NARRATION FOR UNDERSTANDING INHERITANCE: MENDEL, METHOD, AND MAPPING Each of us, unless we re an identical twin, is characterized by a unique combination of traits that makes us different from all other humans. Of course our traits are inherited from our parents, but how? What are the rules and biological mechanisms that govern the passing of traits between generations not only in we humans, but in all living organisms? Before the work of Gregor Mendel in a monastery pea garden in the 1860's little was known about heredity. The theory of heredity that prevailed at the time of Mendel s work held that the traits of parents blended to form intermediate traits in their descendants. However, the naturalist Charles Darwin and others of the time realized that blending theory didn t fit reality, for if blending were the mechanism of heredity, all traits- height, intelligence, weight, would move towards an intermediate or average value and eventually all the members of a species would become identical, a conclusion inconsistent with Darwin s own theory of evolution and the diversity of individuals found in most species including our own. Blending theory is very much like a theory any of us might develop if we were to causally observe parents and their progeny in the living world. Mendel s work on the other hand is a classic example of the scientific method; developing hypotheses based on observation, testing those hypotheses in controlled experiments, and then drawing conclusions. Let s now look at Mendel s work and that of those who followed him. Understanding Inheritance: Mendel s Experiments- Pea Plants Ideal Experimental Subjects Mendel used self-pollinating pea plants in his experiments because their flowers, unlike those of most other flowering plants, are created in such a way that flower petals completely cover each flowers sexual structures so pollen can t be distributed between flowers by wind or insects. But, pea plants can be cross-fertilized by hand. Mendel would carefully open the flower of the plant to be fertilized, and cut off its male sexual structures so that the flower couldn't self-fertilize. Then he would carefully collect pollen from a flower belonging to a second plant with which he wished to breed the first plant, and then apply this pollen to the female reproductive structure of the first flower. Understanding Inheritance: Mendel s Experiments: Single Trait Experiments Mendel hypothesized that as he transferred pollen from the male reproductive structures of pea plants with white flowers to the female reproductive structures of pea plants with purple flowers, he was bringing together discrete physical units, one from each parent, which formed pairs that together would determine flower color in the next generation of pea plants. Mendel referred to these discrete physical units as factors ; today we call them genes and know (unlike Mendel) that they are made up of segments of DNA found on even larger pieces of DNA called chromosomes. The different forms that a gene can take are referred to as alleles. In Mendel s flower experiment there were two alleles one for purple flowers and one for white.
Understanding Inheritance: Mendel s Experiments: Single Trait Experiments- Mendel's Law of Dominance As the parent or P generation plants in Mendel s experiment were true breeding for their respective flower colors (that is if they self-fertilized all their progeny would have flowers of an identical color) Mendel probably suspected that the sperm in the pollen of the white flowered plants contained only white alleles and the eggs of the purple flowered plants only purple alleles. Thus when they were crossfertilized Mendel probably speculated that the next or F1 generation would inherit one purple alleles and one white alleles. How, Mendel wondered, would the alleles interact? Would they blend to form lavender colored flowers? Would there be a mixture of purple and white flowers on each plant? As it turned out all the next generation F1 plants had purple flowers. Mendel developed a hypothesis to explain why all the F1 generation plants had purple flowers even though he believed each F1 plant had a white alleles as well as a purple alleles. Mendel s hypothesis stated that their may be two or more alleles, in this case purple and white, and that one alleles, called the dominant alleles, can completely mask the expression of the other, recessive, alleles. This hypothesis is now known as Mendel's Law of Dominance. In the case of flower color, Mendel proposed, purple alleles are dominant over the white. Understanding Inheritance: Mendel s Experiments: Single Trait Experiments- Mendel s Law of Segregation Now that Mendel had a F1 generation in which he believed all the plants had one purple allele and one white allele what would the next or F2 generation created by the self-fertilization of the F1 generation look like? Mendel s hypotheses here was that the gene pairs that determine a trait separate during gamete or sex cell formation and thus each gamete whether sperm or egg receives only one alleles. This became known as Mendel s Law of Segregation. In the case of the F1 generation all the plants were believed to have one purple alleles and one white. Therefore when the gene pairs divided to form sex cells, according to Mendel s hypothesis, one sex cell would receive a purple alleles and the other a white. Thus, whether sperm or egg, half of all sex cells would receive a purple alleles and the other half a white. Understanding Inheritance: Mendel s Experiments: Single Trait Experiments- Punnett Squares Illustrate Genotypes The F2 generation predicted by the Law of Segregation, can be illustrated graphically using a Punnett square. A Punnett square consists of a large square the top of which is labeled with the possible alleles for flower color the eggs can have, in this case uppercase P for the dominant purple alleles and lowercase p for the recessive white alleles. Drawing a line between the labels forms a column below each. On the left hand side the two different sperm types are labeled, again using uppercase P for the purple alleles and lowercase p for white and then drawing a line between the labels to form a row to the right of each. The result is a Punnett square made up of four smaller squares. The combination of alleles in each smaller square is determined by taking the egg alleles that labels its column and "fertilizing" it with the sperm alleles that labels it's row. This results in three different combinations of alleles on our Punnett square; 1-smaller square with two dominate alleles represented by upper case P upper case P, 2-smaller squares with one dominate and one recessive alleles represented by an upper case P lower case p, and 1-smaller square with two recessive alleles represented by lower case p, lower case p. Each of these combinations of alleles defines an individual s genotype.
