1. EXTENSION OF MENDELIAN
GENETICS
1. Codominance
2. Incomplete dominance
3. Multiple alleles
4. Lethal alleles
5. Epistasis
6. Polygenic inheritance
7. Linked genes
8. Crossover values and
gene mapping
9. Sex linked genes

2. INTRODUCTION
• Mendelian inheritance describe
– patterns that obey two laws
• Law of segregation
• Law of independent assortment
– Includes simple Mendelian inheritance
• A single gene with two different alleles
• Alleles display a simple dominant/recessive relationship

3. INTRODUCTION (cont)
• Simple Mendelian
inheritance
– Traits affected by a
single gene
– Two alleles exist for
this gene
– 3:1 phenotypic ratio
in the F2 generation

4. INTRODUCTION (cont)
• Dominant alleles are usually indicated either by:
– an italic uppercase letter (D)
• Recessive alleles are usually indicated either by:
– an italic lowercase letter (d)
• If no dominance exists, italic uppercase letters
and superscripts are used to denote alternative
alleles (R1, R2, CW, CR).

5. MENDELIAN INHERITANCE
• Alternative forms of a gene are called alleles.
• Mutation is the source of alleles.
• The wild-type allele is the one that occurs
most frequently in nature and is usually, but
not always, dominant.

6. MENDELIAN INHERITANCE (cont)
• Wild-type alleles (dominant) are the most
prevalent alleles in a population
– Encoded protein is generally
• Functional
• Made in the proper amount
– Confer various phenotypes
• e.g., Purple flowers, round seeds, etc.

7. MENDELIAN INHERITANCE (cont)
• Mutant alleles (recessive) have been altered by
mutation
– Tend to be rare in natural populations
– Commonly defective in ability to express functional
protein
• Encoded protein is often:
– Produced in reduced amount
(decreased synthesis)
– Less functional (decreased function)
– Often inherited in a recessive fashion
– Confer various phenotypes
• e.g., White flowers, wrinkled seeds, etc.

8. MENDELIAN INHERITANCE (cont)
• Genetic disease are caused by mutant alleles
• In many human genetic diseases, the recessive
allele contains a mutation. What kind of
genetic disease that you know of containing
recessive alleles? (submit in two weeks time)

9. MENDELIAN INHERITANCE (cont)
• Simple dominant/recessive relationship
– Fairly common among many genes
– One copy of the dominant allele is sufficient to
produce the dominant phenotype
• Recessive allele does not affect the phenotype of
heterozygotes

10. • Production of functional protein is reduced by 50%
– 50% is adequate to provide a normal phenotype
– Homozygotes produce more wild-type protein
than necessary

11. LETHAL ALLELES
• Gene is an essential for survival
– An estimated 1/3 of all genes are essential for
survival
• Mutant allele is a lethal allele
• Has potential to cause death
• Inherited in a recessive manner
• Absence of a specific protein may result in a lethal
phenotype

12. LETHAL ALLELES (cont)
• Many lethal alleles prevent cell
division
– These will kill an organism at an
early age
• However, some lethal alleles
exert their effect later in life
– e.g. Huntington disease causes a
progressive degeneration of the
nervous system
• Age of onset is generally 30 – 50
• Progressive degeneration of nervous
system, dementia and early death

13. LETHAL ALLELES (cont)
• Some lethal alleles exert their effect only under
certain environmental conditions
– “Conditional lethal alleles”
– e.g., Temperature-sensitive (ts) lethals
• May kill developing Drosophila larva at 30oC
• Larva will survive if grown at 22oC
• Why do you think this is the case? (submit in two weeks
time)

14. LETHAL ALLELES (cont)
• Two types of Lethal alleles
– Dominant lethal alleles
– Recessive lethal alleles

15. Dominant LA
• The LA modify the 3:1 phenotypic ratio into 1:1
• The individuals with a dominant LA die before
they can produce the progeny. Therefore, the
mutant dominant LA is removed from the
population in the same generation in which it
arose.
• Huntington’s disease is caused by a dominant LA
and even though it is not described as lethal, it is
invariably lethal in that the victim experiences
gradual neural degeneration for some years
before death occurs.

16. Recessive LA
• They maybe of two kinds (i) one which has no
obvious phenotypic effect in heterozygotes and
(ii) one which exhibit a distinctive phenotype
when in heterozygous condition.
• Recessive LA don’t cause death in the
heterozygous form because a certain threshold of
protein output is maintained. In the homozygous
form, the protein output doesn’t meet the
threshold, causing death.
• Eg diseases; cystic fibrosis, Tay-Sachs disease,
sickle cell anemia, and brachydactyly.

