1. Genetic Inheritance Problems - Exercise 9
Objectives
-Know how to apply basic genetic terms.
-Know how to do compute Punnett squares of monohybrid and dihybrid crosses.
-Know how to do sex-linked crosses.
-Be able to apply Incomplete Dominance and Codominance.

2. Genetic Inheritance Problems
Genetics is the study of the hereditary material of life.
The hereditary material (known as genes) is encoded
as molecules of DNA on chromosomes. Genes can
also be symbolized as letters, called alleles. Alleles
are alternate forms of genes found at a particular
sport on a chromosome. The place where a gene or
allele is found is called the locus.
Hereditary material (genes) in DNA on chromosomes.

3. In diploid animals, alleles exist in pairs
In diploid animals, alleles exist in pairs. Before alleles and chromosomes are passed from parents (P1 generation) to offspring (F1 generation), the allelic and chromosomal pairs are separated by the process of meiosis. The result of meiosis in animals is the production of haploid gametes – egg and sperm. The alleles of the haploid gametes are then combined during reutilization to produce the diploid offspring (zygote).

4. Basic Terms in Genetics

5. Alleles Dominant - expressed when paired with a different allele
Represented by an uppercase letter (RR) or (Rr)
Recessive - no effect when paired with a dominant allele
Represented by a lowercase letter (rr)

6. Alleles in an Individual
Homozygote - same two alleles (AA or aa)
Heterozygote - two different alleles (Aa)
Genotype - the genetic makeup (Is its genetic makeup)
Phenotype - observable characteristics (Is its physical appearance)

7. Useful Genetic Vocabulary
An organism that is homozygous for a particular gene
Has a pair of identical alleles for that gene
Exhibits true-breeding
An organism that is heterozygous for a particular gene
Has a pair of alleles that are different for that gene

8. Genetic Cross When two individuals are mated
P generation - parent generation
F1 generation - first generation
F2 generation - second generation
The hybrid offspring of the P generation
Are called the F1 generation
When F1 individuals self-pollinate
The F2 generation is produced

9. In a typical breeding experiment
Character - a heritable feature, such as flower color
Trait - a variant of a character, such as purple or white
flowers
In a typical breeding experiment
Mendel mated two contrasting, true-breeding varieties, a process called hybridization
The true-breeding parents
Are called the P generation

10. Extensions of Mendel’s Laws
Many alleles do not show complete dominance
Incomplete dominance
Codominance
Epistasis
Environmental effects
Polygenic traits

11. Incomplete Dominance: One does not completely cover the other
Incomplete Dominance: One does not completely cover the other. Halfway between two extremes, so blending of one another.
  -Classic example: A red and white flower is cross, so you end up with a pink flower.
Codominance: There are equally strong, so nothing over powers the other. Equal in strength, you see both phenotypes.
-Classic example: Cow having brown and white spots (Roam).

12. Incomplete Dominance The phenotype of F1 hybrids is
Heterozygote is an intermediate
Horses
Snapdragons
The phenotype of F1 hybrids is
somewhere between the phenotypes
of the two parental varieties
P Generation
F1 Generation
F2 Generation
Red
CRCR
Gametes CR CW 
White
CWCW
Pink
CRCW
Sperm Cw
1⁄2
Eggs
CR CR
CR CW
CW CW

13. INCOMPLETE DOMINANCE
Characterized by an absence of complete dominance in one allele.
This manifests as a “blending” of traits, or a “hybrid” phenotype.
Common in flower color genes.

14. red flowers X white flowers
4 O’ CLOCKS
red flowers X white flowers
RR X rr
[ R1R1] [R2R2]
Rr [R1R2] F1
pink
¼ RR [R1R1] red
½ Rr [R1R2] pink
¼ rr [R2R2] white
1:2:1 genotypic
1:2:1 phenotypic
When 2 heterozygotes
are crossed

16. IDENTIFYING CHARACTERISTICS OF INCOMPLETE DOMINANCE
Traits are blended.
Crossing two heterozygous individuals in a monohybrid cross produces a 1:2:1 genotypic ratio and 1:2:1 phenotypic ratio.
Incomplete dominance is an apparent exception to Mendel’s
First Law because a different phenotypic ratio is obtained.
The alleles are in fact segregating according to Mendel’s first
law, the mechanism by which the phenotype is produced is
different than in pea plants.

17. Codominance In codominance The human blood group MN
Two dominant alleles affect the phenotype in separate, distinguishable ways
The human blood group MN
Is an example of codominance
Also ABO blood groups

18. CODOMINANCE
A codominant gene in a heterozygous individual will express the phenotype of both alleles. The phenotype of both alleles are expressed independently.
ABO blood groups in humans are an example. The I gene (isoagglutinogen) has three alleles (A, B, and O).
The A and B alleles are dominant to O and codominant to each other.

