1. Genetic Crosses and Probability
Chapter 9
Sec. 9.2

2. Genotype and Phenotype
Genotype- the genetic makeup of an organism (Represented by alleles, capital letters for dominant and 2 lowercase letters for recessive) Ex. PP, Pp, or pp
Phenotype- the observable, physical appearance of an organism (results from the genotype)
Ex. The phenotype of a PP or Pp is Purple
The phenotype of a pp is white

3. Genetics Vocabulary continued…
Homozygous- when both alleles of a pair are the same for a gene
Homozygouse Dominant- PP
Homozygous Recessive- pp
Heterozygous- when the 2 alleles in the pair are different for a gene
Heterozygous for flower color: Pp

4. What is Probability?
Probability is the likelihood that a particular event will occur.
It does not ALWAYS happen (think Deal or No Deal)
Can be written as a decimal, percentage, ratio, or fraction
How do we use the principles of probability in our daily lives?
Ex.
Horse racing
NCAA March Madness
Superbowl Coin Toss
Atlantic City $$$$$

5. Determining probability
Number of times the event is expected
Number of times it could have happened
What is the probability that you picked an odd number when picking from 1-10? There are five odd numbers between 1 and 10. The probability is 5:5.
Or you can express it as a fraction: 5/10. Since it's a fraction, why not reduce it? The probability that you will pick an odd number is 1/2.
Probability can also be expressed as a percent...1/2=50% Or as a decimal...1/2=50%=.5

6. The Principles of Probability
The Principles of probability can be used to predict the outcomes of genetic crosses
Alleles segregate by complete randomness
Similar to a coin flip!

7. What is a Punnett Square?
A punnett square is a diagram showing the gene combinations that might result from a genetic cross
The letters in punnett squares represent alleles.
A capital letter is used to show the dominant alleles.
A lower case letter is used to show the recessive alleles.
Monohybrid cross- a cross between individuals that involve one pair of traits

8. How do you fill in a Punnett Square?
Use the genotype to determine the gametes formed. Ex. a Gg parent will have 2 gametes because of segregation: G or g
Parent 1 gametes on the top, parent 2 gametes on the side
If both parents are heterozygous for green color…
Parent 1 G g

9. How do you fill in a Punnett Square?
Parent 1 G g
Parent 2

10. How do you fill in a Punnett Square?
Parent 1 G g
Parent 2 GG Gg gg

11. How do you fill in a Punnett Square?
Parent 1 G g
Parent 2 GG Gg gg

12. What do the alleles mean?
Homozygous Dominant
Homozygous recessive
Heterozygous Dominant
 GG
 gg
 Gg

13. To find the ratio of offspring in a punnett square…
Count the number of different offspring
For a genotypic ratio count the different genotypes
For a phenotypic ratio count the different phenotypes

14. G g GG Gg gg Genotypic ratio 1 GG 2 Gg 1 gg
Parent 1 G g
Parent 2 GG Gg gg
1 GG
2 Gg
1 gg
So the genotypic ratio is 1:2: 1

15. G g GG Gg gg Phenotypic ratio 3 green plants 1 yellow plant
So the ratio is 3:1
Parent 1 G g
Parent 2 GG Gg gg

16. Ex 1: Homozygous x Homozygous Let’s cross a pure bred purple colored flower and a white flower. Purple is a dominant color and white is a recessive color.
Cross: PP x pp P P
(purple x white) p Pp

17. Ex 2: Homozygous x Heterozygous A guinea pig who is homozygous dominant for black fur crosses with a guinea pig who is heterozygous for this trait (black fur)
Cross: BB x Bb B B B b BB Bb

18. Ex 3: Heterozygous x Heterozygous In rabbits, the allele for black coat color is dominant (B) over the allele for brown coat color (b). Cross 2 rabbits that are heterozygous for coat color B b
Cross: Bb x Bb B b BB Bb bb

19. Design your own peas…. An interactive Pea Breading experiment
Choose two plants to breed and see the resulting possibilities for the next generation

20. Independent Assortment
Does the segregation of one pair of alleles affect the segregation of another pair of alleles?
Does the gene that determines round or wrinkled peas have anything to do with pea seed color?
NO!
Genes for different traits can separate independently during the formation of gametes
So one pair of alleles do not influence each other’s inheritance.

21. Complete Dominance
In Mendelian traits if you are homozygous dominant OR heterozygous you will show the dominant trait
This is called having complete dominance
Example, Rolling your tongue:
RR and Rr can roll their tongue
rr cannot roll their tongue

22. Exceptions to the dominance rules
Incomplete dominance
Codominance

23. Incomplete Dominance- The phenotypes MIX when the genotype is heterozygous. Two or more alleles contribute to phenotype, resulting in a phenotype in the middle between the dominant and recessive trait.
R R
Neither allele is completely dominant over the.
Ex. Four o’clock flowers
RR=RED (when all red flowers crossRED)
rr= WHITE (when all white flowers cross WHITE) or rr
When a RED crosses with a white we get  PINK or Rr
Which allele is dominant? NONE!
Incomplete dominance is like mixing paint, you get a blend of the 2 colors Rr r

25. Codominance- when both phenotypes/alleles for a gene are shown in heterozygous offspring
Both alleles of the gene contribute to the phenotype of the organism.
Neither allele is dominant or recessive
Codominance is like mixing glitter together, you can still see the individual glitter colors
Example: Horses
The genes for white and red coat color is called ROAN
Homozygous Dominant RR=red coat
Homozygous Recessive R’R’=white coat
Heterozygous RR’= roan (both red and white hairs can be seen)

26. Dihybrid Crosses
These are done when we want to determine the inheritance of 2 traits at the same time. For example, pea color and pea texture…
If the parents genotypes are heterozygous for both traits… (use Y for color and R for texture)
Parents genotype is: YyRr
Note: Remember to use 1 letter for EACH trait

27. The heterozygous parents will give off gametes
YyRr parents will give off 4 different gametes because of independent assortment and segregation
YR Yr yR yr

28. Dihybrid Crosses Cross: RrYy X RrYy
Dihybrid Cross – a cross involving 2 different traits
Pea Shape:
Round (R) Dominant
Wrinkled (r) recessive
Pea Color:
Yellow (Y) Dominant
Green (y) Recessive
RY Ry rY ry
RRYY
RrYY
RrYy
RRYy
RRyy
Rryy
rrYY
rrYy
rryy RY Ry rY ry

29. So when we cross 2 heterozygous parents…
…you get the same ratio all the time… 9:3:3:1