3. At the end of the topics students should able
to:
1. Define terminologies used in genetic inheritance.
2. State Mendel’s first law (law of segregation) and
second law (law of independent assortment).
3. Calculate genotypic and phenotypic ratio.
4. Explain codominant alleles, incomplete dominant
alleles, multiple alleles, linked genes and sex-linked
genes.
5. Calculate genetic distance (map unit).
4. Genetic inheritance
Terms &
concepts
Mendel’s
experiment
Pisum sativum – garden
pea
Monohybrid
crosses
Dihybrid crosses
Law of
segregation
Law of
independent
assortment
Deviation from
Mendel’s Law
Codominance
Incomplete
dominance
Multiple allele
Polygene
Lethal allele
Linked gene
Genetic
mapping
Pedigree
analysis
5. What is a gene?
A. A factor that controls a heritable
characteristic
B. Something on a chromosome
C. Information stored in a segment of DNA
D. Something that encodes a protein
6. ▪ Inheritance is how traits, or characteristics, are
passed on from generation to generation.
▪ Chromosomes are made up of genes, which
are made up of DNA.
▪ Genetic material (genes, chromosomes, DNA) is
found inside the nucleus of acell.
▪ Particulate Hypothesis of Inheritance.
▪ Parents pass on to their offspring separate and
distinct factors (today called genes) that are
responsible for inherited traits.
7. ▪ Gene - A piece of DNA that encodes a particular trait. EX
a gene for eye-color
▪ Allele-an alternate form of a gene.
E.g. - allele for blue eyes and allele for brown eyes
▪ Locus — the location of a gene on a chromosome.
Plural =loci
▪ Hybrid -The offspring of genetically dissimilarfrom
parents.
▪ Phenotype-the physical appearance of an individual,
determined by his or her genotype. (black, brown, short,
tall, etc).
▪ Genotype-the genetic composition of an individual or
combination of alleles. (BB, Bb, bb)
8. Homozygous - containing a pair of the same alleles.Can be
▪ Homozygous recessive - two recessive alleles e.g. bb, or
▪ Homozygous dominant – two dominant alleles e.g. BB
▪ Heterozygous - containing two different alleles. E.g. Bb
▪ Carrier – an individual who has a recessive allele of a gene that does not have an
effect on their phenotype.
▪ Dominant allele — expressed whether alone or in pairs.
Symbolized by a capital letter.
E.g. Brown eye allele=B
▪ Recessive allele — expressed only in the absence of a dominant
allele. Symbolized by lowercase.
E.g. blue eye allele=b
9.
10. ▪ Austrian Monk.
▪ Is considered “The Father of Genetics"
▪ Experimented with “pea plants”.
▪ Used pea plants because they:
were available
reproduced quickly
showed obvious differences in the traits
Understood that there was something that
carried traits from one generation to the
next- “FACTOR”.
11.
12. 1. Law of Dominance:
- In a cross of parents that are pure for contrasting traits, only
one form of the trait will appear in the next generation.
- Offspring that are hybrid for a trait will have only the dominant
trait in the phenotype.
What happens when the F1’s are crossed?
13. The F1 crossed produced the
F2 generation and the lost trait
appeared with predictable
ratios.
This led to the formulation of
the current model of
inheritance.
● In a hybrid union, the allele which expresses itself phenotypically =
dominant allele while the other allele which fails to express itself
phenotypically = recessive allele.
● The hybrid individual shows phenotypically only the dominant character.
● The law of dominance is often described as Mendel’s first law of
inheritance.
14.
15. 2. Law of Segregations:
-During the formation of gametes (eggs or sperm), the
two alleles (hereditary units) responsible for a trait
separate from each other.
-Alleles for a trait are then "recombined" at fertilization,
producing the genotype for the traits of the offspring.
3. Law of IndependentAssortment:
- Alleles for different traits are distributed to sex cells(&
offspring) independently of one another.
16. ▪A hybrid produced by crossing t
wo individuals with different
alleles at one genetic locus of
interest.
▪In Mendel's experiments with
garden peas (Pisum sativum) .
▪ Earliest and simplest breeding
experiment.
