BIO 120 Grossmont College Dihybrid Cross Laboratory Model Lab Report

NOTE TO TUTOR: Please do question 1 which is the definitions and then start from question 7 and do the tables.

As usual use basic English/ Common language and short answers not long ones.

Question 1. Before starting the lab work, define the terms below. If necessary, look these up in your biology textbook and print the definitions in the spaces below:

1. allele

1. trait

1. gene

1. homozygous

1. heterozygous

1. genotype

1. phenotype

1. genetics

7.Complete the “offspring” column of Table 1 by writing the genotype derived by combining the two gametes in fertilization. To make similar genotypes easily identifiable, always group alleles for the same trait together, and write the letter for any dominant allele first. (e.g., Ab + ab = Aabb).

Table 1

Dihybrid Cross Laboratory Model:

OBSERVED Genotypes of Offspring from the Cross

AaBb X AaBb

Note: Combine the first and second gametes to obtain the genotype of each offspring. For each offspring, group the alleles with the same letter together5, placing the capital letters first

Breeding	First Gamete*	Second Gamete	Genotype of Offspring
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.

1. Count the observed number of each offspring genotype in Table 1. Tabulate these observed numbers according to the genotype list in Table 2.

1. Determine the phenotype for each offspring genotype and fill in the phenotype column of Table 2. Use your answers to Question 4 (above) to help you identify the phenotypes.

Table 2

Tabulation of Offspring OBSERVED from the Dihybrid Cross

AaBb x AaBb

Genotype	Number Observed	Phenotype
AABB
AABb
AAbb
AaBB
AaBb
Aabb
aaBB
aaBb
aabb

1. Count the observed number of each offspring genotype in Table 1. Tabulate these observed numbers according to the genotype list in Table 2.

1. Determine the phenotype for each offspring genotype and fill in the phenotype column of Table 2. Use your answers to Question 4 (above) to help you identify the phenotypes.

1. Using the numbers from Table 2, add up the total observed offspring of each of the six possible phenotypes. Record these values in the “observed” column of Table 3.

Table 3

Phenotype	Number Observed by your Team (from Table 2)	Class Average Observed	Number Expected from Punnett Square (see Table 4)
curly hair, brown eyes
wavy hair, brown eyes,
straight hair, brown eyes
curly hair, blue eyes
wavy hair, blue eyes
straight t hair, blue eyes

Send your Table to Number Observed to Instructor Before Friday 11/6/20

1. The instructor will give you the class averages by Friday 11/6/20, copy the class averages for observed numbers into the proper column of Table 3.

1. To determine the expected numbers for the offspring phenotypes, complete the Punnett Square below (Table 4):

1. Write the genotypes of the 4 parental gametes in the indicated areas for each parent. (Recall you identified these in Question 3.) The first one has been done for you.

1. In each of the 16 boxes of the Punnett Square, combine the row and column gametes to give the genotype of the offspring. Remember to group alleles with the same letter together, with the capital letters (dominant alleles) first.

1. Add up all of the members of each phenotype in the Punnett Square and write the totals in the “expected” column of Table 3.

Table 4

Punnett Square: Calculating

EXPECTED Offspring Frequencies from the Dihybrid Cross

(Parent #1) AaBb x AaBb (Parent #2)

Note: Combine the first and second gametes to obtain the genotype of each offspring. For each offspring, group the alleles with the same letter together, placing the capital letters first.

		GAMETE Genotypes – Parent #2
GAMETE
Genotypes –
Parent #1

Question 5. Which were closer to your expected numbers for the offspring of the dihybrid cross: your team’s observed numbers or the class averages? Which observed numbers are more reliable predictors of population values?

Question 6. Determine the probability of getting any of the 4 different kinds of gametes possible from each dihybrid individual. For help, refer to the following:

1. The probability of any gamete genotype is a fraction:

probability =

1. List all the gamete genotypes for an AaBb individual and indicate their probabilities:

Question 7. A married couple both happen to be doubly heterozygous (dihybrid) for eye color and hair form. They have only one child who has blue eyes and straight hair. Determine the probability of this couple producing a child with the aabb genotype. Follow these steps:

1. Write the probability of an ab gamete from an AaBb parent:

1. At the time of fertilization, the probability of specific gametes getting together is the product of the individual probabilities for each gamete (this is known as the multiplicative law). Therefore, we can calculate the aabbprobability using the following equation:

P (offspring with aabb ) = P(ab sperm) P(ab egg ) =

1. Examine your Punnett Square (Figure 4).

How many total boxes are there?

How many of these are the aabb genotype?

What fraction of the offspring are expected to be aabb?

1. Your probability calculation should agree with the Punnett Square proportion for the aabbgenotype – does it?

Question 5. Which were closer to your expected numbers for the offspring of the dihybrid cross: your team’s observed numbers or the class averages? Which observed numbers are more reliable predictors of population values?

Question 6. Determine the probability of getting any of the 4 different kinds of gametes possible from each dihybrid individual. For help, refer to the following:

1. The probability of any gamete genotype is a fraction:

probability =

1. List all the gamete genotypes for an AaBb individual and indicate their probabilities:

Question 7. A married couple both happen to be doubly heterozygous (dihybrid) for eye color and hair form. They have only one child who has blue eyes and straight hair. Determine the probability of this couple producing a child with the aabb genotype. Follow these steps:

1. Write the probability of an ab gamete from an AaBb parent:

1. At the time of fertilization, the probability of specific gametes getting together is the product of the individual probabilities for each gamete (this is known as the multiplicative law). Therefore, we can calculate the aabbprobability using the following equation:

P (offspring with aabb ) = P(ab sperm) P(ab egg ) =

1. Examine your Punnett Square (Figure 4).

How many total boxes are there?

