Genetics: Simple and Complex Genetic Inheritance

LAB 10

Genetics: Simple and Complex Genetic Inheritance

I. Objectives:

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:

• 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)
• Understand the difference between monozygotic and dizygotic twins and appreciate the contribution twin studies have made to genetics
• Be able to define the term concordance and determine whether a trait is influenced more by genes or environment using % concordance
• Understand the importance of Drosophila as a model organism in genetics research
• Be able to identify the stages of the Drosophila lifecycle
• Be able to identify various mutations in live Drosophila using a dissecting microscope

II. Safety Considerations

• None

III. Introduction

Simple and Complex Genetic Inheritance

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. 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 polydactyly 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.

The Biology of Twinning

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 about28% 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 in 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 twinsare 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.

Twin Studies and Concordance

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 (of twins) in which both twins express the same form of a trait. Comparing the concordance values of a trait 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 / MZ twins / DZ twins
Acne / 14 / 14
Alzheimer's Disease / 78 / 39
Anorexia nervosa / 55 / 7
Autism / 90 / 4-5
Bipolar disorder / 33-80 / 8
Cleft lip and/or palate / 40 / 3-6
Hypertension / 62 / 48
Schizophrenia / 40-50 / 10
Blood type / 100 / 66
Eye color / 99 / 28
Mental retardation / 97 / 37
Hair color / 89 / 22
Handedness (left or right) / 79 / 77
Epilepsy / 72 / 15
Diabetes / 56 / 22

Interpreting the Date in the Chart:

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!

Pre-lab Question 1: For the following traits, use the % concordance table to predict whether each is influenced more by genes, the environment or both (i.e., genes and environmental factors influence the trait in roughly equal amounts)?

1. Acne - ______

2. Autism - ______

3. Blood type - ______

Introduction to Drosophila Melanogaster

Drosophila Melanogaster, otherwise known as the common fruit fly, is an important model organism for studying genetics and developmental biology. It’s short life cycle (approximately 10 days from egg to adult) and the fact that each fly produces many offspring (as many as 500 eggs) makes it particularly convenient. Furthermore, it is relatively easy to introduce and observe mutations, as well as culture flies in the lab.

The fruit fly goes through 4 stages in its short lifespan of approximately 3-4 weeks (for flies reared in the lab). Eggs are laid and hatch into larvae one day later. The larva molt twice before the larval cuticle hardens into a dark brown pupa. During metamorphosis, which occurs during the pupal stage, larvae tissues degenerate and reorganize to form an adult fly. The adult fly reaches full maturation several hours after emerging from the puparium (the casing where metamorphosis occurs) and is capable of laying eggs only 2 days later.

The fruit fly has been an important genetic model since the early 1900s, with Thomas Hunt Morgan’s experiments that used physical, chemical and radiational methods to mutate drosophila genes. He and his students would then cross-breed the flies and look for heritable mutations that resulted in observable phenotypes. Many spontaneous mutations have also be found and characterized in drosophila. The normal fly is called a "wild type" and any fly exhibiting a phenotypic mutation is called a "mutant". Mutant flies are given names that generally denote the type of mutation the fly exhibits. For example, the mutant "ebony" has a much darker body than the wild type fly.

In 2000, the drosophila genome was sequenced, making it the second multicellular genome to be completely mapped (the first being C. elegans). It has since served as an important organism for comparative genomics.

IV. Things to Do

PART A. DETERMINING YOUR PHENOTYPE AND POSSIBLE GENOTYPE FOR SIMPLE TRAITS

Pre-lab Question 2:

A)Under what condition(s) is it possible to determine a person's genotype unambiguously?

B)In what case(s) is it impossible to know?

C)What is being symbolized by the upper-case and lower-case letters?

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.
2. Record your results in the table above and include it in your notebook. We will discuss the pooled class results.
3. Do you know your blood type? If you do, can you determine your genotype knowing your phenotype? What pattern of inheritance does ABO blood type display? (Record these answers in your lab notebook.)

Trait and Alleles / Possible Genotypes/
Phenotype / Possible Genotypes/
Phenotype / Possible Genotypes/
Phenotype / Your Genotype/
Phenotype
Hair Line
W= peak
w = no peak / WW
(peak) / ww
(no peak) / Ww
(peak)
Ear Lobes
L = free
l = attached / LL
(free) / ll
(attached) / Ll
(free)
Darwin's Ear
D = has point
d = no point / DD
(point) / dd
(no point) / Dd
(point)
Freckles
F - freckles
f = no freckles / FF
(freckles) / ff
(no freckles) / Ff
(freckles)
Hair Form
C = curly
C' = strait / CC
(curly) / C'C'
(straight) / C'C
(wavy)
Mid-digital Hair
H = hair on second (middle) joint of one or more fingers
h = hair absent on all fingers / HH
(hair on one or more fingers) / hh
(hair absent) / Hh
(hair on one or more fingers)
Color vision
V = normal
v = red green color blind / male XVY
female XVXV
(normal) / male XvY
female XvXv
(color blind) / female XVXv
(normal)
Taster (PTC)
T = taster
t = non-taster / TT
(taster) / tt
(non-taster) / Tt
(taster)
Tongue Rolling
R = can roll
r = cannot roll / RR
(can roll) / rr
(cannot roll) / Rr
(can roll)
Excretion of methyl mecarptan (from asparagus)
E = excretor
e = non-excretor / EE
(excretor) / ee
(non-excretor) / Ee
(excretor)
Hand clasping
H = left thumb on top
h = right thumb on top / HH
(Left on top) / hh
(right on top) / Hh
(Left on top)
Eye color
B = non-blue
b = blue / BB
(non-blue) / bb
(blue) / Bb
(non-blue)

PART B. EXHIBITS: TWINS

Several exhibits have been placed at the side of the room that concern twinning and twins. Look at the photos and record your observations for each exhibit in your notebook (answering the questions for each).

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?

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?

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?

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.)

PART C. DROSOPHILA LIFECYCLE

1. Observe the contents of the jar containing live drosophila on your workbench.

2. Identify individual flies in the following stages of their lifecycle: 1) egg, 2) larva, 3) pupa, and 4) adult. Use the manual on your bench for assistance. Draw and describe each stage.

PART D. DROSOPHILA MUTANTS

In this section you will examine wildtype and mutant flies using a dissecting scope. The mutant flies have a variation of a specific gene that results in an observable phenotypic change. You will look for the following mutations:

- singed bristles (on top of pronotum)- sepia (brownish) eyes

- vestigial wings (short wings)- dumpy wings (shortened and angled)

- white eyes - mini (smaller than normal size)

- apterous (no winges)

1. Examine the slide containing wildtype drosophila. Determine the sex using your manual for assistance and record this in your lab notebook.

2. Obtain each of the slides with mutant drosophila (labeled A-G). Examine each under the microscope and identify the mutation from the list above. You can use your manual for assistance.

3. Fill out the following chart and include it in your lab notebook.

Labeled slide / Type of Mutation Observed
A
B
C
D
E
F
G
Wildtype / Indicate Male or Female

V. Lab Clean-Up

• 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.
• Make sure microscopes are put up according to your instructor’s directions.

BIO 2 Lab Manual, Spring 2010Lab 10, Page 1