Individuals that have two identical alleles for a trait are said to be homozygous for a trait. In our example, upper case P, upper case P and lower case p, lower case p are both homozygous genotypes. Heterozygous individuals are those that have two different alleles for a trait. In our example individuals with an upper case P, lower case p genotypes are heterozygous for the trait of flower color. Phenotype refers to an individuals actual physical characteristics, in this case whether they have purple or white flowers. In our example, the phenotype characterized by purple flowers is produced by both the uppercase P, uppercase P and the uppercase P, lowercase p genotypes. The phenotype characterized by white flowers is produced only by the lowercase p, lowercase p genotype. In order to calculate the probability of a given genotype or phenotype occurring, the number of squares reflecting a particular genotype or phenotype is divided by the total number of squares making up the Punnett square which in our example is four. For example, our Punnett square predicts an F2 generation in which 3 out 4 plants have a phenotype characterized by purple flowers and 1 out of 4 a phenotype characterized by white flowers. As it turns out Mendel s F2 generation had a distribution of phenotypes close to that predicted by the Punnett square, approximately three out of four plants had purple flowers and about one out of four white. But how could Mendel be sure of the genotypes? The Punnett square predicts that 1 out of 4 F2 pea plants will have a genotype of upper case P, uppercase P, 1 out of 4 a lower case p, lower case p and 2 out of 4 a genotype of uppercase P, lowercase p, but genotype can t always be ascertained by observation. In white flowered pea plants the phenotype as well as the genotype is obvious, because in accordance with the Law of Dominance if a pea plant has even one purple alleles it s flowers will be purple, so white flowered plants must be homozygous for the recessive alleles. But what about purple flowered pea plants? Here phenotype doesn t reveal genotype. Mendel hypothesized that the genotype of purple flowered plants could be determined by cross fertilizing them with the homozygous white flowered plants. The Punnett square predicts that crossing a homozygous purple flowered pea plant with a homozygous white flowered plant would result in all the progeny having purple flowers, just as the F1 generation created by crossing the true breeding purples with the true breeding whites of the P generation did. On the other hand, the Punnett square predicts that crossing a heterozygous purple flowered plant with a homozygous white would yield a generation in which half have white flowers and half purple. The Punnett square of Mendel s F2 generation predicted that one third of the F2 purple flowered pea plants were homozygous and two out of three heterozygous. In Mendel s test crosses one third of the purple flowered plants tested homozygous and two third s heterozygous just as predicted by the Punnett square, suggesting that Mendel s hypothesis about the segregation of genes during the formation of sperm and eggs was correct. Understanding Inheritance: Mendel s Experiments: Two Trait Experiments- The Law of Independent Assortment Mendel moved on to experiments involving two traits, in one case the traits of pod color (green as opposed to yellow) and pod shape (inflated as opposed to constricted). One of the questions Mendel believed such an experiment would help answer was whether or not the inheritance of one trait is independent from the inheritance of another.
