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LAB

8

The Patterns of Inheritance

BIO 106 Life Sciences

Brightpoint Community College

After completing this exercise, you will be able to:

· Define the following terms: homozygous and heterozygous, dominant and recessive alleles, genotype, and phenotype.

· Derive the possible alleles in each gamete of a diploid organism.

· Calculate phenotypic ratio and genotypic ratio using Punnett squares.

· Predict the effect of incomplete dominance and codominance on inheritance.

· Describe how phenotype is determined by genotype.

Checklist of Materials to Gather Before Starting Lab

· Computer with Internet Connection

· Colored pens or markers for drawing by hand

· Paper (if you do not have a printer)

INITIAL KNOWLEDGE CHECK

Indicate your level of knowledge for the following terms by putting an “x” in the box that best describes your understanding of the terms in Table 2-1.

· Unfamiliar: Indicates no knowledge or awareness of the term.

· Basic Understanding: Suggests a basic, surface-level understanding of the term's meaning.

· Proficient: Implies a solid grasp and ability to use the term correctly in context.

· Expert: Signifies a deep, comprehensive understanding and ability to apply the term in various complex situations.

LAB DATA QUESTION 1. Table 8-1: Assess your understanding of the concepts.

Term	Unfamiliar	Somewhat Familiar	Proficient	Expert
Gene
Allele
Genotype
Phenotype
Complete Dominance
Incomplete Dominance
Codominance

INTRODUCTION

Mendelian genetics was first investigated by Gregor Mendel, a Moravian monk, in the 1800s. Mendel's groundbreaking research on inheritance utilized the garden pea, Pisum sativum, a species that typically self-fertilizes, with pollen from a flower fertilizing the eggs within the same flower. Given that each pea plant possesses both male and female reproductive structures, it can produce both types of gametes necessary for reproduction—pollen and eggs. 

The plants Mendel used for his initial crosses were referred to as the P (parental) generation. He then planted the seeds obtained from these crosses, growing them to become the first filial (F1) generation, meaning the first generation of offspring. After observing the traits in the F1 generation, Mendel allowed them to self-pollinate. The seeds from this generation were used to grow the second filial (F2) generation. The trait ratios observed in the P, F1, and F2 generations formed the foundation of his genetic principles. He derived two generalizations from these experiments, which later became known as Mendel's Principles of Heredity or Mendelian inheritance. Mendel discovered that when he crossed purebred white flower and purple flower pea plants (the parental or P generation), the offspring (known as the F1 generation) was purple-flowered. When he self-fertilized the F1 generation pea plants, he obtained a purple flower to white flower ratio in the F2 generation of 3 to 1. Prior to Mendel's experiments, the prevailing belief was that offspring inherited traits through a blending of their parents' traits, for example a plant with red flowers crossed with a plant with white flowers would produce offspring with pink flowers. However, Mendel's cross-pollination experiments revealed that offspring of purebred plant varieties resembled one parent or the other, rather than exhibiting a blend of traits from both parents. Mendel suggested that plants carried two copies of the flower-color trait, with each parent passing one of their two copies to their offspring, where they united.

For each biological trait, an organism inherits two alleles, one from each parent, which may be the same or different. Subsequently, Mendel introduced the concept of "factors," now referred to as genes, to explain hereditary traits. He identified these factors as the underlying cause for the inherited variations in traits. Each organism inherits two versions of a gene, known as alleles, one from each parent. These alleles can either be identical or distinct. Organisms with two identical alleles for a specific trait are termed homozygous, while those with differing alleles are termed heterozygous. Mendel hypothesized that allele pairs separate randomly during the meiosis (production of eggs and sperm). When sperm and egg unite at fertilization, each contributes its allele, restoring the paired condition in the offspring. This is called the Law of Segregation. Additionally, Mendel discovered that during gamete formation, each allele pair separates independently from every other allele pair. An individual's genotype consists of their unique genes, whereas their phenotype—the observable traits—is a combination of their alleles and environmental factors. When an individual inherits different alleles for a trait, this condition is termed heterozygosity. In such pairs, the dominant allele dictates the organism's appearance, overshadowing the effect of the recessive allele, which does not alter the phenotype if a dominant allele is present. The Law of Dominance explains this interaction by looking at how genotypes show up as phenotypes. One example of this is how recessive traits only show up when there is no dominant allele present. This law does not address how traits are transmitted, but rather the expression of genes. The genotype of an individual is made up of the many alleles it possesses. The environment and an individual's alleles together determine their phenotype, or physical appearance. If the two alleles of an inherited pair differ (the heterozygous condition), one determines the organism's appearance and is called the dominant allele, while the other has no noticeable effect on the organism's appearance and is called the recessive allele. The Law of Dominance, which hides the phenotypic effects of the recessive white flower allele, is not a transmission law but concerns the expression of the genotype.