How many of these are the aabb genotype?

What fraction of the offspring are expected to be aabb?

1. Your probability calculation should agree with the Punnett Square proportion for the aabbgenotype – does it?

Question 10. Do you suppose there are other Mendelian characteristics which could be added to a heredity wheel?

How many different phenotypes would there be if you added just one more trait to the wheel? Two more traits?

Question 11. Describe how the variety of human phenotypes illustrates one of the fundamental biological requirement for evolution by natural selection. (Read about the requirements for natural selection in your textbook if necessary.)

Use the information in Figure 2 to help you evaluate your blood type. Indicate your test results below.

Did the “blood agglutinate?” (indicate + or -)

Antiserum	Mr. Smith	Ms. Jones	Mr. Green	Ms. Brown
anti-A
anti-B
anti-D (Rh)
“blood” phenotype
comments

1. Questions and Practice Problems

Question 12. Blood transfusions aim to give the patient a temporary supply of erythrocytes until his body can manufacture enough of its own. It is desirable that the donor and the recipient of the transfusion be of the same blood type. But it has been found that a person of blood type O can safely give blood (in limited quantity, a pint seems always to be safe) to persons of any other blood type. Thus, type O is sometimes called the universal donor.

How might this be explained? (Hint: refer to Figure 2.)

Question 13. What complications might arise if large quantities of blood from a donor of type O were introduced into a recipient of any blood type other than O?

Question 14. If persons of type O are sometimes called universal donors because they can donate to all other types, what blood type might be called the universal recipient? Why? (Refer to Figure 2.)

Question 15. Explain the fact that blood types A and B each have two genotypes.

Question 16. What blood types might occur among the children of a marriage between a person of blood type AB and a person of blood type O? Fill in the Punnett Square: show the gamete genotypes of each parent, then determine the offspring blood types.

What is the probability of the parents having a child with type A blood? Question 17. If you are blood type O and your father is also blood type O, what type or types must your mother be? (Write your father’s gamete genotypes and your own genotype on the Punnett Square. Then identify what you know about your mother’s genotype.)

Possible genotypes of mother:

Could these same parents have a child with blood type AB? Explain.

Question 18. Can a person of blood type A who marries a person of blood type B have type Ochildren? (Explain your answer and show your work.)

Question 19. If a homozygous Rh+ man fathered children with a homozygous Rh- woman, what fraction of the offspring would be Rh+? (Show your work: remember to first determine the egg and sperm genotypes.)

Question 20. If an Rh- woman gave birth to an Rh- child, what could you conclude about genotype of the father?

Question 21. Hemophilia is a hereditary disease characterized by poor clotting of the blood. As a result, hemophiliacs bleed excessively when injured. A certain kind of hemophilia is sex-linked and recessive. Sex-linked means that the allele for hemophilia is found on the X chromosome. Although recessive, the hemophilia allele (Xh) will determine the phenotype of the individual unless the individual is a female with a normal allele (XH) on her second X chromosome.

Problem: A “normal” woman whose father was a hemophiliac marries a normal man. What genotypes and phenotypes are expected in the children and in what proportions? (When you are working with sex-linked traits, it is a good idea to include both types of sex chromosomes in your Punnett Square.)

Question 22. It has been observed that there are more hemophiliac children of one sex than the other born in the general population. Explain.

Question 23. The frequencies of the various blood groups have historically been quite stable in well-defined populations, i.e., they tend to remain unchanged in time, and characteristic of each group. The following chart shows these frequencies among several populations. Refer to Table 5 for theBiology 120 Class Frequencies and complete Table 6 by calculating the percentages of each blood type in your class.

Table 6. Comparison of Blood Group Frequencies

Population	Blood Group Frequencies
	O	A	B	AB	Rh+	Rh-
Japanese	25%	39%	24%	12%	99%	1%
Whites (USA)	45%	38%	12%	5%	85%	15%
Blacks (USA)	47%	28%	20%	5%	93%	7%
Aborigines (Australia)	34%	66%	0	0
Eskimos (Labrador)	49%	51%	0	0
Pueblo Indians (New Mexico)	88%	12%	0	0	98%	2%
Biology 120 Class (frequencies)
Biology 120 Class (percents)

Question 24. Is the frequency distribution for the Biology 120 class similar to any of the others? Comment on this.

Question 25. Imagine that one of your lab partners thinks that recessive phenotypes must be “weaker” than dominant phenotypes. As a result, the student concludes that there must always be fewer recessive genes in the population. The student cites as an example the fact that there are fewer blue-eyed people than brown eyed people. Explain how you would use the information in this lab exercise to set your partner straight.

Question 26. Suggest some factors which might act to bring about changes in the relative proportions of the various blood groups in a population. (Hint: consider reasons why the ethnic composition of an area might change.)

Question 27. What factors might act to keep the blood group frequencies in a population fairly constant over time?

Question 28. A man whose blood group genotype is AO marries a woman with Type AB blood. Assume both parents are also heterozygous for the Rh factor. Construct a Punnett Square which shows the genotypes of all possible offspring. Then organize the data: list all possible phenotypes, and the probability of this couple having any one of those phenotypes.

1. Parent genotypes: X

1. Punnett Square:

GAMETES from- Parent # 2

GAMETE
from-
Parent # 1

1. Expected offspring phenotypes and probabilities are: