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遺傳學Genetics
20/04/10
M.T.Yeung
Variations and Inheritance
A.
Terminology
Dominance & Recessive:
showing their effect in both homozygous or heterozygous and mask the effect of another
contrasting character which is said to be recessive and only shows its effect when homozygous.
e.g. The factor (gene) for tallness is dominant to the factor for dwarfness which is described as
recessive.
Alleles : one of a pair of genes occupying the same locus on homologous chromosomes which separate
during meiosis; different alleles would affect the expression of the same character.
e.g. Genes for height exists in 2 different allelic forms:
T :
an allele for tallness
t :
an allele for dwarfness
Homozygous : containing 2 identical alleles at the corresponding loci of the homologous chromosomes.
(e.g. TT, tt) (Homozygote)
Heterozygous : containing 2 different alleles at the corresponding loci of the homologous
chromosomes.(e.g. Tt) (Heterozygote)
Genotype :
genetic constitution (genetic make-up) of an organisms (e.g. TT, Tt, tt)
Phenotype :
the external visible appearance of an organism resulting from certain inherited character.
--- includes all the morphological, structural, anatomical, biochemical and physiological
characteristics of the organism (e.g. tallness, dwarfness)
Backcross :
cross of a hybrid with one of its parents
B.
Discontinuous Variation
sharp and clear cut variation where are no 'in-betweens'
e.g.
tongue rolling, blood groups
1.
Monohybrid Inheritance (Monohybrid Cross)
- inheritance of single pairs of contrasting characters.
- controlled by a single gene
-
a.
variation due to various combination between different alleles of a gene that resulted from gene
mutation
Mendel's Experiments on Monohybrid Inheritance
Gregor Mendel (1822-1884): "Father of Genetics"
He had selected 7 contrasting traits (discontinuous variation) in his crossing experiments:
i
form of seed（Smooth／wrinkled）
ii.
colour of cotyledons (yellow / green)
iii.
colour of seed coat (grey-brown / white)
iv.
form of pods (inflated / constricted)
v.
colour of unripe pods (green / yellow)
vi.
position of flowers (axial / terminal)
vii.
length of stem (long / short)
遺傳學Genetics
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20/04/10
M.T.Yeung
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遺傳學Genetics
20/04/10
Example 1: tall (2 m) & dwarf (0.25-0.5 m)
Pure breeding plants:
remain tall after self-pollination (selfing) for at least 2 generations
----------------> pure line (homozygote).
The Cross:
Parental generation (P):
phenotype
Pure-breeding
tall pea plant
genotype
(TT)
Gametes
x
(All) T
(All) t
First filial generation (F1)(Progeny) :
phenotype
genotype
Gametes
Pure-breeding
dwarf pea plant
(tt)
T
all tall
(Tt)
t
Ｘ
T
Second filialgeneration (F2)：
genotype
TT
Tt Tt
phenotype
Tall (787)
phenotypic ratio
3
t
tt
Dwarf (277)
:
1
Conclusion：
- There is no blending
 No intermediate effect in outward appearance
- Definite particles (genes) are transmitted from parents to offsprings
Other Examples:
M.T.Yeung
頁 4 /14
遺傳學Genetics
b.
20/04/10
M.T.Yeung
Mendel's First Law (The Law of Segregation)
The characteristic of an organism are determined by internal factors which occur in pairs. Only one
of each pair of such factors can be presented in a single gamete.
Modern interpretation of the law:
The factors mentioned by Mendel are genes. Any character is controlled by a pair of genes (each member
present on one of a pair of homologous chromosome) and in the formation of gametes, each gene segregates
from its member and passes into a different gamete (during Anaphase I of meiosis) so that each gamete has
one and only one of each type of gene.
c.
Test Cross :
= Since an expressed dominant character can have two possible genotypes, hence, whether a dominant
organism is homozygous / heterozygous dominant can be determined by a test cross.