17. brachydactyly

18. Recessive LA (cont)
• One coat colour of ranch foxes is caused by recessive
lethal gene. This gene causes a death if both recessive
alleles are possessed by the same individual.
• It occasionally arise by mutation from a normal allele.
• However, in many cases lethal genes became operative at
time the individual become sexually mature.
• Complete lethality, thus is the case where no individual
of a certain genotype attain the age of reproduction.

19. LETHAL ALLELES (cont)
• Lethal alleles may produce ratios that
seemingly deviate from Mendelian ratios
– e.g., “Creeper” phenotype in chickens
• Shortened wings and legs
• Creeps rather than walking normally
• Creeper chicken are heterozygous
Creeper Cp - shortlegged,
- autosomal semi-lethal
incomplete dominant

20. All “creeper” birds are heterozygous
• Creeper x Normal • Creeper x creeper  2:1
1:1 phenotypic ratio • Creeper allele is a
– Creeper phenotype is recessive lethal
dominant – Creeper homozygotes are dead

22. TWO ALLELES
Codominance
Incomplete Dominance

23. CODOMINANCE
• Any given gene may have more than two
alleles and the phenotype of both alleles are
in heterozygote.
• Two alleles are expressed
• Do not blend the phenotype
• ‘Co’ – means together
• Co-dominance = both alleles are dominant
– Example: different blood types and group antigens

24. • Also an example of multiple alleles, will be
explain further later

25. CODOMINANCE (cont)
• Different from Segregation Law
• As both of the alleles are dominant, we are
going to use ‘R’ for the colour of cow’s hair
• RR = all red hair
• RW = all white hair
• RRRW = red and white hair
• RR RR x RW RW = 100% RRRW

26. INCOMPLETE DOMINANCE
• Heterozygotes sometimes display a phenotype
intermediate between the homozygotes
– e.g., Flower color in the four-o’clock,
snapdragons, carnations, etc.
– Homozygous red (CRCR) x homozygous white (CWCW)
• F1 offspring (CRCW) are heterozygous
and pink
• F2 offspring display 1:2:1 phenotypic
and genotypic ratios

27. 1:2:1 phenotypic ratio
but NOT the 3:1 ratio
50% of the CR observed as in simple
protein is not Mendelian Inheritance
sufficient to
produce the red
phenotype

28. • Many traits appear to be dominant
– Closer examination shows that some are actually
incompletely dominant
– e.g., Seed shape in Mendel’s peas
• RR and Rr genotypes produce round seeds
• rr genotypes produces wrinkled seeds
– Decreased starch deposition

30. EXERCISE
1. Predict the phenotype ratios of offspring when a
homozygous white cow is crossed with a roan (red
and white) bull.
2. What is the phenotypes and genotypes for parent
cattle be if a farmer wanted only cattle with red fur?
3. A cross between a black cat and a tan cat produces a
tabby pattern (black and tan fur together)
a) What pattern of inheritance does this illustrate?
b) What is the percentage of kittens will have tan fur if a
tabby cat is crossed with a black cat?

31. THREE ALLELES
Multiple alleles

32. MULTIPLE ALLELES
• Individuals possess two copies of each gene
– At most, they possess two different alleles
• Means there are same/more than three alleles
• The situation exclude the dominant and
recessive effects
• All the alleles show own effects in inheritance
• Eg. Blood type and hair colour

33. MULTIPLE ALLELES (cont)
• Coat color in rabbits is determined by alleles of
the “C” gene
– Four different alleles
exist
• C = full coat color
• cch = chinchilla
• ch = himalayan
• c = albino
– Any particular rabbit
possesses only two
alleles

34. MULTIPLE ALLELES (cont)
• Dominant/recessive relationships
between coat color alleles
– C is dominant to cch, ch, and c
– cch is recessive to C, but dominant to
ch, and c
– ch is recessive to C and cch, but
dominant c
– c is recessive to C, cch, ch
C > cch > ch > c

35. MULTIPLE ALLELES (cont)
• Four different alleles
– C (full coat color)
– cch (Chinchilla - partial defect in coloration)
– ch (himalayan- pigmentation only in certain parts)
• Temperature-sensitive conditional allele
– c (albino- is a defective allele producing no
protein necessary for pigment production)

