Monohybrid crosses

How to Proceed

• Read through the introductory materials below.
• Open the Unit 6 Experiment Answer Sheet and complete the following Experiment exercises this unit:
  • Experiment 6 Exercise 1 – Monohybrid crosses (~45 min)
  • Experiment 6 Exercise 2 – Dihybrid crosses (~30 min)
  • Experiment 6 Exercise 3 – Inheritance of Human Traits (~30 min)
• Save your completed Unit 6 Experiment Answer Sheet and submit it no later than Sunday midnight (CT).

Monhybrid and Dihybrid Crosses – Introduction

Mendel crossed true-breeding pea plants in order to develop and understanding of how traits are inherited. True-breeding means that if a plant was crossed with itself, it always generated offspring that looked like the parent. Although Mendel didn’t know this at the time, it meant that the parent plant was homozygous or had two copies of the same allele that controlled the appearance of the trait.

Mendel noticed that when he crossed two true-breeding plants exhibiting different versions of a trait (e.g., green and yellow); the offspring (F1) always looked like only one of the parent plants. We know now that the F1 individuals looked like the parent that carried the dominant trait. But what surprised Mendel, was that when he crossed the F1 individuals with each other, the F2 offspring exhibited BOTH traits! Based on this observation, he concluded that the F1 individuals were hybrids, meaning they carried both alleles for a given trait. Only the dominant trait was expressed in the F1 individuals and the recessive trait, although present, was masked.

A monohybrid cross is when you are interested in crossing individuals that vary in only a single trait (e.g., flower color, seed color, stem length). In a dihybrid cross, we are crossing individuals that differ at two traits (e.g., flower color and seed color, flower color and stem length). Obviously, the more traits that vary, the more complex the crosses become!

By examining the distribution of the various traits obtained following different types of crosses, Mendel was able to describe the general pattern of genetic inheritance. Be sure to review the online lecture this unit on Genetics and pp 146-153 in your book before starting these first two exercises.

We will be using the following website for the first exercise. Be sure that you can access it and use it before beginning:

Glencoe-McGraw Hill. No date. Punnett Squares
http://glencoe.mcgraw-hill.com/sites/dl/free/0078759864/383934/BL_05.html (Links to an external site.)

You will need to complete the Tables and answer the questions in the Unit 6 Experiment Answer Sheet for Exercises 1 and 2.

Inheritance of Human Traits – Introduction

Some human traits are controlled by a single gene that has only two alternative alleles. If a characteristic is determined by the dominant allele, one or both parents express that trait and many of the children will as well. Dominant characteristics will most likely be present in every generation, since the expression of these traits requires only one of the dominant alleles in order to be expressed. If the characteristic is determined by the recessive allele, then neither parent may express the trait nor few of the children. This is because two copies of the recessive allele must be present in order for the recessive trait to be expressed. If a trait is X-linked recessive; meaning the gene for the trait is found on the X chromosome, it will be expressed primarily in males.

The application of human genotypes in medicine and genetic counseling is becoming more and more necessary as we discover more about the human genome. Despite our increasing ability to decipher the chromosomes and their genes, an accurate family history remains one of the best sources of information concerning the individual. In this exercise you will determine your genotype for certain characteristics that are controlled by a single gene with two alleles based on your phenotype. We will not be looking at any X-linked traits in this exercise.

Use the information about the traits of interest in the Unit 6 Experiment Answer Sheet to answer the questions found there.

WEEK 6 EXPERIMENT ANSWER SHEET Please submit to the Week 6 Experiment dropbox no later than Sunday midnight.

SUMMARY OF ACTIVITIES FOR WEEK 6 EXPERIMENT ASSIGNMENT

· Experiment 6 Exercise 1 – Monhybrid Crosses

· Experiment 6 Exercise 2 – Dihybrid Cross

· Experiment 6 Exercise 3 – Inheritance of Human Traits

Experiment 6 Exercise 1: Monohybrid Crosses

You will be conducting monohybrid crosses using fruit flies. Open in the following website:

Glencoe-McGraw Hill. No date. Punnett Squares http://glencoe.mcgraw-hill.com/sites/dl/free/0078759864/383934/BL_05.html

Procedure

A. Open the above website and click on the VCR to listen to the introduction. Close the window when done.

B. Click on the Lab Notebook on the lab bench. A breeding scenario will be presented to you that you will need to carry out. Here are the possible phenotypes and genotypes you will using:

a. Normal wings (LL or Ll) or vestigial wings (ll)

b. Gray body (GG or Gg) or black body (gg)

C. Enter the Scenario number in Table 1 below.

D. Based on the Scenario, use the down arrows beneath the Parent 1 and Parent 2 boxes to select the appropriate parents. Look carefully at the flies so that you know which ones to select.

E. Before proceeding, click on the Check Parents button. If necessary, make corrections. If you are correct, the maternal and paternal alleles will be added to the Punnett Square.

F. Next, drag the correct allele combinations and the corresponding fly types to the boxes in the Punnett Square.

G. When you are done, click on Check Offspring. If necessary, make corrections.

H. Record your data in Table 1 below. An example has been given, but note that the example is not using the genotypes and phenotypes used in this exercise.

I. Click on Reset. Repeat steps B – H four more times for a total of five crosses. If a scenario is presented that has already been completed, click Reset again. DO NOT REPEAT a given scenario!

