Genetics: the study of the traits of organisms.
What is a gene pool?
Sexual reproduction involves the combining of a sperm cell and an egg cell from two
individuals. The offspring of this "cross" will have characteristics from both "parents".
Organisms that are produced by crossing individuals with different traits are known as hybrids.
In most cases, hybrid organisms are more successful than those that are produced from a very
limited gene pool. This is sometimes called hybrid vigor. While gene sharing is well understood
today, before Gregor Mendel the process was a mystery.
Gregor Mendel is known as the father of modern genetics. Mendel's genius was that he noticed
differences in peas being grown in his garden and applied the scientific method to determine the
cause of these differences. He published a paper in 1865 with these three conclusions to his
* Principle of Dominance and Recessiveness - One factor in a pair may mask the effect of
the other.
* Principle of Segregation - The two factors for a characteristic separate during the formation
of eggs and sperm.
* Principle of Independent Assortment - The factors for different characteristics are
distributed to reproductive cells independently.
At the time, chromosomes and the process of meiosis were unknown. Mendel's work was
considered obscure and unimportant until 1900, when Walter Sutton proposed the Chromosome
Important genetic terms:
Genotype - the genetic makeup of an organism.
Phenotype - the external appearance of an organism.
Homozygous trait - the genes for that trait are the same.
Heterozygous trait - the genes for that trait are not the same.
Monohybrid cross - a cross between individuals with one pair of contrasting genes.
Dihybrid cross - a cross between individuals with two pairs of contrasting genes.
Parents - the two organisms whose genes produce offspring.
F1 generation - the offspring from parents.
F2 generation - the offspring produced by crossing two F1 individuals.
To understand genetics, one must understand probabilities.
The traits that are passed from parents to offspring are determined by the combination of genes
that are in the gametes. This is a random event in nature. The probability of a particular event
occuring is the fraction of outcomes in which that event occurs.
What is the probability of the number 5 coming up when a dice is rolled?
* A single roll of a dice has the possible outcomes 1, 2, 3, 4, 5, 6.
* There are a total of 6 possible outcomes, one of those being the number 5.
* The probability of the number 5 coming up is 1/6.
What is the probability of rolling an even number with a dice?
* The even numbers are 2, 4, 6.
* Each of these has a 1/6 chance of occuring.
* This gives us three chances in six of rolling an even number. 3/6 = 1/2
What is the probability of cutting an ace from a deck of shuffled cards?
Introduction to Punnett Squares:
A Punnett Square is a chart drawn to determine the probable results of a genetic cross. To be able
to draw a Punnett Square, you must know the genotype of both parents.
Sample Problem 1. In roses, red flowers are dominant over white flowers. What are the
possible offspring when a homozygous red rose is crossed with a homozygous white rose.
Nine steps are needed to solve problem 1:
Write genotype for each parent.
* Use a capital case letter of the dominant trait to represent a dominant gene.
o R - for the dominant trait red.
* Use a small case letter of the dominant trait to represent a recessive gene.
o r - for the recessive white.
* If no dominant trait is present, use a small letter of each trait.
* Both parents are homozygous in the sample problem. The genotypes for the cross are RR X rr
Write the possible gametes of one parent across the top.
Write the possible gametes of the other parent down the side.
Draw the lines of your Punnett Square. This is the first time you know how many squares are
inside the Punnett Square.
Combine the gametes from each parent in the squares. Always write capital letters first.
Count the number of each possible combination present in your Punnett Square. These are the
offspring genotypes.
1 Rr
The phenotypes are the way the offspring appear.
1 red
Since the genes are not the same, this trait is heterozygous.
Divide the number of each combination type by the total combinations.
You now have the probability ratio for the offspring of the parents.
One out of one, or all, of the offspring will be heterozygous red,
Sample Problem 2. Draw a Punnett Square showing the possiblities when two of the Rr
offspring above are crossed.
What genotypes are predicted by this square?
What phenotypes are predicted by this square?
Sample Problem 3. In carnations, neither red nor white is dominant. What are the possible
results when two pink carnations are crossed?
Since neither trait is dominant, no capital letters are used in the Punnett Square. Use the
first letter of each trait to indicate that gene in the square.
Test your knowledgeof Mendel's Laws.
Concept Understanding:
1. Mendel's paper explaining his experiment has been translated into English. Read the first
three sections to answer the following questions:
1. How long did Mendel run the experiment?
2. Why did Mendel think members of the family Leguminosae would be best for the
3. What is the scientific name (genus and species) of the plant Mendel used?
4. What seven characteristics in Mendel's plants were chosen for comparison during his
2. Answer the following questions about gene cloaning:
1. What are restriction enzymes used for?
2. What is the shape of a DNA plasmid?
3. What type of cells, through cloning, are being used to produce human insulin?
3. What does a capital letter represent in a Punnett Square?
4. When you see a Punnett Square with no capital letters in it, what do you know about the
traits represented in the square?
Research Links:
* Glossary of Genetic Terms - National Human Genome Research Institute
* Java Genetics - BioCrypt
* Genetics 320 - University of Arizona
* The Interactive Fly - Society for Developmental Biology
Lab 1: Bunny Babies
Students use their knowledge of how traits are inherited to determine the traits for Easter Egg
At fertilization the genetic traits of an individual are determined. Fertilization occurs when two
gametes, an egg and a sperm, join together. Each of the cells contributes a set of traits to the
offspring. In this activity you will use coins to represent the egg and sperm. The parents are
heterozygous for each of the possible traits for our bunny baby. You will flip the coins to
determine which trait your bunny baby will inherit. You will then build a model of your bunny
Two coins
Plastic egg
Glue gun
Construction paper
Sharpie Marker
Glue sticks
Punnet Square Practice: worksheet in science folder under Uplands.
Good worksheets:
Uses SpongeBob etc...
Lab 2:
Genetic make-up of Superheros
Lab 3:
Students see what genetic traits they have compared to others.
More info:
Life Science Standards - 6A, 8A
English Content Standards - 1A, 2A, 2B, 3A, 4A, 5A, 10A, 11A, 11B, 12B, 13A
Math Standards - 1A, 1B, 1C, 3B, 5A, 7A, 7B, 8C
Scientific Inquiry Standards - 12A, 13A, 14A, 15A
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