Laboratory Assignment 5 Genetics PDF

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This is the 5th lab with answers....

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LABORATORY V – Genetic Inheritance OBJECTIVES When you have completed this exercise, you will be able to: ● define the following terms: gene, genome, allele, genotype, phenotype, dominant, recessive ● describe the similarities and differences between the inheritance patterns of traits that are determined by genes on autosomes and those that are determined by genes on sex chromosomes.. ● know how to use a Punnett square to predict the probability of certain genotypes or phenotypes in offspring. ● know how to interpret a pedigree. ● understand sex-linked inheritance and understand why males are more likely than females to inherit an X-linked trait.

INTRODUCTION Each gene in our collection of genes, our genome, is found at a particular position on one of the different types of chromosomes that we possess. We look like our parents because we have inherited genes from them. Genes are segments of DNA in chromosomes that provide the instructions to make proteins in our cells that allow them to develop and function. For example, there is a gene that has the instructions for how to make an enzyme called tyrosinase; this enzyme (a protein) is made in the melanocytes of our skin, and is important for making the pigment, melanin. Our Genome Our genome consists of over 20,000 genes. Females have 2 copies of every single gene (see Figure 1). The story is more complex for males, since they have one X and one Y chromosome. Males have 2 copies of each gene found on autosomes, but the situation for the sex chromosomes is different. There are about 1000 genes on the X chromosome (colored in Figure 2) that males have only one copy of because these genes are found only on the X chromosome (differential region of the X). In addition, there's a region on the Y chromosome that contains about 100 genes that are specific to the Y chromosome (differential region of the Y, in purple below). For these 2 regions, males have only 1 copy of each gene. In contrast, males do have 2 copies of each of the 18 genes found on the homologous regions of the X and Y chromosomes (in yellow below) (source: http://ghr.nlm.nih.gov/chromosome=X, http://ghr.nlm.nih.gov/chromosome=Y)

Figure 1. Human female karyotype. Variation and Heredity

Figure 2. Comparison of the human sex chromosomes. The X chromosome is larger and contains more genes than the Y chromosome. Most of the genes of the X and Y chromosomes are unique to each chromosome but there is a homologous region (in yellow) between them.

The genes on a chromosome may exist in more than one form, called alleles (Figure 3). Individuals generally inherit two alleles for each trait, one received from the mother in the egg and the other from the father in the sperm. An exception to this rule are the traits that are determined by the sex chromosomes, as discussed above. Each one of us is unique due to our unique combination of distinct alleles. The combination of alleles for a trait represents an individual’s genotype (such as BB or Bb). If the two inherited alleles represent different forms of the gene (different alleles), an individual is said to be heterozygous for that gene (ex: Bb). If they are identical, an individual is said to be homozygous for that gene (ex: BB or bb). The allele that is expressed in heterozygous individuals is referred to as dominant and is represented by a capital letter (ex: B). The allele not expressed in heterozygous individuals is called recessive. Recessive alleles are represented with lowercase letters (ex: b). For example, freckles is a dominant trait. The genotype of people with freckles is either FF or Ff. The genotype of people without freckles is ff. The actual appearance of a specific trait is called the phenotype. Note that the phenotype is normally expressed in words, such as brown eyes, curly hair, freckles, dark skin etc. Your genes are responsible, along with some influence from the environment, for determining your phenotype.

Figure 3. A pair of homologous chromosomes with the locus for eye color. In this example, the alleles (alternative versions of a gene) are not identical. For each locus, the alleles may be identical (homozygous) or different (heterozygous).

