1 Heredity Lab Mendelian Genetics Part 1: Terminology Beginning students of biology always learn about Mendelian genetics. Inevitably, the study of inheritance always leads to additional questions. In fact, Mendelian inheritance patterns are exceedingly rare, especially in humans. We now know that inheritance is much more complex, usually involving many genes that interact in varied ways. Nonetheless, a clear understanding of basic inheritance patterns that follow Mendel’s original observations will provide a springboard for understanding current scientific exploration. Inheritance patterns that follow Mendelian rules are as follows: • Traits are governed by single genes • There are two alternate forms of a gene, known as alleles • Alleles are expressed as dominant and recessive It just so happened that the traits Gregor Mendel observed in his pea plants did indeed conform to these rules. After collecting and analyzing his data, Gregor Mendel developed two laws of inheritance: The Law of Segregation and the Law of Independent Assortment. 1. Describe these laws below: A. The Law of Segregation B. The Law of Independent Assortment 2. Before you can work with problems involving Mendelian inheritance, you need to be comfortable with the following terms: Define them and give examples. DNA Chromosome Gene Locus
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Heredity Lab
Mendelian Genetics
Part 1: Terminology
Beginning students of biology always learn about Mendelian genetics. Inevitably, the study of
inheritance always leads to additional questions. In fact, Mendelian inheritance patterns are
exceedingly rare, especially in humans. We now know that inheritance is much more complex,
usually involving many genes that interact in varied ways. Nonetheless, a clear understanding
of basic inheritance patterns that follow Mendel’s original observations will provide a
springboard for understanding current scientific exploration.
Inheritance patterns that follow Mendelian rules are as follows:
• Traits are governed by single genes
• There are two alternate forms of a gene, known as alleles
• Alleles are expressed as dominant and recessive
It just so happened that the traits Gregor Mendel observed in his pea plants did indeed
conform to these rules. After collecting and analyzing his data, Gregor Mendel developed
two laws of inheritance: The Law of Segregation and the Law of Independent Assortment.
1. Describe these laws below:
A. The Law of Segregation
B. The Law of Independent Assortment
2. Before you can work with problems involving Mendelian inheritance, you need to be comfortable with the following terms: Define them and give examples.
DNA
Chromosome
Gene
Locus
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Diploid
Haploid
Allele
Dominant
Co-Dominant
Recessive
Genotype
Phenotype
Homozygous
Heterozygous
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Part 2: Mendel’s First Law- Law of Segregation
The Law of Segregation states that alternative alleles of a trait segregate independently
during meiosis.
Using a technique known as Punnett Square analysis, we will see how Mendel analyzed his
monohybrid crosses to come up with the Law of Segregation.
Procedure
Carefully follow each step to create a Punnett square analysis. You can use these SAME
general procedures to analyze EVERY Punnett Square you do!
Problem: In pea plants, height is coded for by the “T” gene. The dominant allele (T) codes for the tall phenotype while the recessive allele (t) codes for the short phenotype. Make a cross between a true breeding tall pea plant and a true breeding short pea plant.
Things to consider and how to solve the problem;
1. What are the phenotypes of the parent plants? The parents are considered the P
Determine the genotypes of each parent plant. 1st Parent _____ 2nd Parent_____
2. Imagine each parent goes through MEIOSIS to produce gametes. List the genotype(s) of the possible gametes that each parent would produce.
3. Create a Punnett square that displays the GENOTYPES of the possible offspring. Also
label the PHENOTYPES of the possible offspring. These offspring are considered the F1
(first filial) generation. Remember, parents only give up one allele for each trait, so there
is only 1 allele above each box. When you “multiply” alleles the offspring will each have 2;
one from each parent.
Genotypes of parent
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4. Now allow the F1 generation to self-pollinate. What are the possible gametes that each F1 parent can produce?
5. Create a Punnett square crossing two F1 offspring that displays the Genotypes of the possible offspring. Also label the PHENOTYPES of the possible offspring. These offspring are considered the F2 (second filial) generation. Genotypes of parent
Note: Always reduce the phenotypic and genotypic ratios to their lowest terms.
