Gregor Mendel conducted breeding experiments with pea plants in the mid-1800s. Through his experiments, he discovered the laws of inheritance and developed the principles of genetics. Mendel observed that traits are passed from parents to offspring in predictable ratios. He found that for each trait, individuals have two versions of the gene, called alleles. One allele is dominant and expressed, while the recessive allele has no observable effect. When gametes are formed, the alleles separate and each gamete carries one allele. This explains how offspring can exhibit traits that are not seen in the parents. Mendel's work formed the basis of classical genetics and laid the foundation for modern understanding of heredity and inheritance patterns.

1. Genetics

2. Mendel
&
Simple Patterns of Inheritance

3. Human Traits
• Your physical traits resemble those of your
parents.
• Heredity = the passing of traits from
parents to offspring

4. Gregor Johann Mendel
• Mendel was an Austrian
monk who conducted
breeding experiments
with garden peas.
• Developed rules which
accurately predict
patterns of heredity
• Genetics = the branch
of biology that focuses
on heredity

5. Useful Features in Peas
1.
Many traits that have two clearly different forms (no
intermediate forms)
Mating can be easily controlled
2.
–
–
3.
Self-fertilization
Cross-fertilization
Small, grows easily, matures quickly & produces many
offspring

6. Self-Pollination

7. Cross-Pollination
Cross= mate or breed two
individuals

8. Nature’s Pollinators
• Insects (bees)
• Vertebrates (birds &
bats)
• Wind
• Water

9. Mendel’s Observations:
Traits are Expressed as Simple Ratios
• First experiments
involved monohybrid
crosses
• Monohybrid cross = a
cross that involves
one pair of contrasting
traits

10. Mendel’s Experiments: Step #1
• Self-pollinate each pea
plant for several
generations
• True-breeding= all
offspring display only one
form of a particular trait
• P generation = parental
generation, the first two
individuals crossed in a
breeding experiment

11. Mendel’s Experiment: Step #2
• Cross-pollinate the P
generation plants with
contrasting forms of a
trait
• F1 generation = the first
filial generation, the
offspring of the P
generation
• Characterize and count
plants

12. Mendel’s Experiment: Step #3
• Allow the F1 generation
to self-pollinate
• F2 generation = the
second filial generation,
the offspring of the F1
generation plants
• Characterize and count
plants

13. Mendel’s Results
• F1 generation showed only one form of the trait
(e.g. purple flowers)
– The other form of the trait disappeared (e.g. white
flowers)
– Reappeared in the F2 generation
• 3:1 ratio of the plants in the F2 generation

14. Expressing Ratios: Mendel’s F2
Generation
705 purple-flowered plants; 224 white-flowered
plants
• Reduce the ratio to its simplest form:
705/224 = 3.15 Purple
224/224= 1 White
– Ratio can be written in a few different ways:
• 3:1
• 3 to 1
• 3/1

15. Are offspring simply a blend of their
parents’ characteristics?
• Before Mendel’s experiment: this was the
theory (blending hypothesis)
– Ex. Tall x Short = Medium Mendel’s Conclusion
– Offspring have two genes for each trait (one gene
from each gamete)

16. Mendel’s Hypotheses:
Hypothesis #1
• For each inherited trait, an individual has
two copies of the gene
– One gene from each parent

17. Mendel’s Hypotheses:
Hypothesis #2
• There are alternative
versions of genes
• Alleles = different
versions of genes; an
individual receives one
allele from each parent

18. Mendel’s Hypotheses:
Hypothesis #3
• When two different alleles occur together, one
may be completely expressed, while the other
may have no observable effect on the
organism’s appearance.
• Dominant = the expressed form of a trait
• Recessive = the trait which is not expressed
when the dominant form of the trait is present

19. Mendel’s Hypotheses:
Hypothesis #4
• When gametes are
formed, the alleles
for each gene in an
individual separate
independently of
one another.
– Gametes = one
allele for each
inherited trait
– Fertilization – each
gamete contributes
one allele

20. Representing Alleles
• Letters are often used to represent alleles
• Dominant Alleles = capitalize the first letter
of the trait
– Purple flowers = P
• Recessive Alleles = first letter of the
dominant trait, in lowercase
– White flowers = p