EXERCISE 1. Mendelian Characteristics in Humans. Analyze your phenotype and genotype for human Mendelian characters. For this lab, we'll be looking at traits that follow a simple inheritance pattern (ex: freckles). These are called Mendelian characters because they were discovered by a person named Mendel. But you should know that most human traits have a much more complicated inheritance pattern. For example, human height is thought to be determined by at least 6 different genes, and can be influenced by the environment (ex: how well you eat). Table 1 highlights the relationship between genotype and phenotype and lists the terms used to describe the three possible genotypes for freckles. Table 1A. Relationship between Genotype and Phenotype for Mendelian characters. Complete the table Genotype

Example

Phenotype (Recessive or Dominant)

Example

Homozygous dominant

FF

Dominant

Freckles

Homozygous recessive

ff

Recessive

No freckles

Heterozygous

Ff

Dominant

Freckles

Exercise 1. The following are characters inherited in a Mendelian fashion in humans. You will be observing several traits, each determined by a single gene on an autosomal (non-sex) chromosome. Determine your phenotype for each character and complete Table 1B. Use the information provided below the table to explore these characters and how they are inherited. Table 1B: My Phenotypes and Genotypes for some Mendelian Characters

Character 1. Earlobes 2. Widow’s Peak 3. Ear wax

Phenotype

Is the phenotype dominant or recessive?

Possible Genotypes

Free earlobe

Dominant

FF or Ff

Straight hairline

Recessive

ww

Wet ear wax

Dominant

WW or Ww

1. Earlobes An individual either has free earlobes (Free) or attached earlobes (Attached). Free earlobes is determined by the presence of a dominant allele F. Free

Attached

3. Widow’s Peak An individual either has a protrusion in their hairline (Widow’s Peak) or not (Straight Hairline). The presence of a Widow’s Peak is determined by the presence of a dominant allele W

Widow’s Straight Peak Hairline 4. Ear wax An individual will produce either wet (W) or dry (w) ear wax. The ability to produce wet ear wax is determined by the presence of a dominant allele, W. If you produce dry ear wax, your genotype is ww.

EXERCISE 2. Punnett Squares. Use a Punnett square to predict the probability of a particular genotype or phenotype. Punnett squares can be used to predict the probability of inheriting a particular genotype or phenotype for Mendelian characters from parents of known genotypes. For example, if we know that a man and a woman are both heterozygous for freckles we could predict the probability that they would have a child with freckles. The construction of a Punnett square for this example is illustrated in Figure 4. Each individual has a 50% chance of packaging F or f in their gametes. If we look at the possible combinations of zygotes they can produce, we see that there is a 25% chance of producing a homozygous dominant zygote, a 25% chance of producing a homozygous recessive, and a 50% chance of producing a heterozygous zygote. This means that every time this couple produces a child, there would be a 75% chance that the child has freckles.

Figure 4. Punnett square for two individuals who are heterozygous for the gene that governs freckles.

Exercise 2. Fred has a widow’s peak and is homozygous dominant. His wife Frieda has a straight hairline. They are having a child together and want to know the possibility of their child having a widow’s peak. Construct a Punnett square to help them out. Complete the missing fields below:

WW [Fred’s genotype]

ww

X

[Frieda’s genotype]

Possible Sperm: W Ww

P o s s i b l e What is the

W Ww

w w Ww

Ww

w probability that their child will have a widow’s peak?

What is the probability that their child will be heterozygous? EXERCISE 3. Sex-linked Characters in Humans.

100 %

100 %

Characters controlled by genes on these chromosomes are referred to as sex-linked. An X-linked character is control by a gene on the X chromosome. Men inherit X-linked character differently from autosomal characters because they inherit only one allele for each gene on the X chromosome. A Ylinked character is governed by a gene on the Y chromosome and only affects men. Table 3 highlights the relationship between genotype and phenotype for a recessive X-linked disorder. Females need two copies of the recessive allele and can be protected from the disorder by having one dominant allele. Males need one copy of the recessive allele. X-linked recessive traits are more common in males than in females. Table 3. Relationship between Genotype and Phenotype in Males and Females for a Recessive X-linked Disorder. Complete the table Female Genotypes

Female Phenotypes

Male Genotypes

Male Phenotypes

XDX D

Normal

X DY

Normal

X DXd

Normal

XdY

Affected

X dX d

Affected

Consider the following example. Hemophilia is an X-linked recessive disorder of the blood. A female who has hemophilia and a normal male are having a child. The Punnett square is illustrated in Figure 5. The chance of a female with hemophilia is 0% while the chance of a male with hemophilia is 100%.

Figure 5. Punnett square for the inheritance of hemophilia for an affected female and a normal male.

