Mendelian Genetic Activities



L3 Biology Mendelian Genetic Activities

Introduction

The Augustinian monk Gregor Mendel (1822-1884) conducted his experiments within the confines of a quiet monastery garden in almost complete anonymity. His findings were written down, but only after his death were his contributions to genetics recognized. Mendel chose the common garden pea, Pisum sativum, as his experimental model. It was an excellent choice. The plants were commercially available, easy to cultivate and grew rapidly. Different varieties had clearly different characteristics that “bred true”—reappearing consistently in crop after crop. Also the sexual structures of the plant are completely enclosed within the petals of the flower; consequently the flower normally self-pollinates (sperm from the flower’s own pollen fertilizes its egg cells) essentially creating genetic clones of the parent. If pea flowers were pollinated by insects, the large number of hybrid offspring and the genetic variety created would have clouded Mendel’s results and made the clear patterns that led to Mendel’s conclusions impossible to detect.

Many other people may have noticed genetic effects just as Mendel did, but it was Mendel’s application of mathematical interpretation to his results and his clear replicable reporting of the experiments that was revolutionary for its time.

Mendel’s experiments

Mendel’s experiments were an extension of the efforts of a long line of biologists and farmers that studied the effects of unifying (hybridizing) two different parental stocks. It had long been thought that heredity was purely a blending process, with offspring being a “dilution” of different parental characteristics. It was Mendel who demonstrated the appearance and disappearance of traits follows specific laws which could be determined merely by counting the different characteristics of offspring produced from any particular set of crosses.

Mendel found seven traits that he could observe clearly being passed from parents to offspring: seed form and color, flower position and color, pod form and color, and stem length (height of plant). Mendel began each experiment by intentionally dusting the pollen from one parent onto the stigma of the flower of another variety. He called this first cross the “parental generation”; abbreviated P. The first generation of offspring of the parents was called F1 (meaning family number one). In every case Mendel found that the F1 generation showed the same trait as only one of the parents. This is called the dominant phenotype or trait. When the F1 offspring were allowed to self-fertilize (essentially crossing two F1 organisms) Mendel observed a return of some of the missing traits, which he called the recessive trait. The second generation is known as F2.

Table 1: A summary of results of Mendel’s original crosses.

| |Original Crosses P | F2 (Second Generation) |

|Trait |Dominant X recessive |Dominant |recessive |Ratio (use two decimal |

| | | | |places) |

|Seed form |Round x wrinkled |5474 |1850 | |

|Seed color |Yellow x green |6022 |2001 | |

|Flower position |Axial x terminal |651 |207 | |

|Flower color |Purple x white |705 |224 | |

|Pod form |Inflated x constricted |882 |299 | |

|Pod color |Green x yellow |428 |152 | |

|Stem length |Tall x dwarf |787 |277 | |

Mendel’s Principles of Heredity

Mendel’s First Law: The principle of segregation.

The hypothesis that every individual carries a pair of factors (alleles) for each trait and the members of the pair segregate (separate) during the formation of gametes is known as Mendel’s First Law. When alleles of the same pair are the same, the organism is said to be homozygous for that trait; when the alleles are different, the organism is said to be heterozygous for that trait. In the genetic make-up or genotype of the trait, each allele exists independently as a discreet unit, even when the alleles come from a heterozygous pair and the actual trait seen, called the phenotype, may not have reflected the recessive allele. The recessive allele will segregate from its dominant partner when gametes are formed. Only if two recessive alleles come together—one from the male and one from the female—will the recessive trait be seen as the phenotype.

Mendel’s Second Law: The principle of independent assortment.

This principle states that members of each pair of alleles for a trait not only segregate, but do so randomly. These alleles are said to assort independently.

Mendel formulated this idea following a second set of experiments where he studied the inheritance of two traits at once (e.g. one parent plant had seeds that were round and yellow, and another parent had seeds that were wrinkled and green.) As seen in Table 1, the round and yellow traits are dominant, and the wrinkled and green are recessive. As you would expect, all seeds produced in the F1 generation showed the dominant phenotype. When the F1 seeds were allowed to self-pollinate, 556 seeds were produced in the F2 generation. The results are shown in Table 2 below.

Table 2: Results from the F2 generation following a cross of round yellow and wrinkled green parents in the P generation.

|Traits |Round, yellow |Wrinkled, yellow |Round, green |Wrinkled, green |

|Number of seeds showing each |315 |101 |108 |32 |

|trait | | | | |

Tally and compare separately the round vs. wrinkled ratio and the yellow vs. green ratio to see if the traits follow a Mendelian 3:1 ratio.

|# round |# wrinkled |Ratio (2 decimal places) |

| | | |

|# yellow |# green |Ratio (2 decimal places) |

| | | |

These experimental results did not contradict Mendel’s early experiments. Round and wrinkled still appeared in the same 3:1 ratio and so did yellow and green. But the round and yellow traits and the wrinkled and green traits, which had originally combined in one parent plant each, sorted themselves out as if they were independent of one another.