As Mendel crossed pea plants that were homozygous for green and inflated pods, dominate traits (upper case G and I), with pea plants that were homozygous for yellow and constricted pods, recessive traits (lower case g and i), Mendel knew that all the F1 generation plants would have the same genotype (GgIi) and phenotype (green and inflated) regardless of whether or not dominant and recessive pod traits where inherited independently, as only one genotype can be created when cross-fertilizing two homozygous individuals. But Mendel knew that the F2 generation would look very different depending on whether or not the pod traits of green and inflated, yellow and constricted were passed on to gametes together or independently. If pod traits were inherited together then in the F2 generation all the pea plants would continue to have one of two phenotypes- green inflated pods or yellow constricted pods. However, if the alleles for green and inflated, yellow and constricted separated independently of one another during gamete formation then the F2 generation should have four phenotypes- green inflated pods, yellow constricted pods and in addition green constricted and yellow inflated pods. The alleles for the traits Mendel was studying separated independently during gamete formation and thus produced a F2 generation with the four phenotypes just mentioned. Based on these observations Mendel formed the Law of Independent Assortment which states that the gene pairs for different traits segregate independently of one another during the formation of sex gametes. Mendel of course knew nothing about chromosomes, how they are made up of genes, and the process of meiosis by which chromosomes are divided and gametes formed. As chance would have it all the genes for the traits Mendel studied were on different chromosomes. If the genes had been located on the same chromosome he certainly would have had different results that would have forced him to modify, as later geneticists did, the Law of Independent Assortment. Understanding Inheritance: Sutton s Hypothesis: Chromosomes Made of Genes Observing sperm formation in grasshoppers through a microscope William Sutton, thought that perhaps the chromosomes he was watching separate during meiosis were the genes or factors that Mendel had hypothesized the existence of nearly forty years earlier. After all only one copy of each chromosome was going into each newly formed sperm cell and Mendel had predicted that each sex gamete would receive only one copy of the gene for a given trait. But there was one major difficultly- there weren't enough chromosomes to account for all the different traits of the typical living organism. So Sutton developed a simple hypothesis-chromosomes are made up of many genes. Understanding Inheritance: Sutton s Hypothesis: Chromosomes Made of Genes- Sex Chromosomes Have at Least One Gene The first support of Sutton s hypothesis came in the 1930's. Microscopists noted that in mammals and certain insects, that while males have the same number of chromosome pairs as females, the males always had one pair of chromosomes that appeared to be mismatched, while all the female chromosome pairs match perfectly. In males one chromosome in the mismatched pair, dubbed the X chromosome by geneticists, looked identical to the equivalent X chromosomes in female pairings. But the chromosome with which it was paired dubbed the Y chromosome was usually much smaller than the X. Since whether an individual had a pair of X chromosomes or a pair made up of one Y chromosome and one X
chromosome, determined the sex of an individual, these chromosomes became known as sex chromosomes. Sex chromosomes appeared to support Sutton s hypothesis as they demonstrated that the gene for at least one very important trait, an individual s sex, was carried on a chromosome. Experiments, by Thomas Morgan would soon prove that chromosomes carry more than one gene. Understanding Inheritance: Morgan s Work- Sex-Linked Traits in Fruit Flies While observing fruit flies, Morgan discovered a male that had white eyes instead of the typical red ones. He mated the white eye male with a homozygous red eyed female. The entire F1 generation whether male or female had the dominant red eyes, just as Mendel would have predicted. However, the F2 generation held a surprise. Just as Mendel would have predicted three quarters of the offspring had red eyes and one quarter had white eyes, but there was a catch, only males had white eyes. One half of the males had red eyes and the other half white eyes. If sex and eye color were being inherited independently of one another as Mendel would have predicted then one half the white-eyed individuals should be female. Obviously, eye color was somehow linked to sex. In order to explain his results Morgan hypothesized that while there is a gene for eye color located on the X chromosome, there is no corresponding gene on the Y chromosome. Thus, females which receive two X chromosomes, also receive two genes for eye color, while male which receive one X and one Y chromosome, receive only one gene for eye color. Looking at Punnett squares based of Morgan s hypothesis helps explain the results of his experiment. Morgan started with a white eyed male that produced only two types of sperm: sperm containing the X-chromosome and along with it the alleles for white eyes and Y-chromosome sperm that contain no alleles for eye color. The female fruit fly being homozygous for red-eyes produced only eggs carrying the alleles for red eyes. In the F1 generation the female offspring are created when the Xchromosome /white eyes sperm unite with the X-chromosome/red eye eggs. Because all the females inherit one white eye gene and one red eye gene, they are all heterozygous for eye color. The F1 generation males are created when Y-chromosome/no eye color gene sperm unite with X chromosome/red eye eggs. Thus all males have red eyes as a result of the single eye color gene they receive via the X-chromosome of their female parent. Crossing the males and females of the F1 generation produces an F2 generation in which one