1. What is the phenotypic ratio of the F1 generation?
2. What is the genotypic ratio of the F1 generation?
3. What is the phenotypic ratio of the F2 generation?
4. What is the genotypic ratio of the F2 generation?
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Part 3: Probability
Do the expected and observed phenotypic and genotypic ratios always match up in real life? In the case of flipping coins, we would expect to see heads 50% of the time and tails 50% of the time. But, does this always occur? Let’s explore! Materials- 2 coins
Procedure 1. Working with a partner, take two coins and assume that heads represent the dominant allele (A)
and tails represents the recessive allele (a). The genotype for each coin is heterozygous (Aa).
2. Assume that each coin represents one parent. When a single coin is flipped, one gamete is formed (through the process of meiosis). If the flipped coin is on heads, then the gamete has the dominant allele (A). When both coins are flipped simultaneously, there will be two possible gametes which can combine through fertilization to form a zygote. Each time you flip both coins, you will record the “genotype” of the offspring.
3. Flip the coins 100 times and record your results in the chart below.
Expected results
(After 100 flips)
Your results (# of flips
with each outcome)
Class results
Genotype Expected
count
Ratio
(4*count)
total flips
Tally Observed
count
Ratio Observed
count
Ratio **
AA
Aa
aa
Total
flips
100 -- Total flips 100 Total
flips
Calculate the ratios using this formula:
Genotypic Ratio = # of possible combinations (4) x # of flips of a given genotype (from tally)
total number of flips counted (100)
**Note: If calculating class totals, the denominator in this equation is equal to the total of all flips counted by all
students in the class.
4. Record your results on the board and when all groups have entered their totals, calculate the “Class Totals” column.
5. What is the expected genotypic ratio for a cross between two Aa coins?
6. Did the observed and expected genotypic ratios match? Why or why not?
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Part IV: Make a Baby
Purpose: To demonstrate the principles of Mendelian genetics and sex determination, including the
concepts of allele, phenotype, genotype, dominant, recessive, codominant, homozygous and heterozygous
by creating a simulated baby.
Materials: Two pennies, art supplies, paper.
Procedure:
1) Working with a partner, determine the genotype of the baby by flipping pennies. "Mom" flips one penny
to choose an allele for her egg and "Dad" flips the other to choose an allele for his sperm. (Note that
the gender of the baby is a special case and is determined by dad alone. Boys are XY and girls are XX.
Mom can give only an X but dad can give either an X or a Y.)
2) Record the alleles which resulted from the coin flips, and put "sperm and egg" together. (You cannot
pick the traits you want; life doesn't work that way!) Write down baby's genotype for each trait in
9. PKU is an inherited disease determined by a recessive allele. If a woman and her husband are both carriers, what is
the probability of each of the following? Show your work.
a. All three of their children will be normal _____________________________________
b. One or more of the three children will have the disease. ________________________
c. All three children will be afflicted with the disease. ____________________________
d. At least one child will be normal. _________________________________________
10. In an examination of Mendel’s principles, strain of light brown mice was crossed with a strain of dark brown mice. All F1 were dark brown. In the F2, 42 were dark brown and 15 were light brown. Is this consistent with the law of segregation? Explain.
25. The pedigree below shows the ABO blood group for a family.
What is the genotype for the following individuals?
Individual Genotype Individual Genotype
(a) (f)
(b) (g)
(c) (h)
(d) (i)
(e)
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26. The following pedigree traces a form of deafness in a family. This deafness is a recessive trait. Using the letters N for normal and n for deafness, provide the genotypes for the individual indicated in the chart that follows.
Individual Genotype Individual Genotype
I 1 III 5
II 2 III 9
II 6 IV 4
II 7 IV 12
27. The pedigree below traces brachydactyly, a condition in which fingers are abnormally short, through several
generations of a family. Those individuals afflicted with brachydactyly are shaded. Use this pedigree to
answer the questions that follow.
a. Examine the children produced by individuals 6 & 7 in Generation II. How do you explain the fact