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:
Fly Lab JS (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:
Fly Lab JS. No date.
A. Select a trait to modify from the left side of the page. For the monohybrid crosses, you will only change one trait at a time.
B. Select a trait for the male and then click “Select for Mating”. Repeat for the female.
C. You are now ready to mate the flies by selecting “Mate” from the menu near the top of the screen.
D. You will now analyze your results by clicking on “Analyze”. Click on the box “Ignore Sex of Flies”
E. Record the data in the table below.
F. Repeat five times to complete the table.
Table 1. Results of crosses.
Wild Type (WT)
1. How might you use these data to determine which traits were recessive and which were dominate (2 pts)?
2. Is it possible to determine the genotype for each cross? 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.
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
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).
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.
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
E or e
W or w
Widow’s peak Straight
T or t
Able to roll
Unable to roll
R or r
Right thumb on top
Left thumb on top
C or c
No cleft chin
F or f
Can fold tongue backwards
Cannot fold tongue backwards
H or h
Straight thumb (cannot bend backwards)
Hitchhiker’s thumb (can bend it backwards)
B or b
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.
Unattached OR Detached
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
Correctly perform and record the outcome of five monohybrid crosses (Table 1).
Demonstrate an understanding of the possible outcomes of monohybrid crosses with respect to genotypes and phenotypes (Questions 1-2).
Determine the correct parental gametes and conduct a dihybrid cross.
Correctly evaluate the outcome of a dihybrid cross (Questions 1-2).
Correctly recognize one’s phenotype and assigns the correct genotype (Table 2).
Demonstrate an understanding of dominant and recessive traits, genotype vs phenotype and homozygous vs heterozygous (Questions 1-6).
Updated October 2013