Objectives

In this lab, you will explore how Mendel's principles of inheritance apply to the transmission of traits from parents to offspring. By examining different allele combinations, you'll gain insights into the genetic mechanisms that determine the phenotype. This hands-on approach will help solidify your understanding of genotype-phenotype relationships and the predictability of genetic inheritance patterns.

Mendel’s Monohybrid Cross

Mendel's monohybrid cross, which involved crossing pea plants that differed in only one trait, demonstrated the Principle of Segregation and the concept of dominant and recessive alleles. Specifically, Mendel crossed pea plants that were purebred for different traits, such as round seeds (dominant) and wrinkled seeds (recessive). In the first filial generation (F1), all the offspring exhibited the dominant trait, indicating that it masked the expression of the recessive trait. However, in the second filial generation (F2), the recessive trait reappeared in a ratio of approximately 3:1 (Figure 8.1). This ratio demonstrated that traits segregate independently during gamete formation, with each parent contributing one allele for each trait to their offspring, and that dominant alleles mask the expression of recessive alleles when present. This experiment laid the foundation for understanding the inheritance of traits and formed the basis for our modern understanding of genetics and heredity.

Setting up a Monohybrid Cross

You have two choices for learning how to complete a monohybrid cross. If you prefer a video, then watch Monohybrids and the Punnett Square Guinea Pigs. If you prefer step by step directions, continue reading below.

Step by Step Directions:

To demonstrate how to set up a monohybrid cross, we will use seed color in pea plants, denoting the dominant gene for yellow seeds as "Y" and the recessive gene for green seeds as "y". Crossing a true-breeding pea plant that produces yellow seeds with a true-breeding pea plant that makes green seeds allows us to predict the genotypes in their offspring. When true beeding organisms are crossed, all offspring will have the same phenotype as the parents. This means that both alleles are the same in the true breeding plants are the same, so the genotype is (YY) or (yy).

1) Identify the genotypes in the Parental (P) generation (Figure 8.1):

Yellow True breeding Genotype: YY,

Figure 8.1 Mendel’s Monohybrid Cross.

In the P generation, pea plants that are true breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype. This cross produces F1 heterozygotes with a yellow phenotype. Punnett square analysis can be used to predict the genotypes of the F2 generation.

Green True breeding Genotype: yy

2) Determine the gametes produced by each parent (P generation):

Yellow Gametes: Y Female Gametes: y

3) Cross the P1 generation (parents) to produce the F1 generation (children). Refer to Figure 8.1

Draw a 1 x 1 grid, one row and one column for each gamete.

Punnett Square F1 Generation	Y
Y	Yy yellow

Capital letters always go first and write in the phenotype.

Use the Punnett square to determine the expected genotypic ratios of the offspring.

1Yy

Use the Punnett square to determine the phenotypic ratio for the offspring.

1 yellow

Cross the F1 generation (children) to produce the F2 generation (grandchildren) Refer to Figure 8.1.

4) Identify the genotypes in the F2 generation

Male Plant: Yy, Female Plant: Yy Gametes are not separated by commas

5) Determine the gametes produced by each F1 parent.

Male Gametes: Y, y Female Gametes: Y,y Each gamete is separated by a comma.

(CONTINUE TO NEXT PAGE)

6) Cross the F1 generation (children) to produce the F2 generation (grandchildren). Refer to Figure 8.1

a. Draw a 2 x 2 grid, one row and one column for each gamete.

