Genetics Problems: Predicting Genotype and Phenotype Using Punnett Square

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BIOL 1014 Lab 9 Genetics Problems
Skills: Punnett Square Use and Genetics Terminology
Objective: Use Punnett Squares to accurately predict genotype and phenotype percentages for offspring involving various types of inherited traits. There is additional information about this procedure in section 12.2 in your textbook. The problems below will have multiple parts/questions, please make sure that you answer all parts/questions in each problem.
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Vocabulary
Allele – Alternate forms of a gene.
Cross – Mating of two parent organisms
Dominant Trait – A genetic trait is considered dominant if it is expressed in an organism who has only one copy of that gene. In genetic terms, a dominant trait is one that is phenotypically expressed in heterozygotes. (Example: Both RR and Rr have round seeds.)
F1 Generation – The first set of offspring from the original parents.
F2 Generation – The second generation of offspring.
Gene – Section of a chromosome that carries the information for a specific trait.
Genotype – The fundamental genetic makeup of an organism for a specific trait; the combination of the two alleles located on the homologous chromosomes that determines the expression (phenotype) of the trait. The genotype is represented by two letters, which are in upper and lower case to represent different alleles. (Example: R is genotype for the dominant trait of a round pea seed and r is the genotype for the recessive trait of a wrinkled pea seed.)
Heterozygous – The genotype has a dominant gene and a recessive gene for the same trait. (Rr)
Homozygous – The genotype has both letters being exactly the same, meaning the organism either has 2 genes for a dominant or a recessive trait rather than one of each. (RR or rr)
Offspring – New organisms resulting from a cross.
Phenotype – The way a trait physically expresses itself in an organism. (Example: wrinkled seeds vs. smooth
seeds, red flowers vs. white flowers.)
Probability – The chance/possibility that a specific event will occur. Expressed mathematically using percentages or ratios.
Punnett Square – A mathematical tool that helps predict the offspring outcome (both genotypes and phenotypes) of a cross between two parents with known genotypes.
Recessive Trait – Refers to an allele that causes a phenotype that is only seen in homozygous genotype and never in a heterozygous genotype. If a genetic trait is recessive, the organism needs to inherit two copies of the gene for the trait to be expressed. Thus, both parents must be carriers of the trait in order for the offspring to express that trait. (Example: rr is expressed as wrinkled seeds.)
BIOL 1014 Lab 9 Genetics Problems
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Practice Problem
Gregor Mendel was an Austrian Monk who is considered the father of genetics. By completing many observations and keeping meticulous records, Mendel was able to categorize several inherited traits in the common garden pea plant. This was a hearty plant that reproduced quickly which made it a good candidate for studies of this type. The table on the right summarizes a number of traits Mendel observed.
In his genetics experiments with the garden pea in the mid-1800’s, Mendel found that in peas, red flower color (R), is dominant to white flower color (r). Give the phenotypic and genotypic probabilities for the offspring resulting from a cross of a homozygous red flower (RR) and a heterozygous red flower (Rr).
Notice, in assigning letters to traits, conventionally the letter chosen is the first letter of the DOMINANT trait. To represent the recessive gene, the same letter is used but lower case. In Incomplete Dominance examples, both letters are capital and represent both different traits. (See question 5)
A Warning! - Be careful about MS Word’s habit of auto-capitalization when completing this assignment, make sure your letters are appropriately capital or lower case.
Step 1
Draw a Punnett Square
X









Step 2
Identify the parent genotypes: In this case, RR and Rr. Place the parents’ genotypes into the Punnett square, one parent across the top, and the other down the side. The RED X indicates that nothing needs to into that particular box.
X

R

R

R



r



Step 3
Combine the genotypes for each box, much like a multiplication table, placing the allele at the top and on the left of each box into the box. (Traditionally, the alleles are written with the capital letter, if any are present, written first.)
X

R

R

R

RR

RR

r

Rr

Rr

Step 4
Calculate the Genotypic and Phenotypic Ratio for the Offspring.
For Genotypic Ratio – Sum each genotype present and give ratio out of total present, reduce if necessary.
In this example: 2 – RR, 2 – Rr.
So the Genotypic Ratio is 2 RR / 2 Rr, which can be simplified to 1RR / 1Rr, this indicates that 50% of offspring will have the genotype RR and 50% will have the genotype Rr.
For Phenotypic Ratio – Examine each genotype and determine what traits will be expressed in the offspring.
In this exam...