Mendelian Genetics

How do parents pass on genetic material to their offspring?

THROUGH MENDEL’S MANY YEARS AND HUNDREDS OF PIECES DATA, HE THREW OUT MANY OLD IDEAS ABOUT HEREDITY AND CAME UP WITH
FOUR HYPOTHESES THAT TURNED INTO TWO LAWS THAT STILL HOLD TRUE TODAY. THE FIRST HYPOTHESIS STATED THAT INDIVIDUALS HAVE TWO
COPIES OF THEIR GENES, ONE FROM EACH PARENT. THE SECOND HYPOTHESIS SAYS THAT THERE EXIST TWO DIFFERENT VERSIONS OF THE SAME
GENE REPRESENTED BY LETTERS. WE NOW CALL THOSE VERSIONS ALLELES. THE THIRD HYPOTHESIS STATES THAT IF TWO DIFFERENT ALLELES OCCUR
TOGETHER, ONE MAY BE EXPRESSED WHILE THE OTHER IS NOT. WE SAY ONE IS DOMINANT AND THE OTHER IS RECESSIVE. HIS FOURTH AND FINAL
HYPOTHESIS STATES THAT WHEN GAMETES ARE FORMED, ALLELES FOR EACH TRAIT SEPARATE INDEPENDENTLY DURING MEIOSIS. FROM THESE
HYPOTHESES WHICH HAVE BEEN PROVEN TRUE TIME AND AGAIN, WE NOW HAVE TWO LAWS THAT CAN BE ATTRIBUTED BACK TO MENDEL’S
RESEARCH. THE FIRST LAW IS CALLED THE LAW OF SEGREGATION AND IT SAYS THAT BECAUSE EACH INDIVIDUAL HAS TWO DIFFERENT ALLELES, IT
CAN PRODUCE TWO DIFFERENT TYPES OF GAMETES. IF THE GENE IS REPRESENTED BY THE LETTER R, IT CAN PRODUCE R ALLELE OR r ALLELE, WHICH
REPRESENTS DIFFERENT FORMS OF THE SAME TRAIT. THE SECOND LAW IS CALLED THE LAW OF INDEPENDENT ASSORTMENT AND IT STATES THAT
GENES FOR DIFFERENT TRAITS ARE INHERITED INDEPENDENTLY OF EACH OTHER. FOR EXAMPLE, IF A PERSON HAS GENE A AND GENE B ON THE
SAME CHROMOSOME, THEY ARE BOTH INHERITED WITHOUT BEING TIED TO THE OTHER.

Model 1—Mendelian Inheritance

Learning Objective: I can identify classic principles of inheritance.

Homozygous dominant, homozygous

Genotype Phenotype
recessive, or heterozygous?

HH Homozygous dominant 1.

hh 2. Short Hair

3. 4. Heterozygous 5.

1. Based on the information given at the top of the page: in your own words, define gene, allele, dominant, and
recessive.

2. Consider the table in Model 1.

a. What phenotypic trait is being expressed by the gene represented?
 b. What are the two alleles that can be produced by this gene?

c. Which phenotype is dominant and which is recessive?

d. Which allele represents the dominant trait and which represents the recessive trait?

3. Complete Model 1 by filling in blanks 1-5.

4. Work together to define phenotype and genotype.

5. Describe the relationship between genotype and phenotype.

6. What are all possible genotypes that would lead to a guinea pig having long hair? Explain why more than 1
genotype leads to the long-haired phenotype.

Model 2—Solving Monohybrid Punnett Squares

Learning Objective: I can solve classic genetic problems using monohybrid Punnett Squares.

Example 1: Example 2:

is bred with is bred with

H H

h Hh Hh

h Hh Hh

Resulting in 100% offspring. Resulting in ___________ offspring.

 1. Consider Example 1 from Model 2. Given a cross between a Guinea pig with genotype HH and one with
genotype hh, what is the likelihood of offspring having long hair?

2. Complete the cross between two short haired Guinea pigs shown in Example 2 from Model 2.
a. What is the likelihood of offspring having long hair?

b. What is the likelihood of offspring having short hair?

3. What if the long-haired Guinea pig from Example 1 had genotype Hh—what would the likelihood of offspring
having long hair be? Use a Punnett Square to answer.

4. Give an example of two crosses (HH x HH, Hh x HH, etc) that would result in the same in the same phenotypic
likelihoods in the offspring? Explain why.

5. Why is the Law of Segregation important as you think about Mendelian Inheritance?

Model 3—Solving Dihybrid Crosses

Learning Objective: I can solve dihybrid genetic problems using Punnett Squares and the multiplication
rule.

Genotype Phenotype
HHBB Long brown hair
1. Short white hair
HhBb 2.
hhBB 3.
hhBb 4.
5. or 6. Long white hair
7. or 8. Short brown hair
1. Recall the Law of Independent Assortment. Define it here.

2. Consider the table in Model 3.

a. How many traits are being represented?

b. Which gene codes for which trait?

c. For each trait represented, which phenotype is dominant? Which is recessive?

d. Why are two genotypes possible for the long white haired phenotype and the short brown haired
phenotypes?

3. Complete Model 3 by filling in the blanks for 1-8.

4. The multiplication rule (in terms of Mendelian Inheritance) states that the likelihood of an individual expressing
two traits simultaneously is equal to the likelihood of that individual expressing trait 1 multiplied by the
likelihood of them expressing trait 2. Write this out as a mathematical equation.

5. Using Punnett Squares and the multiplication rule, given a parent who is homozygous dominant for the hair
length trait and heterozygous for the hair color trait and a parent who is homozygous dominant for both traits,
what is the likelihood of offspring having long white hair?

6. Why was it important for us to recall the Law of Independent Assortment before we learning about dihybrid
crosses?
Model 4—Complex Inheritance Patterns
Learning Objective: I can differentiate between complex inheritance patterns.

NOT ALL TRAITS FOLLOW MENDEL’S LAWS. THEY FOLLOW DIFFERENT INHERITANCE PATTERNS THAT STRAY AWAY FROM THE SIMPLICITY OF
DOMINANT AND RECESSIVE ALLELES. FOUR SUCH EXAMPLES ARE INCOMPLETE DOMINANCE, CODOMINANCE, EPISTASIS, AND PLEIOTROPY.

Complex Pattern Genotype Phenotype

Incomplete dominance BB Brown hair
bb White hair
Bb Blonde hair
Codominance BB Brown hair
bb White hair
Bb Mix of brown and white hairs
Epistasis HH Hairy
hh Naked
Hh Hairy
Pleiotropy HH Long hair, blue eyes
hh Short hair, brown eyes
Hh Long hair, blue eyes

1. Which complex inheritance pattern(s) in Model 4 result in possible phenotypes not seen in the examples of
Mendelian Inheritance?

2. Given the information in Model 4, come up with a definition for codominance.

3. In Guinea pigs, blonde hair color occurs in the offspring of brown and white homozygotes. What cross would
produce offspring in the ratio of 1 brown : 2 blonde : 1 white?

4. In a cross between a naked (hhbb) guinea pig and a hairy brown haired (HhBb) guinea pig, what effect would the
expression of the hh gene have on the expression of the hair color gene in offspring?

5. In which of the examples listed in Model 4, does 1 gene seem to affect multiple phenotypic traits?

6. How do the examples given in Model 4 break Mendel’s Laws?