➢ Calculate the phenotype ratio when taking both traits into account (hint: divide the number of each observed phenotype by the number observed for the double-recessive trait).

: : : 1

Adapted from Ward’s Natural Science Establishment: Principle of Mendelian Genetics, 1995.

Activity 1

In this activity you will see how the laws of chance relate to biological occurrences. Mendel realized the role of chance in heredity and it was a radical concept for its time.

Materials (per team of two students)

Two coins (which can be different types).

Background information and Questions

Fortunately for casino operators, many people do not truly understand the statistics behind random chance. For an event that is truly random (contrary to the rumor, coins flips really are random), the previous results have no bearing on the next outcome. A coin that has flipped tails up three times in a row doesn’t “know” that it is “due” so the next flip is still random.

What are the odds of a normal coin landing heads up after being flipped?

What is the probability (odds) of two coins both landing heads up after being flipped? (Hint: the probability of a sequential event is the probability of each event separately, multiplied together.)

What is the probability of three coins all landing heads up after being flipped?

Imagine three coins are flipped, but one at a time. The first two coins both land heads. What are the odds of the last coin landing heads up?

Procedure

1. Complete the Punnett squares below. (A = any generic dominant trait; a = any generic recessive trait)

a) Aa x Aa cross b) Aa x aa cross

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a) Record the genotype and phenotype ratios for the Aa x Aa cross.

b) Record the genotype and phenotype ratios for the Aa x aa cross.

1. Create a Punnett square to predict the outcome of tossing two coins (assume heads is one allele and tails is the other, neither is dominant).

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3. Flip or toss your two coins 100 times (Yes, you have to do it 100 times). Put tallies in the top boxes and then count the tallies and write the number in the total row of the table.

| |Heads/Heads |Heads/Tails |Tails/Tails |

| | | | |

|Actual total | | | |

|Predicted | | | |

3a Fill in the predicted number (based out of 100 flips) of each category based on your Punnett square from question #1.

➢ Do the actual results match your prediction? Explain why they do or do not.

4. If you toss the coins 1000 times instead of 100, would you expect the actual and predicted numbers to match more closely than you saw after 100 flips, be the same as after 100, or become very different? Explain.

Activity 2

This activity investigates crosses between pea plants that are different in two trait characteristics (a dihybrid cross) with each pair having one dominant and one recessive allele—and how each gene pair acts independently of the other. Phenotypes in the resultant generations will be, on average, in a ratio of 9:3:3:1, demonstrating the independence, or independent assortment, of these two gene pairs.

Materials needed (per team of two students)

• Two (2) four-sided pyramidal dice—each numbered point represents a possible gene pair that the parents could pass on.

|Die number |Genotype |Phenotype |

|1 |RY |Round Yellow |

|2 |Ry |Round green |

|3 |rY |wrinkled Yellow |

|4 |ry |wrinkled green |

1. Which traits (seed form and seed color) are dominant? _______________ _________________

Which traits are recessive? ____________________ ____________________

2. What is the genotype of the F1 generation following Mendel’s P cross of a pure dominant pea plant (RRYY) with a double recessive plant (rryy)? Hint: this is easy, think about what alleles each parent will contribute and put them together in a new genotype (

What would be the only phenotype observed from this cross?

3. Complete the Punnett square showing possible outcomes for the F2 generation (cross RrYy x RrYy).

|RRYY | | | |

| | | | |

| | | | |

| | | |rryy |

4. Calculate the expected phenotypic ratio based on the outcome of the Punnett square.

: : : 1 green wrinkled

5. Use the two four-sided dice to generate some random data that simulates the uniting of gene pairs in a new organism (zygote). Read the dice by reading the number on the point of each die that is facing up. The two numbers correspond to a pair of alleles as shown in the chart in the materials section. Thus a roll of 1, 4 would represent RY and ry coming together as the genotype RrYy. The phenotype for this combination is Round Yellow, so that is the column where the mark is written.

➢ Toss the dice about 80 times and record the phenotypes generated by each roll in the chart below (use a tally mark ‘ for each one). If you accidentally go over 80, use all data you have.

| |Round Yellow |Round green |wrinkled Yellow |wrinkled green |

| |1,1 1, 2 1,3 1,4 2,3 |2,2 2,4 |3,3 3,4 |4,4 |

| | | | | |

|Totals ( | | | | |

Use your totals and calculate the phenotypic ratio observed from the dice toss (use 2 decimal places).

: : :

6. Put your data on the board as directed by your teacher. Total all the class data and recalculate the phenotypic ratio for the whole class total.

➢ Explain (referencing rules of probability as related to sample size) any differences seen from group to group and overall compared to the predicted outcome. In other words, why is the class total (or maybe certain individual groups) closer to the “perfect” 9:3:3:1 ratio than others are?

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