21. Combinations of Alleles
• Homozygous = if two alleles of a particular
gene present in an individual are the same
• Heterozygous = if two alleles of a particular
gene present in an individual are different
• Example: yellow peas (Y) are dominant to green peas
(y)
– Homozygous for yellow peas = YY
– Heterozygous for yellow peas = Yy
yy
YY
Yy

22. Heterozygous Alleles - Ff
• Only the dominant allele
is expressed
– Recessive allele is
present but not expressed
• Example: Freckles
– Freckles, F = Dominant
allele
– No Freckles, f =
Recessive allele
– Individuals who are
heterozygous for freckles
(Ff) have freckles

23. Genotype vs. Phenotype
• Genotype = the set
of alleles that an
individual has
– Uppercase letter is
always written first
• Phenotype = the
physical
appearance of a
trait (determined by
which alleles are
present)

24. Laws of Heredity:
The Law of Segregation
• Law of
Segregation= the
two alleles for a trait
segregate
(separate) when
gametes are formed

25. Laws of Heredity:
The Law of Independent Assortment
• Law of Independent Assortment = the alleles of
different genes separate independently of one another
during gamete formation
– Example: alleles for plant height separate independently of
the alleles for flower color
http://www.sumanasinc.com/webconten
– Applies to:
• Genes on different chromosomes
• Genes that are far apart on the same chromosome

26. Punnett Squares &
Probabilities

27. Punnett Squares:
Predicting Expected Results in Crosses
• Punnett Square = a diagram that predicts
the expected outcome of a genetic cross
by considering all possible combinations
of gametes in the cross

28. Punnett Squares
• Possible gametes from one parent are written along the top of
the square
• Possible gametes from the other parent are written along the
left side of the square
• Letters inside the boxes = possible genotypes of the offspring

29. Monohybrid Crosses
• Homozygous for
yellow seeds (YY) x
Homozygous for
green seeds (yy) =
– Only yellow
heterozygous
offspring (Yy)

30. Monohybrid Crosses
• Heterozygous for
yellow seeds (Yy) x
heterozygous for
yellow seeds (Yy) =
– ¼ YY (Homozygous
dominant)
– 2/4 Yy
(Heterozygous)
– ¼ yy (Homozygous
recessive)
• 1 YY: 2 Yy: 1 yy
genotypic ratio

31. Let’s Solve a Punnett Square!!!
•Inflated pod shape is DOMINANT
• Constricted pod shape is RECESSIVE

32. Step #1: Choose a Letter to
Represent your Alleles
• Inflated pea pod = I
(dominant)
• Constricted pea pod = i
(recessive)

33. Step #2: Write the Genotypes
of the Parents
• Parent #1 = Ii
= Heterozygous Inflated
• Parent #2 = Ii
= Heterozygous Inflated

34. Step #3: Determine the Possible
Gametes
• Parent #1 = Ii
=
I
or
i
• Parent #2 = Ii
=
I
or
i

35. Step #4: Enter the possible
gametes at the top and side of the
Punnett Square
I
I
i
i

36. Step #5: Write the alleles from the
gametes in the appropriate boxes
I
i
I
II
Ii
i
Ii
ii

37. Step #6: Determine the phenotypes
of the offspring
I
i
Inflated Pods
Inflated Pods
I
II
Ii
Inflated Pods
i
Ii
ii
Constricted
Pods

38. Step #7: Determine the genotype
and phenotype ratios
I
i
Inflated Pods
Inflated Pods
I
II
Ii
Inflated Pods
i
Ii
ii
Constricted
Pods
Genotype Ratio:
1 II : 2 Ii : 1 ii
Phenotype Ratio:
3 Inflated : 1 Constricted

39. Probabilities Can Also Predict the
Expected Results of Crosses
• Probability = the likelihood that a specific
event will occur
– Words – 1 out of 1
– Decimal – 1
– Percentage – 100%
– Fraction – 1/1
Probability = number of one kind of possible
outcome ÷ total number of all possible
outcomes

40. Probability in Pea Plants: Gamete
• Parent = Yy
– Can either contribute a yellow allele (Y) or a
green allele (y)
• ½ chance that the gamete will have Y
• ½ chance that the gamete will have y

