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Chapter 7: Mendelian Inheritance

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1 Chapter 7: Mendelian Inheritance
Family resemblance: how traits are inherited Lectures by Mark Manteuffel, St. Louis Community College

2 Section 7-1 Opener Identical twins inherit the same genetic material. (These twins were not told what to do with their hands when the photographer took the photo. Note the similarities in their hand positions.)

3 7.1 Family resemblance: your mother and father contribute equally to your genetic makeup.
Can a gene be cruel? Of course not. But if one could, consider this candidate: in humans, there is a gene for an enzyme called FMO3 (flavin-containing monooxygenase-3), which breaks down a chemical in the body that smells like rotting fish. Some unfortunate individuals inherit a defective FMO3 gene and can’t break the noxious chemical down. Instead, their urine, sweat, and breath excrete it, causing them to smell like rotting fish. Worst of all, because the odor comes from within their bodies, they cannot wash it away no matter how hard they try. Called fish odor syndrome, this disorder often causes its carriers to suffer ridicule, social isolation, and depression. For those born with this malady, beyond their own suffering, there looms a scary question: “Will I pass this condition on to my children?” They might also wonder how they came to have the disorder, particularly if neither of their parents suffered from it.

4 Selective Breeding: Observing Heredity
Observing heredity is easy. Elucidating how it works is not. Fortunately, that has not been a huge problem for plant and animal breeders. For thousands of years before the mechanisms of heredity were discovered and understood, plant and animal breeders understood that there is a connection from parents to offspring across generations. This recognition was enough to enable them to systematically create strains of crops, livestock, and even pets with desirable traits (Figure 7-4 Have a cow!). Once breeders recognized the existence of heredity, they began selecting those individuals with the desired traits to breed with each other, in the hope that their offspring would have the desirable trait.

5 More than 9,000 single-gene traits have been identified in humans.
Traits that are determined by the instructions a person carries at one gene are called single-gene traits. More than 9,000 single-gene traits have been identified in humans. Mechanisms of heredity are like a puzzle; they become a bit easier to grasp when we focus on the smallest aspects of family resemblances. Virtually everyone with a cleft chin, for instance, has a similarly dimpled parent. Everyone who has earlobes that are not attached directly to their neck has at least one parent with the same feature. Everyone who has a widow’s peak has a parent with the same hairline. Everyone who is farsighted has a similarly farsighted parent. The list goes on and on. In fact, there are more than 9,000 human traits that exhibit such straightforward patterns of inheritance. Traits that are determined by the instructions a person carries at one gene are called single-gene traits.

6 When Mendel turned to these questions, there were no obvious answers and the prevailing state was confusion. There were a couple of existing ideas, but they had obvious problems. One postulated that perhaps an entire, pre-made human—albeit a very, very tiny one—was contained in every sperm cell (Figure 7-6 The “pre-made human”). That had been a popular theory since the late 1600s. It didn’t take much imagination to see why it ran into difficulty, though, since children resemble both their mothers and their fathers, not just their fathers. Another popular idea was that offspring reflect a simple blending of their two parents’ traits. This theory couldn’t explain how brown-eyed parents could give birth to blue-eyed children, though. Or why one tall and one short parent sometimes produced a tall child, rather than always producing children with intermediate heights.

7 7.3 Mendel learned about heredity by conducting experiments.
What do parents “give” their offspring that confers similarity? In other words, how is it that a physical entity is inherited and how is it passed from parent to offspring? It wasn’t until the mid-1800s that any real headway had been made on these questions. At that time Gregor Mendel, a monk living in what is now the Czech Republic, began some studies that not only shed light on these questions but practically answered them completely. Mendel understood the essence of the genetics puzzle and set out to piece it all together. It’s easy to control fertilization in plants.

8 True-Breeding 3) A third critical feature of Mendel’s approach was that he began his studies by first repeatedly breeding together similar plants, until he had numerous distinct populations, each of which was unvarying for a particular trait. He described these plants as true-breeding for that trait because they always produced offspring with the same variant of the trait as the parents. True-breeding round-pea plants always produced round peas when they were crossed together. True-breeding purple-flowered pea plants always produced purple-flowered offspring, while true-breeding white-flowered pea plants always produced white-flowered offspring. It was a lot of prep work to establish these populations but once he had them, he was in a position to set up all of the different crosses that enabled him to piece together the genetics puzzle. Figure 7-8 part 1 White or purple? True-breeding = offspring always have the same trait as their parents. Traits were easily observed.