-
by crossing with the double recessive homozygote
-
applicable for both plants and animals
Example： Fruit fly (Drosophila)
wild type：
non-wild type：
i.
ii.
long-wings (VV or Vv)
vestigial wings (vv)
If the fruitfly is homozygous dominant:
P
Genotype
Phenotype
Gametes
F1
Genotype
Phenotype
Ratio
VV
long-wings
all V
x
vv
vestigial wings
all v
all Vv
all long-wings
1
If the fruitfly is heterozygous dominant:
P
Genotype
Phenotype
Gametes
Vv
long-wings
V
v
F1
Genotype
Phenotype
Ratio
Vv
long-wings
1
x
vv
vestigial wings
all v
:
vv
vestigial wings
1
Conclusion：
(1)
If all the offsprings are dominant
===> the dominant parent must be homozygous dominant.
(2)
If the ratio of
dominant offsprings : recessive offsprings１：１
===> the dominant parent must be heterozygous dominant.
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遺傳學Genetics
d.
20/04/10
M.T.Yeung
Types of monohybrid crosses and the ratios in their offspring
Monohybrid inheritance in human and other organisms transmitted in a Mendalian manner
e.
Modification of the 3:1 phenotypic ratio
(i)
Incomplete dominance／semidominance：
the phenotype in heterozygotes is intermediate between the 2 homozygotes i.e. when both alleles are
equally effective, allelic interaction occurred.
Example： Snapdragon -- colour of petals
P
red (CRCR)
Gametes:
CR
F1
Gametes:
CR
white (CWCW)
CW
x
all pink（CRCW）
x
CW
CR
CW
selfing
red（CRCR）
1
F2
ratio

:
pink（CRCW）
2
Phenotypic ratio = genotypic ratio
:
white（CWCW）
1
頁 6 /14
遺傳學Genetics
(ii)
20/04/10
M.T.Yeung
Co-dominance :
Both members of an allelic pair show an independent effect when heterozygous and both contribute
to the phenotype.
---
differs from incomplete dominance in that there is no intermediate effect.
Example： Human blood group (ABO system)
(iii)
Multiple Alleles:
when the gene exist in  3 allelic forms in a pair of homologous chromosomes.
Example： Human blood group (ABO system)
3 allelic forms:
IA
dominant
IB
dominant
i
recessive
Genotype
IAIA, IAi
IBIB, IBi
IAIB
ii
Phenotype (blood gp.)
A
B
AB
O
Antigen on RBC
A
B
A and B
-
Antibody in plasma


, 
2. Dihybrid Inheritance (Dihybrid Cross):
a cross between individuals differing in 2 gene pairs.
(a) Example：
P
Phenotype Pure-breeding tall pea plant with
coloured flowers
x
Genotype
TTCC
Gametes
F1
all TC
all
Genotype
Phenotype
Gametes
F2 Genotype
Phenotpye
dwarf plant with white
flowers
ttcc
tc
TtCc
All tall, coloured
TC
Tc
tC
T_C_
tall coloured
Phenotypic ratio
9
(96)
tc
x
（selfing）
T_ cc
tall white
:
3
(31)
TC
Tc tC
ttC_
dwarf coloured
:
3
(34)
tc
^ttcc
dwarf white
:
1
(11)
This ratio is only for unlinked genes i.e. genes located in different pairs of chromosomes.)
TC
Tc
tC
tc
TC
TTCC
TTCc
TtCC
TtCc
Tc
TTCc
TTcc
TtCc
Ttcc
tC
TtCC
TtCc
ttCC
ttCc
tc
TtCc
Ttcc
ttCc
^ttcc
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遺傳學Genetics
(b)
20/04/10
M.T.Yeung
Mendel's Second Law (Law of Independent Assortment)
When Mendel study the inheritance of 2 characters simultaneously, he draw a conclusion and stated the
Mendel's Second Law:
each of a pair of contrasted characters may be co-exist (or combine) randomly with either of another pair
Modern interpretation:
During gamete formation in meiosis, each member of a pair of alleles may combine randomly with any
member of another pair of alleles. (Only true for unlinked genes.)
e.g.
Each of the T or t gene may combine randomly with any of the C or c alleles.
Probability for an offspring to be
tall :
dwarf :
coloured flowers :
white flowers:
 Tall & coloured
Tall & white :
dwarf & coloured
(c)
:
3/4
3/4
:
Ex:
1. Different pairs of factors (alleles) are independent
of each other. They assort randomly in the
formation of gametes.
