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Lab 11

HEREDITY AND MENDELIAN GENETICS

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INTRODUCTION

For thousands of years farmers and herders have been selectively breeding their plants and animals to produce more useful hybrids. It was somewhat of a hit or miss process since the actual mechanisms governing inheritance were unknown. Knowledge of these genetic mechanisms finally came as a result of careful laboratory breeding experiments carried out over the last century and a half.

By the 1890's, the invention of better microscopes allowed biologists to discover the basic facts of cell division and sexual reproduction. The focus of genetics research then shifted to understanding what really happens in the transmission of hereditary traits from parents to children. A number of hypotheses were suggested to explain heredity, but Gregor Mendel, a little known Central European monk, was the only one who got it more or less right. His ideas had been published in 1866 but largely went unrecognized until 1900, which was long after his death. His early adult life was spent in relative obscurity doing basic genetics research and teaching high school mathematics, physics, and Greek in Brno (now in the Czech Republic). In his later years, he became the abbot of his monastery and put aside his scientific work.

Common edible peas

While Mendel's research was with plants, the basic underlying principles of heredity that he discovered also apply to people and other animals because the mechanisms of heredity are essentially the same for all complex life forms.

Through the selective cross-breeding of common pea plants (Pisum sativum) over many generations, Mendel discovered that certain traits show up in offspring without any blending of parent characteristics. For instance, the pea flowers are either purple or white--intermediate colors do not appear in the offspring of cross-pollinated pea plants. Mendel observed seven traits that are easily recognized and apparently only occur in one of two forms:

1. flower color is purple or white 5. seed color is yellow or green

2. flower position is axial or terminal

6. pod shape is inflated or constricted

3. stem length is long or short 7. pod color is yellow or green

4. seed shape is round or wrinkled

This observation that these traits do not show up in offspring plants with intermediate forms was critically important because the leading theory in biology at the time was that inherited traits blend from generation to generation. Most of the leading scientists in the 19th century accepted this "blending theory." Charles Darwin proposed another equally wrong theory known as "pangenesis". This held that hereditary "particles" in our bodies are affected by the things we do during our lifetime. These modified particles were thought to migrate via blood to the reproductive cells and subsequently could be inherited by the next generation. This was essentially a variation of Lamarck's incorrect idea of the "inheritance of acquired characteristics."

Mendel picked common garden pea plants for the focus of his research because they can be grown easily in large numbers and their reproduction can be manipulated. Pea plants have both male and female reproductive organs. As a result, they can either self-pollinate themselves or cross-pollinate with another plant. In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations. This was the basis for his conclusions about the nature of genetic inheritance.

Reproductive structures of flowers

In cross-pollinating plants that either produce yellow or green peas exclusively, Mendel found that the first offspring generation (f1) always has yellow peas. However, the following generation (f2) consistently has a 3:1 ratio of yellow to green.

This 3:1 ratio occurs in later generations as well. Mendel realized that this was the key to understanding the basic mechanisms of inheritance.

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He came to three important conclusions from these experimental results:

1. that the inheritance of each trait is determined by "units" or "factors" that are passed on to descendants unchanged (these units are now called genes).

2. that an individual inherits one such unit from each parent for each trait.

3. that a trait may not show up in an individual but can still be passed on to the next generation.

It is important to realize that, in this experiment, the starting parent plants were homozygous for pea color. That is to say, they each had two identical forms (or alleles) of the gene for this trait--2 yellows or 2 greens. The plants in the f1 generation were all heterozygous. In other words, they each had inherited two different alleles--one from each parent plant. It becomes clearer when we look at the actual genetic makeup, or genotype, of the pea plants instead of only the phenotype, or observable physical characteristics.

Note that each of the f1 generation plants (shown above) inherited a Y allele from one parent and a G allele from the other. When the f1 plants breed, each has an equal chance of passing on either Y or G alleles to each offspring.