9 Homologous chromosomes have alternative alleles.

10 Homozygous for that gene…
Homologous chromosomes may bear either the same alleles or different ones at a given locus. Homozygous for that gene… When an organism has identical alleles for a gene. (Ex. PP or pp) Heterozygous for that gene… When an organism has different alleles for a gene. (Ex. Pp)

11 A dominant trait masks the effect of a recessive trait.
Dominant traits determine the phenotype. Here’s where Mendel’s meticulous and methodical experiments paid off. First, he started with true-breeding white-flowered plants. Then he got some true-breeding purple-flowered plants. He wondered: which color wins out when the white-flowered plant is crossed with the purple-flowered plant? The answer was definitive: purple wins. All of the offspring were purple, every time. For this reason, Mendel called the purple-flower color trait dominant, and he considered the white-flower color trait to be the recessive trait. In general, a dominant trait masks the effect of a recessive trait when the individual carries both the dominant and the recessive versions of the instructions for the trait. Figure 7-8 White or purple?

12 Three Ideas Mendel Used for Explaining This Pattern of Inheritance
Each parent puts into every sperm or egg it makes a single set of instructions for building the trait. Offspring thus find themselves with two copies of the instructions for any trait (called alleles). The actual trait produced by an individual depends on the two copies of the gene that they inherit from their parents. homozygous and heterozygous Mendel’s story for explaining this pattern of inheritance incorporated three ideas which then helped him to make predictions about crosses he hadn’t yet done: 1) Rather than passing on a trait itself, each parent puts into every sperm or egg it makes a single set of instructions for building the trait. That is the instruction set that today we call a gene.

13 7.4 The Principle of Segregation: you’ve got two copies of each gene but put only one copy in each sperm or egg. Mendel’s Test: a monohybrid cross… One odd and recurring result spurred Mendel to figure out the mechanism by which traits could be passed from parent to offspring. Just as brown-eyed parents can have blue-eyed children, sometimes traits that weren’t present in either parent pea plant would show up in their offspring. Crosses that entailed the fertilization of plants with purple flowers by pollen from other plants with purple flowers produced mostly purple-flowered offspring, for instance, but sometimes they produced plants with white flowers. How was that possible? Where did the whiteness come from?

14 7.5 Observing an individual’s phenotype is not sufficient for determining its genotype.
An organism’s genetic makeup – designated by alleles (alternative forms of genes on homologous chromosomes) Phenotype An organism’s physical traits – appearances, may also manifest in behaviors. Assign letters to represent alleles… Capital letters = dominant traits – determine phenotype Lower-case letters = recessive traits – may be hidden

15 Things are not always as they appear. Take skin coloration for example
Things are not always as they appear. Take skin coloration for example. Humans and many other mammals have a gene that contains the information for producing melanin, one of the chemicals responsible for giving our skin its coloring (Figure An albino deer stands out from its peers). Unfortunately, there is also a defective, non-functioning version of the melanin gene that is passed along through some families. An individual who inherits two copies of the defective version of the gene cannot produce pigment and has a condition known as albinism, a disorder characterized by little or no pigment in the eyes, hair, and skin. It is impossible to tell whether a normally pigmented individual carries one of the defective alleles just by looking—one would need to do a genetic analysis to discover this information.

16 It is not always possible to determine an individual’s genotype from its phenotype.
We can trace the possible outcomes of a cross using a Punnett square. In Figure 7-11a we illustrate the cross between a true-breeding pigmented individual, PP, with an albino, pp. On the top of the square we list, individually, the two alleles that one of the parents produces, and on the left side of the square we list the two alleles that the other parent produces. We split an individual’s two alleles (remember segregation) up because, although the individual carries two alleles, there is only one of the alleles in each sperm or egg cell that they produce. In the four sections of the Punnett square, we enter the genotypes of the possible offspring resulting from our cross. In the cross illustrated in Figure 7-11a, every possible offspring would be heterozygous and would be normally pigmented because it receives a dominant allele from the pigmented parent and a recessive allele from the albino parent. In Figure 7-11b, we trace the cross between two heterozygous individuals. Note that each parent produces two kinds of gametes, one with the dominant allele and one with the recessive allele. This cross has four possible outcomes: one quarter of the time the offspring will be homozygous dominant (PP), one quarter of the time the offspring will be homozygous recessive (pp), and the remaining half of the time the offspring will be heterozygous (Pp). Phenotypically, three-quarters of the offspring will be normally pigmented (PP or Pp) and one quarter will be albino (pp).