 Mendel’s _____ Law
3/4
1/4
3/4
1/4
x
x
3/4
=
2. 9 During gamete formation, the two members of
/
each pair of factors (alleles) separate and each
1
6 gamete receives on member of each pair of factors
(alleles)
 Mendel’s _____ Law
1/4 = 3/16
1/4 x 3/4 =
3
/
Dihybrid Test Cross:
1
= by crossing with a homozygous recessive (ttcc) 6individual.
dwarf & white
:
1/4 x 1/4 =
1
Tall & coloured : Homozygous (TTCC) /Heterozygous
(TtCc) ?
/
1
If homozygous (TTCC)：
6
P
Phenotype
===>
Ratio
9tall
: 3 :coloured
3:1
=
( 3 : 1 )2
TTCC
x
dwarf white
ttcc
Genotype
Gametes
F1
TC
tc
Genotype
TtCc
Phenotype
tall and coloured flowers
Phenotypic ratio:
1
If heterozygous (TtCc)：
P
Phenotype
Genotype
Gametes
F1
tall coloured
TtCc
TC
Tc
tC
Genotype
TtCc
Phenotype tall and
coloured flowers
Phenotypic ratio 1
:
x
tc
Ttcc
tall and
white flowers
1
:
dwarf white
ttcc
tc
ttCc
dwarf and
coloured flowers
1
:
Conclusion： If the offsprings have
4 phenotypes in the ratio of 1: 1: 1: 1 ==> Parent : heterozygous dominant
only 1 phenotype
==> homozygous dominant
ttcc
dwarf and
white flowers
1
頁 8 /14
遺傳學Genetics
20/04/10
M.T.Yeung
(d) Single character determined by gene interaction
sometimes the 2 gene pairs concerned in dihybrid inheritance may interact to determine a single
character
e.g. Types of chicken comb
(gene interaction can be more complicate that lead to a deviation of 9:3:3:1 ratio
e.g. Epistasis and other Polygenic inheritance)
The Chi-Square Test
In any genetic experiment, how can we decide if our data fits any of
the Mendelian ratios we have discussed? A statistical test that can
test out ratios is the Chi-Square or Goodness of Fit test.
Chi-Square Formula
A Chi-Square Table
 2 value of various
Degrees
of
Probability
Freedom 0.9 0.5 0.1 0.05 0.01
1
0.02 0.46 2.71 3.84 6.64
2
0.21 1.39 4.61 5.99 9.21
Degrees of freedom (df) = n-1 ( where n is the number of classes )
3
4
By statistical convention, we use the 0.05 probability level as our
5
critical value. If the calculated chi-square value is less than the
0 .05 value (i.e.  2 < 7.82 for df=3 with a probability greater than
0.05), we accept the hypothesis. If the chi-square value is greater than the 0.05
value (i.e. > 7.82 for df=3 with a probability lesser than 0.05), we reject the
hypothesis.
0.58 2.37 6.25
7.82 11.35
1.06 3.36 7.78 9.49 13.28
1.61 4.35 9.24 11.07 15.09
accept
reject
Example:
Let's test the following data to determine if it fits a 9:3:3:1 ratio.
Class no.
Observed Values
Expected Values
1
315 Round, Yellow Seed
(9/16)(556) = 312.75 Round, Yellow Seed
2
108 Round, Green Seed
(3/16)(556) = 104.25 Round, Green Seed
3
101 Wrinkled, Yellow
Seed
(3/16)(556) = 104.25 Wrinkled, Yellow
4
32 Wrinkled, Green
(1/16)(556) = 34.75 Wrinkled, Green
-----------
556 Total Seeds
556.00 Total Seeds
Number of classes (n) = 4
∴
df = n-1 = 4-1 = 3

Chi-square value = 0.47
According to the Chi-Square table at df = 3 , we see the chi-square value  =0.47 is smaller than 0.58 (i.e.
2
 2 is much smaller than
the calculated probability (>0.90) is
much greater than 0.05, then we accept the hypothesis that the data fits a 9:3:3:1 ratio.
7.82 ) and the probability is greater than 0.90 (i.e. >0.05 ). Threrefore, because
頁 9 /14
遺傳學Genetics
20/04/10
M.T.Yeung
3. Linkage
(a) Linkage group
-
Each chromosome bear many genes.
i.e. The genes are linked on the same chromosome.
e.g. Drosophila melanogaster: only 4 pairs of chromosomes, but > 10,000 genes
-
Chromosomes are inherited as unit and the homologous pair segregates during meiosis.