19. ABO GENOTYPES A IAIA or IAIO A ANTI-B B IBIB or IBIO B ANTI-A
PHENOTYPE GENOTYPE ANTIGEN ANTIBODY
A IAIA or IAIO A ANTI-B
B IBIB or IBIO B ANTI-A
O IOIO NONE BOTH
AB IAIB A and B NONE

20. A ANTIGEN
B ANTIGEN
BLOOD TYPE A
BLOOD TYPE B
NO ANTIGENS
BOTH A +B ANTIGENS
BLOOD TYPE AB
BLOOD TYPE O

21. Codominance ABO Blood Groups Both alleles are expressed
Seen in blood types
IAIA or IAi = type A
IBIB or IBi = type B
ii = type O
IAIB = type AB
ABO Blood Groups
The ABO blood group in humans Is determined by multiple alleles.

22. GENES
Genes are discrete units of heredity determining biological characteristics of living things.
Genes exist in pairs in diploid organisms.
Alleles are alternate forms of the same gene, each is on a different homologous chromosome.

23. GENOTYPE AND PHENOTYPE
The genotype is the genetic constitution of the individual, in other words, the genes (and alternate forms) that are carried.
Alternate forms of the same gene are called alleles.
The phenotype is the observable trait (characteristic) produced by the genotype (gene).

24. MENDEL’S FIRST LAW SEGREGATION
Governs the behavior of alleles.
3 important observations from Mendel’s crosses.
[1] Hybridization between two traits showed only one trait in the offspring.
white flowers X purple flowers
purple flowers

25. Basic Patterns of Inheritance
Mendel started with true breeding plants
Recessive trait skipped a generation

26. Mendel’s First Law: Law of Segregation

27. Phenotype versus Genotype3 1 2
Phenotype
Purple
White
Genotype PP
(homozygous) Pp
(heterozygous) pp
Ratio 3:1
Ratio 1:2:1

28. yellow seeds X green seeds [parental generation] YY yy [P1]
yellow seeds [first filial generation]
Yy [F1]
¾ yellow seeds [F2]
¼ green seeds
[second filial generation]

29. GENERATION DESIGNATIONS
The parental generation (P1) is the first generation of the controlled cross.
The first filial generation (F1) is the result of crossing the parental generation.
The second filial generation (F2) is produced from the crossing of the F1 progeny.

30. PUNNETT SQUARE
Graphical means of visualizing a monohybrid cross and applying probability to the outcome.
E.G. cross 2 heterozygous individuals [Yy] Y y
Genotypic
Ratio=1:2:1
¼ YY
½ Yy
¼ yy
Phenotypic
Ratio=3:1
¾ yellow seeds
¼ green seeds YY Yy Y
yellow
yellow Yy yy y
yellow
green

31. MENDEL’S SECOND LAW INDEPENDENT ASSORTMENT
Governs the behavior of different genes.
Mendel started with two hypotheses.
[1] All traits from 1 parent would be transmitted together and only two types of offspring would result.
[2] Traits would be inherited independently and there would be more than two types of offspring.

32. Independent assortment
Using the information from a dihybrid cross, Mendel developed the law of independent assortment
Each pair of alleles segregates independently during gamete formation
Mendel identified his second law of inheritance
By following two characters at the same time
Crossing two, true-breeding parents differing in two characters
Produces dihybrids in the F1 generation, heterozygous for both characters

33. Mendel’s Second Law: Law of Independent Assortment

34. Characteristics Studied

35. Dihybrid Cross – two characters
YYRR
P Generation
Gametes YR yr 
yyrr
YyRr
Hypothesis of
dependent
assortment
independent
F2 Generation
(predicted
offspring)
1⁄2
1 ⁄2
3 ⁄4
1 ⁄4
Sperm
Eggs
Phenotypic ratio 3:1 Yr yR
9 ⁄16
3 ⁄16
1 ⁄16
YYRr
YyRR
Yyrr
YYrr
yyRR
yyRr
Phenotypic ratio 9:3:3:1
315
108
101 32
Phenotypic ratio approximately 9:3:3:1
F1 Generation
RESULTS
CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and
seed shape sort into gametes independently of each other.
EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.