17 7.7 A test-cross enables us to figure out which alleles an individual carries.
Suppose you are in charge of the alligators at a zoo (Figure 7-14 part 1 A test-cross can reveal an unknown genotype). Some of your individuals come from a population in which white, albino alligators have occasionally occurred, although none of your alligators are white. Because white alligators—those having two recessive pigmentation alleles, mm (the letter “m” refers to the color pigment melanin)—are popular with zoo visitors, you would like to produce some via a mating program. The problem is that you cannot be certain of the genotype of your alligators. You would like to produce white alligators via a mating program. The problem is that you cannot be certain of the genotype of your alligators. 17

18 In either case their phenotype is normal coloration.
They might be homozygous dominant, MM, or they might be heterozygous, Mm. In either case their phenotype is normal coloration. How can you figure out which of these two possibilities is the actual genotype? This is a challenge to animal breeders, but not an insurmountable one. Genes may be invisible, but their identity can be discovered by a simple tool called the test-cross. Figure 7-14 part 1 A test-cross can reveal an unknown genotype. 18

19 The phenotypes of the offspring reveal the unknown genotype.
In a test-cross, an individual with a dominant phenotype and an unknown genotype is mated with a homozygous recessive individual. The phenotypes of the offspring reveal the unknown genotype. In the test-cross, you take an individual exhibiting a dominant trait but whose genotype is unknown. You cross (i.e., mate) that individual with an individual that is homozygous recessive and look at the phenotypes of the offspring. There are two possible outcomes, and they reveal the genotype of your unknown-genotype alligator (Figure 7-14 part 2 A test-cross can reveal an unknown genotype). If your alligator is homozygous dominant (MM), it will contribute a dominant allele, M, to every offspring. Even though the albino alligators will contribute the recessive allele, m, to all its offspring, the offspring will all be heterozygous, Mm, and none of them will ever be white. If, on the other hand, your unknown-genotype alligator is heterozygous, Mm, half of the time it will contribute a recessive allele, m, to the offspring. In every one of those cases, the offspring will be white and homozygous recessive. So the cleverness of the test-cross is that when you cross your unknown genotype organism with an individual showing the recessive trait, if it sometimes—half the time, on average—produces offspring with the recessive trait its genotype must be heterozygous. If it never produces offspring with the recessive trait it must be homozygous for the dominant allele. 19

20 Pedigree: a type of family tree
7.8 Using pedigrees to decipher and predict the inheritance patterns of genes. Pedigree: a type of family tree People want to know things about the future, such as: what is the likelihood that I will have a child with a particular genetic disease, say hemophilia? Or, what is my own risk of developing a genetic disease such as Huntington’s disease later in my life? Geneticists who study these and other diseases want to know how they are passed along. Are they recessive or dominant? Are they carried on the sex chromosomes or one of the other chromosomes? A pedigree is a type of family tree that can help to answer these questions. 20

21 Analyzing Which Individuals Manifest the Trait and Which Do Not
In a pedigree, information is gathered from as many related individuals as possible across multiple generations (Figure Family tree). Starting from the bottom, each row in the chart represents a generation, listing all of the children in their order of birth and whether or not they express a particular trait. Working up the pedigree, their parents are indicated and above them, their parents, for as far back as the data are available. Squares represent males and circles represent females and these shapes are shaded to indicate that an individual exhibits the trait of interest. Sometimes the genotype (as much of it as is known) is also listed for each individual. By analyzing which individuals manifest the trait and which do not, it may be possible to deduce the pattern of inheritance for the trait—or, at least, rule out certain patterns. For example, if an individual exhibits a trait that neither of their parents exhibits, the trait is recessive. For dominant traits, all affected individuals must have at least one parent who exhibits the trait. In contrast, an individual can exhibit a recessive trait even if both parents are unaffected. In this case, the individual’s parents would have to be heterozygous for that trait, each carrying one dominant gene and one recessive gene. Similarly, it is sometimes possible to determine whether a trait is carried on the sex chromosomes or one of the non-sex chromosomes (i.e, the autosomes). Traits that are controlled by genes on the sex chromosomes are called sex-linked traits. Recessive sex-linked traits, for example, appear more frequently in males than females while dominant sex-linked traits appear more frequently in females. These patterns may become obvious only upon inspection of a large pedigree. 21