With all of the seven pea plant traits that Mendel examined, one form appeared dominant over the other. In other words, it masked the presence of the other allele. For example, when the genotype for pea color is YG (heterozygous), the phenotype is yellow. However, the dominant yellow allele does not alter the recessive green one in any way. Both alleles can be passed on to the next generation unchanged. Mendel's observations from these experiments can be summarized in two principles:

1. the law of segregation

2. the law of independent assortment

Segregation of alleles in the production of sex cells

According to the Law of segregation, for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring. Which allele in a parent's pair of alleles is inherited is a matter of chance. We now know that this segregation of alleles occurs during the process of sex cell formation (i.e., meiosis).

According to the Law of independent assortment, different pairs of alleles are passed to offspring independently of each other. The result is that new combinations of genes present in neither parent are possible. For example, a pea plant's inheritance of the ability to produce purple flowers instead of white ones does not make it more likely that it will also inherit the ability to produce yellow peas in contrast to green ones. Likewise, the principle of independent assortment explains why the human inheritance of a particular eye color does not increase or decrease the likelihood of having 6 fingers on each hand. Today, we know this is due to the fact that the genes for independently assorted traits are located on different chromosomes.

MONOHYBRID CROSSES

Garden peas have both male and female parts in the same flower and are able to self-fertilize. For his experiments, Mendel chose parental plants that were true-breeding, meaning that all self-fertilized offspring displayed the same form of a trait as their parent. For example, if a true breeding purple- flowered plant is allowed to self-fertilize, all of the offspring will have purple flowers.

When parents that are true-breeding for different forms of a trait are crossed—for example, purple flowers and white flowers—the offspring are called hybrids. When only one trait is being studied, the cross is called a monohybrid cross. We'll look first at monohybrid problems.

A Monohybrid Problems With Complete Dominance

1. Most organisms are diploid; that is, they contain homologous chromosomes with genes for the same traits. The location of a gene on a chromosome is its locus (plural: loci). Two genes at homologous loci are called a gene pair. Chromosomes have numerous genes, as illustrated in Figure 11-1. Genes may exist in different forms, called alleles. Let's consider one gene pair at the F locus. There are three possibilities for the allelic makeup at the F locus:

One chromosome Its homologue

Gene A Gene B Gene C Gene F

| |[pic] |

|FF | |

|ff | |

|Ff |& |

2. During fertilization, two gamete nuclei fuse, and the diploid condition is restored. Give the diploid genotype produced by fusion of the following gamete genotypes.

|Gamete genotype X Gamete genotype → |Diploid genotype |

|f |f | |

|F |f | |

|F |F | |

3. Now let's attach some meaning to genotypes. As you see from the previous problems, the genotype is an expression of the actual genetic makeup of the organism. The phenotype is the observable result of the genotype, that is, what the organism looks like because of its genotype. (Although phenotype is determined primarily by genotype, in many instances environmental factors can modify phenotype).

Human earlobes are either attached or free (Figure 11-2). This trait is determined by a single gene consisting of two alleles, F and f. An individual whose genotype if FF or Ff has free earlobes. This is the dominant condition. Note that the presence of one or two F alleles results in the dominant phenotype, free earlobes. The allele F is said to be dominant over its allelic partner, f. The recessive phenotype, attached earlobes, occurs only when the genotype is ff. In the case of complete dominance, the dominant allele completely masks the expression or affect of the recessive allele.

When both alleles in a nucleus are identical, the nucleus is homozygous. Those having both dominant alleles are homozygous dominant.

When both recessives are present in the same nucleus, the individual is said to be

homozygous recessive for that trait.

When both the dominant and recessive alleles are present within a single nucleus, the individual is heterozygous for that trait.

a. Suppose a man has the genotype FF. What is the genotype of his gamete (sperm) nuclei?

b. Suppose a woman has attached earlobes. What is her genotype?

c. What allele(s) does her gametes (ova) carry?

d. These two individuals produce a child. Show the genotype of the child by doing the cross:

Sperm genotype

X Ovum genotype

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Child’s genotype

i. What is the phenotype of the child? (That is, does this child have attached or free earlobes?)