Punnett Square F2 Generation	Y	y
Y	YY yellow	Yy yellow
y	Yy yellow	yy green

b. Capital letters always go first

c. Write in the phenotype.

d. Warning: Auto capitalization sometimes changes “yy” to “Yy”. Double check or turn it off.

7) Use the Punnett square to determine the expected genotypic ratios of the offspring.

1YY: 2Yy: 1Yy

8) Use the Punnett square to determine the phenotypic ratio for the offspring.

3 yellow: 1 green

· Ratios represent the chance that offspring will be born with these phenotypes. For example, there is a 75% chance that the offspring will have the YY phenotype and 75% chance they will be green plants. The actual results may vary since this is a prediction.

· Ratios must always be simplified. For example, a ratio of 4:2 can be divided by 2 and simplified to 2:1.

Problem 1: Round seeds (R) are completely dominant over wrinkled seeds (r). You cross a plant that is homozygous for round seeds (male) with a plant that is heterozygous for round seeds (female). This example has been filled in for you.

QUESTION 1. Male Plant Genotype _____ Female Plant Genotype _____

QUESTION 2. Male plant Gametes _____

QUESTION 3. Female Plant Gametes _____

Write down the phenotypes as you complete the problem

Punnett Square

QUESTION 4. Use the Punnett square to determine the expected genotypic ratios of the offspring.

QUESTION 5. Use the Punnett square to determine the phenotypic ratio for the offspring.

Problem 2: Green pods (G) are completely dominant over yellow pods (g). You cross a plant that is heterozygous green pods (male) with another plant that has homozygous for yellow pods.

QUESTION 6. Male Plant Genotype _____ Female Plant Genotype ______

QUESTION 7. Male plant Gametes _____

QUESTION 8. Female Plant Gametes ______

Write down the phenotypes in the Punnett square as you complete the problem

Punnett Square

QUESTION 9. Use the Punnett square to determine the expected genotypic ratios of the offspring.

QUESTION 10. Use the Punnett square to determine the phenotypic ratio for the offspring.

Problem 3: Inflated pods (I) are completely dominant over constricted pods (i). You cross two heterozygous (one male and one female) pea plants.

QUESTION 11. Male Plant Genotype _____ Female Plant Genotype ______

QUESTION 12. Male plant Gametes _____

QUESTION 13. Female Plant Gametes ______

Write down the phenotypes in the Punnett square as you complete the problem

Punnett Square

QUESTION 14. Use the Punnett square to determine the expected genotypic ratios of the offspring.

QUESTION 15. Use the Punnett square to determine the phenotypic ratio for the offspring.

Problem 4: Round seeds (R) are completely dominant over wrinkled seeds (r). If a male pea plant is heterozygous for round seeds, can any of this plant’s offspring have round seeds? Wrinkled seeds? Think about the different genotypes (females) with which you can cross the heterozygous parent: homozygous dominant, heterozygous, and homozygous recessive.

QUESTION 16. What is the genotype of the male plant? ____

QUESTION 17. Unknown female plant, list the three possible genotypes:

____, ____, ____

· Use the same “Dad” but use a different “Mom” for each cross.

· Write down the phenotypes as you complete each square

Punnett Square

Punnett Square

Punnett Square

QUESTION 18. Look at the three Punnett squares. Which female genotype(s) will only give you round seeds when crossed to a heterozygote?

Female genotype:

QUESTION 19. Look at the three Punnett squares. Which female genotype(s) will give you wrinkled seeds when crossed to a heterozygote?

Female genotype:

QUESTION 20. Look at the three Punnett squares. What is the genotypic ratio for the offspring when both plants are heterozygous for round seeds?

Genotype ratio:

Problem 5: Yellow seeds (G) are completely dominant over green seeds (g). You cross two pea plants (male and female) that are heterozygous for green seeds.

QUESTION 21. Male Plant Genotype _____ Female Plant Genotype ______

QUESTION 22. Male plant Gametes_____ Female Plant Gametes ______

Write down the phenotypes in the Punnett square as you complete the problem

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Punnett Square