22 Section 7-2 Opener Because of the role of chance in genetics, we cannot always predict the exact outcome of a genetic cross. Sometimes genetics is a bit like gambling. Even with perfect information, it can still be impossible to know the outcome with certainty. It’s like flipping a coin: you can know every last detail about the coin, but you still can’t know whether the coin will land on heads or tails when flipped. The best you can do is define the probability of each possible outcome. 22

23 Probability has a central role in genetics for two reasons:
7.6 Chance is important in genetics. Probability has a central role in genetics for two reasons: The first is a consequence of segregation. In segregation, each gamete receives only one of the two copies of each gene. It is impossible to know which allele goes into which gamete. The second reason is that fertilization, too, is a chance event. All of an individual’s sperm or eggs are different. Any of these gametes may be the gamete involved in fertilization. Probability has a central role in genetics as well, for two reasons. The first is a consequence of segregation, the process Mendel described, in which each gamete that an individual produces receives only one of the two copies of each gene the individual carries in most of the other cells. As a result, it is equally likely that the haploid gamete—the sperm or egg—will include one or the other of the two alleles that the individual carries. It is impossible to know which allele it will be. The second reason probability plays a central role in genetics is that fertilization, too, is a chance event. All of the sperm and eggs produced by an individual are different from one another and any one of those gametes may be the gamete involved in fertilization. Thus, knowing everything about the alleles a parent carries is not enough to be able to determine with certainty which alleles their offspring will carry. 23

24 Punnett Squares are very useful in predicting probability.
In the albinism cross, two events also must occur to produce a homozygous recessive offspring. First, the mother’s gamete must carry the recessive allele (“a”, the only type of allele she carries as an albino) and, second, the father’s gamete must carry the recessive allele (“a”). The overall event of a homozygous recessive offspring is 1 * 0.5 or 0.5. This is a general rule when determining the likelihood of complex events occurring: if you know the probability of each component that must occur, you simply multiply all of them together to get the overall probability of that complex event occurring (Figure Using probability to determine the chance of inheriting albinism). 24

25 Section 7-3 Opener Phenotypic diversity has multiple sources and is all around us. (The photo shows schoolchildren from London.) As Mendel saw it, the world of genetics was straightforward and simple. We should be so lucky. Unfortunately, the world in which each trait is coded for by a single gene with two alleles—one completely dominant and one recessive—with no environmental effects at all doesn’t quite capture the complexity of the world beyond Mendel’s pea plants.

26 7.9 Incomplete dominance and codominance: the effects of both alleles in a genotype can show up in the phenotype. Incomplete dominance — a heterozygote displays a characteristic somewhere between the characteristics (a blended version of the trait) of the two homozygotes. Codominance — a heterozygote displays characteristics of both homozygotes equally. In the next sections, we’ll build up a more complex model of how genes influence the building of bodies. We’ll begin with the observation that sometimes the phenotype of heterozygous individuals differs from either of the homozygotes, and instead reflects the influence of both alleles rather than a clearly dominant allele. 26

27 Incomplete dominance, in which the heterozygote appears to be intermediate between the two homozygotes. Incomplete dominance is when the heterozygote appears to be intermediate between the two homozygotes. Flower color of snapdragons follows this pattern (Figure Pink snapdragons demonstrate incomplete dominance). True-breeding lines of red flowers are crossed with true-breeding lines that produce only white flowers. We would expect all red or all white flowers if one allele were dominant over the other. Instead, such crosses always produce plants with pink flowers. Interestingly, when we cross two plants with pink flowers, we get 1/4 red-flowered plants, 1/2 pink-flowered plants, and 1/4 white-flowered plants. The white flowers have the genotype CWCW and produce no pigment. The red flowers have the genotype CRCR and produce a great deal of pigment. The letter C refers to the fact that the gene codes for color and the superscript W or R refers to an allele producing no-pigment or red-pigment. The pink flowers receive only one of the pigment producing CR alleles and one of the no-pigment CW alleles and so produce an intermediate amount of pigment. Ultimately, the intensity of pigmentation just depends on the amount of the pigmentation chemical made by the color gene. 27