36. • This is caused by tyrosinase; producing melanin
• Eumelanin: black pigment and phaeomelanin
(orange/yellow pigment)

37. MULTIPLE ALLELES (cont)
• cchc x Cch
cch c
C Ccch Cc
ch Cchch chc
• Genotypes:
– 1 Ccch :1 Cc : 1 Cchch : 1 chc
• Phenotypes:
– 2 Full: 1 Chinchilla: 1Himalayan

38. CONDITIONAL ALLELES
• ch is a temperature-sensitive conditional allele
– Results in pigmentation only in certain parts of the
body
• Encoded enzyme functions only in cooler areas of the body
– Ends of extremities, tail, paws, nose, ears
– Similar temperature-sensitive
alleles are found in other animals
• e.g., Siamese cat

39. MULTIPLE ALLELES (cont)

40. MULTIPLE ALLELES (cont)
• Red blood cells contain carbohydrate chains
on their plasma membranes
– “Antigens”
• Recognized by immune system’s antibodies
– A, B, and O antigens determine human blood type
• Synthesized by three alleles of a single gene
• IA, IB, and i

41. MA: RED BLOOD CELLS
• What are antigens? (submit in two weeks time)

42. • The “i” gene produces an enzyme
– Glycosyl transferase
– Attaches sugar “branches” to carbohydrate “trees”
present on the surface of red blood cells
• i allele encodes a defective enzyme
– No sugar branches are attached
• IA and IB alleles encode enzymes with different
substrate specificities
– Different sugar “branches” are attached

43. MA: RED BLOOD CELLS (cont)
• i is recessive to both IA and IB
– ii  type O blood
• IA and IB are codominant
– IAIB  AB blood
• Possesses both A and B antigens

44. MA: RED BLOOD CELLS (cont)
• Blood typing is essential for safe blood
transfusions
• The donor’s blood must be an appropriate match
with the recipient’s blood
• Eg. If a type O individual received blood from a
type A, type B or type AB blood
– Antibodies in the recipient blood will react with
antigens in the donated blood cells
– Donated blood will agglutinate
– Life threatening situation: clogging vessels

45. EXERCISE
Cross a heterozygous type A with a heterozygous
type B.
1. ___IAi____ X ____IBi___
2. ___IAIB____ X ____ii___
What is the genotypes and phenotypes?

46. EXERCISE
• A woman with type O blood and a man who is
type AB are expecting a child. What are the
possible blood type of the kid?
• What are the possible blood type of a child who’s
parents are both heterozygous for ‘B’ blood type?
• Jill is blood type O. She has two brothers (who
always tease her) with blood type A and B. What
are the genotypes of her parents with respect to
this traits?

47. Continue….

48. GENES COMPLEX INTERACTION
1. Epistasis
2. Polygenic Inheritance

49. EPISTASIS
Defined as:
• An inheritance pattern in which the alleles of
one gene mask the phenotypic effects of the
alleles of another genes

50. EPISTASIS
• Epistasis, first defined by the English geneticist,
William Bateson in 1970, is the masking of the
expression of a gene at one position in a
chromosome, or locus, at one or more genes at
other position.
• Epistasis is the phenomenon where the effects of
one gene are modified by one or several other
genes, which are sometimes called modifier
genes.

51. EPISTASIS (cont)
• The genes whose phenotype is expressed is said
to be epistatic, while the phenotype altered or
suppressed is said to be hypostatic.
• Epistasis can be contrasted with dominance,
which is an interaction between alleles at the
same gene locus.
• Epistasis is often studied in relation to
Quantitative Trait Loci (QTL) and polygenic
inheritance

52. Example: Walnut Comb
• rr and pp to be epistatic
to this phenotype.
• rr and pp mask a walnut
comb.

53. P1 Pea Comb X Rose Comb
PPrr ppRR
F1 All Walnut Combs
PpRr
When these F1 birds are crossed, all
four phenotypes are observed:
F2 PR Pr pR pr
PR PPRR PPRr PpRR PpRr
Pr PPRr PPrr PpRr Pprr
pR PpRR PpRr ppRR ppRr
pr PpRr Pprr ppRr pprr