36. DIHYBRID CROSS WITH GENOTYPES
A cross involving two traits.
round,yellow seeds X wrinkled, green
All round, yellow [F1]
R=round
r=wrinkled
Y=yellow
y=green
RRYY
rryy
RrYy

37. MENDEL’S EXPERIMENT THE DIHYBRID CROSS
The dihybrid cross, a cross involving two traits.
round,yellow seeds X wrinkled, green
All round, yellow [F1]
9/16 round, yellow
3/16 wrinkled, yellow
3/16 round, green
1/16 wrinkled, green
Phenotypic
ratio=9:3:3:1

39. Recessively Inherited Disorders
Many genetic disorders
Are inherited in a recessive manner
Recessively inherited disorders
Show up only in individuals homozygous for the allele
Carriers
Are heterozygous individuals who carry the recessive allele but are phenotypically normal

40. Inheritance of Sex-Linked Genes
The sex chromosomes
Have genes for many characters unrelated to sex
A gene located on either sex chromosome
Is called a sex-linked gene
Other sex-linked conditions
Some recessive alleles found on the X chromosome in humans cause certain types of disorders
Color blindness
Duchenne muscular dystrophy
Hemophilia

41. Sex-linked genes follow specific patterns of inheritance

42. Questions - Page 7 - Lab Book
1. State Mendel’s First Law. What part of meiosis is the basis for this law?
2. State Mendel’s Second Law. What part of the meiotic process is the basis for this law?
3. Why do we use a Punnett squares to solve genetic problems?
-Can Punnett squares give us precise outcomes of an
offspring?

43. Questions - Page 7 - Lab Book
1. State Mendel’s First Law. What part of meiosis is the basis for this law?
Two alleles from a heritable character separate during gamete formation and
end up in different gametes, during Anaphase I.
2. State Mendel’s Second Law. What part of the meiotic process is the basis for this law?
Each pair of alleles segregates independently of other pairs of alleles during
gamete formation, during metaphase I.
3. Why do we use a Punnett squares to solve genetic problems?
Shows all the possibilities of the combination of alleles in an offspring that
results from a cross whether its monohybrid or dihybrid.
-Can Punnett squares give us precise outcomes of an offspring? No, it gives you possibilities of what could happen not actual outcomes.

44. Questions If an allele for tall plants (T) is dominant to
short plants (t), what offspring would you
expect from a TT x Tt cross?
½ tall; ½ short
¾ tall; ¼ short
All tall

45. Questions If an allele for tall plants (T) is dominant to
short plants (t), what offspring would you
expect from a TT x Tt cross?
½ tall; ½ short
¾ tall; ¼ short
All tall
The correct answer is C.
The only way to have offspring that are short is if both parent plants carry the short (t) allele.
Since one parent is homozygous dominant, no short offspring can be produced.

46. Questions Fur color in rabbits shows incomplete dominance.
FBFB individuals are brown, FBFW individuals are
cream, FWFW individuals are white. What is the
expected ratio of a FBFW x FWFW cross?
3 white: 1 brown
3 white: 1 cream
2 white: 2 cream

47. Questions Fur color in rabbits shows incomplete dominance.
FBFB individuals are brown, FBFW individuals are
cream, FWFW individuals are white. What is the
expected ratio of a FBFW x FWFW cross?
3 white: 1 brown
3 white: 1 cream
2 white: 2 cream
The correct answer is C.
In order to produce brown rabbits, each parent would have to have at least one copy of the brown allele, since brown is homozygous (FBFB).
Although a 3:1 ratio is possible of the offspring, the expected ratio cannot be 3:1 (see Punnett square in Lecture notes).

48. Questions - Monohybrid Cross
Height in pea plants is determined by the genes T (dominant) and t (recessive).
Cross a homozygous tall pea plant with a dwarf pea plant and determine the probability of producing a tall plant.

49. Questions - Monohybrid Cross
Height in pea plants is determined by the genes T and t.
Cross two heterozygous tall plants and determine the probability of producing a dwarf plant.

50. Questions
Note that blood type genotypes may be written using an "I" before the A and B, such as IAIA and IBi, etc. In this problem I’m not using "I".
Hazel has type B blood (genotype BO) and Elijah has type O blood (genotype OO). If they have children, what is the probability that they will have a type B child? What is the probability they will have a type A child?
In this problem you are given the genotypes so you know both genes for each blood type.

51. Questions - Dihybrid Cross
When a genetic cross involves the consideration of two factors (such as shape and colour in pea seeds), the cross is called a "dihybrid".
Cross a completely heterozygous round/yellow seeded plant with a completely homozygous round/green seeded plant.
Then determine the probability of obtaining a round/yellow seeded plant in the offspring.
R = round seeds, r = wrinkled seeds Y = yellow seeds, y = green seeds

52. Questions - Page 7 - Lab Book

53. Questions - Page 7 - Lab Book

54. Questions - Page 7 - Lab Book

55. Questions - Page 7 - Lab Book