Exercise 3A. Test Your Partner for Red-Green Color Blindness (Ishihara Color Vision Test). The ability to distinguish the colors red and green is due to the dominant allele C on the Xchromosome (XC). Lack of this ability is due to the recessive allele Xc. As an X-linked recessive disorder, red-green color blindness is 16 times more common in males than in females. Follow the link below to perform an online version of the Ishihara Color Vision Test Note: The online test consists of 38 pseudoisochromatic plates, each of them showing either a number or some lines. For each plate you have to either enter the number or you have to choose the number of lines you can see. If you don't see a number or a line(s), just leave the input field empty. https://www.color-blindness.com/ishihara-38-plates-cvd-test/#prettyPhoto/1/ 2. After conducting the test, what is your possible phenotype [check]: ☐ Normal (none-weak)

or

☐ RGCB (moderate to strong)

Exercise 3B. Genetic Analysis of Results 1. Based on the results of the test in 3A, what is your genotype (or possible genotypes). Include your sex chromosomes XCXc, XCXC 2. Based on this genotype or possible genotypes, which of the following combinations of genotypes are possible combinations for the genotypes of your parents? To find out, do all 6 crosses and see if your genotype is one of the possible genotypes of the children. For example, if you are a colorblind male (XcY), after doing the first cross below (XCXC x XCY), you would conclude that these couldn’t be the genotypes of your parents since no XcY would be produced. Mother

Father

Possible? (yes or no)

X CXC

X CY

yes

X CXC

XcY

yes

XCX c

X CY

yes

XCX c

XcY

yes

X cXc

X CY

yes

X cXc

XcY

no

N.B. The recessive allele has been underlined. 3. If a female has RGCB. Which parent must also have the disorder? Circle one:

Father

or

Mother

or

Both

or

Neither

4. If a male has RGCB. Which parent must also have the disorder? Circle one:

Father

or

Mother

or

Both

or

Neither

EXERCISE 4. Human Disorders and Pedigrees. Use a pedigree to determine a particular genotype for a human disorder. Many human disorders follow Mendelian inheritance. Being normal (i.e. most common phenotype) or having a disorder, can be considered a character where the traits (phenotypes) are: affected or not affected. Some disorders are dominant disorders while some are recessive disorders. Table 4 highlights the relationship between genotype and phenotype for dominant and recessive autosomal disorders. Table 4. Relationship between Genotype and Phenotype for a Dominant Disorder and Recessive Disorder Complete the table Genotype

Example

If Dominant Disorder Phenotype is:

If Recessive Disorder Phenotype is:

Homozygous dominant

DD

Affected

Normal

Homozygous recessive

dd

Normal

Affected

Heterozygous

Dd

Affected

Normal

A pedigree is a chart that illustrates the occurrence of a particular trait over many generations in a family tree (Figure 6). These figures are useful to geneticists studying the inheritance patterns of particular traits such as disorders and diseases. A pedigree can also help you determine an unknown genotype for an individual in a family. In a pedigree, sometimes carriers are indicated by half-filled symbols. The term carrier refers to individuals that are heterozygous for a recessive disorder. These individual do not display signs of the disorder (are normal) but carry the disorder allele and can pass it on to offspring. However, when the disorder is dominant, the term carrier does not apply to heterozygotes because these individuals display signs of the disorder. Figure 6. Pedigree. In this example, the trait being followed (filled symbols) is a recessive trait (inherited in a recessive manner). Unaffected individuals are identified with empty symbols while carriers are identified with half-filled symbols.