half of the females are homozygous for red eyes, inheriting one red-eye gene from their mother's Xchromosome and the other from their father's X chromosome and one half are heterozygous, inheriting the white-eye gene from their mother and the red eye gene from their father. One half of the males are redeyed, inheriting a red-eye gene from the mother's X chromosome and no eye color gene from their father's Y chromosome. The other one-half are white-eyed inheriting the white-eye gene from their mother's Xchromosome and no eye color gene from their father's Y-chromosome. The outcome of Morgan s Punnett square is consistent with the actual breeding outcomes. The Morgan hypothesis isn t just about predicting eye color in fruit flies; it is also relevant in explaining the inheritance of a number of genetic diseases in humans. Genes that are located on one sex chromosome but not the other are referred to as sex-linked. Nearly all sex-linked genes are found on the
X-chromosome and are often referred to as X-linked genes. Humans carry genes related to color blindness, hemophilia, and muscular dystrophy exclusively on the X-chromosome. The fact that males have only one copy of the genes for these traits puts them at much greater risk for these illnesses than females as they have no chance of inheriting a second "good" gene that will mask the expression of a "bad" gene they may inherit. Understanding Inheritance: Beyond Mendel- Linkages in Non-Sex Chromosomes Sex chromosomes are a special case, all the rest of an individual organisms chromosomes called autosomes match up very well with the chromosome with which they are paired. Since Sutton and Morgan it has been well established that each chromosome has a large number of genes on it, some with identical alleles to the chromosome with which it is paired, in other cases different alleles. As we know Mendel hypothesized that genes sort independently, but how can genes on the same chromosome sort independently if it is chromosomes, rather than genes, that assort independently during meiosis? The answer is that Mendel was partially correct, genes that are on different chromosomes do always assort independently and the genes for most the traits Mendel studied were on different chromosomes. But, it would appear that genes on the same chromosome would always be linked or inherited together. There is, however, a phenomenon, crossing-over, that you may remember from your study of meiosis that prevents all the genes on a chromosome from always being linked together. Understanding Inheritance: Beyond Mendel- Linkages in Non-Sex Chromosomes- Crossing-Over During prophase I of meiosis homologous chromosomes intertwine. The points at which they cross over one another form chiasmata. At chiasmata, chromosomes may exchange sections with one another, in a process called crossing over. If one gene is located on one side of a chiasmata, and another on the other side, after crossing-over the two genes are no longer linked. If two genes are on the same side of the chiasmata after cross-over they remain linked. Obviously the closer two genes are to one another on a chromosome the more likely they are to be linked or in other words to be inherited together. There is almost always at least one or two cross-overs per pair of homologous chromosomes during meiosis. If genes are located far apart on a chromosome they are likely to be frequently separated during crossing over and act almost as if the were independent of one another, just as genes on separate chromosomes do. However, if two genes are very close together on a chromosome, say right next to each other on the end of a chromosome, they will almost never be separated from one another during the process of cross over and thus almost always be inherited together. In light of this information Mendel's Law of Independent Assortment should be modified to say; genes located on different chromosomes always assort independently during meiosis. As a result of crossing over, genes on the same chromosome may or may not tend to sort independently, depending on how far apart the two genes are on the chromosome, the further apart they are the more likely they are to sort independently. There are also a number of other variations regarding inheritance that Mendel's work didn't deal with- Incomplete Dominance, Multiple Alleles and Co-dominance, Polygenic Inheritance, and Environmental Effects on Gene Expression. Understanding Inheritance: Beyond Mendel- Incomplete Dominance
Mendel ran into a rather straight-forward situation with the pea plants he studied, the genes for all the traits he studied were either/or, completely dominant or recessive. Probably a good thing as it made the argument against blending stronger. But, interestingly there are occasions in nature when something very much like blending appears to occur. For example, when homozygous red-flowered snapdragons are crossed with homozygous white-flowered snapdragons, the flowers of the F1 generation aren't all red or all white, because the alleles for neither flower color is dominant, the flowers of the now heterozygous plants are pink. This pattern of inheritance is called incomplete dominance. The reason we say that blending only appears to have occurred, is that in the F2 generation both red flowers and white flowers return along side the pink flowers of the remaining heterozygous plants. If real blending had occurred there would be no red or white as they would never form again, as the genes for red and white flowers would have blended to form pink flower genes. Understanding Inheritance: Beyond Mendel- Multiple Alleles, Co-Dominance and Blood Type Mendel was not only lucky because the traits he had to deal with were all clearly dominant or recessive, but also because none of the traits he experimented with ever appeared in more than two different forms. But more than two alleles are common for a large number of genes in nearly all species, however any individual can only have two. A common example of multiple alleles in humans occurs in blood type. Three different alleles are responsible for the formation