LAB 10:
This lab will help students understand the differences between simple and complex genetic inheritance and how the environment can influence gene expression. At the end of today's lab, students should:
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Know how to distinguish between heritable traits that are under the control of a single gene (simple) and those that are under the control of many genes
(complex)
Be able to define the term concordance
Be able to list several heritable human traits that exhibit simple inheritance
Be able to list several heritable human traits that exhibit complex inheritance
Understand the difference between monozygotic and dizygotic twinning
Appreciate the importance of twin studies in studying heritable human traits
Be able to describe how whole organism cloning is performed
 None
In humans, as well as all other organisms, some phenotypic traits are under the control of a single gene while others are under the control of two or more genes. Traits that are under the control of single genes are called simple traits because they follow simple (Mendelian) rules of inheritance and they can usually be easily and predictably followed through generations.
Traits that are under the control of many genes, on the other hand, are called complex traits because their inheritance is much more complex and difficult to predict.
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In addition, the expression of genes can be influenced by the environment. Therefore, some traits that are under the control of a single gene do not behave according to Mendel's predictions. For example, polydactyly (extra fingers and/or toes) is controlled by a single gene in mammals, where the polysactyly allele is dominant to the normal allele. However, an individual who inherits the polydactyly allele may or may not express it. The environmental influences on the expression of this allele are not well understood, but presumably occur at some point during embryogenesis.
When a trait is controlled by more than one gene and the expression of those genes is also influenced by the environment, the trait is said to be multifactorial. One example of this kind of trait is human skin color. Skin pigmentation is under the control of at least six genes.
However, exposure to sunlight can influence the expression of those genes, giving a normally light-skinned person a darker skin color. Another example is Alzheimer's Disease (AD), which is under the control of at least 5 genes. AD clearly runs through families, usually as a dominant illness. However, persons without a clear genetic disposition to the illness also get the disease, indicating that interplay between genes and the environment is important. As you will see in the discussion below, this idea is strengthened by data gathered from sets of identical and fraternal twins raised apart or together.
Twins can be either monozygotic (identical) or dizygotic (fraternal). Monozygotic (MZ) twins arise from a single fertilized egg, which then begins to divide and splits into two embryos sometime during the first weeks of gestation. MZ twins account for about 28% of all twin births and are always the result of a random event that is not under genetic control.
Therefore, MZ twins do not "run through families." MZ twins share the same genotype but do not have exactly the same phenotype. Therefore, they are living examples of how the environment influences gene expression.
Dizygotic (DZ) twins originate from two independent fertilization events. The mother ripens two eggs in a cycle instead of just one, and each egg is then fertilized independently by different sperm. As a result, DZ twins and are no more related to one another than any two siblings (on average, they share about ½ of their genes) and can therefore differ in sex (unlike
MZ twins, who are always the same sex). Moreover, differences between the phenotypes of
DZ twins are due to both genetic factors and environmental influences on gene expression. DZ twinning accounts for about 72% of all twin births and can "run through families" because there are genes that control the tendency of a woman to ovulate from more than one ovary
(or release more than one egg from a single ovary) during a menstrual cycle.
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In MZ twinning, about one third of the time the embryo splits very early in development (by day 5) and results in twins with separate amnions and chorions (the two sacs that surround babies during fetal development). Thus, the twins are separated from one another by two membranes as they develop.
About two-thirds of MZ twins result from a split of the embryo between days 5 and 7. In these cases, the babies share a chorion but have separate amnions and are separated by a single membrane (the aminion) throughout development.
Rarely, (~1% of all MZ twinnings), the embryo splits after both the chorion and amnion membranes form. Thus, they share both membranes and are not separated during
development. These unusual twinning events are the ones that can result in conjoined
twins. http://www-medlib.med.utah.edu/WebPath/PLACHTML/PLAC101.html
If a MZ embryo splits as late as between days 13-15, conjoined twins result. The term
“Siamese twins” is also sometimes used to describe conjoined twins because of a famous pair
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of such twins, Chang and Eng, who were born in Thailand (then called “Siam”) in 1811. Chang and Eng were joined by a band of tissue from the naval to the breastbone, lived 63 years, married two different women, and fathered 22 children. (With today’s medical technologies, twins separated in this manner are easily separated without harm.)
Abigail and Brittany Hensel are living conjoined twins whose common embryo split between day 9 and 14. Each girl has her own neck, head, heart, stomach, gall bladder, brain, and nervous system, one leg, and one arm (a third arm between their heads was surgically removed).The girls share a large liver, a single bloodstream, all the organs below the naval, three lungs, and three kidneys. You will be watching a video about these remarkable twins in today's laboratory.
