is that the reciprocal crosses did not give the same results. That is, a red-eyed female crossed with a white-eyed male gave different results from the cross of a red-eyed male and a whiteeyed female. At first, this would appear to be a contradiction of Mendel, but in fact it is not. A new concept was needed to explain these results and that concept was linkage. Simply put, linkage is the observation of two things happening together (being linked) and may be described as the absence of independence. Since Mendel s rules assumed independence, Morgan s fly results are outside of what Mendel addressed. What are the two features which are linked in these experiments? They are the determination of gender and the color of the eye. I will expand this topic with results from some other experiments Morgan did which are not described in your book. I will include actual data as well, since that will allow us to understand even more about this system. Here are the crosses. For convenience, I will write red female for red-eyed female, etc. Cross A Parents: Red female X white male
F1: All red, males and females Cross B Parents: F1 female (from Cross A) X F1 male (from Cross A) F2: Red White Male 1011 782 Female 2459 0 Cross C Test cross (not in Morgan s scheme) Parents: F1 female (from Cross A) X white male F1: Red White Male 132 86 Female 129 88 Cross D Parents: F1 female (from Cross C) X F1 male (from Cross C) Offspring: All females red, all males white. Comments on this new series of crosses 1. Cross A and Cross B are the same as A and B in Figure 4.19. The others are new. 2. The offspring in Cross B (called F2) are usually described as all females are red, ½ of males are red, ½ are white. The actual data show this is not quite the case in two senses. First, there are substantially fewer males (1793 vs 2459 about 73%) and second, there are fewer white males than red males (about 77%). This illustrates a point not made in your textbook, which is that there is some undefined general deleterious effect of the white allele. 3. The offspring in Cross C do not show a survival difference between male and female, but definitely show the harmful effect of white once more. 4. Conclusion: the general patterns are as expected, but analysis of the details can raise questions. 5. It would be very valuable for students to work through these 4 crosses while keeping track not only of alleles but chromosomes. Morgan s work to this point is impressive, but is still a little cumbersome to interpret, so it is not surprising that not everyone immediately accepted the idea of chromosomes as the physical location of alleles. The more compelling evidence came from Calvin Bridges, working in Morgan s lab. Bridges repeated some of Morgan s work, but with much larger groups of flies. This allowed him to find rare events. In particular, he discovered that when white-eyed females were mated with red-eyed males, about 1 time in every 2000 crosses he did not see the expected results (which were all red-eyed females and all white-eyed males). What he found in these rare instances were males with red eyes and females with white eyes. Furthermore, these unusual flies were found in roughly equal proportions.
The hypothesis Bridges put forth to explain these rare flies was that they arose from nondisjunction of X chromosomes. This is the name given to the failure of chromosomes to separate properly during meiosis. ( Disjoin means to separate, so nondisjunction means failure to separate.) Figure 4.20 shows the consequences of this hypothesized nondisjunction, namely that the female will produce unbalanced gametes. Instead of having one X chromosome in each of her eggs, she will put two X s in half of the eggs and no X s in the other half. In the eggs with two X s, these chromosomes are still attached instead of being separate as in normal female cells. When these unbalanced eggs are fertilized by sperm with the normal complement of chromosomes, the results are offspring that either have an extra X or no X. As shown in Figure 4.20 (a), there will be four types of offspring, with sex chromosome composition of : XXX, XXY, XO OY where O stands for blank, so an XO fly has only an X, and an OY fly has only a Y. What are the effects of this imbalance? As shown in Table 4.1, it is possible for animals to survive most of these combinations, although there may be harmful consequences. There are some interesting differences between the effects in humans and flies, which will be discussed later, but the only configuration which is always lethal is OY. This reflects the fact that the X chromosome contains a great deal of information unrelated to gender and no animal can live with its complete absence. What about the genetic aspects of nondisjunction? Figure 4.20, both parts (a) and (b), shows that it is necessary to follow the chromosome distribution as well as the allele distribution to understand the situation in detail. Part (b) of that figure in particular is an excellent diagram of a Punnett Square which does just that. I will leave the details to the student, since that figure is so clear, but examine it carefully to understand how it is an excellent model to explain the data that Bridges gathered. It should be noted that this unusual complement of chromosomes can happen for any chromosome, not just the X. It is just that it was discovered with the X. This accidental discovery was valuable because it happened in a fly with white eyes in the laboratory where the whole question was being investigated. As described above, the results of Morgan and Bridges showed that traits unrelated to gender were carried on a sex chromosome, and we termed this combination linkage. Chapter 5 extends this concept of linkage in a much more general way by examining other genes, some of which are carried on the X chromosome and some of which are not. As an introduction to this discussion, it is important to consider some of the simple mathematical aspects to the distribution of genes. Flies have about 5000 genes and 4 different chromosomes. Humans have about 20,000 genes and 23 different chromosomes.
These numbers are not precisely known and are still subject to change. Since genes are carried on chromosomes, then it must be true that each chromosome has a very large number of genes, from hundreds to thousands. It is this reality that makes the study of linkage so important and is the subject of chapter 5.