This document provides information about Gregor Mendel and his experiments with pea plants that formed the basis of classical genetics and heredity. It discusses Mendel's work with traits controlled by single genes, including his discovery of dominant and recessive alleles and his laws of segregation and independent assortment. It also describes more complex patterns of inheritance beyond simple Mendelian genetics, including incomplete dominance, codominance, multiple alleles, epistasis, pleiotropy, and polygenic and sex-linked traits. The document uses examples like coat color in cats and human genetic disorders to illustrate these concepts.

1. War m up
 Match the items on the left with one item on
the right
1. HH A. heterozygous
2. Curly hair B. homozygous
3. Hh C. phenotype
4. Genotype D. tt

2. Hel pf ul
Crash Course Biology
 Hank Green
Bozeman Biology
 Paul Anderson

3. Inheritance
Ch. 11

4. Main Topics
 Gregor Mendel’s work
 Mendel’s Laws
 Dominant/recessive
 Heterozygous/homozygous
 Alleles
 Codominance and incomplete dominance
 Epistasis, Pleiotropy, Multifactorial Inheritance,
Polygenic Traits

5. The father of genetics
 Gregor Mendel is
considered the Father of
Genetics
 Born in 1822
 Studied math & physics
at an Austrian university
 He was the first person to
study how traits are
passed along from one
generation to the next.
 He did his work with the
pea plant
Who’s your
daddy?

6. Mendel’s Garden
 Analyzed
observable traits
of peas growing
in his monastery
garden.

7. Mendel’s Garden
 Eight years & 20
volumes of data
and analysis on 7
distinctive traits
 Published in 1865

8. Why peas?
 The garden pea was a good choice for a
variety of reasons. The garden pea:
 is easy to raise
 produces large numbers of offspring
 reproduces quickly
 has flowers which are self fertilizing but can be
easily crossed to other varieties

9. Experimental
Approach
 Can also be
cross-fertilized
by human
manipulation

10.  Mendel cross-fertilized true-breeding garden pea
plants having clearly contrasting traits

12. Allele for purple flowers
Homologous
pair of
chromosomes
Allele for white flowers
Locus for flower-color gene

13. Mendel's
Theory of Segregation
 Diploid organisms inherit two genes
per trait
 Each gene segregates from the other
during meiosis so that each gamete
will receive only one gene per trait

14. How can the Chances of an Offspring’s Traits
be Determined?
 The chance of an offspring showing a certain trait can
be determined by using the Punnett square.
 The table contains spaces for the parent’s gametes
and the possible offspring from that mating.
 The alleles are represented by their letters.
 Genes come in pairs and must be separated during
gamete formation.
 These gametes (letter) of each pair are placed in each
of the outside spaces.
 They are then combined to form the possible
offspring.

15. Punnett Square: Bb X Bb
bbBbb
BbBBB
bBGametes

16. Monohybrid Crosses
 Mendel's first
experiments
 One trait
 Monohybrid crosses
have two parents that
are true-breeding for
contrasting forms of a
trait.

17.  All the offspring from the
first cross showed only 1
form of the trait
 This trait seemed
“stronger” so he called it
DOMINANT
 When he crossed the
offspring from the first
cross, the other form of
the trait reappeared, but
only 1/4 of the time
 This trait seemed
“weaker” so he called it
recessive

18. Predicting the Outcome
Why does one form of the trait disappear
in the
first generation (F1),
only to show up in the
second generation (F2)??

19.  Artificial selection: populations could evolve (i.e.
change) if members show variation in heritable traits
 Variations that improved survival chances in the wild
would be more common in each generation
 This idea is known as natural selection
Prevailing Theories

20. Mendel’s Experiments
 Natural selection did not fit with prevailing view
of inheritance-blending
 Blending would produce uniform populations;
such populations could not evolve

21. Mendel’s Experiments
 Many observations did not fit blending
 A white horse and a black horse did not
produce only gray horses

22. Test (Back) Crosses
 To support his concept of segregation,
Mendel crossed F1 plants (Pp) BACK with
homozygous recessives (pp)
What ratio would
Mendel have gotten?
He didn’t know the letter
combination of the F1
plants. The test (back)
cross allowed him to
figure it out