▪ Organisms showed contrasting
traits in character were crossed.
17.
18. 1. they were easy to grow
2. they had a short life cycle
3. their pollination could be
controlled
4. they have easily
observable
characteristics.
19. He studied 7 characteristics, each of which has two
contrasting alternatives.
seed shape : round / wrinkled pod color : yellow / green
seed color : yellow / green flower color : purple / white
pod shape : inflated / constricted flower position : axial / terminal
plant height : tall / dwarf
20. Totest the particulate hypothesis, Mendel crossed true-breeding
plants that had two distinct and contrasting and found that the
resulting offspring ALWAYSlooked like just one of the parents, not
a combination of the two.
What is meant by “true breeding?”
21. True breed organism– homozygous dominant or
homozygous recessive.
▪ A true breeding organism or a purebred, is an organism that always
•2 true-breeding purple flower(PP) is crossed with white flower(pp).
•When self-pollinate or crossed among themselves – all offspring produced
will be identical to the parent which all have purple flower.
22. ▪ This 1st crossing is calledhybridisation
▪ True breeding parents are called – P generation
▪ Their offspring are called – F1 generation or (First Filial
Generation)
▪ Next he allowed the F1 generation to self-pollinate
▪ This will produce the Second Filial Generation orF2
generation.
What happened ??
23. Conclusion: Recessive trait had not been destroyed or
deleted in F1 generation, but merely masked by dominant
trait.
24. ▪ 4 concepts:
1. Alternative versions of genes account for variations
in inherited characters.
Gene for flower colour exists in 2 versions:
▪ Purple and white i.e. P and p.
▪ These alternative versions of a gene are called Alleles.
▪ Location of a gene on a chromosome is called Locus.
25. Alleles: alternative versions of a gene.
The gene for a particular inherited character resides at a specific
locus (position) on homologous chromosome.
For each character, an organism
inherits two alleles, one from each
parent.
26. 2.
The 2 alleles at a particular locus may be identical (true-breeding
plants,TT or tt) or it may differ as in the F1 hybrids(Tt)
3. If the 2 alleles at a locus differ, then one, the dominant allele
determines the organism’s appearance, the other, the
recessive allele, has no noticeable effect on the organism.
4. The 2 alleles for a heritable character segregate (separate)
during gamete formation and end up in different gametes.
This is the Law of Segregation orThe 1st Mendel’sLaw.
27. ▪ Two alleles for a gene segregate during gamete formation and are
rejoined at random, one from each parent, during fertilization.
28. ▪ Used to determine the outcome of a cross between
two individuals.
29. How does a genotype ratio differ from the phenotype ratio?
30. When any individual produces gametes, the
alleles separate, so that each gamete receives
only one member of the pair of alleles.
▪ An exception to this rule is linked genes
Gametes:
31.
32. Is designed to reveal whether an organism that displays the
dominant phenotype is homozygous or heterozygous.
A cross between a recessive homozygous and an
organism of dominant phenotype, but unknown genotype.
If an organism displays a dominant characteristic, it may
possess 2 dominant alleles (homologous) or a dominant
and recessive allele for that characteristic (heterozygous)
To find out which in the case, the organism is crossed with
one displaying the recessive characteristic.
If all the offspring show the dominant characteristic then
the organism is homozygous, but if half show the recessive
characteristic, then the organism is heterozygous.
33.
34. A mating between individuals of the parental
generation (P) and the first generation (F1).
▪ A backcross can be made to the dominant parental
type or to the recessive parental type.
▪ If the backcross is to the dominant parent, all
offspring show the dominant phenotype.
▪ If the backcross is to the recessive parent (a
testcross), ½ the offspring have the dominant
phenotype and ½ have the recessive phenotype.
35. ▪ A back cross=test cross if a parent is homozygous
recessive.
36. A cross reversing the roles of males and females to
confirm the results obtained from an earlier cross.
@
A cross with the phenotype of each sex reversed as
compared with the original cross.
37. ▪ Each pair of alleles segregates independently during
gamete formation.
Allele for one gene will be found within a resulting gamete
independently of the allele for a different gene in the same
gamete.