41. Probability of the Outcome of a Cross
• Consider the offspring of two parents who
are heterozygous for freckles:
– Mom = Ff
• Possible gametes from mom = F
• ½ probability of mom contributing F
• ½ probability of mom contributing f
or
f
or
f
– Dad = Ff
• Possible gametes from dad = F
• ½ probability of dad contributing F
• ½ probability of dad contributing f

42. Probability of the Outcome of a Cross
– Mom F + Dad F = FF
(1/2) x (1/2) = ¼ probability of FF (freckles)
– Mom F + Dad f = Ff
(1/2) x (1/2) = ¼ probability of Ff (freckles)
– Mom f + Dad F = Ff
(1/2) x (1/2) = ¼ probability of Ff (freckles)
– Mom f + Dad f = ff
(1/2) x (1/2) = ¼ probability of ff (no freckles)

43. Probability of the Outcome of a Cross
• Genotype Probabilities:
– ¼ FF (freckles)
– ¼ Ff + ¼ Ff = ½ Ff (freckles)
– ¼ ff (no freckles)
• Phenotype Probabilities:
– ¼ FF + ½ Ff = ¾ freckles
– ¼ ff = ¼ no freckles

44. Pedigrees

45. Studying Pedigrees:
How are traits inherited???
• Pedigree= a family history that shows how
a trait is inherited over several generations

46. Why are pedigrees helpful/useful?
Common Genetic Disorders:
• Pedigrees are
helpful if
couples are
concerned
that they might
be carriers of
genetic
disorders
Angelman Syndrome
Canavan Disease
Charcot-Marie-Tooth Disease
Cri du Chat Syndrome
Duchenne Muscular Dystrophy
Fragile X Syndrome
Gilbert's Syndrome
Joubert Syndrome
Klinefelter Syndrome
Krabbe Disease
Lesch–Nyhan Syndrome
Myotonic Dystrophy
Neurofibromatosis
Noonan Syndrome
Pelizaeus-Merzbacher Disease
Phenylketonuria
Porphyria
Prader-Willi Syndrome
Retinoblastoma
Rubinstein-Taybi Syndrome

47. Pedigree Symbols

48. Pedigree Numbers
• Roman
numerals (I, II,
III, IV) represent
= Generations
•Regular
numbers
(1,2,3,4)
represent =
Individuals in
each
generation

49. Pedigree Symbols – Male and Female
= Normal Male
= Normal Female
Horizontal line between a male and
female indicates
MATING/MARRIAGE
= Male with trait
= Female with
trait
Branching vertical lines point to
OFFSPRING

50. Autosomal vs. Sex-linked Traits
• Autosomal Trait = appears in both sexes
equally, alleles appear on the autosomal
chromosomes
• Sex-linked Trait = a trait whose allele is
located on the X chromosome
– Appears mostly in males
– Mostly recessive
– Female only exhibits the condition if she
inherits two recessive alleles

51. Human Chromosomes
• Humans have 46 chromosomes:
– 1 pair of sex chromosomes (X and Y)
– 22 pairs of autosomes
• Females have 2 X chromsomes (XX).
• Males have an X chromsome and a Y
chromosome (XY)

52. Karyotypes
• A Karyotype is a test to identify and
evaluate the size, shape, and number of
chromosomes in a sample of body cells.

53. Dominant vs. Recessive Trait
• Autosomal Dominant
Traits = each
individual with the
trait will have a
parent with the trait
• Autosomal
Recessive Traits =
an individual with the
trait can have one,
two, or neither parent
who exhibit the trait

54. Recessive Disorder: Albinism
• Albinism = a genetic disorder in which the
body is unable to produce an enzyme
necessary for the production of melanin (dark
color to hair, skin, scales, eyes, and feathers)

55. Genetic Disorders = Carriers
• Carriers = individuals
who are heterozygous
for a recessive
inherited disorder, but
do not show
symptoms of the
disorder
– Can pass the recessive
allele for the disorder to
their offspring

56. Red-Green Color Blindness:
A Sex-Linked Recessive Disorder
• X-linked recessive
disorder
• Among Caucasian
individuals:
– 8% of males
– 0.5% of females
• Difficulty
distinguishing
between shades of
green and red

57. Red-Green Color Blindness:
A Sex-Linked Recessive Disorder
Males have the disorder more often than females because they
only have one X chromosome.
• Unaffected female =
• Affected female =
• Carrier female =
• Unaffected male = XRY
• Affected male = XrY
XRXR
XrXr
XRXr