4. In garden peas, purple flowers are dominant over white flowers. Let R represent the allele for purple flowers, r the allele for white flowers.

a. What is the phenotype (color) of the flowers with the following genotypes:

|Diploid genotype |Gamete genotype |

|RR | |

|rr | |

|Rr | |

NOTE: ALWAYS BE SURE TO DISTINGUISH CLEARLY BETWEEN UPPER AND LOWERCASE LETERS.

b. A white-flowered garden pea is crossed with a homozygous dominant purple-flowered plant.

i. Name the genotype(s) of the gametes of the white-flowered plant?

ii. Name the genotype(s) of the gametes of the purple-flowered plant?

iii. Name the genotype(s) of the plant produced by the cross?

iv. Name the phenotype(s) of the plant produced by the cross?

c. The Punnett square is a convenient method of performing the mechanics of a cross. The circles along the top and side of the Punnett Square represent the possible gamete nuclei. Insert the proper letters indicating the genotypes of the gamete nuclei for the above cross in the circles and then fill in the Punnett Square for all the possible genetic outcomes.

Gametes of white-flowered plant

Gametes of purple-flowered plant

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d. A heterozygous plant is crossed with a white-flowered plant. Fill in the Punnett Square and give the genotypes and phenotypes of all the possible genetic outcomes of the offspring.

Gametes of white-flowered plant

Gametes of purple-flowered plant

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Genotypes: Phenotypes:

(Draw a line from the genotype to its respective phenotype).

For the remaining problems, you may wish to draw your own Punnett squares on a separate sheet of paper.

5. In mice, black fur (B) is dominant over brown fur (b). Breeding a brown mouse and a homozygous black mouse produces all black offspring.

a. What is the genotype of the gametes produced by the browned-furred parent?

b. What is the genotype of the browned-furred parent?

c. What is the genotype of the black-furred parent?

d. What is the genotype of the black-furred offspring?

By convention, P stands for the parental generation. The offspring are called the first filial generation, abbreviated F1. If these F1 offspring are crossed, their offspring are called the second filial generation, designated F2. Note the following diagram.

P x P

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F1

and

F1 x F1

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F2

6. If two of the F1 mice are bred with one another, what will the phenotype of the F2 be, and in what proportion?

7. The presence of horns on Hereford cattle is controlled by a single gene. The hornless (H) condition is dominant over the horned (h) condition. A hornless cow was crossed repeatedly with the same horned bull. The following results were obtained in the F1 offspring:

8 hornless cattle and 7 horned cattle

a. What are the genotypes of the parents?

Cow: Bull:

8. In fruit flies, red eyes (R) are dominant over purple eyes (r). Two red-eyed fruit flies were

crossed, producing the following offspring:

76 red-eyed flies

24 purple-eyed flies

a. What is the approximate ratio of red-eyed to purple-eyed flies?

b. Based upon your experience with previous problems, what two genotypes give rise to this ratio?

c. What are the genotypes of the parents?

d. What is the genotypic ratio of the F1?

e. What is the phenotypic ratio of the F1?

Monohybrid Crosses With Incomplete Dominance

1. Petunia flower color is governed by two alleles, but neither allele is truly dominant over the other. Petunias with the genotype R1R1 are red-flowered, those that are heterozygous (R1R2) are pink, and those with the R2R2 genotype are white. This is an example of incomplete dominance. (Note that superscripts are used rather than upper- and lowercase letters to describe the alleles).

a. If a white-flowered plant is crossed with a red-flowered petunia, what is the genotypic ratio of the F1?

b. What is the phenotypic ratio of the F1?

c. If two of the F1 offspring were crossed, what phenotypes would appear in the F2?

d. What would be the genotypic ratio in the F2 generation?

Monohybrid Problems Illustrating Codominance

Another type of monohybrid inheritance involves the expression of both phenotypes in the heterozygous situation. This is called codominance.

One of the well-known examples of codominance occurs in the coat color of Shorthorn cattle. Those with reddish-gray coats are heterozygous (RR’) and result from a mating between a red RR Shorthorn and one that's white (R’R’). Roan cattle do not have roan-colored hairs, as would be expected with incomplete dominance, but rather appear roan because of both red and white hairs being on the same animal. Thus, the roan coloration is not a consequence of blending of pigments within each hair. Because R and R’ alleles are both fully expressed in the heterozygote, they are codominant.

