Extranuclear inheritance

Extra nuclear inheritance

1.  Cytoplasmic Inheritance
 Introduction
 Definition
 Cytoplasmic inheritance
 Organelle chromosome
 Infectious inheritance
 Extra nuclear inheritance
Types of Extra nuclear inheritance
 Examples
 Quantitative Inheritance
 History
 Definition
 Introduction to inheritance Genetics
 Phylogenic inheritance
 Examples
 Skin color
 Human eye color


2. Cytoplasmic Inheritance


Chromosome theory of inheritance suggested that Mundelein
factors or genes were located on chromosomes. This theory was
proved to hold good through a variety of experimental evidences.
Since the chromosome complement in male and female gametes
obtained from same individual would be similar, reciprocal
crosses (♀A x ♂B: ♀B x ♂A) should give same results. The only
exception to this expectation, earlier studied in Sex Linked, Sex
Influenced and Sex Limited Traits is sex linked inheritance.
Differences in reciprocal crosses involving sex linked characters
can be easily explained on the basis of transmission of sex
chromosomes.


Introduction:
There are many exceptions to the rule in genetics. One of them is that not all
inherited characters are determined by genes located in the nucleus. A small
minority are controlled by genes located in cell organelles in the cytoplasm
i.e. cytoplasmic genes, and these of course are exceptions to the chromosome
theory of inheritance. Since they are extra chromosomal (i.e. outside the
chromosomes), such genes are not subject to the normal rules of Mendelian
heredity.
The recent emphasis in evolutionary biology on looking at genes as the unit of
selection has focused almost exclusively on nuclear genes.it is a neglected fat
that an important component of hereditary material in organsim is non-nuclear,
and perhaps more importantantly,I,e this non-nuclear genetic material is
inherited in ways that can be radially at varience with nuclear patterens of
inheritance,this creates the potential for conflict between nuclear & cytoplasmic
genes,partiularly with respect to sex, reproduction, the allocation of parental
investment !

3. Cytoplasmic inheritance is by no means a rare or aberrant phenomenon but is
rather a regular part of the life of every eukaryotic organism (as well as large
population in prokaryotes)
Since chromosomes divide in a very precise manner both during mitosis
as well as during meiosis, it is easy to draw a parallelism between
chromosomes and genes. Cytoplasm, however, does not divide in such a
precise manner during cell division. Female gamete usually contributes
more cytoplasm to the zygote.

Consequently, for characters having cytoplasmic control, differences in
reciprocal crosses would be observed. Inheritance in these cases would
be mainly of maternal type as shown in Figure 18.1. As can be seen in
the figure, if two strains A and B respectively having
genotypes AA and BB and cytoplasm’s a and b are crossed reciprocally,
we will get two hybrids AB (a) and AB (b) (cytoplasm is indicated in
parentheses). In case of maternal effect, AB (a) and AB (b), despite
having same nuclear genotype will differ. AB (a) will resemble strain A
or AA (a) and AB (b) will resemble strain B or BB (b). Since such effects
are solely produced by cytoplasm of the egg, they are described as
maternal effects. However, maternal effects are often produced due to
effect of genes through cytoplasm. In other words, properties of
cytoplasm depend on nuclear genes. Such cases can be distinguished
from those, where extra chromosomal or cytoplasmic hereditary units
are present and function either independently or in collaboration with
nuclear genetic system. This is called extra
chromosomal or cytoplasmic or organelles inheritance

. We know that in chromosomes, DNA is the sole genetic material
and is the storehouse of genetic information. Discoveries of
presence of DNA in cell organelles found outside the nucleus is a
strong evidence to suggest that genetic information does exist in
cytoplasm also. It will be seen in this section, that two important
and essential organelles i.e. plastids (in plants only) and
mitochondria located in cytoplasm carry DNA. These organelles

4. control extra chromosomal inheritance in many cases through
their DNA,
Which carries genetic information?
DEFINATION:
Inheritance:
1. The acquisition of characters or qualities by transmission from parent to
offspring.
2. That which is transmitted from parent to offspring.

Cytoplasmic inheritance:
The acquisition of traits or conditions controlled by self-
replicating substances within thecytoplasm, such as mitochondria or chloro
plasts, rather than by genes on the chromosomesin the nucleus. The phen
omenon occurs in plants and some animals but has not beendemonstrated
in humans.
OR
A form of non-Mendelian inheritance in which a trait was transmitted from
the parent to offspring through nonchromosomal, cytoplasmic means.

5. Now the question arrieses that WHAT IS IN THE
CYTOPLASM THAT COULD CONTAIN DNA???
 The Mitochondria:
 Animal mitochondrial genomes are 13-18 kb in size.
 Fungal mitochondrial genomes are ~75 kb.
 Higher plant mitochondrial genomes are 300-500 kb.
 Each mitochondrion has 5-20 copies of the mitochondrial
chromosomes.
 Human cells have a range of numbers of mitochondria:
 Liver cells have 1000 mitochondria per cell.
 Skin cells have 100.
 Egg cells have up to 10 million.
 Human mitochondria have 37 genes.
 The Chloroplast :
 Chloroplast genomes are 130-150 kb in size.
 Chloroplasts have more genes than mitochondria (110 vs. 37).
Most genes are involved in photosynthesis.
 Corn has 20-40 chloroplasts per cell, with each chloroplast
having 20-40 chromosomes (can make up 15% of DNA)

Endosymbiotic hypothesis –– Free living prokaryotes ancestors
of chloroplasts and mitochondria invaded plant and animal cells but
provide useful function
and so a symbiotic relationship developed over time.