54. Ratio Description Name(s) of Relationship
(Used by Some Authors)
9:3:3:1 Complete dominance at both gene pairs; new phenotypes result Not named because the ratio
from interaction between dominant alleles, as well as from interaction between looks likeindependent assortment
both homozygous recessives
9:4:3 Complete dominance at both gene pairs; however, when 1 Recessive epistasis
gene is homozygous recessive, it hides the phenotype of the other gene
9:7 Complete dominance at both gene pairs; however, when either Duplicate recessive epistasis
gene is homozygous recessive, it hides the effect of the other gene
12:3:1 Complete dominance at both gene pairs; however, when one gene is dominant, it Dominant epistasis
hides the phenotype of the other gene
15:1 Complete dominance at both gene pairs; however, when either gene is dominant, it Duplicate dominant epistasis
hides the effects of the other gene
13:3 Complete dominance at both gene pairs; however, when eithergene is dominant, it Dominant and recessive epistasis
hides the effects of the other gene
9:6:1 Complete dominance at both gene pairs; however, when eithergene is dominant, it Duplicate interaction
hides the effects of the other gene
7:6:3 Complete dominance at one gene pair and partial dominance at the other; No name
when homozygous recessive, the first gene is epistatic to the second gene
3:6:3:4 Complete dominance at one gene pair and partial dominance at the other; No name
when homozygous recessive, either gene hides the effects of the other gene;
when both genes are homozygousrecessive, the second gene hides the effects of
the first
11:5 Complete dominance for both gene pairs only if both kinds ofdominant alleles are No name
present; otherwise, the recessivephenotype appears

55. POLYGENIC INHERITANCE
• This term is use to refer to inheritance of
quantitative traits, traits which are influenced by
multiple genes and not just one.
• Because many traits are spread out across the
continuum, rather than being divided into black
and white differences, polygenic inheritance
helps to explain the way in which these traits are
inherited and focused.
• A related concept is pleiotropy, an instance
where one gene influences multiple traits
(phenotypes).

56. • However, in the 20th century, people were well
aware that most traits are too far complex to
be determined by a single gene, and the idea
of polygenic inheritance was born.

57. POLYGENIC INHERITANCE (cont)
• One easily understood example
of polygenic inheritance is height. People are not
just short or tall; they have a variety of heights
which run along a spectrum.
• Furthermore, height is also influenced by
environment; someone born with tall genes
could become short due to malnutrition or
illness, for example, while someone born with
short genes could become tall through genetic
therapy.

58. POLYGENIC INHERITANCE (cont)
• Basic genetics obviously
wouldn't be enough to
explain the wide diversity of
human heights,
but polygenic inheritance
shows how multiple genes in
combination with a person's
environment can influence
someone's phenotype, or
physical appearance.

59. POLYGENIC INHERITANCE (cont)
• Polygenic traits are a result of additive effects
of contribution of each genes in loci and
therefore they do not follow typical
dominance and recessive patterns.
• The second aspect of polygenic genes are, the
traits are determined by environmental
variations. It means that an individual can be
genetically same, but can differ in their
physical appearance.

60. POLYGENIC INHERITANCE (cont)
• Phenotypes like high blood pressure (hypertension) are
not the result of a single "blood pressure" gene with
many alleles (a 120/80 allele, a 100/70 allele, a 170/95
allele, etc.)
• The phenotype is an interaction between a person's
weight (one or more obesity genes), cholesterol level
(one or more genes controlling metabolism), kidney
function (salt transporter genes), smoking (a tendency
to addiction), and probably lots of others too. Each of
the contributing genes can also have multiple alleles.

61. • Skin colour is another example of
polygenic inheritance, as are
many congenital diseases.
• Because polygenic inheritance is
so complex, it can be a very
absorbing and frustrating field of
study.
• Researchers may struggle to
identify all of the genes which
play a role in a particular
phenotype, and to identify places
where such genes can go wrong.
• However, once researchers do
learn more about the
circumstances which lead to the
expression of particular traits, it
can be a very rewarding
experience.

62. DON’T GET CONFUSED!
• Multiple alleles=more than two forms of the
same gene in the population e.g blood type
• Polygenic traits=more than one gene
contributes to the phenotype

63. POLYGENIC INHERITANCE (cont)
Examples in Humans
• Weight
• Height
• Eye colour
• Intelligence
• Behaviour
• Skin colour

64. • In pleiotropy, on the other hand, one gene is
responsible for multiple things.
• Several congenital syndromes are examples of
pleiotropy, in which a flaw in one gene causes
widespread problems for a person.

65. • For example, sickle cell anemia is a form of
pleiotropy, caused by a distinctive mutation in
one gene which leads to a host of symptoms.
• In addition to causing mutations, pleiotropy
also occurs in perfectly normal genes,
although researchers tend to use it to track
and understand mutations in particular.

