High school biology worksheet

Biology 30—Lab III.1 Page 4

BIOL200-27 Single Trait Lab Page 3

FINDING GENOTYPES AND PHENOTYPES FOR ONE TRAIT

In genetics, it is possible to calculate the results that should appear in offspring if the genotypes of both parents are known. These are called expected results. Expected results can be calculated by mathematics or use of Punnett squares. Thus, expected results are specific numbers and are not the result of random events. Observed results are those that appear in offspring in actual crossings. They are due to chance combinations of certain genes.

Expected and observed results may not always agree exactly, but there should be some agreement. Expected results are used to predict the results of a cross before the cross is done. If the expected results indicate that a certain type of offspring is likely, the cross can be carried out with some certainty that the type of offspring will appear in the observed results.

In this investigation, you will

a) substitute properly marked coins for gamete cells,

b) toss the marked coins 100 times to represent 100 offspring,

c) determine the expected numbers of genotypes for 100 offspring and compare them with the observed numbers of genotypes obtained through 100 coin tosses,

d) determine the numbers of expected phenotypes for a genetic cross, and compare them with the numbers of observed phenotypes obtained through coin tossing.

Part A: Determining Numbers of Expected Genotypes

How many of each genotype combination are expected in the offspring of a cross if both parents are Ss for a trait?

Use the Punnett square below to determine the genotypes. Record the number of each genotype Record the number of each genotype in column A of Table 1. If there are 100 offspring multiply each number in column A by 25. Record this number in column B of Table 1.

	S	s
S
s

Part B: Determining Numbers of Observed Genotypes

Cover both sides of two pennies with adhesive tape. Trim off any excess tape with scissors. Print an S on one side of each coin and an s on the other side of each coin.

Place both coins in cupped hands, shake, and then toss the coins onto your desk. Read and record the letter combination in column C (Toss Results) of Table 1. Make a slash in the proper row of column C to indicate the letter combination. Repeat this process until the coins have been tossed 100 times. Record the coin combinations for each toss in Table 1.

Record in column D the totals for each.

Table 1: Expected and Observed Genotypes
Gene Combination	(A) Expected Genotype For 4 Offspring	(B) Expected Genotype For 100 Offspring	(C) Toss Results	(D) Observed Genotype For 100 Offspring
SS
Ss or sS
ss

Part C: Determining Numbers of Expected Phenotypes

Assume that S represents the dominant gene for normal skin pigment. Assume that s represents a recessive condition called albinism, no skin pigment. From the Punnett square, list in column A of Table 2 the number of offspring expected to have normal skin colour (SS or Ss) and the number expected to be albino (ss).

Calculate the number expected to have each trait if there are 100 offspring. Do this by multiplying column A figures by 25. Record these numbers in column B of Table 2.

Part D: Determining Numbers of Observed Phenotypes

From your data in column D of Table 1, total and record in Column C of Table 2 the number of offspring who will have normal skin pigment (SS, Ss or sS) and those who will be albino (ss).

Table 2: Expected and Observed Phenotypes
Phenotype Possible	(A) Expected Phenotype For 4 Offspring	(B) Expected Phenotype For 100 Offspring	(C) Observed Phenotype For 100 Offspring
Normal Skin
Albino

Analysis

1. What does each side of each coin represent?

2. How does the chance of a coin landing on each side compare to the chance that a gamete cell will receive a particular gene at meiosis?

3. Why must two coins be used to determine the genotypes for the offspring?

4. What does the use of two coins compare to at fertilization?

5. Compare the expected genotypes of 100 offspring with the observed genotypes.

6. What is the advantage of comparing the 100 expect offspring with the 100 observed offspring rather than comparing only four expected offspring with four observed offspring?

7. Compare the expected phenotypes for 100 offspring with the observed phenotypes.

Genotype & Phenotype For One Trait

Genotype & Phenotype For One Trait Page 2 of 4