28 Hypercholesterolemia
Is a human trait that is incompletely dominant LDL (carries cholesterol) LDL receptor (mops up LDL) Cell Normal HH Mild disease Hh Severe disease hh Heterozygous Figure 9.19

29 Both phenotypes are expressed equally in the heterozygote.
Codominance, in which the heterozygote displays characteristics of both homozygotes. Codominance is when the heterozygote displays characteristics of both homozygotes. In this situation, the alleles can be thought of as codominant because neither masks the effect of the other. An example of codominance occurs with sickle-cell disease (Figure With codominance, a heterozygous individual shows features of both alleles), a condition in which individuals produce defective red blood cells that change their shape, becoming sickled upon losing the oxygen they carry. Sickle-cell disease involves a mutated version of the sickle-cell allele for hemoglobin, HbS, the oxygen-carrying molecule within red blood cells. Individuals with two copies of the normal hemoglobin allele, HbA HbA, are not affected by the disease. Individuals that are heterozygous, HbS HbA, appear to be only mildly affected by the disease. Individuals that are homozygous for the sickle-cell allele, HbS HbS, are severely affected by the disease. In other words, the heterozygotes exhibit an intermediate condition between the two homozygotes. Both phenotypes are expressed equally in the heterozygote. 29

30 Some genes may have more than two alleles.
7.10 What’s your blood type? Some genes may have more than two alleles. Multiple allelism, in which a single gene has more than two alleles. Each individual still carries only two alleles. What is your blood type? It can be O, A, B, or AB. Each of these different blood types indicates something about the physical characteristics of your red blood cells and has implications for blood transfusions—both giving and receiving blood. 30

31 Inheritance of the ABO Blood Groups
A, B, and O alleles The A and B alleles are both completely dominant to O. The A and B alleles are codominant to each other. Individuals can be one of four different blood types: A, B, AB, and O. Inheritance of the ABO blood groups is the simplest example of multiple allelism because there are only three alleles. We can call these A, B, and O. The A and B alleles are both completely dominant to O, so an individual is considered blood type A whether she has the genotype AA or AO. Similarly, an individual with the genotype BB or BO is considered to be blood type B. If you carry two copies of the O allele, you are blood type O. The A and B alleles are codominant with each other, so that the genotype AB gives rise to blood type AB. 31

32 Consequently, with these three alleles in the population, individuals can be one of four different blood types: A, B, AB, and O. Figure Multiple allelism. 32

33 Why are people with type O blood considered “universal donors”
Why are people with type O blood considered “universal donors”? Why are those with type AB considered “universal acceptors”? Individuals with only B antigens on their red blood cells produce antibodies that attack A antigens. Blood type O individuals, who have neither A nor B antigens on their red blood cells, produce both antibodies that attack both A and B antigens. Individuals with blood type AB don’t produce either type of antibody (or else they would have antibodies that attack their own blood cells). From this information, we can deduce which blood types can be used in transfusions. This is shown in Figure 7-21 (Mapping blood compatibility). 33

34 7.11 Multi-gene Traits How are continuously varying traits such as height influenced by genes? When babies are born, the parents are often curious about how tall their child will grow to be. Old wives’ tales suggest a couple of ways for predicting height: if the baby is a boy, they say to add five inches to the mothers’ height and average that with the father’s height. Or if it is a girl, subtract five inches from the father’s height and average that with the mother’s height. Alternatively, the lore says to just take the child’s height at two years and double it. 34

35 Polygenic inheritance is the additive effect of two or more genes on a single phenotype.
This hypothetical example shows a range of possible skin colors in the F2 Generation based on just 3 alleles. aabbcc (very light) AABBCC (very dark) AaBbCc Eggs Sperm Figure 9.22

36 7. 12 Pleiotropy: How can one gene influence multiple traits
7.12 Pleiotropy: How can one gene influence multiple traits? What is the benefit of “almost” having sickle cell disease? Just as multiple genes can influence one trait, some individual genes can influence multiple unrelated traits. In fact, this may be true of nearly all genes. For example, earlier we described the sickle cell allele for hemoglobin production. This allele causes sickle cell disease when in the homozygous state, but in the heterozygous state it has the effect of conferring resistance to the parasite that causes malaria. The resistance is due to the fact that the malarial parasite—which lives in red blood cells—cannot survive well in the cells that carry the defective version of the hemoglobin gene. Because the person who is heterozygous for sickle cell anemia has a significant number of such red blood cells, their bloodstream is just not a hospitable environment for the malarial parasite. Figure From one gene, multiple traits. 36

37 7.13 Why are more men than women color-blind?
Sex-linked traits differ in their patterns of expression in males and females. If a man is color-blind, did he inherit this condition from his mother, his father, or both parents? Traits coded for by the sex chromosomes have different patterns of expression in males and females. One example of this phenomenon is red-green colorblindness. On the X chromosome in humans, there is a gene that carries the instructions for producing light-sensitive proteins within the eye. These pigments make it possible to distinguish between the colors red and green. There is a rare allele for this gene, however, that produces a non-functioning version of the protein. Having some nonfunctioning protein is not a problem as long as the person carries another, normal version of the gene and produces some of the functioning protein. But this poses a problem for men because they have only one X chromosome. In contrast, females inherit two X chromosomes. 37

38 Hemophilia is another sex-linked (X-linked) trait
Hemophilia is another sex-linked (X-linked) trait. Women are often carriers. Here’s the problem: men only get one chance to inherit the normal version of the gene. It must be on the X chromosome they inherit from their mother. Women get two chances. Although a woman may inherit a defective allele from one parent, she can still inherit the normal allele from the other parent. As long as she inherits the normal gene from one parent, she will have normal color vision. As we would predict, then, the frequency of red-green colorblindness is significantly greater in males than females (Figure 7-24 Sex-linked traits such as color-blindness are not expressed equally among males and females). Approximately 710% of men exhibit red-green colorblindness, while less than 1% of women are red-green colorblind. While males exhibit sex-linked recessive traits more frequently than do women, the situation is reversed for sex-linked dominant traits. For those cases, because females have two chances to inherit the allele that causes the trait, they are more likely to show it than are males, who have only one chance to inherit the allele. 38

39 7.14 Environmental effects: identical twins are not identical.
Genotypes are not like blueprints that specify phenotypes. Many phenotypes are a product of the genotype in combination with the environment.

40 Could you create a temporarily spotted Siamese cat with an ice pack?

41 The Role of Environment
Many human characteristics result from a combination of heredity and environment. Eye color appears to be entirely genetic. The height of an individual is partially genetic, but can also be influenced by health and diet during childhood and adolescence. Often characteristics such as susceptibility to heart disease, cancer, alcoholism, and schizophrenia are influenced by both genes and environment.

42 The inheritance pattern of one trait doesn’t usually influence the inheritance of any other trait.
Section 7-4 Opener Different genes influence red hair and freckles, so why are they often inherited together?

43 7.15 Most traits are passed on as independent features: Mendel’s law of independent assortment.
Sometimes you can be right about something for the wrong reason. This happened to Gregor Mendel. He didn’t know that genes were carried on chromosomes, so he believed that they were all just free-floating entities within cells. Given this perspective, it made sense to him that the inheritance pattern of one trait wouldn’t influence the inheritance of any other trait. He believed all genes behaved independently. 43

44 Mendel tested this using a dihybrid cross.

45 Genes on the same chromosome are sometimes inherited together.
7.16 Red Hair and Freckles Genes on the same chromosome are sometimes inherited together. 45

46 The alleles for two genes may be inherited and expressed together when they are close together on the same chromosome. Why are linked genes inherited together? To answer this, we must revisit the behavior of chromosomes during the production of gametes, discussed in the previous chapter. Imagine that you have a child. Let’s focus on the gamete of yours that took part in the fertilization to produce that child. When you made that sperm or egg by meiosis, only one of your two copies of each chromosome ended up in the new gamete. It may have been the one from your mother or it may have been the one from your father. In either case, all of the alleles that were on the chromosome from that one parent of yours will be passed on as a group to the child that results from the fertilization involving that gamete. This process continues generation after generation. The linked alleles never get split up unless, during meiosis, recombination occurs between them, moving one or more to the other chromosome in the pair and become linked with the alleles on that chromosome. When alleles are linked closely on the same chromosome, Mendel’s second law doesn’t hold true. It is very surprising—and was fortunate for Mendel—that of the seven pea traits he studied, none of them were on the same chromosome. For this reason they all behaved as if they weren’t linked, and in his crosses of different pea plants Mendel never noticed any linked genes. Figure Gene linkage. 46

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