23. Dominant phenotype,
unknown genotype:
PP or Pp?
If PP,
then all offspring
purple:
p p
P
P
Pp Pp
Pp Pp
If Pp,
then 1
2 offspring purple
and 1
2 offspring white:
p p
P
P
pp pp
Pp Pp
Recessive phenotype,
known genotype:
pp
His back
crossed
supported
his idea of 2
“factors” for
each
individual,
and the idea
that those
“factors” are
segregated

24. Dihybrid
Crosses
 Mendel also
performed
experiments
involving two traits

25. Predicting the Outcome
What is the predicted PHENOTYPIC
ratio
and the predicted
GENOTYPIC ratio
that Mendel saw?

26. Predicting the Outcome
 The F2 results showed 9/16
were tall and purple-
flowered and 1/16 were
dwarf and white-
flowered-as were the
original parents;
however, there were 3/16
each of two new
combinations: dwarf
purple-flowered and tall
white-flowered.

27. Out comes
Monohybrid crosses
Both parents
HETEROZYGOUS
3:1 phenotype
Dihybrid crosses
Both parents
HETEROZYGOUS
9:3:3:1 phenotype

28. Theory of Independent Assortment
 Each gene of a pair tends to assort into
gametes independently of other gene pairs
on non-homologous chromosomes

29. Theory in
Modern Form
 Genes located
on non-
homologous
chromosomes
segregate
independently
of each other

30. Practice with your neighbor
 For the following questions
 Work with your neighbor to answer
the question.
 Answer the multiple choice question
then,
 Use your notes to determine which one
of Mendel’s principles it demonstrates

31. 1. A father carries 2 alleles for the gene
for widow’s peak. He carries one
dominant allele and one recessive
allele. His gametes will
a. All contain the dominant allele
b. All contain the recessive allele
c. ½ will get the dominant allele and ½ will get the
recessive allele
d. Each gamete will get both the dominant and the
recessive allele

32. Which principle does question
number one best demonstrate?
 Principle of Segregation
The dominant allele goes to one gamete and
the recessive allele goes to another
gamete

33. 2. A mother that is homozygous dominant
for bushy eyebrows (BB) and
heterozygous for round ears (Rr). The
gametes she can make will
a. All have a B and a R in them
b. ½ will have a B and ½ will have a R or a r in
them
c. ½ will have a B and a R and ½ will have b and
r
d. ½ will have B and R and ½ will have B and r

34. What principle does number 2
demonstrate?
 The Principle of Independent Assortment
 All gametes will have a B, since mom only has B.
 The big B can be with the big R or the big B can
be with the little r.

35. 3. In meiosis, a diploid cell divides
twice to form 4 haploid gametes.
Each gamete contains:
a. A complete set of DNA identical to the parents
b. A ½ set of DNA, with just one copy of each
chromosome
c. Homologous pairs of chromosomes
d. Multiple copies of chromosomes, depending on
which ones moved during meiosis

36. Which one of Mendel’s Principles
does number 3 demonstrate?
 Principle of Segregation
 All the homologous pairs of chromosomes
separate so that there is just one of each
pair in each gamete.

37. 4. When Mendel crossed a true
breeding green pea plant (GG) with a
true breeding yellow pea plant (gg),
the offspring plants were
a. All green
b. All yellow
c. ½ green and ½ yellow
d. Green and yellow mixed

38. Which one of Mendel’s principles
does number 4 demonstrate?
 Principle of Complete Dominance
 All offspring were Gg, and the dominant
allele (G) masked the recessive allele (g)

39. 5. Mendel wanted to know if the color for
pea seeds was linked to the shape of the
pea seeds. He crossed a green, wrinkled
seed plant (Ggrr) with a yellow, smooth
seed (ggRr) plant. The offspring produced
were:
a. All green and wrinkled
b. All yellow and wrinkled
c. All green and smooth
d. All yellow and smooth
e. Some of each of the above

40. Which one of Mendel’s Principles
does number 5 demonstrate?
 Principle of Independent Assortment
 The green trait can go with the smooth or
the wrinkled trait
 The yellow trait can go with the smooth or
the wrinkled trait

41. Mendel’s Work
 The work that Mendel did
helped explain patterns of
inheritance in eukaryotes.
 But Mendel worked with
traits that had a clear
dominant/recessive
pattern.
 Also, the traits he worked
with were all controlled by
a single gene.