2. If a roan Shorthorn cow is mated with a white bull, what will be the genotypic and phenotypic ratios in the F1 generation?

Genotypic ratio: Phenotypic ratio:

3. List the parental genotypes of crosses that could produce at least some:

White offspring: Roan offspring:

Monohybrid, Sex-linked Problems

In humans, as well as in other primates, sex is determined by special sex chromosomes. An individual containing two X chromosomes is a female, while an individual possessing an X and a Y chromosome is a male. (Rare exceptions of XY females and XX males have recently been discovered).

4. I am male/female (circle one).

a. What sex chromosomes do you have?

b. In terms of sex chromosomes, what type of gametes (ova) does a female produce?

c. What are the possible sex chromosomes in a male's sperm cells?

d. The gametes of which parent will determine the sex of the offspring?

The sex chromosomes bear alleles for traits, just like the other chromosomes in our bodies. Genes that occur on the sex chromosomes are said to be sex-linked. More specifically, the genes present on the X chromosome are said to be X-linked. There are many more genes present on the X chromosome (described as X-linked) than are found on the Y chromosome (describe as Y-linked).

The Y chromosome is smaller than its homologue, the X chromosome. Consequently, some of the loci present on the X chromosome are absent on the Y chromosome.

In humans, color vision is X-linked; the gene for color vision is located on the X chromosome but is absent from the Y chromosome. Figure 11-3 illustrates the appearance of duplicated sex chromosomes, each consisting of two sister chromatids.

N locus N locus absent

X chromosome Y chromosome

Figure 11-3 Diagrammatic representation of a sex-linked trait, N.

5. Normal color vision (XN) is dominant over color blindness (Xn). Suppose a color-blind man fathers children of a woman with the genotype XNXN.

a. What is the genotype of the father?

b. What proportion of daughters would be color-blind?

c. What proportion of sons would be color-blind?

6. One of the daughters from the above problem marries a color-blind man.

a. What proportion of their sons will be color-blind? (Another way to think of this is to ask what the chances are that their sons will be color-blind.)

b. Explain how a color-blind daughter might result from this couple.

An Observable Monohybrid Cross

Examine the monohybrid genetic corn demonstration. This illustrates a monohybrid cross between plants producing purple kernels and plants producing yellow kernels. Note that all the first-generation kernels (F1) are purple, while the second-generation ear (F2) has both purple kernels and yellow kernels. Count the purple kernels and then the yellow kernels. purple: yellow.

When reduced to the lowest common denominator, is this ratio closest to 1:1, 2:1. 3:1, or 4:1?

.

This is called the phenotypic ratio.

7. A corncob represents the products of multiple instances of sexual reproduction. Each kernel represents a single instance; fertilization of one egg by one sperm produced each kernel. Thus, each kernel represents a different cross.

a. What genotypes produce a purple phenotype?

b. Which allele is dominant?

c. What is the genotype of the yellow kernels on the F2 ear?

d. Suppose you were given an ear with purple kernels. How could you determine its genotype with a single cross?

DIHYBRID INHERITANCE

All the problems so far have involved the inheritance of only one trait; that is, they monohybrid problems. Now we’ll examine cases in which two traits are involved: dihybrid problems.

Examine the demonstration of dihybrid inheritance in corn. Notice that not only are the kernels two different colors (one trait), but they are also differently shaped (second trait). Kernels with starchy endosperm (the carbohydrate-storing tissue) are smooth, while those with sweet endosperm are shriveled. Notice that all four possible phenotypic combinations of color and shape are present in the F2 generation.

Dihybrid Problems

1. Let’s consider these two traits:

In humans, a pigment in the front part of the eye masks a blue layer at the back of the iris. The dominant allele P causes production of this pigment. Those who are homozygous recessive (pp) lack the pigment, and the back of the iris shows through, resulting in blue eyes. (Other genes determine the color of the pigment, but in this problem, we'll consider only the presence or absence of any pigment at the front of the eye).

