1. Monohybrid and Dihybrid
Honors Biology-Ms. Kim

2. What is a genotype?
A. Brown Hair
B. Freckles
C.Tt
D. All of the above

3. Homozygous Dominant
A. aa
B. Aa
C. AA
D. Blue Eyes

4. If I crossed a 2 True Breeding Plants with different traits, their offspring would be…
A. All Heterozygous
B. Hybrids
C. AA, Aa, aa
D. Both A and B

5. What is a carrier?
The heterozygous genotype that does not express the phenotype when disorders are caused by recessive alleles

6. What is the probability that 2 carriers for Cystic Fibrosis will have a child with Cystic Fibrosis/
100%
75%
50%
25%

7. What is Probability? The chance that a specific event will occur
Probability = number of ways a specific event can occur
number of total possible outcomes

8. How is Probability used in Genetics?
Used to explain the chance of an offspring inheriting a specific trait
Each box represents ¼ or 25 %

9. What is a Punnett Square?
Way to predict ALL possible outcomes of a cross

10. How do you read a Punnett square?
Axes represent possible gametes from each parent
Boxes represent possible genotypes for offspring

11. Genetics Vocab (pt 2)
Monohybrid cross  cross where parents differ in only one trait (Rr x rr)
Dihybrid cross  cross where parents differ in two traits (RrHh x rrHH)
Punnett square – a diagram that shows the gene combinations that might result from a genetic cross of two parents

12. Monohybrid Cross Dihybrid Cross
a cross between 2 individuals that looks at 1 trait
Ex: Just looking at the possibility of getting freckles
a cross between 2 individuals that looks at the possibilities of inheriting 2 DIFFERENT traits at one time
Ex: looking at the possibility of getting freckles AND dimples in the SAME offspring
Dihybrid Cross

13. Monohybrid Punnett Square
Mom’s genotype (Hh) x Dad’s genotype (hh)
Tall Short
Mom’s allele # Mom’s allele #2
H h
Dad’s
Allele # h
Allele # h Hh hh

14. Genotype Outcome (Ratio) vs. Phenotype Outcome (Ratio)
Genotype Possibilities = the GENOTYPE probabilities (expected results) of offspring
Ex: 50% Hh and 50% hh (0:2:2)
Phenotype Possibilities= the PHENOTYPE probabilities (expected results) of offspring
Ex: 50% Tall 50% Short (2:2)

15. Genetics Vocab (pt 3)
Dominant – allele that appears more frequently. It masks the recessive.
Represented by a capitol letter (R=red)
Recessive – allele that appears less frequently (b/c it is repressed when paired with a dominant allele)
Represented by a lower case letter (r=white)
Genotype – a description of the genetic make-up of an individual (TT, Rr)
Phenotype – a description of what an individual LOOKS like (tall, red)

16. Genetics Vocab (pt 4) Homozygous – two identical alleles for a trait
AA – HOMOZYGOUS dominant
aa – homozygous recessive
Heterozygous – two different alleles for a trait
Aa – HETEROZYGOUS one of each allele

17. Solving Punnett Squares
H Biology

18. Punnett squares Step 1 R = round r = wrinkled
STEP 1  Define the alleles
If a homozygous round pea plant is crossed with a heterozygous round pea plant, what will their offspring look like?
Why are we using the same letter? Why not use “R” for round and “W” for wrinkled?
R = round
r = wrinkled

19. Step 2 RR x Rr Define the parents
If a homozygous round pea plant is crossed with a heterozygous round pea plant, what will their offspring look like?
RR x Rr

20. Step 3 Draw the Punnett square

21. Step 4
Cross the parents to find the probability of offspring
1. Bring the top letter down and the side letter over… R R RR Rr R r

22. Step 5 RR Rr R R R r Find the genotype and phenotype of the offspring
Genotype: genetic make-up (letters) R R RR Rr R
Phenotype: physical characteristics r

23. Finished Product RR Rr R R R r Key: R=round r=wrinkled
Genotype: genetic make-up (letters) R R RR Rr
2 RR: 2Rr
50% RR
50% Rr R
Phenotype: physical characteristics r
4 (round): 0 (wrinkled)
100% Round

24. Practice #2
In pea plants, round seeds are dominant over wrinkled. A plant that is homozygous dominant for round seeds is crossed with a heterozygous plant.
Key:
Cross:
R = round
r = wrinkled
RR x Rr
Genotype:
Phenotype:

25. Example: Heterozygous x Heterozygous
Do the following cross: Mom’s genotype (Hh) x Dad’s genotype (Hh) Tall Tall Draw a Punnett Square and determine the offspring’s genotype and phenotype. Give the probabilities and ratios for each

26. Example: Heterozygous x Heterozygous
Mom’s genotype (Hh) x Dad’s genotype (Hh)
Tall Tall
H h H h
Genotype ratio = 25% HH, 50% Hh, 25% hh (1:2:1)
Phenotype ratio = 75% Tall, 25% short (3:1) HH Hh hh