42. Different Patterns of Inheritance
 As we now know,
many traits do not
follow Mendelian
Inheritance patterns.

43. Degr ees of Domi nance
 Complete Dominance - BB and Bb =
same phenotype
 Incomplete Dominance - Bb has in-
between phenotype
 Codominance - Bb has both B and b
phenotype

44. Co-dominance
 When both
alleles are
expressed
equally in the
heterozygous
individual.
 A and B blood type alleles are
co-dominant, because a person
with AB genotype will have
both A and B blood proteins.
 Black and orange color in cats
are co-dominant, because a
heterozygous female will have
both orange and black hair.

45. Incomplete Dominance
 Both alleles are blended
together in the
heterozygous individual.
 Dominant allele cannot
completely mask the
expression of another

46. Multiple Alleles
 More than 2 versions
(alleles) for a single trait
 can be completely
dominant or
codominant

47. Bl ood Types
Genotype of
offspring
Phenotype of
offspring
A
iA
iB
AB
iA
i A
iA
iA
iB
iB
B
iB
i B
ii o

48. Rh f act or
Rh factor Possible genotypes
Rh+
Rh-
+/+ or +/-
-/-

49.  So far we’ve only looked at how a
single gene pair affects phenotype
 More often - multiple genes involved
 2 primary cases:
 1. 2 or more genes affect a single trait
 2. 1 gene affects the phenotype of
another gene

50. Epi st asi s ( s t a nd i ng up o n)
- 2 or mor e genes af f ect a
si ngl e t r ai t
 Labs can be black, yellow, or chocolate

51.  Black is dominant to chocolate
 BB and Bb = black
 bb = chocolate

52.  AND - another gene P codes for
whether or not any pigment is put into
the hair
 PP and Pp = hair has pigment and dog
will be black (BB or Bb) or brown (bb)
 pp = no hair pigment and dog will be
yellow, regardless of the “b” alleles

53.  So in this case, the P gene “stands upon” the
B gene
 P is epistatic to B
 We don’t get the classic 9:3:3:1 but some
other version of it

55. Pl ei ot ropy
 A single gene can
have multiple effects
on phenotype
 e.g. pleiotropic alleles
--> multiple symptoms
of sickle cell anemia
(pain, jaundice,
infections, fatigue, etc)

56. Pol ygeni c Inheri t ance
 2 or more genes affect a single
phenotypic trait
 Eye color, skin color, height

57.  Skin color is controlled by at least 3
separate gene pairs
 Genotype AABBCC would be very dark
skin
 Genotype aabbcc would be very light skin
 Any other combination would be
intermediate

58.  And, of course, skin color is also
influenced by your environment -
multifactorial inheritance

59. X-linked traits
 genes found on the X
chromosome.
 show different inheritance
patterns in men than in
women.
 X-linked traits may show
dominant/recessive or
codominant patterns.

60. Sex- l i nked
genes
• An organism’s sex is
an inherited
phenotypic character
determined by the
presence or absence
of certain
chromosomes
• Mammals like
humans have an XX
or XY system of
inheritance
• Other organisms
have other systems

61. Genes on t he sex
chr omosomes ar e
cal l ed sex- l i nked
genes
• Some diseases on the X
chromosome:
• Color blindness
• Rare in females, mild disease
• Duchenne muscular dystrophy
• 1 in 3500 males in US gets it
• Lack the gene for the muscle protein
dystrophin
• Muscles get weaker and lose
coordination
• Usually don’t live past 20s
• Hemophilia
• Lack the protein to cause clotting
• Don’t clot normally

62. Bar r
bodi es
• In mammalian females, 1 of the 2 X
chromosomes is inactivated during embryonic
development
• The inactive X condenses into what is called a
Barr body (we can see it under the
microscope)
• If she is heterozygous for a sex-linked trait,
she will be a mosaic for that trait

63. • Some cells have
the maternal X
inactivated
• These cells have
the orange color
• Some cells have
the paternal X
inactivated
• These cells have
the black color
• All cells in the
ovaries have active
X chromosomes

64. Y- l i nked t r ai t s
 Y-linked traits called holandric
inheritance.
 Y-chromosome is small and does not
contain many genes
 Deletions on y chromosome  male
infertility
 SRY gene  sex determining region

65. The cur i ous case of
t he guevedoces
 Deficient in an enzyme that converts
testosterone to dihydrogen testosterone, so
don’t develop male genitalia as embryos.

