1. 11-3 Exploring Mendelian Genetics
Key Concepts:
What is the principle of independent assortment?
What inheritance patterns exist aside from simple dominance?
Vocabulary:
Independent assortment
Incomplete dominance
Codominance
Multiple alleles
Polygenic traits

2. Independent Assortment
Two-factor cross of Parent Generation:
Crossed true breeding plants (Parents)
Round Yellow x Wrinkled Green
F1 phenotypes= all yellow, round
Which alleles are dominant?
Round shape and Yellow color.
Why is this called a two factor cross?
Because the experiment tests two characteristics controlled by two factors= genes.

3. Summarization of Mendel’s two factor cross (F1 Generation)
Mendel crossed plants that were homozygous dominant for round yellow peas (RRYY) and plants that were homozygous recessive for wrinkled green peas (rryy).
All of the F1 offspring were heterozygous dominant for round yellow peas (RrYy).

4. The Two-Factor Cross F2
Mendel knew all of the F1 plants were RrYy
but how would these alleles interact if the F1 plants were crossed?
Would the two dominant alleles stay together (R and Y) or would they segregate independently?
In his experiment, F2 plants produced 556 seeds.
315 were round and yellow (looked like Parental)
32 were wrinkled and green (looked like Parental)
209 had combinations of the phenotypes
Therefore, the alleles for seed shape segregated independently of the alleles for seed color.

5. This phenomenon is known as…
Independent assortment
Definition: genes for different traits can segregate independently during the formation of gametes (meiosis).

7. Principle of Independent Assortment
If the genotype of a plant is TtYy, what are the four possible combinations of gametes?
TY, tY, Ty, ty
These are FOUR possible gametes, each with two alleles!!!

8. Independent Assortment..in other words…
Does the segregation of one pair of alleles affect the segregation of another pair of alleles?
EX: does the gene that determines seed shape have anything to do with the gene that determines seed color?
The answer: NO
If genes are not connected, then they should separate (segregate) independently: Independent Assortment

9. F2 generation

10. F2 Generation 9:3:3:1 Can you see
the ratio of phenotypes in the Punnett square?
9 round, yellow
3 round, green
3 wrinkled,yellow
1 wrinkled, green

11. How do you figure out the possible gametes?
Remember FOIL (from math???)
S s Y y
The gametes mom will produce: SY, Sy, sY, sy
Ex: Mom’s genotype

12. If you wanted to cross the offspring of the F1 generation…
P generation
F1 generation
If you wanted to cross the offspring of the F1 generation…
What would the gametes be?
SsYy= SY, sY, Sy, sy
NEXT—try the cross!!!

13. SsYy x SsYy
SY Sy sY sy
SY SSYY Sy
sY ssYY
Sy Ssyy

14. Predictions…
Since both parents are heterozygous for both traits, you can predict that the offspring ratios should be…
9:3:3:1

15. A summary of Mendel’s Principles
Inheritance is determined by genes
Some genes are dominant and some are recessive
Each sexually reproducing adult has 2 copies of a gene.
These genes are segregated during gamete formation.
Alleles for different genes USUALLY segregate independently (independent assortment)

16. Beyond Dominance and Recessive Alleles
Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles OR multiple genes.
Examples of genes that are different than being totally “Dominant” or “Recessive:”
1. Incomplete dominance
2. Codominance
3. Multiple Alleles
4. Polygenic Traits

17. 1. Incomplete Dominance
One allele is not completely dominant over another.
The heterozygous phenotype is somewhere between the 2 homozygous phenotypes .
What does this mean?
Let’s take a look at some of Mendel’s studies.
He crossed a homozygous red plant with a homozygous white plant.
What do you think would be the expected results?...

