1. Unit 5 Genetics Lesson 3 Solving Mendelian Genetics Problems
John Levasseur
Some slides from:
Springfield Central High School

2. Questions to Ponder from Lesson 2
 Who was Gregor Mendel?
What did Mendel study at university ?
Why did Mendel study peas?
What is molecular genetics?
What is Mendelian genetics?
What are Mendel’s two laws? What do they mean?
What form of cell division proves Mendel’s first law?
What are genes?
How does math help explain genetics?
Solve Tt X Tt
Solve Aa X AA
Solve TtPp X TtPp
Solve TTPp X Ttpp

3. Example of One of Mendel’s Crosses
Recall that peas have a gene for seed color and this gene has two (forms) alleles: yellow and green color
Mendel cross pollinated a pea plant that he knew could only produce yellow peas with a pea plant that he knew could only produce green peas. 
His results were a collection of pea seeds that grew into pea plants that all produced yellow peas. 
Mendel’s next move was to cross pollinate a group of all these yellow producing pea plants. 
His results were seeds that grew pea plants 3 out of 4 which produced yellow seeds and one out of four produced green seeds. 
From the results of his crosses with 7 different traits he selected, Mendel knew there must be 2 “factors” (we now know they are alleles of a gene) that control the trait. 
Mendel also realized that one of these factors could disappear or be masked. He called the masked trait recessive and the unmasked trait dominant. 
Mendel based his Law of Segregation on the observations he made from the Yy x Yy cross. 
Mendel correctly realized that yellow seed (Yy) producing pea plants had one dominant “factor” from its yellow seed producing parent and one recessive “factor” from its green seed producing parent. 
These pea plants (Yy) can produce two different types of gametes-yellow seeds (Y) and green seeds (y). 
These gametes can combine as fertilization takes place, in four different combinations (YY, Yy, Yy, yy). 

4. Explain Mendel’s First Law.
Solve this cross
Mendel’s first law tells us that pairs of chromosomes will separate when the organism makes sex cells (gametes)
So what will happen to these pairs of chromosomes during meiosis?
Rr X Rr
How is this reported on a punnett square?
Explain Mendel’s First Law.

5. Another Mendelian Cross
The parent generation:
A true breeding tall pea plant crossed with a true breeding short.
The cross TT x tt produces all tall plants.
The (TT) tall plant only produces (T) tall gametes.
The (tt) short plant one produces (t) short gametes.
The F1Generation
Tall gametes (T) and short gametes (t) can only combine to make Tt.
The Tt offspring of the parent generation are known as the F1 generation.
The F1 generation plants can make two different gamete types (T) tall and (t) short.
The F2 Generation
These gametes can combine to form four types of offspring in a 1:2:1 ratio:
One homozygous dominant
Two heterozygous
One homozygous recessive
The phenotype ratio will be 3:1
Three tall and one short

6. Homozygous recessive, heterozygousB b
The genotype of the offspring in the red box is _______ bb
Define:
Homozygous dominant,
Homozygous recessive, heterozygous
The offspring in the red box is
homozygous heterozygous
homozygous

7. G g G g GG Gg gg The parents in this cross are _____________ 75
Homozygous Heterozygous
Heterozygous G g GG Gg gg
Define
Define:
Phenotype, genotype, ratio.
What is the genotype ratio?
What is the phenotype ratio?
; Phenotyp
If G is dominant for green pods and g is recessive for yellow pods, what percentage of the offspring will have green pods? _______% 75

8. Sample Problems on Monohybrid Cross:
One of Mendel’s monohybrid experiments involved pea pod shape gene. Pea seed pods can be inflated (the dominant allele) or pinched (the recessive allele).
Parent Generation
Mendel crossed homozygous dominant inflated (II) with homozygous recessive pinched (ii).
What was the result of this cross?
Tell genotype and phenotype ratio.
F1 Generation
The results of the parent’s generation cross are known as the F1 generation.
Mendel crossed his F1 generation with themselves.
Solve a cross between your F1 results.
F2 Generation
The results of crossing the F1 with themselves produces the F2 generation.
What will be the F2 genotype and phenotype ratios for this trait?

9. The 6 Possible Monohybrid Crosses
There are 6 possible monohybrid crosses for any gene that has 2 alleles.
Let’s makeup a gene that produces a protein.
We have a gene for the “AWESOME” protein
There are two alleles A and a
A is dominant and produces the “AWESOME” protein phenotype
a is recessive and produce a mutation of the AWESOME gene known as the “Awful” protein phenotype
Solve all 6 and give phenotype and genotype ratios.
(AA x AA)
(AA x Aa)
(Aa x Aa)
(Aa x aa)
(aa x aa)
(aa x AA)

10. Mendel’s Dihybrid Crosses
Dihybrid crosses (2 genes)
Mendel went on to show that the 7 different traits are inherited independently; this is Mendel’s second law which is the Law of Independent Assortment.
Mendel showed this with a dihybrid cross. Mendel considered two traits at the same time.
Let’s consider two traits, seed shape and seed color.