of four different types of human blood: A, B, AB, and O. The three alleles are usually designated I-a, I-b, and i. These three alleles are responsible for directing the synthesis of the glycoprotein "identification markers" that protrude from the surface of red blood cells. I-a and I-b control the synthesis of the glycoproteins A and B respectively. The i alleles produce no glycoproteins at all. These three alleles produce one of six different genotypes in a human being: I-a, I-a; I-b, I-b; I-a, I-b; I-a, i; I-b, i; or i, i. Both the I-a and I-b alleles are dominant to the i allele and thus form A glycoprotein and B glycoprotein respectively if in a heterozygous pairing with an i alleles. However, if I-a and I-b alleles are paired, both A glycoprotein and B glycoprotein protrude from the surface of an individuals blood cells. Since both A and B glycoproteins are present in approximately equal amounts and maintain their own unique characteristics, they are said to be codominant to one another while the snapdragon example is considered incomplete dominance because the traits of red and white are no longer discernable as individual colors. Being I-a, I-b and thus having AB blood makes one a universal blood recipient. For obvious reasons the body of an individual with AB blood wouldn't make A glycoprotein or B glycoprotein antibodies, that would cause type A, B, or AB to clump up and form clots. Not having those antibodies allows AB individuals to receive blood from A, B, AB, and type O individuals, as type O blood has no glycoproteins to be attacked by antibodies. Since they have no glycoproteins to be attacked by antibodies O individuals are known as universal donors, but since they have both A and B antibodies, type O individuals can only receive blood from other type O individuals. Understanding Inheritance: Beyond Mendel- Polygenic Inheritance Mendel's work also didn't look at traits that are controlled by more than one gene, of which there are many. Two examples in humans are eye color and the darkness of ones skin. Eye color is controlled
by at least two and skin color by at least three separate incompletely dominant genes. The greater the number of genes that control a trait the subtler the variations in a trait. If the alleles are incompletely dominant, as they are in the case of eye and skin color, that increases the number of variations further. The genes that control eye and skin color do so by controlling the amount of the yellowish-brown pigment melanin produced on the front of the iris or in the skin. An iris with no melanin in it would naturally be blue, reflecting blue light just as deep bodies of water do. The eyes of babies whose eyes will eventually become darker often start out blue because they haven't yet started producing melanin. If we presume that eye color is controlled by two genes, with two incompletely dominant alleles, then individuals can potentially have five different eye colors, depending on the number of melanin producing alleles the individual inherits from their parents. An individual that inherited no melanin producing alleles would have light blue eyes, an individual with one melanin alleles deep blue or green eyes, an individual with two melanin alleles deep green or light brown, an individual with three melanin alleles medium brown, and an individual with four melanin alleles would have dark brown eyes. Variations in skin color occur in a like manner except that three or more genes create seven or more shades of skin color. But environmental factors like exposure to the sun also affect skin color. Understanding Inheritance: Beyond Mendel- Environmental Effects on Gene Expression Not only skin color but the expression of all traits weight, height, personality, intelligence, athletic and musical abilities, etc. are influenced to a greater or lesser degree by environmental factors. However, we humans are unique in that we can control our environment and thus the expression of our genes to a much larger extent than other living organisms. Thus, while the genes we inherit from our parents certainly influence who and what we are, they do not define our destiny. Though scientists certainly don t know exactly how much complex traits such as intelligence and musical ability are influenced by our genes and how much by our environment they certainly know that their development and expression can only be maximized through hard work, determination, and exposure to an environment conducive to their development. Review In this program we have learned individuals differ in a trait as a result of inheriting different forms or alleles of a gene. In some cases one alleles is dominant over other alleles, in other cases different alleles are incompletely dominant or co-dominate, while in the case of sex-linked traits males (because of their small Y chromosomes) inherit only the single alleles carried on the X chromosome they receive from their mother. Some traits such as eye or skin color are influenced by two or more pairs of genes and environmental factors play a role in the expression of all traits. The independent assortment of chromosomes and a phenomenon called crossing over, which occur during the creation of sex gametes contribute to the diversity of genotypes and phenotypes in a species population. We now live in a day and age in which geneticists play outside natures rules of inheritance and sexual reproduction, defining the genotypes and phenotypes of living organisms by transferring genes directly between individuals, often individuals from different kingdoms life. Sheep whose mammary
glands secrete human enzymes and hormones for pharmaceutical companies, potatoes whose genome includes fish genes that make them resistant to freezing, and soybeans that synthesize plastics, are just a few examples of the once unlikely organisms being engineered by geneticists. In order to have any hope of understanding the methods being used to engineer life in the new millennium, we must first understand the discoveries made by a monk in a monastery pea garden nearly a hundred and fifty years ago which led us here.