19. Incomplete Dominance R= Red r= White P: RR x rr
F1: what is the F1 generation going to look like (phenotype)?
F2: What is the F2 generation going to look like (phenotype)?
Do the crosses now in your notes

20. Incomplete Dominance R= Red r= White P: RR x rr F1: All Rr (All Pink)
F2: 1 Red: 2 Pink: 1 White r
Rr Rr
Rr Rr
Which allele is dominant in
pink offspring?……….neither
****notice the ratio for incomplete
dominance 1:2:1
review:
what was the ratio for independent
assortment?
9:3:3:1
R r
RR Rr
red pink
Rr rr
pink white R r

21. 2. CoDominance!
Definition: both alleles for a trait contribute to the phenotype of the organism.
Examples
The alleles for red (RR) and white (WW) hair in cattle are co-dominant.
Cattle with both alleles have brown/white patterning or roan (RW).
In certain varieties of chickens the alleles for black and white feathers are co-dominant.
Chickens with both alleles appear speckled.

23. What is the difference between incomplete dominance and codominance?
Incomplete dominance = heterozygous phenotype is somewhere in between the 2 homozygous phenotypes.
For example, in (Rr), the r allele is not active, but R cannot produce its full effect when it is combined with r.
RR = red
Rr = pink
rr = white
1:2:1 ratio for F2 generation

24. What is the difference between incomplete dominance and codominance?
Codominance = heterozygous phenotype has characteristics of both alleles for that trait. …
Both alleles are active and are expressed together (both act like dominant genes).
For example, cross between red hair (RR) and white hair (WW), the calf will be roan (RW) both red and white hairs.
RR = red WW = white RW= red & white

25. AND…One more time!!!!!!!! (how fun)

26. Incomplete Dominance RED Flower x WHITE Flower ---> PINK Flower
Remember: Incomplete Dominance in the form of an example like this:
RED Flower x WHITE Flower ---> PINK Flower
With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a totally different from the parental traits.

27. Codominance ”Co-" is "together".
Cooperate = work together
Coexist = exist together
In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.
remember codominance in the form of an example like this:
red x white ---> red & white hair

28. Codominance
With codominance, a cross between organisms with two different phenotypes produce offspring with a third phenotype in which both of the parental traits appear together.

29. 3. Multiple Alleles Definition: Genes with more than two alleles
The alleles are found on the same location on the chromosome
Remember: You only inherit TWO alleles (one from mom, one from dad)

30. 3. Multiple Alleles Example 1: Example 2:
Rabbits coat color is determined by a single gene with four alleles.
Example 2:
Human Blood Types: 3 alleles (A, B, O)
Phenotypically Type A Blood (genotype = AA or AO)
Phenotypically Type B Blood (genotype = BB or BO)
Phenotypically Type AB Blood (genotype = AB)
Phenotypically Type O Blood (genotype = OO)

31. There are four possible coat colors in rabbits. This does not mean
that an individual can have more than two alleles but that there are
more than 2 possible alleles that can exist in a population.
wild type (C):          
chinchilla (cch):          
himalayan (ch):          
albino (c):          

32. 4. Polygenic Traits Traits that are controlled by two or more genes
Example:
Eye color in fruit flies is controlled by three genes
Human skin color is controlled by more than 4 different genes
Shows a wide range of phenotypes as result

33. 4. Polygenic Traits
Human skin color is controlled by 4 different genes
Dark skinned people have alleles that code for melanin at all gene positions for skin color.
Lighter skinned people have few gene positions with alleles that code for melanin

34. Application of Mendel’s Principles
1900s Thomas Hunt Morgan decided to look for a model organism to advance the study of genetics and he picked…….
Drosophila melanogaster!
The fruit fly is the perfect organism for genetic study for the following reasons:
They are small
Easy to keep in the laboratory
Reproduce rapidly

35. How many chromosomes are there ?

36. Why Study Mendel’s Principles?
His studies with plants can be applied to humans.
They can be used to study human inheritance and traits.
They can help us calculate the probabilities of certain traits appearing in the next generation. Therefore helping with the study of genetic diseases and how they are passed down to offspring.