 Genes on the same chromosome (linked genes) tend to stay together during gamete formation and
will not be assorted independently (NOT follow Mendel's 2nd Law)
All the genes present on the same chromosome ==> Linkage Group
In general the no. of linkage groups found in a particular species corresponds to the no. of different types
of chromosomes (the haploid no.) characteristic of the species.
e.g. Maize :
10 pairs of chromosomes ==> 10 different linkage groups.
If a linkage group contains a large no. of genes ==> the chromosome is large.
If a linkage group contains a small no. of genes ==> the chromosome is small.
Example: Linkage in fruitflies
P
V V
x
A A
long-winged & broad abdomen
v v
a a
vestigial-winged ＆narrow abdomen
Gametes:
F1
V
A
v
a
long-winged
broad abdomen
Gametes:
(selfing)
F2
V
A
Phenotypic ratio
V
V v
V
A
A a
A
long-winged & broad abdomen
3
v
a
v v
a a
vestigial-winged & narrow abdomen
:
1
頁 10 /14
遺傳學Genetics
(b)
20/04/10
M.T.Yeung
Crossing-over
During linkage, genes in the same chromosome will remain together all the time. However, during meiosis,
chromosomes are pairing and undergoing synapsis (in contact with each other at certain points called
chiasmata along their length)

Homologous chromosomes may exchange entire segments of chromosomal material
===> Crossing-over.
By crossing-over, linked genes separated and recombine in another fashion.
Example
In maize a single 'ear' is covered with several hundreds kernels, each representing a seed/fruit.
P
C C
c c
S S
s s
Coloured
Colourless
smooth kernels
x
shrunken kernels
Gametes
C
S
c
s
F1 (Test Cross)
C c
S s
Coloured
smooth kernels
Gametes
C
S
F2
C c
S s
Coloured
smooth
kernels
4032
Ratio
c
s
48.25%
x
x
C
c
s
S
After crossing-over
c c
s s
Colourless
shrunken kernels
c
s
c c
s s
Colourless
shrunken
kernels
4035
C c
s
s
Coloured
shrunken
kernels
149
48.25%
1.78%
c c
S
s
Colourless
smooth
kernels
149
1.78%
Majority of offspring have same combination of characteristics as parents (parental combination):
48.25% + 48.25%
= 96.5%
However, in the formation of a small proportion of gametes, there is exchange of segments (crossing over)
between the locus of genes.

After fertilization, they give rise to new combination of genes (recombination) in the offspring:
Significance of Crossing-over (Recombination):
allows new kind of genetic recombination
-->
genetic variation
-->
evolution
頁 11 /14
遺傳學Genetics
20/04/10
M.T.Yeung
4. Sex Determination
In dioecious species, usually 1 pair of chromosome is important to determine the individual's sex
===> Sex Chromosomes
chromosomes of a homologous pair are identical in size and shape in all individual of a species ===>
Autosomes
except sex chromosomes ===> Heterosomes (which are non-identical pair in either sex)
In human the 23rd pair is Heterosomes
female : 2 X-chromosomes
==> produce only 1 kind of gametes
(X-chromosome)
==> homogametic sex
male : 1 X-chromosome
1 Y-chromosome
==> 2 kinds of gametes: sperms: X-ch.
sperms: Y-ch.
==> heterogametic sex
Y-chromosome is smaller and short, not empty, but carries some genes sometimes governs male fertility and
maleness(e.g. Drosophila and human), without these genes a female is resulted.
Egg (X-chromosome)
---->
Zygote (XX)
Daughters
= 1
:
or
x
Zygote (XY)
Sons
1
Sperm (Y-ch. or X-ch.)
頁 12 /14
遺傳學Genetics
5.
20/04/10
M.T.Yeung
Sex-linkage (Inheritance related to sex)
Y-chromosome:
X-chromosome:
contains only a few genes concerning with maleness.
contains many genes for other traits other than those determining sex.
Sex-linked genes:
genes located in sex-(X)chromosome
==> inherited together with sex chromosome.