▪ Mendel experimented this using 2 pairs of contrasting
traits at a time i.e. dihybrid cross.
▪ This law applies only to genes (allele pairs) located on
different chromosomes.
▪ Genes located near each other or in the same
chromosome tend to be inherited together and have
more complex inheritance patterns.
38. ▪ Each pair of alleles segregates independently
during gamete formation.
39. It applies to genes
that lie on separate
chromosomes.
It does not apply when
genes lie on the same
chromosome.
40. ▪ The inheritance of two separate traits in a
single cross
▪ for example: RRYY x rryy
Let:
R represent round seed
r represent wrinkled seed
Y represent yellow seed
y represent green seed
Seed shape
Seed colour
49. RY
F1 genotypes:
Gametes: RY Ry
RrYy
rY ry
RrYy x
Ry rY ry x
♂
♀
RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy
What is the phenotypic ratio?
50. RY
F1 genotypes:
Gametes: RY Ry
RrYy
rY ry
RrYy x
Ry rY ry x
♂
♀
RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy
R - round seed
r - wrinkled seed
Y - yellow seed
y - green seed
F2 phenotypes:
round yellow › :
round green £ :
wrinkled yellow Õ :
wrinkled green ¯ :
R_Y_
R_yy
rrY_
rryy
51. RY
F1 genotypes:
Gametes: RY Ry
RrYy
rY ry
RrYy x
Ry rY ry x
♂
♀
RY Ry rY ry
RY RRYY › RRYy › RrYY › RrYy ›
Ry RRYy › RRyy RrYy › Rryy
rY RrYY › RrYy › rrYY rrYy
ry RrYy › Rryy rrYy rryy
F2 phenotypes:
round yellow › :
round green £ :
wrinkled yellow Õ :
wrinkled green ¯ :
R_Y_
R_yy
rrY_
rryy
R - round seed
r - wrinkled seed
Y - yellow seed
y - green seed
52. RY
F1 genotypes:
Gametes: RY Ry
RrYy
rY ry
RrYy x
Ry rY ry x
♂
♀
RY Ry rY ry
RY RRYY › RRYy › RrYY › RrYy ›
Ry RRYy › RRyy RrYy › Rryy
rY RrYY › RrYy › rrYY rrYy
ry RrYy › Rryy rrYy rryy
F2 phenotypes:
round yellow ›:
round green £ :
R_Y_
R_yy
wrinkled yellow Õ :rrY_
wrinkled green ¯ : rryy
R - round seed
r - wrinkled seed
Y - yellow seed
y - green seed
53. RY
F1 genotypes:
Gametes: RY Ry
RrYy
rY ry
RrYy x
Ry rY ry x
♂
♀
RY Ry rY ry
RY RRYY › RRYy › RrYY › RrYy ›
Ry RRYy › RRyy £ RrYy › Rryy £
rY RrYY › RrYy › rrYY Õ rrYy Õ
ry RrYy › Rryy £ rrYy Õ rryy ¯
F2 phenotypes:
9 round yellow ›:
3
round green £ :
3 wrinkled yellow Õ :
1 wrinkled green ¯ :
R_Y_
R_yy
rrY_
rryy
54. The phenotype ratio: 9 : 3 : 3 : 1
1) Parents are BOTH
heterozygous for
both genes.
This ratio indicates:
2.The two genesare
on separate
chromosomes.
x
55.
56.
57. What are the possible genotypes of this individuals gametes?
58.
59. Bronze colour in turkey is controlled by a dominant allele, R, whereas
red colour is controlled by are recessive allele, r.
The dominant allele , H, produces normal feather while the recessive
allele, h, produces ‘hairy` feather.
In a cross between homozygous bronze ‘hairy`- feathered turkey and
homozygous red , normal- feathered turkey, what are the fractions of
F2 progeny with;
a) Rrhh genotype?
b) Bronze and ‘hairy`-feathered phenotype?
c) rrHH genotype?
d) Red and normal-feathered phenotype?
e) RrHh genotype?
f) Bronze and normal-feathered phenotype?
g) RrHH genotype?
h) RRHh ghenotype?