Dimpled chins (D = allele for dimpling) are dominant over undimpled chins (d = allele for lack of dimple).

a. List all possible genotypes for an individual with pigmented iris and dimpled chin.

b. List the possible genotypes for an individual with pigmented iris but lacking a dimpled chin.

c. List the possible genotypes of a blue-eyed, dimpled-chinned individual.

d. List the possible genotypes of a blue-eyed individual lacking a dimpled chin.

2. An individual is heterozygous for both traits (eye pigmentation and chin form).

a. What is the genotype of such an individual?

b. What are the possible genotypes of that individual's gametes?

If determining the answer for the last question was difficult, recall that the principle of independent assortment states that genes on different (non-homologous) chromosomes are separated out independently of one another during meiosis. That is, the occurrence of an allele

for eye pigmentation in a gamete has no bearing on which allele for chin form will occur in that same gamete.

There is a useful convention for determining possible gamete genotypes produced during meiosis from a given parental genotype. Using the genotype PpDd as an example, here's the method:

2

PpDd

Follow the four arrows to determine the four-gamete genotypes.

c. Suppose two individuals heterozygous for both eye pigmentation and chin form have children. What are the possible genotypes of their children?

You can set up a Punnett square to do dihybrid problems just as you did with monohybrid problems. However, depending upon the parental genotypes, the square may have as many as sixteen boxes, rather than just four. Insert the possible genotypes of the gametes from one parent in the top circles and the gamete genotypes of the other parent in the circles to the left of the box.

Gametes of one parent

Gametes of other parent

d. Possible genotypes of children produced by two parents heterozygous for both eye pigmentation and chin from are? (Hint: there are 9)

e. What is the ratio of the genotypes?

f. What is the phenotypic ratio?

3. You would probably agree that it is unlikely that a family will have sixteen children. In fact, one of the most useful facets of problems such as these is that they allow you to predict what the chances are for a phenotype occurring. Genetics is really a matter of probability, the likelihood of the occurrence of any particular outcome.

To take a simple example, let’s consider flipping a coin by asking the question: What is the probability of flipping heads twice in a row? The chance of flipping heads the first time is ½. The same is true for the second flip. The chance (probability) that we’ll flip heads twice in a row is ½ x ½ = ¼. The probability that we could flip heads 3 times in a row is ½ x ½ x ½ = 1/8.

Now apply this example to the question of the probability of having a certain genotype. Look at your Punnett square in problem 2c. The probability of having a genotype is the sum of all occurrences of that genotype. For example, the genotype PPDD occurs in one of the sixteen boxes. The probability of having the genotype PPDD is 1/16.

a. What is the probability of an individual from problem 2c having the genotype:

ppDD PpDd PPDd

b. Returning to eye color and chin form (2c), state the probability that three children born to these parents will have the genotype ppdd.

c. What is the probability that three children born to these parents will have dimpled chins and pigmented eyes?

d. What is the genotype of the F1 generation when the father is homozygous for both pigmented eyes and dimpled chin, but the mother has blue eyes and no dimple?

e. What is the phenotype of the individual(s) you determined in letter d above?

4. A pigmented-eyed, dimpled-chinned man marries a blue-eyed woman without a dimpled chin. Their first-born child is blue-eyed and has a dimpled chin.

a. What are the possible genotypes of the father?

b. What is the genotype of the mother?

c. What alleles may have been carried by the father's sperm?

5. Suppose a dimple-chinned, blue-eyed man whose father lacked a dimple marries a woman who is homozygous recessive for both traits.

a. What would be the expected genotypic ratio of children produced in this marriage?

b. What would be the expected phenotypic ratio?

6. In his original work on the genetics of garden peas, Mendel found that yellow seed color (YY, Yy) was dominant over green seeds (yy) and that round seed shape (RR, Rr) was dominant over shrunken seeds (rr). Mendel crossed pure-breeding (homozygous) yellow, round-seeded plants with green, shrunken-seeded plants.

a. What would be the genotype and phenotype of the F1 produced from such a cross?

b. If the F1 plants were crossed, what would be the expected phenotypic ratio of the F2?

Multiple Alleles

The major blood groups in humans are determined by multiple alleles, that is, there are more than two possible alleles, any one of which can occupy a locus.