Note that the scenario number you need to record in the Table below is the number associated with the specific scenario you completed.

Table 1. Results of crosses.

	Parent Genotypes	Offspring Genotypes	Offspring Phenotype
Scenario #	Parent 1	Parent 2	#	%	#	%
Example	Rr	rr	2 Rr 2 rr	50% Rr 50% rr	2 red 2 white	50% red 50% white

Questions

1. Which type of cross gave you the greatest number of genotypes? Was the number of phenotypes the same as the number of genotypes or different? If different, why (2 pts)?

2. Can the genotype for a gray-bodied fly be determined? If so, how? (3 pts)?

Experiment 6 Exercise 2: Dihybrid Cross

We will continue to use flies for our crosses, but this time we will examine the inheritance of TWO different traits: body color (gray or black) and wing type (long or vestigial). As with our first crosses, the gray body color is dominant (GG or Gg) over the black body color (gg). And the long wing type is dominant (LL or Ll) over vestigial (ll). Be sure you have reviewed our online Genetics lecture and this week’s reading before proceeding. An example of a dihybrid cross is shown on p 150 in your book.

Recall our flies from the previous exercise. We have the following traits:

· Gray body (GG or Gg) is dominant over black body (gg)

· Long wings (LL or Li) is dominant over vestigial wings (ll)

We will cross a gray bodied fly with long wings which has the genotype GGLl with a gray bodied fly with long wings with a genotype of GgLl . Note that even though the phenotypes are the same, the genotypes of the two parents are different.

Identify the four possible gametes produced by these two individuals. Note that each gamete must consist of two alleles (G or g and L or l):

Parent 1 Parent 2

 

GGLl GgLl

Parent 1 (GGLl) Gametes: _______ ______ _______ _______ (1 pts)

Parent 2 (GgLl) Gametes: _______ ______ _______ _______ (1 pts)

Create a Punnett square to show the outcome of a cross between these two individuals (GGLl and GgLl) using the gametes you identified above (3 pts).

		Parent 1
Parent 2

Questions

1. What are the possible F1 genotypes (these must now have four alleles) and their percentages (4 pts)?

2. Recall that GG and Gg individuals are gray bodied, while gg individuals are black bodied and that LL and Li individuals have long wings, while ll individuals have vestigial wings. What are the phenotypes of the resulting offspring and what are the percentages of these phenotypes (2 pts)?

Experiment 6 Exercise 3: Inheritance of Human Traits

Read over the Inheritance of Human Traits Introduction under the Week 6 Experiment link in our course before beginning.

Procedure

A. For each of the heritable traits describe below, determine which form you have (dominant form or recessive form). This is your phenotype.

B. Record your phenotype information in Table 2 below. Then, enter the possible genotype(s) you have based on your phenotype.

C. Answer the questions found following Table 2 below.

Description of Heritable Traits

Trait	Possible Alleles	Dominant Form	Recessive Form	Examples
Ear lobes	E or e	Detached (Free)	Attached
Hairline	W or w	Widows peak	Straight	Widow’s peak Straight
Tongue rolling	T or t	Able to roll	Unable to roll
Hand folding	R or r	Right thumb on top	Left thumb on top
Chin	C or c	Cleft chin	No cleft chin
Tongue folding	F or f	Can fold tongue backwards	Cannot fold tongue backwards
Thumb	H or h	Straight thumb (cannot bend backwards)	Hitchhiker’s thumb (can bend it backwards)
Little Finger	B or b	Bent inwards	Straight
Mid-digital hair	M or m	Hair on fingers	No hair on fingers

An example is shown as to what should be entered in RED. Please correct the entry for “Ear lobes” based on your personal data. For the Genotypes, please use the letters provided above (8 pts).

Table 2. Your phenotypes and genotypes.

Trait	Phenotype	Genotype
Ear lobes	Unattached OR Detached	EE/Ee OR ee
Hairline
Tongue Rolling
Hand Folding
Chin
Tongue Folding
Thumb
Little Finger
Mid-digital Hair

Questions

1.  Which traits did you have that were dominant (1 pts)?

2.  Which traits did you have that were recessive (1 pts)?

3. What does it mean to be homozygous for a trait? Cite source(s) used (1 pts).

4.  What does it mean to be heterozygous for a trait? Cite source(s) used (1 pts).

5.  Define genotype and phenotype? Cite source(s) used (1 pts).

6.  Which traits do you know for sure that you were homozygous (1 pts)?

Week 6 Experiment Grading Rubric

Component	Expectation	Points
Experiment 6 Exercise 1	Correctly perform and record the outcome of five monohybrid crosses (Table 1).	5
	Demonstrate an understanding of the possible outcomes of monohybrid crosses with respect to genotypes and phenotypes (Questions 1-2).	5
Experiment 6 Exercise 2	Determine the correct parental gametes and conduct a dihybrid cross.	5
	Correctly evaluate the outcome of a dihybrid cross (Questions 1-2).	6
Experiment 6 Exercise 3	Correctly recognize one’s phenotype and assigns the correct genotype (Table 2).	8
	Demonstrate an understanding of dominant and recessive traits, genotype vs phenotype and homozygous vs heterozygous (Questions 1-6).	6
TOTAL		35 pts

 

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