65. SY sy
SSYY x ssyy
Parents:
Gametes:
F1:
SsYy
(100% smooth & yellow)
x
66. x ssyy
Parents:
Gametes:
F1:
sy
SsYy
SY Sy sY sy
SsYy Ssyy ssYy ssyy
1/4 smooth :
& yellow
1/4 smooth : 1/4 wrinkled : 1/4 wrinkled
& green & yellow & green
x
S – smooth (seed shape)
s – wrinkled (seed shape)
Y – yellow (seed colour)
y – green (seed colour)
67. SY
SsYy x ssyy
Parents:
Gametes:
F1:
Sy sY sy sy
Ssyy ssYy ssyy
SsYy
1/4 smooth :
& yellow
1/4 smooth : 1/4 wrinkled : 1/4 wrinkled
& green & yellow & green
x
68. x ssyy
Parents:
Gametes:
F1:
sy
SSYy
SY Sy
SsYy Ssyy
1 smooth
& yellow
: 1 smooth
& green
x
S – smooth (seed shape)
s – wrinkled (seed shape)
Y – yellow (seed colour)
y – green (seed colour)
70. x ssyy
Parents:
Gametes
F1:
sy
SsYY
:
SY sY
SsYy ssYy
1 smooth
& yellow
: 1 wrinkled
& yellow
x
S – smooth (seed shape)
s – wrinkled (seed shape)
Y – yellow (seed colour)
y – green (seed colour)
72. i. Codominant Alleles
ii. Incomplete Dominant Alleles
iii. Multiple Alleles
iv. Polygenes and Polygenic
Inheritance
v. Dominant Lethal Alleles
vi. Linked Genes
vii. Sex-Linked Genes
73. • When neither allele for a gene is recessive
• Example: Blood type
• AllelesA and B are both dominant (bothare
expressed)
• i is recessive to allelesA and B
• One letter is chosen and the possible alleles are
written in upper case letters to illustrate
codominance Phenotypes Genotypes
A IAIA orIAi
B IBIB or IBi
AB IAIB
O ii
74. Neither allele is dominant.
Heterozygous shows combined effects of both
alleles i.e. intermediates/blending.
Can produce other phenotype that is totally
different from P.
75. Complete dominance Incomplete dominance
The dominant allele
completely masks the
recessive one
Neither allele is
dominant
RR rr
Rr
RR Rr
rr
76. Red White
Pink
R allele:
is partially
dominant
Neither allele is dominant.
Heterozygous shows combined effects of both alleles i.e.
intermediates/blending.
Can produce other phenotype that is totally different from P.
77. F2 Phenotypic ratio:
1 white : 2 pink :1 red
genotypic ratios are the
same
F2 Genotypicratio:
1 rr : 2 Rr :1 RR
A ratio of 1:2:1 (in F2) is characteristic of
INCOMPLETE DOMINANCE
78. ▪ MultipleAlleles
Single trait may have more than 2 alleles ie multiple
alleles/alternatives genes
Fruit fly is one example – eye colour
ABO blood group – 3 alleles IA, IB, i
79. When the inheritance/expression of a characteristic
is controlled by more than one gene.
Genes that responsible for continuous variation in
human and animals ie height, skin colour.
Determined by a large number of genes at different
loci.
Proteins produced by these genes will interact with
each other to produce continuous variation in an
organism.
Environment and diet will effect how genes being
expressed.
80.
81. ▪ LethalAlleles
Allele that has negative affects on the survival of a
homozygote i.e. causes death
Genes which result in the premature death of the
organism; dominant lethal genes kill heterozygotes,
whereas recessive lethal genes kill only homozygotes
▪ Two types:
▪ Dominant LethalAllele
▪ Autosomal dominant lethal disorder (both homo n hetero will be
affected)
▪ Usually will not live long enough to reproduce
▪ Huntington’s disease,TaySach’sdisease
▪ Recessive LethalAllele
▪ Affect the survival of the organism usually as early as fetal stage
▪ Inviable (affect only homozygote)
82.