66. LINKED GENES
• The dihybrid cross we previously did assumed the
genes were on different pairs of chromosomes.
• Now, we want to look at an example where the
genes involved are on the same chromosome.

67. LINKED GENES (cont)
• One such example is the flower colour and
pollen shape experiment done
by Bateson and Punnett.
• In the plants that they studied, the genes for
pollen shape and flower colour are located on
the same chromosome (pair) as each other,
thus are inherited together.

68. Dihybrid cross Linked genes cross

69. • If the parents are PPLL × ppll, the first parent
will only make gametes with PL and the
second with pl, which doesn't seem too
different so far.
• From these parents, the F1 generation would
all be PpLl.

70. • However, when calculating what the F2 generation
will be, since the genes are located on the same pair
of chromosomes, then theoretically, the only
possible gametes are PL and pl (not Pl or pL).
PL pl
PL PPLL PpLl
pl PpLl ppll
• The phenotype ratio for this cross is 3:1, not 9:3:3:1
as would be expected for a “normal” dihybrid cross.
Because these genes are on the same chromosome
pair, they are called linked genes.

71. SEX LINKED GENES
• This is sex-linked genes, genes located
on one of the sex chromosomes (X or Y) but
not the other.
• Since, typically the X chromosome is longer, it
bears a lot of genes not found on the Y
chromosome, thus most sex-linked genes
are X-linked genes.

72. • One example of a sex-linked gene is fruit fly eye colour.
• An X chromosome carrying a normal, dominant, red-eyed
allele would be symbolized by a plain X, while the recessive,
mutant, white-eyed allele would be symbolized by X' or Xw.
• A fly with genotype XX' would normally be a female with
red eyes, yet be a carrier for the white-eyed allele.
• Because a male typically only has one X chromosome, he
would normally be either XY and have normal, red eyes, or
X'Y and have white eyes.
• The only way a female with two X chromosomes could have
white eyes is if she would get an X' allele from both parents
making her X'X' genotype. The cross between a female
carrier and a red-eyed male would look like this:
X Y
X XX XY
X’ XX’ X’Y

73. SEX LINKED GENES (cont)
• Typically, X-linked traits show up more in males than
females because typical XY males only have one X
chromosome, so if they get the allele on their X
chromosome, they show the trait.
• If a typical XX female is a carrier, 50% of her sons will
get that X chromosome and show the trait. In order for
an XX female to exhibit one of these X-linked traits,
most of which are recessive mutations, she would
have to have two copies of the allele (X'X'), which
would mean that her mother would have to be a
carrier and her father have the trait so she could get
one allele from each of them.

74. • The Punnett square would predict that ½ of their sons (¼ of their children) would be hemophiliacs
and ½ of their daughters (¼ of their children) would be carriers. Their children married other
royalty, and spread the gene throughout the royal families of Europe.

75. SEX LINKED GENES (cont)
SEX LIMITED TRAIT
– Affects a structure or function of the body that is
present in only male or only female
– E.g. Growth of beard or breast
SEX INFLUENCED TRAIT
– An allele that is dominant in one sex and recessive in
the other
– E.g. Baldness
– Heterozygous male is bald, heterozygous female is not

76. CROSSING OVER
• Even though the alleles for different genes may
be linked along the same chromosome, the
linkage can be altered during meiosis.
• In diploid eukaryotic species, homologous
chromosomes can exchange pieces with each
other, a phenomenon called crossing over.
• This event occurs during prophase of meiosis I.

77. Combination alleles = genetic recombinant

79. • Chiasma is the point where
chromatids become criss-crossed
and the chromosome exchange
segements.
• New combinantion arise from
crossing over resulting in
recombination and passed during
the gamete formation.
• When meiosis is over 2
chromosome separated to become
individual chromatids and produce
4 genetically different chromosome.

80. GENE MAPPING
• Also known as chromosome mapping, to
determine linear order and distance of
separation among genes that are linked to
each other along the same chromosome.

82. GENE MAPPING (cont)
• Why it is useful?
– Allow geneticist to understand overall complexity
and genetic organization of a particular species.
– The genetic map of a species portrays underlying
basis for the inherited traits that an organism
display.
– Help in cloning process too

83. GENE MAPPING (cont)
• Genetic map benefits:
– Locate human gene that causing diseases, this
information can be used to diagnose and someday
treat inherited human diseases.
– Predict likelihood that a couple will produce
children with certain inherited diseases.
– Also important in agriculture – help in improving
crops/animal through selective breeding.

84. The End