27. Working Backwards…The Testcross
Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
Genotype is not obvious…could be HH or Hh
Cross an individual with the dominant phenotype with individual that is recessive for the same trait

28. Conduct a test cross, where the unknown dominant individual is crossed with the known recessive .
H _ ?_ x hh

29. Mom’s genotype (H?) x Dad’s genotype (hh)
Test Cross
Mom’s genotype (H?) x Dad’s genotype (hh)
Tall Short
H ? h
If all the offspring are ALWAYS tall…Mom has to be HH
If some offspring are short…Mom has to be Hh Hh ?h

30. Assuming genes follow Mendelian Genetics
DIHYBRID CROSSES:
Assuming genes follow Mendelian Genetics
(complete dominance)

31. Dihybrid Crosses
crosses involving crossing 2 DIFFERENT traits at one time
Example: Mate 2 parents and look at the probability of seeing 2 traits, such as:
eye color AND hair color
freckles AND dimples

32. How do You Do Dihybrid Crosses?
Setting up a complex Punnett Square OR
2 separate monohybrid
1 square for EACH trait
use PROBABILITY RULES and MULTIPLY

33. What is a dihybrid cross?
Cross that shows inheritance of two different traits
For example: homozygous round & yellow crossed with a homozygous wrinkled & green seed
RRYY x rryy

34. Setting up a Dihybrid #1- read the problem & list all 4 alleles
For example: R=round, r=wrinkled, Y=yellow, y=green
#2 – Create the parental genotypes (4 letters each)
Example: RRYY (Round, yellow) x rryy (wrinkled, green)
#3 – Using the “foil” method, determine the sets of gametes (up to 4 possibilities)
Example:
1. RRYY RrYy
RY RY, Ry, rY, ry

35. Setting up a Dihybrid
#4 – Fill in the tops and sides of punnett square with gamete combinations
Example:
1. RRYY 2. RrYy
RY RY, Ry, rY, ry
#5 - Genotype and Phenotype as usual
RY Ry rY ry RY
RY Ry rY ry ry RY
RRYY
RRYy
RrYY
RrYy

36. Dihybrid Final Product
R=round, r=wrinkled, Y=yellow, y=green
RRYY =
RRYy =
RrYY =
RrYy =
So…we can say that all of our offspring (100%) will be round and yellow!
RY Ry rY ry RY
RRYY
RRYy
RrYY
RrYy
Round and Yellow
Round and Yellow
Round and Yellow
Round and Yellow

37. Dihybrid Example Problem #1
Round is dominant over wrinkled
Yellow is dominant over green
Two pea plants produce offspring. One is round and heterozygous for yellow seed color. The other is wrinkled and heterozygous for yellow seed color.
STEP 1:
Parental genotypes =
Possible gametes
RRYy x rrYy
rY, ry
RY, Ry

38. Dihybrid Example Problem #1
STEP 2:
Set up the dihybrid cross using the gametes from before…
RY Ry rY ry
RrYY
RrYy
Rryy

39. Dihybrid Example Problem #1
STEP 3: Determine the genotype and phenotype!
RY Ry
Genotype: Phenotype:
1 RrYY: 2 RrYy : 1 Rryy
RY Ry rY ry
RrYY
RrYy
Rryy
3 Round, yellow
1 Round, green

40. Dihybrid Example Problem #2
Black fur is dominant to white fur
Long hair is dominant to short hair
Two guinea pigs mate. The dad is homozygous for black fur and long hair. The mom is also homozygous, but for white fur and short hair.
1) Determine the dominant & recessive traits
2) Determine the possible gametes of each parent
3) What is the only gamete possibility for their offspring?

41. Dihybrid Example Problem #2
1) Key:
Black fur is dominant (B) to white fur (b)
Long hair is dominant (L) to short hair (l)
Two guinea pigs mate. The dad is homozygous for black fur and long hair. The mom is also homozygous, but for white fur and short hair.
2) Determine the possible gametes of each
Dad  ALL BL Mom  ALL bl
3) What is the only gamete possibility for their offspring?
GENOTYPE: 100% BbLl
PHENOTYPE: Black, long-haired

42. Ggbb x ggBb FOIL (FIRST, OUTER, INNER, LAST) Ggbb X ggBb G= Grey hair
g = white hair
B = Black eyes
b = red eyes
FOIL (FIRST, OUTER, INNER, LAST)
Ggbb X ggBb

43. Ggbb x ggBb Gb gb
GgBb
ggBb
Ggbb
ggbb gB gb

44. Ggbb x ggBb
Gb gb Gb gb gB gb
1/16 GgBb
1/16 ggBb
1/16 Ggbb
1/16 ggbb

45. GGbb x ggBb GENOTYPE POSSIBILITIES: GgBb = 4/16 or ¼ = 25%
Genotypic ratio:
1 GgBb : 1 Ggbb : ggBb : 1 ggbb
PHENOTYPE POSSIBILITES:
Grey Hair Black Eyes: 25%
Grey Hair Red Eyes: 25%
White Hair Black Eyes: 25%
White Hair Red Eyes: 25%
Phenotypic ratio:
1: 1: 1: 1