83. ▪ Epistasis occurs when the phenotypic expression of
one gene is affected by another gene.
▪ In epistasis:
two genes interact to control a single phenotype
▪ not producing new phenotypes
▪ one modifies or masks the expression of the other
▪ The gene that masks another is epistatic
▪ The gene that is masked is hypostatic
84. Epistasis : Coat color in mice
Yellow tip
Black
▪ the epistatic gene:
1) controls synthesis of melanin
2) has two alleles:
coloured (M - dominant)
albino (m - recessive)
▪ the hypostatic gene:
1) controls distribution of pigment in the hair
2) its alleles are:
agouti (A - grey, dominant)
black (a - recessive) Black
85. MM or Mm
mm
AA orAa
BLACK
AGOUTI
aa
Colour forms
Albino
Epistatic gene (M):
Melanin production Hypostatic gene (A):
Melanin distribution
86. Epistasis : Coat color in mice
Agouti Black Albino
MMAA
MMAa
MmAA
MmAa
mmaa
mmAA
mmAa
MMaa
Mmaa
87.
88. ▪ Discovered by Morgan in the early 1900
when he crossed Drosophila.
▪ Do not conform to Mendel’s law of
independent assortment.
• Link genes are located on the same
chromosome.
• Genes inherited together during meiosis or
as a single unit.
• Do not sort independently.
But can the linked genes be separated??
▪ Fail to produce the expected 9:3:3:1 ratio in
a breeding situation involving dihybrid
inheritance.
▪ In these situations a variety of ratios are
produced.
89. ▪ Genes for body colour and wing
length are linked.
Body colour:
grey [G]
black [g]
Wing length:
long wings [L]
vestigial (short) wings [l]
Vestigial wings
G g
L l
90. Parental genotypes: GGLL x ggll
Gametes: GL x gl
F1 genotypes: GgLl
If F1 are allowed to interbreed: GgLl xGgLl
Expected F2 phenotypes: 9:3:3:1
However, the F2 showed 3:1
94. ▪ In practice:
this 3:1 ratio is never achieved
reason: total linkage is rare
[since crossing-over can occur]
▪ In reality:
4 phenotypes are produced
95. GgLl x ggll
G – grey body
g – black body
L – long wing
l – vestigial wing
108. 1.approximately equal
numbers of the parental
phenotypes
Grey
Long wing
Black
vestigial
Black
Long wing
Grey
vestigial
965 944 206 185
2. a significantly smaller
number of phenotypes
showing new
combinations of
characteristics also in
equal numbers, called
recombinants
109. ▪ COV – CrossOverValue
Shown by percentage of genes separation
No of recombinant offspring/Total no of offspring x
100%
1% = 1 map unit (mu) = 1 centimorgan (cM)
Can be used to determine the order in which genes
are located on the chromosome
110. Recombinant frequency =
Number of individuals showing recombination X 100
Number of offspring
parental phenotypes:
grey body, long wing 965
black body, vestigial wing 944
recombinant phenotypes:
black body, long wing
grey body, vestigial wing
206
185
Recombinant frequency 100 17%
206 185
965 944 206 185
111. ▪ Purpose:
Todetermine the linear order and distance of separation
among genes that are linked to each other along the same
chromosome
▪ Based to predict amount of crossing over
Greater the distance between two genes on a
chromosome, the more likely crossing over occurs hence
higher % of recombinants
Low percentage of recombinants will indicate that genes
are relatively close together
112. ▪ COV between genes into hypothetical
distances along the chromosome
C? A B C?
4
9 9
A COV of 4% :
between genes
A and B: they are
4 units apart
A COV of 9% :
for a pair of genes A andC:
they are 9 units apart but
does not indicate the
linear sequence of genes
113. ▪ consider the following values involving four
genes, P,Q, R and S:
P –Q = 24%
R – P = 14%
R – S = 8%
S – P = 6%
114. ▪ consider the following values involving four
genes, P,Q, R and S:
P –Q = 24%
R – P = 14%
R – S = 8%
S – P = 6%
115. ▪ consider the following values involving four
genes, P,Q, R and S:
P –Q = 24%
R – P = 14%
R – S = 8%
S – P = 6%
116. ▪ consider the following values involving four
genes, P,Q, R and S:
P – Q = 24%
R – P = 14%
R – S = 8%
S – P = 6%
117.