11. Example of a Mendelian Dihybrid Cross
If we start with true breeding round yellow seeds (genotype RRYY) and cross them with true breeding wrinkled green seeds (genotype rryy) what will be the result?
The result would be a collection of seeds that would grow into plants that could only produce round yellow seeds.
What will be the result if we take the F1 seeds that are round yellow (RrYy) and cross them with themselves (RrYy x RrYy)?
Here we need to FOIL to determine what gametes can be made.
4 different gametes can form:
RY, Ry, rY, ry
Now that we have determined the possible gametes we can set up the Punnett Square and solve the problem.
The results of this RrYy x RrYy cross will be 9 round yellow seed producers, 3 round green seed produces, 3 wrinkled yellow seed produced, and 1 wrinkled green seed producer.

12. FOIL BT BT BT BT BT Bt bT bt bT bt bT bt
How do you determine gametes with dihybrids?
FOIL
What type of gametes can this organism produce?
BBTT
______ ______ _______ _______
BT BT BT BT
What type of gametes can this organism produce?
bbTt
______ ______ _______ _______
bT bt bT bt
What type of gametes can this organism produce?
BbTt
______ ______ _______ _______
BT Bt bT bt

13. TR Tr tR tr TtRr X TtRr Tall = T short = t R= round r=wrinkled
TTRR TTRr TtRR TtRr
TTRr TTrr TtRr Ttrr
TtRR TtRr ttRR ttRr
TtRr Ttrr ttRr ttrr
Solve this dihybrid:
IiRr X IiRr
Tall = T short = t R= round r=wrinkled
seeds seeds

14. How many offspring will be Short AND Round? ______ 3/16
This is an example of a DIHYBRID cross. Which pattern of phenotypes will be seen in the offspring?
9:3:3:1
How many offspring will be
Short AND Round? ______
Short AND wrinkled? ______
3/16
Recessive and dominant
1/16
Recessive and recessive
HINT: 9- dominant for both 3- recessive and dominant
3- dominant and recessive
1- recessive and recessive

15. Dominant Dominant Recessive Recessive
FILL IN THE BLANKS WITH THE WORDS
Dominant OR Recessive
In a DIHYBRID HETEROZYGOUS cross the offspring that are
9/16 are _______________ for both traits.
3/16 are _____________ for one trait and
_____________ for the other.
1/16 are _____________ for both traits.
Dominant
Dominant
Recessive
Recessive

16. Solve for F2 from a Parent Generation of PPTT crossed with pptt.
Gametes

17. Solve for F2 from a Parent Generation of PPTT crossed with pptt
Solve for F2 from a Parent Generation of PPTT crossed with pptt. (answer)
PPTT x pptt
What gametes can be produced?
The law of Independent Assortment says that each of the P’s can be paired with either of the T’s.
What gametes can be produced by PPTT
PT, PT, PT, PT
What gametes can be produced by pptt?
pt, pt, pt, pt
If sex recombined these gametes, what offspring are possible?
Here we can set up a Punnett to solve the possible offspring.
The gametes of one organism (PPTT) go across the top while the gametes of the other organism (pptt) go down the side.
This cross will produce only PpTt offspring
Now what will be the results if a PpTt self pollinates?
First we need to determine the possible gametes that PpTt can produce,
Then we can set up a Punnett square.
We can determine the phenotype ratio by looking at the possible genotypes.
Purple Tall genotypes:
PPTT, PPTt, PpTT, PpTt
Purple tall= 9
Purple short genotypes:
PPtt, Pptt
Purple short=3
White tall genotypes:
ppTT, ppTt
White tall=3
White short genotypes:
Pptt
White short=1

18. Questions to Ponder from Lesson 3
Solve Tt X tt where T =Tall and t = short State phenotype and genotype ratios
Solve for F2 generation from a P generation of PP X pp where P = Purple and p = white State phenotype and genotype ratios
Solve for F2 generation from a P generation of TTPP X ttpp where P = Purple and p = white and T =Tall and t = short. State phenotype ratios
Solve TtPp X ttpp where P = Purple and p = white and T =Tall and t = short. State phenotype ratios
Answer these questions on DP set answer sheet