66. 20.
Orange and black coat color are on the X
chromosome in cats and they are codominant
to each other. Tortoise shell is the codominant
phenotype.
A black female (XB
XB
) mated with an unknown
male. The kittens were:
2 tortoise shell females and 2 black males.
What is the father’s genotype and phenotype?
XO
Y- orange

67. 21.
• Ricket’s is a dominant disorder on the X
chromosome in humans.
• X = normal XR
= affected by rickets
A couple wants to know their chances of having
a child born with Rickets.
The wife is normal, the husband has the
disease.
What are the chances of having an affected
son? An affected daughter?
0% affected son, 100%
affected daughter

68. 22.
• Another couple, same disease. This
time, the wife is affected. Her father was
normal. The husband is not affected.
Same question: chances of an affected
son? Affected daughter?
50% son, 50% daughter

69. 23.
• A tortoise shell female mated with an
unknown male. The kittens were 2
orange females, 1 tortoise shell
females, 1 black male, 2 orange males.
• What is the genotype and phenotype of
the father?
XO
Y- orange

70. Chr omosomal mut at i ons
• In nondisjunction,
pairs of homologous
chromosomes do
not separate
normally during
meiosis
• As a result, one
gamete receives two
of the same type of
chromosome, and
another gamete
receives no copy

71. What r esul t s…
• Aneuploidy - a zygote
produced from a normal
gamete and a gamete
produced by
nondisjunction
• Offspring with this
condition have an
abnormal number of a
particular chromosome

72. What
r esul t s…
• Trisomy - having 3 copies of a particular chromosome
• Monosomy - having just one copy of a particular
chromosome
• Polyploidy - a condition in which an organism has more
than two complete sets of chromosomes
Recent research
has shown that
this Chilean
rodent is a
tetraploid
Very rare among
animals
Common in plants,
some fish, some
amphibians

73. Chr omosomal br eakage
• Breakage of a chromosome can lead to
four types of changes in chromosome
structure:
• Deletion removes a chromosomal
segment

74. Del et i on mut at i on
• Example: retinoblastoma (eye
tumors)

75. Chr omosomal br eakage
• Duplication repeats a segment

76. Dupl i cat i on mut at i on –
f r agi l e X syndr ome

77. Chr omosomal br eakage
• Inversion reverses a segment within a
chromosome

78. Hemophi l i a A – i nver si on
mut at i on pat i ent was gi ven
i nj ect i on i n but t ocks

79. Chr omosomal br eakage
• Translocation moves a segment from
one chromosome to another

80. Tr ansl ocat i on mut at i on
causes Bur ki t t ’ s l ymphoma
Tumor s on hand f r om cancer

81. Why does t hi s happen?
 When would you predict these kinds of
chromosomal errors would occur?
 Why?

82. Down syndr ome
• Trisomy 21 - 3 number 21
chromosomes
• 1 in 700 children in US
• Frequency increases with
age of mother

83. Tr i somy 18 – Edwar d’ s
syndr ome l ow bi r t h wei ght ,
ment al r et ar dat i on, ext r a
f i nger s and t oes

84. Tr i somy of sex
chr omosomes
• Klinefelter
syndrome is the
result of an extra
chromosome in a
male, producing
XXY individuals
• Monosomy X, called
Turner syndrome,
produces X0 females,
who are sterile; it is the
only known viable
monosomy in humans

85.  DNA is also found in
mitochondria and
chloroplasts.
 Mitochondrial DNA is
only passed from
Mother to child.

86. How ar e t r ai t s
i nher i t ed?
 What mode(s) of inheritance would you
predict for the trait of skin color? Why?
 Make a list of all the possible modes of
inheritance we’ve learned about
 Next to each one give a short definition and
an example