In this ABO blood group system, a single gene can exist in any of three allelic forms: IA, IB, or i. The alleles A and B code for production of antigen A and antigen B (two proteins) on the surface of red blood cells. Alleles A and B are codominant, while allele i is recessive.

Four blood groups (phenotypes) are possible from combinations of these alleles (Table 11-1).

|Table 11-1 The ABO Groups |

|Blood Type |Antigens Present |Antibody Present |Genotype |

|O |neither A nor B |A and B |ii |

|A |A |B |IAIA, or IAi |

|B |B |A |IBIB or IBi. |

|AB |AB |neither A nor B |IAIB |

7. Is it possible for a child with blood type O to be produced by two AB parents? Explain!

8. In case of disputed paternity, the child is type O, the mother type A. Could an individual of the following blood types be the father?

O A B AB

SOME READILY OBSERVABLE HUMAN TRAITS

In the preceding problems, we examined several human traits that are simple and that follow the Mendelian pattern of inheritance. Most of our traits are much more complex, involving many genes or interactions between genes. As an example, hair color is determined by at least four genes, each one coding for the production of melanin, a brown pigment. Because the effect of these genes is cumulative, hair color can range from blond (little melanin) to very dark brown (much melanin).

Clearly, human traits are most interesting to humans. A number of traits listed below exhibit Mendelian inheritance. For each, examine your phenotype and fill in Table 11-4. List your possible genotype(s) for each trait. When convenient, examine your parents' phenotypes and attempt to determine your actual genotype.

a b c d e f

Figure 11-5 Some readily observed human Mendelian traits.

1. Mid-digital hair (Figure 11-5a). Examine the joint of your fingers for the presence of hair, the dominant condition (MM, Mm). Complete absence of hair is due to the homozygous-recessive condition (mm). You may need a hand lens to determine your phenotype. Even the slightest amount of hair indicates the dominant condition.

2. Tongue rolling (Figure 5b). The ability to roll one's tongue is due to a dominant allele, T. The homozygous-recessive condition, t. results in inability to roll one's tongue.

3. Widow's peak (Figure 5c). Widow's peak describes a distinct downward point in the frontal hairline and is due to the dominant allele, W. The recessive allele, w, results in a continuous hairline. (Omit study of this trait if baldness is affecting the hairline).

4. Earlobe attachment (Figure 5d). Most individuals have free earlobes (FF, Ff). Homozygous recessives (ff) have earlobes attached directly to the head.

5. Hitchhiker's thumb (Figure 5e). Although considerable variation exists in this trait, we will consider those individuals who cannot extend their thumbs backward to approximately 45° to be carrying the dominant allele, H. Homozygous-recessive persons (hh) can bend their thumbs at least 45°, if not farther.

6. Relative finger length (Figure 5f). An interesting sex-influence (not sex-linked) trait relates to the relative lengths of the index and ring finger. In males, the allele for a short index finger (S) is dominant. In females, it is recessive. In rare cases, each hand may be different. If one or both index fingers are greater than or equal to the length of the ring finger, the recessive genotype is present in males, and the dominant present in females.

|Table 11-4 Summary of My Mendelian Traits |

| | | | |Mom's Possible | |Dad's Possible |My possible or |

|Trait |My Phenotype |My Possible |Mom's Phenotype |Genotype |Dad's Phenotype |Genotype |Probable Genotype |

| | |Genotype(s) | | | | | |

|Mid-digital hair | | | | | | | |

|Tongue rolling | | | | | | | |

|Widow's peak | | | | | | | |

|Earlobe attachment| | | | | | | |

|Hitchhiker's thumb| | | | | | | |

|Relative finger | | | | | | | |

|length | | | | | | | |

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Solve monohybrid and dihybrid cross problems;

Use a chi-square test to determine whether observed results are consistent with expected results;

Determine your phenotype and give your probable genotype for some common traits.

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OBJECTIVES

Allele F

One chromosome

Its homologue

Allele F

Allele f

One chromosome

Its homologue

Allele f

Allele F

One chromosome

Its homologue

Allele f

Figure 11-2 Free and attached earlobes in humans.

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