Studying twins has been a particularly useful way to quantify the relative influence of gene action and the environment on human traits. Concordance is the percentage of pairs in which both twins express the same form of a trait. Comparing the concordance values of a trait
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across many pairs of MZ and DZ twins can be particularly useful, since differences in concordance values indicate the influence of genes on variations in the trait.
If the variation in a trait that is observed in a population is strictly under the control of genes, then the concordance value should be 100% for MZ twins and much less than that for DZ twins. (Although DZ twins share 50% of their genes, the different inheritance patterns of genes prevents DZ twins from having concordance values of 50% for all traits whose variation is strictly under the control of genes.)
% Concordance Values for Some Human Traits
(MZ and DZ twin pairs reared together)
Trait
Acne
Alzheimer's Disease
Anorexia nervosa
Autism
Bipolar disorder
Cleft lip and/or palate
Hypertension
Schizophrenia
Blood type
Eye color
Mental retardation
Hair color
Handedness (left or right)
Epilepsy
Diabetes
MZ twins
14
78
55
90
33-80
40
62
40-50
100
99
97
89
79
72
56
48
10
66
28
37
22
DZ twins
14
39
7
4-5
8
3-6
77
15
22
Examples of Interpreting the Date in the Chart:
 The variation in the appearance of acne is under minimal genetic control. The concordance is low (14%) between MZ twins and is about the same as for DZ twins as it is for MZ twins. Perhaps the diet of the twins (which should be equivalent for twins reared together whether they are MZ or DZ) is more important than genetic influences.
 The variation in self-starvation behavior (Anorexia nervosa) is under the influence of both genes and the environment. MZ are far more likely to share the disorder than DZ twins, but only about 55% of them do.
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 Variation in blood type is almost entirely genetically influenced. MZ twins always share the same blood type and DZ twins share it at a rate consistent with their relatedness (~50% of their genes are shared) and the dominance hierarchy of the alleles in the ABO blood system.
It is important to note that it is the difference between the concordance rate in MZ and DZ twins that is of paramount importance in determining how much influence genes have over the variation observed in a trait. As an example, German Measles is highly contagious
(environmental) but MZ and DZ twins both show a concordance rate of close to 100%. This isn't because getting German measles is due to the influence of genes but because both twins are likely to be exposed!
Cloning is the artificial (human-driven) creation of genetically identical genetic entities. It is a confusing term because it is used to describe making many identical copies of DNA molecules and cells, as well as making genetically-identical copies of whole organisms. The main controversies with cloning, however, are at the whole-organism level.
1. Tissue Plug Cloning in Plants
Cloning Carrots. A plug of tissue is removed and separated into single cells.
Each cell is placed on a separate dish with growth medium. As single cells divide, they form a mass of cells called a callus. Calluses are transferred into a new medium to induce the formation of roots and shoots. The developing plants are then transferred to soil, where they grow into genetically identical copies, or clones, of the original carrot.
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2. Cell Fusion Cloning in Animals
 First successfully performed in sheep in 1986, 11 years before Dolly was cloned.
 The method does not clone an adult animal but clones an embryo in a process somewhat similar to identical twinning.
 This method was adapted for use in cows and many other mammals but could not predict the clones’ phenotype or characteristics.
Cloning Mammals by Cell Fusion
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The process has two stages. First, unfertilized eggs are collected from a donor cow, and the nucleus is removed from each egg by microsurgery. Second, the embryo to be cloned is recovered and separated into single cells. Embryo cells are fused with the donor eggs. Each egg then contains a genetically identical nucleus. These zygotes can be frozen in liquid nitrogen for future use or implanted into the uterus of a surrogate mother to develop. All offspring are clones of the original embryo.
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3. Nucleus Injection Method in Animals
 The first animal cloned from the cell of an adult donor was Dolly the sheep. She was created by a method called “nucleus injection” in 1997.
 The advantage of cloning an adult organism is that scientists can select for certain desirable traits that are present in the adult donor
 This method is now in common use and many different animals have been cloned by nucleus injection, including primates
 The method is illustrated below in mice.
The strategy for Cloning Mice by Injection of Nuclei from Adult Cells. (This same method was used to clone the first cloned mammal, Dolly the sheep.
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Are Cloned Organisms Identical to their Genetic Donor?
Clones created by the nucleus injection method are likely to be somewhat genetically different than their donor. Clones are derived from a single adult cell. This cell has been exposed to mutagens and rounds of DNA replication that may impose upon it a mutation profile different from that of the organism as a whole. Therefore, unexpected diseases and altered phenotypes may show up in the clone(s), because every cell (not just a single cell) in the clone will carry the mutation(s).
In addition, clones are gestated and raised in environments that are likely to be quite different than those of its genetic donor. Therefore, traits that are influenced by the environment are likely to differ in a number of ways.
