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Gregor Mendel was an Austrian monk whose investigations into the mechanisms of inheritance
BIO-110 Lab: Genetics
Submission Instructions:
Type your answers and paste any required pictures directly into this Word document. Submit it via the Blackboard submission link in Word format (docx).
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Objectives:
Students will interpret an individual’s phenotype by recognizing a provided genotype.
Students will perform a series of monohybrid crosses to predict the expected offspring in a pairing.
Students will explain how the phenotypes for incompletely dominant and codominant traits differ from traditional dominant/recessive phenotypes.
Introduction:
Gregor Mendel was an Austrian monk whose investigations into the mechanisms of inheritance earned him the honor of being called the “Father of Genetics”. Reginald Punnett developed a diagram called a Punnett square used to illustrate Mendel’s laws of genetics.
To be able to solve genetic problems, you must first be familiar with the critical terms in order to understand the mechanisms. The first few exercises will familiarize yourself with the following terms: gene, allele, phenotype/genotype, homozygous/heterozygous, and dominant/recessive.
Gene – Unit of heredity existing on the chromosomes; in diploid organisms, typically two versions are inherited-one from each parent.
Allele – one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
Genotype – genetic makeup of an organism (AA, Aa, aa) These 2 letters represent an allele. You get one allele from your mother and one allele from your father.
Phenotype – physiological features, bodily characteristics or behavior (example: brown hair, diabetes, blue eyes)
Homozygous – organism with 2 identical alleles (AA, aa)
Heterozygous – organism with 2 differing alleles (Aa)
Dominant trait – a trait that expresses its phenotypic effect in the heterozygous condition (AA, Aa).
Recessive trait – a trait that can remain hidden in certain generations and is only expressed in the homozygous recessive condition (aa).
Punnett Square Instructions:
View the following video so that you can Learn to Draw a Punnett Square (opens new window). Also, students are welcome to follow these steps. First, you should draw a box and divide it into four squares. Leave room above it and to its left, so that you can label it. Next, you should choose a letter to represent the alleles. Write the dominant allele with any capital letter, and the recessive allele with the same letter in lowercase. Make sure that both versions are visually distinct. Write down the parental genotypes on the top and left-hand side of the graphic. It does not matter which gender is assigned to which side. In both cases, label the first column (or row) with the first allele and the second column (or row) with the second allele. Finally, have each box inherit letters from its row and column. Each box should be diploid (two genes), and it should have one allele from each parent.
Procedure:
Use your textbook to complete Activities I-IV below.
Activity 1:
For each genotype below, indicate whether it is heterozygous (Het) or homozygous (Hom) (2 points each).
AA__________
Bb__________
Cc__________
Ee__________
ff__________
Gg_________
Jj__________
kk__________
Ll__________
Mm________
nn_________
PP __________________
Pp __________________
pp __________________
EE _________________
Ee _________________
ee _________________
__________________Ability to tongue curl
__________________ Ability to tongue curl
__________________Can’t tongue curl
__________________Widow’s peak
__________________ Widow’s peak
__________________No widow’s peak
Widow’s peak occurs when the hairline forms a distinct point in the center of the forehead. This is controlled by a dominant allele. If two individuals without a widow’s peak have a child, what are the chances that this child will have a widow’s peak? Show your cross (Punnett square).
A dominant allele (T) gives an individual the ability to curl his/her tongue in a U-shape. If two homozygous tongue rollers have children, what percentage of their offspring will have this trait? Show your cross and your parental genotypes.
If you crossed two heterozygous plants for flower color (as given above), then what percentage of their offspring will have purple flowers? Show your cross.
If you crossed one heterozygous tongue roller with someone that couldn’t roll their tongue, then what proportion of their offspring would be able to roll their tongue? Show your cross.
In snapdragon flowers, the alleles for petal color are incompletely dominant. Two pink snapdragon flowers are crossed. Determine the genotype of each parent, then complete the cross, and list all possible phenotypes and genotypes of the offspring. Don’t forget to describe the relative proportion for each offspring (i.e. ¼, ½, etc.).
In humans, ABO blood types show evidence of codominance. A man is suspected to be the father of a child with type B blood. The mother has blood type AB (genotype IA IB) and the father has blood type A (genotype IAIA). The child’s blood type is B (genotype IBi). Is it possible that this man is the father of the child? Show your cross.
A person with type A blood is crossed with a person with type B blood. They have offspring with type O blood. What are the genotypes of the parents? Show your Punnett square to support your answer.
What would you expect when a colorblind man marries a colorblind woman? Give both genotypic and phenotypic proportions for the offspring. Show your cross.
Colorblindness is caused by a recessive, X-linked allele carried on the X chromosome. A man with normal vision marries a woman who is a carrier for colorblindness. What fraction of their offspring are likely to be colorblind? Show your cross.
Activity 2:
For each of the genotypes below determine what phenotypes would be possible (2 points each)
Purple flowers are dominant to white flowersFree earlobes are dominant to attached ones
For each phenotype below, list the genotypes. Remember to use the letter of the dominant trait; for example, if purple flowers are dominant to white flowers, you should use P and p (for purple) to represent the dominant and recessive alleles, respectively (2 points each).
Tongue curling is a dominant traitWidow’s peak is controlled by a dominant allele
Activity 3:
You will use this information to complete some genetic problems. For each question, use a Punnett square to help you answer the questions below. Show your Punnett squares. Please use letters whose capital (B) and lowercase (b) variants are distinct (6 points each).
Activity 4:
View the following video to tutorial that demonstrates how to solve Punnett squares for Complex Inheritance Patterns (opens new window).Mendel’s pattern of inheritance works well for dominant and recessive traits, however there are many traits that do not follow this pattern. Use the information below to help you answer questions 28-30 (6 points each).
Incomplete dominance: In incomplete dominance, the heterozygous condition (Aa) expresses a mixture of the two traits. For example: in snapdragon flowers the homozygous dominant (AA) would express red flowers. Homozygous recessive (aa) species expresses white flowers. In the heterozygous condition (Aa), the flower petals are pink in color (a mixture of both red and white).
Codominance: An event in which two alleles are expressed to an equal degree within an organism. A popular example of codominance is human ABO blood typing. Blood types are determined by the presence (dominant allele) or absence (recessive allele) of certain antigens (substances that can trigger an immune response in the body). Therefore, someone with the “A” blood type expresses A-type antigens (genotype IAIA or IAi). The same principle applies for a person with the “B” blood type (genotype IBIB or IBi). However, someone can have access to both dominant alleles. In this case, they would equally express both “A” and “B” type antigens on their red blood cells. This is known as the “AB” blood type (genotype IA IB). Type O blood can only result form the inheritance of two recessive “i” alleles (genotype ii). An individual with this genotype wouldn’t express antigens.
X-linked or sex-linked traits: Each individual has a set of sex chromosomes. A male has an X-chromosome inherited from his mother (XX) and a Y-chromosome inherited from his father (XY). Females have two X chromosomes, one from each parent. As a result, they can display an array of genotypes (homozygous dominant, heterozygous, or homozygous recessive). Males, on the other hand, only have one X-chromosome. Therefore, males show a higher prevalence for sex-linked recessive disorders (i.e., colorblindness, muscular dystrophy, and hemophilia). For females to express an X-linked trait, they must express both recessive alleles. For these crosses, you must include the X and Y chromosomes in your Punnett squares.
Males can be: XAY (normal)orXaY (he will have the trait)
Females can be: XAXA (normal), XAXa (carrier),orXaXa (will have the trait)
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