118. Black body and vestigial wing in Drosophila is controlled by recessive
alleles, b and vg, respectively.The dominant alleles, b+ and vg+, produce
wild-type flies with grey body and normal wing. A homozygous wild type
is crossed to a homozygous fly with black body and vestigial wing.TheF1
progeny (all wild-type) is test –crossed.The F2 generation is asfollows:
¡
▪ 1930 wild type
▪ 1888 black, vestigial
▪ 412 black, normal
▪ 370 grey, vestigial
¡
a) Determine whether the genes controlling body colour and wing type
are linked.
b)If the genes are linked , determine the distance between the two
genes.
c) If the genes are not linked, what is the expected number of individuals
of each phenotype in the F2 generation?
119. a) The genes are linked because the recombinant
phenotype ratio is smaller compared to parental
phenotype.
b)No. of recombinants X 100
Total no. of progeny
412 + 370 x 100 = 17% = 17 map unit
4600
c) 1150
122. In case of defective genes ie colour blindness.
.
Why CB is only common in male and rarely in female?
▪ Father has defective genes onX.
▪ Male only receives X chromosome from his mother.
▪ Female receives X chromosome from her father.
▪ Hence the male won't have any defective genes but the female
would become a carrier.
▪ Eventually, the defective genes would be transmitted to her
sons.
123. •Examples of recessive sex-linked disorders:
Colorblindness – inability to distinguish between certain colors
most common type is red-green color blindness, where red and
green are seen as the same color.
Youshould see 58
(upper left), 18(upper
right), E (lower left)
and 17 (lower right).
126. Pattern Baldness In Humans
Baldness is an autosomal trait and is apparently influenced by sex
hormones after people reach 30 years of age or older.
In men the gene is dominant, while in women it is recessive.A man needs
only one allele (B) for the baldness trait to be expressed, while a bald
woman must be homozygous for the trait (BB).
What are the probabilities for the children for a bald man and
woman with no history of baldness in the family?
127. Normal
A
Females
carry two allele of a gene. If
one allele is defective,
female is still normal as
effect is masked by the
normal allele.
A Normal:A
Sick: a
Sick
Phenotypically
normal / carrier
A
A
a
a
a
a
Normal Sick
128. ▪ Is used to track inheritance patterns in families.
▪ What does a PEDIGREECHART show?
The phenotypes of individuals in several generations of
a family, and provides a basis for attempting to
determine their genotype.
129. Pedigree analysis reveals Mendelian patterns in human
inheritance
•In these family trees, squares symbolize males and circles represent females.
•A horizontal line connecting a male and female (--) indicates a mating, with
offspring listed below in their order of birth, from left to right.
•Shaded symbols stand for individuals with the trait being traced.
130. Key:
AA = affected
Aa = affected
aa = normal
= normal female
= normal male
= affected female
= affected male
Aa Aa
aa Aa aa aa
aa
A?
Aa aa
Aa aa aa
aa
F1 generation
F2 generation
140. aa A?
Aa A?
Aa
Aa Aa A?
A?
Key
aa = affected
Aa = carrier (appears normal)
AA = normal
A?
aa A?
aa
5.
141. 1. Autosomal recessive
e.g. albinism
2. Autosomal dominant
e.g. Huntington’s Disease
[degeneration of brain cells]
3. X-linked recessive
e.g. color-blindness, haemophilia
4. X-linked dominant [Very Rare]
e.g. hypophosphatemia [low level of
phosphate in blood]
142. ▪ Dominant traits- traits that are expressed.
▪ Recessive traits- traits that are covered up.
▪ Alleles- the different forms of a characteristic.
▪ Punnett Squares- show how crosses are made.
▪ Probability- the chances/ percentages that
something will occur.
▪ Genotype- the types of genes (Alleles) present.
▪ Phenotype- what it looks like.
▪ Homozygous- two of the same alleles.
▪ Heterozygous- two different alleles.