58. Red-Green Color Blindness:
A Sex-Linked Disorder

59. Heterozygous vs. Homozygous
• Homozygous Dominant or Heterozygous individuals =
phenotype will show the dominant characteristic
• Homozygous Recessive individuals = phenotype will
show the recessive characteristic
***Heterozygous carriers of a recessive mutation will not
show the mutation, can produce children who are
homozygous for the recessive allele

60. Let’s look at a pedigree for polydactyly:
a dominant trait

61. Let’s look at a pedigree for
phenylketonuria (PKU): a recessive
disorder
The trait
skips a
generation!!

62. Complex Patterns of
Inheritance

63. Traits Influenced by Several Genes
• Polygenic Trait = when several
genes influence a trait
– Genes can be:
• Scattered along the same
chromosome
• Located on different chromosomes
– Independent Assortment = many
different combinations in offspring
• Polygenic traits =
– Eye color, height, weight, hair
and skin color

64. Polygenic Trait – Skin Color

65. Intermediate Traits
• Incomplete Dominance =
an individual displays a trait
that is intermediate
between the two parents
• Examples:
– Red snapdragon + White
snapdragon = Pink
snapdragon offspring
– Curly hair + Straight hair =
Wavy haired offspring
• Neither allele is dominant to
the other

66. Pink Snapdragon - Heterozygous

67. Traits With Two Forms Displayed At
The Same Time
• Codominance = when two
dominant alleles are
expressed at the same
time, both forms of the
trait are displayed
• Example:
– Red Horse + White Horse =
Roan Horse

68. Roan Horse - Heterozygous

69. Traits Controlled By Genes With
Three or More Alleles
• Multiple Alleles = Genes with three or
more alleles
• Example: ABO blood groups in humans
– IA and IB = Dominant to i;
• A & B are two carbohydrates on the surface of red
blood cells
– i = Recessive
– When IA and IB are present together =
Codominant

70. Traits Controlled By Genes With Three
or More Alleles
• An individual can only have two of the possible
alleles for the gene

71. Genotypes of Each Blood Type

72. Traits Influenced By The Environment
• Phenotype is affected
by environmental
conditions
• Hydrangeas (flowers)
range from blue to
pink based upon the
pH of the soil
– Acidic soil = blue
flowers
– Basic soil = pink
flowers

73. Traits Influenced By The Environment
• Siamese Cat
– Fur on ears, nose, paws, and tail is darker than the
rest of the body
– Dark fur at locations which are cooler than normal
body temperature

74. Traits Influenced by the Environment:
Identical Twins

75. Human Examples:
Traits Influenced By The Environment
• Height
– What can influence
height besides genes?
• Skin Color
• Human Personality

76. Traits Caused By Mutation
• Damaged genes/genes which are copied
incorrectly – result in faulty proteins
• Mutations are RARE
• Inherited Mutations cause Genetic
Disorders
• Many mutations are carried in recessive
alleles
– Carrier = heterozygous individual
• Carry the recessive allele but do not exhibit the
disorder

77. Sickle Cell Anemia
• Recessive
• Defective hemoglobin
– Red Blood Cells
– Binds and transports
oxygen
• Sickle-Cell Shape
– Rupture-prone
– Clotting in blood vessels
• Heterozygote Advantage
= protection from malaria

78. Hemophilia
• Recessive
• Sex-linked
– X chromosome
– More males afflicted
than females
• Impairs blood clotting
• English royal family

79. Hemophilia: The Royal Family

80. Huntington’s Disease
• Dominant
• First symptoms
appear in thirties or
forties:
– Mild forgetfulness
– Irritability
• Long-term symptoms:
– Loss of muscle control
– Chorea (physical
spasms)
– Severe Mental Illness
– Death

81. Detecting and Treating Genetic
Disorders
• Genetic Counseling = a form of medical
guidance that informs people about genetic
problems that could affect them or their offspring
• Phenylketonuria (PKU)
– Lacks enzyme to convert phenylalanine into tyrosine
– Can cause mental retardation
– Early intervention involves low-phenylalanine diet
• Gene Therapy = replacing defective genes with
healthy ones