46. Practice
In pea plants, yellow seeds are dominant to green seeds in peas. Round seeds are dominant over wrinkled. Cross two plants that are heterozygous for both traits.
Write the genotypes for the parents.
Then use “FOIL” to determine your possible allele combinations from each parent
Then set up Punnett Square and fill in the boxes
Then figure out the genotypic and phenotypic ratios

47. YR Yr yR yr YR Yr yR yr YyRr x YyRr 1/16 YYRR 1/16 YYRr YyRR YyRr YYrr

48. YR Yr yR yr YR Yr yR yr YyRr x YyRr 1/16 YYRR 1/16 YYRr YyRR YyRr YYrr

49. YyRr x YrRr
GENOTYPE POSSIBILITIES: YyRr = 4/16 or 1/4 = 25% YYRr=2/16 or 1/8 =12.5% YyRR=2/16 or 1/8 =12.5% Yyrr=2/16 or 1/8 =12.5% yyRr=2/16 or 1/8 =12.5% YYrr= 1/16 = 6.25% YYRR= 1/16 = 6.25% yyRR=1/16 = 6.25% yyrr =1/16 = 6.25% PHENOTYPE POSSIBILITES: 9 yellow, round 3 yellow, wrinkled 3 green, round 1 green, wrinled

50. Would you like to know a few SHORT CUTS?

51. Short Cuts for MONOHYBRID CROSSES
Every parent “donates” only 1 allele to each offspring
Law of Segregation
When crossing 2 heterozygous individuals in complete dominance, you will ALWAYS get
1:2:1 GENOTYPE ratio
1 homozygous dominant: 2 heterozygous: 1 recessive
3:1 PHENOTYPE ratio
3 dominant phenotype: 1 recessive

52. Short Cuts for DIHYBRID CROSSES
When crossing 2 heterozygous individuals in complete dominance, you will ALWAYS get
9:3:3:1 PHENOTYPE ratio
9 dominant, dominant phenotype
3 dominant, recessive phenotype
3 recessive, dominant phenotype
1 recessive, recessive phenotype
NOTE: The genotypes have to be ALL heterozygous
Ex: HhFf x HhFf

53. Now…Let’s do Multicharter Problems 
What is the probability of producing an offspring with the genotype AaBBCcDDeeFf in a cross between 2 parents with the following genotypes?
AABbCcDDeeFf X
AaBbCcDdeeFf
½ x ¼ x ½ x ½ x 4/4 x ½
= 4/256 = 1/64 or 1.5% chance

54. Practice
 In humans, curly hair is dominant over straight hair.  A woman heterozygous for hair curl marries a man with straight hair and they have four children. a) Show the parental cross and the possible gametes produced. b) Use a Punnett square to find the possible genotypes for the F1 generation. c) What are the phenotypic and genotypic ratios of the F1? d) What is the probability that the first child will have curly hair?  What is the probability that the third child will have curly hair?

55. Practice
2) The ability to taste the drug phenyl-thio carbamide (P.T.C.) is due to a dominant gene.  A non-taster man marries a taster woman whose father was a non-taster. a) What will be the expected genotypes of their four children? b) What would be the expected phenotypes for ten children? c) What is the probability that their first child has the heterozygous genotype?
d) Give the phenotypic and genotypic ratios of the possible offspring produced

56. More Practice?
The dark wizard, Lord Voldemort, is able to speak parseltongue (pp), which is a recessive trait. Not only can he speak parseltongue, he can also do dark magic (dd). Him and his followers, the Death Eaters, spend a lot of time together practicing dark magic. Over the years, Voldemort and Bellatrix Lestrange fell in love. In order to continue their dream of taking over the wizarding world, they decided to have children. Bellatrix, who can do dark magic, cannot speak parseltongue. However, they only want children if they can speak pareseltongue as well.
What is Voldemort’s genotype? 
What does Bellatrix’s genotype have to be in order to have a child who can do black magic and speak parseltongue?
Create a dihybrid cross using Bellatrix’s genotype of part B, include genotypic and phenotypic ratios.
What is the percent chance that they have a child who can speak parseltongue and do black magic?

57. Practice
Determine the results of a cross between two dolphins.  Female is heterozygous dominant for skin color and has a short tail land the male is heterozygous for both traits.  Grey skin (G) is dominant to white skin (g) and long tails (T) are dominant over short tails (t).  List all phenotype ratios and % for the F1 generation.

58. Practice
In humans, right-handedness (R) is dominant to left-handedness
and hitchhiker thumb (H) is dominant to straight thumb.
Chuck is homozygous right-handed and has a hitchkiker’s thumb. His father has a straight
thumb. Emily is left-handed and can’t bend her thumb back.
What are the phenotype possibilities if Chuck and Emily have children?  
What is the probability that their baby’s genotype will be like:
Chuck?
Emily?