Examples：
(a)
Sex linkage in Drosophila (The Inheritance of Eye-colour)
In Drosophila, ~ 140 genes are sex-linked e.g. eye-colour (red eye dominant over white eye)
(Morgan的果蠅實驗—性染色體與遺傳特徵間的關係)
a red-eyed fly cross with a white-eyed fly
Results depends on which parent is red and which is white (i.e. depends on sex.)
i. If the father is white-eyed:
P phenotype
genotype
red-eyed female
X+ X+
white-eyed male
XwY
X+
gametes
F1 genotype
phenotype
ratio
x
Xw
X+X+
red-eyed female
1
Y
X+Y
red-eyed male
1
:
ii. If the father is red-eyed:
P phenotype
genotype
gametes
F1 genotype
phenotype
ratio
white-eyed female
XwXw
x
Xw
XwX+
red-eyed female
1
red-eyed male
X+Y
X+
:
Y
XwY
white-eyed male
1
female : carries 2 genes, either homozygous (X+X+, XwXw) or heterozygous (X+Xw).
male : having only 1 gene for any sex-linked trait, cannot be either homozygous or heterozygous, but is
termed hemizygous (X+ , Xw).
[C.f. reciprocal cross = A cross, with the phenotype of each sex reversed as compared with the original cross, to
test the role of parental sex on inheritance pattern. A pair of crosses of the type genotype A(female) X
genotype B(male) and genotype B(female) X genotype A(male).]
頁 13 /14
遺傳學Genetics
b)
20/04/10
M.T.Yeung
Sex linkage in man
-
> 50 sex-linked traits : most are recessive sex-linked genes
e.g. haemophilia, red-green colour blindness trait.
Examples：
i. Red-green colour blindness
P phenotype
genotype
colour-blinded male
Xc Y
Xc
gametes
x
XC
Y
XCY
normal male
F1 genotype
phenotype
normal female
XC XC
XC
XC Xc
normal female (carrier of the defected gene)
If one of the daughters (carrier) marries a normal man:
F1 phenotype
genotype
gametes
F2 genotypes
phenotype
normal male
XCY
XC
normal female (carrier of the defected gene)
XC Xc
XC
Y
XC XC XCY
normal female and male
Xc
Xc XC
normal female
(carrier)
XcY
colour-blinded
male
 Red-green color blindness is more common in men than in women: affects 4% human male, but
< 1% female.
ii. Haemophilia: deficiency in formation of thromboplastin
 blood difficult to clot properly.
P phenotype
genotype
normal male x
X+Y
X+
gametes
F1 genotype
phenotype
ratio
normal female, but carrier
X+Xh
X+
Y
X+Y
X+X+
normal male and female
1
:
1
:
Xh
XhX+
normal female
(carrier)
1
:
XhY
male with haemoplilia
1
頁 14 /14
遺傳學Genetics
20/04/10
M.T.Yeung
C.
Continuous Variation (Quantitative Inheritance)
- the variation in a quality between individuals of the same species where there is a gradual transition
between the two extremes of the quality
- quantitative variations: measurable characters e.g. size, height etc.
- difficult to analyze genetically
- phenotypic range appeared to be continuous.
- most measurable characters follow a normal distribution curve ranging from low to high values but
without sharp separation in values between the many classes within the range.
1.
Normal Distribution Curve
mean
Frequency
decreases with
value
Frequency
decreases with
value
Standard deviation:
2.
Explanation / Causes for Continuous Variation
i. Polygene: genes with a small effect on a particular character that can supplement each other to
produce observable quantitative changes.
---> Phenotype is the sum total of the -ve and +ve effects of individual genes
(Polygenic Inheritance)
ii.
3.
refers to the departures from the mean.
Environmental Effects
- produce modifications in the expected phenotypes (quantitative variation), but has no effect on
discontinuous variation.
- As environmental effects  , a greater variety of phenotypes are formed ---> normally distributed.
- Not inherited
-- acquiring such variation cannot change the gene.
Examples:
i.
Skin pigmentation in human:
quantitative characteristics :
from light ---> dark (exclude albino.)
-
influenced by polygenes & environment
sun bath
white man -----------> dark
ii.
Identical twins brought up under different social & educational background
---> different I.Q.
iii.
2 boys grown up under different environment
If well-fed
If unfed
Genetically tall boy
tall
short
Genetically short boy
short
even shorter