Finally, some developmental events are completely random and occur differently in each embryo. Therefore, not only will clones differ from their genetic donors but they are likely to differ from one another as well (though not by much). This is true also of identical twins.
Bioethical Issues
Many people feel perfectly comfortable with the idea of cloning plants and animals but draw the line when it comes to humans. Their arguments are largely based on a strong belief or sense that human individuality will be lost if cloning becomes commonplace. However, MZ twins perceive themselves as individuals and are probably more alike than any clones will ever be, particularly MZ twins that were reared together. Therefore, this argument does not carry much weight - at least scientifically.
A more important objection involves the potential loss of genetic diversity as a result of cloning – whether it be plants or animals (including humans). Genetic diversity keeps populations strong, wards against the evolution of parasites, and protects the species from extinction due to a single environmental change or parasitic attack. Whole fields of cloned plants have been wiped out due to a single viral agent, and the fear is that cloning will create the same kind of susceptibility in farm animals and even humans.
How do you feel about this important issue? Should there be legislation passed that outlaws human cloning (currently there is none)? Would you like to be cloned? Would you like to be able to clone a beloved pet?
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PART A. DETERMINING YOUR PHENOTYPE AND POSSIBLE GENOTYPE FOR SIMPLE
TRAITS
1. Each lab group has been provided with pictures that illustrate various simple traits in humans. Using these tools, score yourself and your lab partners for phenotype and
(possibly) genotype. Under what conditions is it possible to determine a person's genotype unambiguously? In what cases is it impossible to know? What is being symbolized by the upper-case and lower-case letters? What about the letters with apostrophes? Record your results in your lab notebook.
Trait and Alleles
2.
Possible
Genotypes/
Phenotype
WW
(peak)
HH
(hair on one or more fingers) male X V Y female X V X V
(normal)
TT
(taster)
Possible
Genotypes/
Phenotype ww
(no peak)
Possible
Genotypes/
Phenotype
Ww
(peak)
Your
Genotype/
Phenotype
Hair Line
W= peak w = no peak
Ear Lobes
L = free l = attached
Darwin's Ear
D = has point d = no point
Freckles
F - freckles f = no freckles
Hair Form
C = curly
C' = strait
Mid-digit Hair
H = hair on one or more fingers h = hair absent on all fingers
Color vision
V = normal v = color blind
Taster (PTC)
T = taster t = non-taster
Tongue Rolling
R = can roll r = cannot roll
Excretion of methyl mecarptan
(from asparagus)
E = excretor e = non-excretor
LL
(free)
DD
(point)
FF
(freckles)
CC
(curly)
RR
(can roll)
EE
(excretor) ll
(attached) dd
(no point) ff
(no freckles)
CC'
(straight) hh
(hair absent) male X female X tt rr ee
(nonv v
Y
X excretor) v
(color blind)
(non-taster)
(cannot roll)
Ll
(free)
Dd
(point)
Ff
(freckles)
C'C'
(wavy)
Hh
(hair on one or more fingers) female X
Tt
Rr
Ee
V X
(normal)
(taster)
(can roll)
(excretor) v
Record your results in the table on the board. We will discuss the pooled class results.
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PART B. EXHIBITS: TWINS AND CLONES
1 Several exhibits have been placed at the side of the room that concern twinning and twins. Exhibit A is a series of pictures of a set of identical twins, Molly and Betsy, at various ages. Compare their phenotypes as infants, children, and adults. What do you observe? How do you interpret your observations?
2. Exhibit B is a photograph of fraternal twins whose parents are both half African-
American and half Caucasian. How do you explain the differences in phenotypes between these two twins? Do you think the differences are mostly genetic or environmental? Why?
3. Exhibit C is a photograph of seven sets of identical twins. When the photograph was taken, the twins were asked to sit beside their twin. No other instructions were given.
Carefully examine and compare the twins, looking at hair color, skin color, facial expression, posture, hand position, and other characteristics. What does this photograph seem to indicate about the influence of genes on phenotype in humans?
4. Exhibit D shows a pair of identical twins, Sherry and Terry. Sherry is taller than Terry and has always been more athletic and a better student than her sister. In interviewing Sherry and Terry, Terry says she has always felt like the "shadow twin" of her sister. What do you think might account for the differences in these twins? (They were reared together and still live together as college students.)
5. Exhibit E shows a photograph of "CC" (the first cloned cat) and her genetic donor,
"Rainbow." Note the difference in the appearance of their coat colors. This is not due to a mutation in CC but, rather, to the strong influence of non-genetic factors on coat color in female cats.
PART C. VIDEO: "JOINED FOR LIFE: THE HENSEL TWINS"
 Make sure all of the visual tools you used for phenotyping and genotyping are neatly laid out for the students in the next lab section.
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