Watch the following video titled ”Explore autosomal recessive trait and X-linked recessive trait tracking in pedigrees with the Amoeba Sisters!” for an introduction to understanding pedigrees https://www.youtube.com/watch?v=Gd09V2AkZv4 Once you fully understand what a pedigree is, following the link below for an online pedigree simulation to practice constructing pedigrees http://www.zerobio.com/drag_gr11/pedigree/pedigree_overview.htm ● Step 1: Read the Instructions to understand how to use the simulation

● Step 2: Click on the first pedigree “Tongue Rolling”

● Step 3: Click “Show Problem” (lower right of the panel) to read the genetic problem

● Step 4: Once you understand the genetic problem, click “Hide Problem” and proceed to drag and drop the “Symbols” from the right side panel to construct your pedigree. ● Step 5: Once your pedigree is completed, you can check your pedigree with the correct answer by clicking “Show Pedigree”

After successfully completing the tongue rolling pedigree, try to construct the other three pedigrees (”Eye Color”, “Diabetes” and “Hypertension”) Pedigree

Successfully Completed (Yes / No)

Tongue Rolling

Yes

Eye Color Diabetes Hypertension

yes yes no

Exercise 4A. Gaucher’s disease Gaucher’s disease is a rare inherited metabolic disorder characterized by anemia, mental and neurologic impairment, yellowish pigmentation of the skin, enlargement of the spleen, and bone deterioration. This disease is inherited as an autosomal recessive trait and is caused by a mutation in a particular gene that makes it non-functional. The mutant allele (g) must be present in both copies of the homologous chromosomes to give rise to the disease. i) Fill in the missing genotypes (below the symbol) in the pedigree in Figure 7A. If two genotypes are possible, list both. Also, identify the carriers with half-shaded symbols.

1.

Generation 1 : Male: GG

Female: gg

Generation 2 : Daughter: Gg Male husband: Gg Generation 3: Son male: Gg Son male: gg Daughter female: gg Son male: Gg

Figure 7A. Gaucher’s disease pedigree. N.B. Individuals with Gaucher’s disease are shaded. For some reason, the pedigree image does not appear properly in the document.

ii) In a hypothetical situation, individual 1. is having a child with a man with Gaucher’s disease, what is the chance that their child will inherit the disease? 50% Show your work below: Individual 1 genotype: Gg Male genotype: gg Punnett square : Gg

Gg

gg

gg

Exercise 4B. Huntington’s disease Huntington's disease is a neurological disorder resulting in impaired motor function. This disease is inherited as a dominant autosomal trait. The mutant allele (H) results in an abnormal protein. This defective protein is toxic to neural tissue, resulting in the characteristic symptoms of the disease. Hence, one copy suffices to confer the disorder. The disorder itself is not fatal, but as symptoms progress, complications reducing life expectancy increase. i) Fill in the missing genotypes (below the symbol) in the pedigree in Figure 8B. If two genotypes are possible, list both.

Generation 1: male: hh, female: Hh Generation 2: female: hh, male: Hh, female: Hh, male: hh, male: hh, male: hh, male: hh Generation 3: male: Hh, female: hh, male: hh, male: Hh Figure 8B. Huntington’s disease pedigree. N.B. Individuals with Huntington’s disease are shaded. For some reason, the pedigree image does not appear properly in the document. ii) In a hypothetical situation, individual 1. is having a child with a woman without Huntington’s disease, what is the chance that their child will inherit the disease? 50% Show your work below:

H

h

h

Hh

hh

h

Hh

hh

EXERCISE 5. Identify the Genetic Disorder! Do some research to answer the questions and identify the genetic disease. Disorder 1: This is a rare inherited disease that progressively destroys nerve cells (neurons) in the brain and spinal cord. The most common form of this disease becomes apparent in infancy. Infants with this disorder typically appear normal until the age of 3 to 6 months, when their development slows and muscles used for movement weaken. As the disease progresses, children with this disease experience seizures, vision and hearing loss, intellectual disability and paralysis. Children with this severe infantile form of the disease usually live only into early childhood. 1. What disease is being described? Tay-Sachs disease 2. Which gene is mutated in this disease? HEXA gene 3.

Is this gene found on an autosome or sex chromosome? autosomal

4. Is this disorder dominantly or recessively inherited? recessive 5. This gene normally directs the synthesis of a protein that is found in which organelle? lysosomes 6. If 2 people who are both heterozygous for this disease have a child, what is the probability that the child will have the disease? Show your work. The probability of two heterozygous people having a child with the disease is 25%.

D

d

D

DD

Dd

d

Dd

dd

Disorder 2: This disease is caused by a mutation in a gene that has the instructions on...