Differences in sexes:
 The transmission of cytoplasm differs between sex
cells:
– Male contribution: Sperm or pollen transfer little or no
cytoplasm to the egg
– Female contribution: Egg contributes almost all of the
cytoplasm to the zygote.

6. Organelle chromosomes:
• A zygote inherits its organelles from the cytoplasm of the egg:
Maternal inheritance.
• The pattern of inheritance is not associated with meiosis or mitosis
because the organelles are in the cytoplasm not the nucleus.
• Organelles (Chloroplasts and mitochondria) have circular chromosomes.
1. Maternal inheritance:
Thetransmission of traits or conditions controlled by cytoplasmic factors wit
hin he ovumthat are not selfreplicating and are determined by genes within
the nucleus. An example ofsuch a characteristic is the direction of coiling in
the shells of snails. Also called maternal effect!
Example:
 The classic study of maternal inheritance was performed by
Correns on the four o'clock plant.
 This plant can have either green, variegated (white and green) or
white leaves.
 Flower structures can develop at different locations on the plant
and the flower color corresponds to the leaf color.
 When Correns crossed the different colored flowers from
different locations on the female plant with pollen obtained from
flowers of the three different colors, the progeny that resulted
from the cross always exhibited the color of the leaf of the
female.
 That is, regardless of whether the pollen was from a leaf that
was green, variegated or white. If the female flower came from a
region where the leaves where green, all the progeny were
green.
 Similar results were seen when the female was from a region on
the plant where the leaves were either variegated or white.
 In comparison to traits controlled by maternal effects, those
traits controlled by maternal inheritance, the female phenotype
is always expressed in its offspring.

7. Variegation in four o'clock plant and maternal inheritance:

Female Male
Progeny
Phenotype
Green
Green, Variegated
or White
Green
Variegated
Green, Variegated
or White
Variegated
White
Green, Variegated
or White
White
 The results can be explained in the following manner.
 All of the organelle DNA that is found in an embryo is from the
female.
 The egg cell is many times larger than the pollen cells, and contain
both mitochondria and chloroplasts.
 Pollen is small and is essentially devoid of organelles, and thus
organelle DNA.
 So any trait that is encoded by the organelle DNA will be contributed
by the female.
 In the case of the four o'clock plant, the different colors of the leaves
is a result of the presence or absence of chlorophyll in the
chloroplast, a trait that can be controlled by the chloroplast DNA.
 Thus, green shoots contain chloroplasts that have chlorophyll, the
chloroplasts in the white shoots contain no chlorophyll, and the
variegated shoots contain some chloroplasts with chlorophyll and
some without chlorophyll.
 Thus, depending upon the location in the plant where the flower
comes from, the egg can have chloroplast with chlorophyll, without
chlorophyll or a mixture of the two types of chloroplasts.
 This is the biological basis of maternal inheritance.
Infectious inheritance: the cytoplasm of animal and plants are
complex, in addition to the normal cellular components numerous
parasites an effect the cytoplasm and replicate
There, these parasites include bacteria & viruses and this is
termed as infectious inheritance.

8. Example:
Infectious Heredity in Drosophila:
Affected flies produce predominantly female offspring if reared at 21°C or
lower. The condition is transmitted only to daughters, not to the small
number of males produced.The responsible element is a protozoan. When
ooplasm from affected individuals or the protozoan itself is injected into
oocytes of normal individuals, the temperature-sensitive, altered sex ratio
condition results.
Maternal Effect: The maternal genome has a strong effect on early
developmental events in the newly formed individual after fertilization.
Numerous transcripts are synthesized off the maternal genome during
oogenesis. These RNA transcripts are not immediately translated; instead,
they are preserved in the oocyte. Numerous transcriptsare synthesized off
the maternal genome during oogenesis. These RNA transcripts are not
immediately translated; instead, they are preserved in the oocyte. When
the oocyte has been fertilized, these transcripts are translated to provide
the proteins necessary to drive metabolism and the initial developmental
events in the zygote. These transcripts serve to support the new individual
until its own, unique genome is activated and can drive cell function,
Extra nuclear inheritance:
 Transmission of genes that occur outsidethe nucleus
 Found in most eukaryotes and iscommonly known to
occur incytoplasmic organelles such as
mitochondria and chloroplasts from cellular parasites
like viruses or bacteria.

Types of extra nuclear inheritance:
1. VEGETATIVE SEGREGATION
2. Uniparental inheritance
3. Biparental inheritance

VEGETATIVE SEGREGATION:
Vegetative segregation results from: Results from random replication
and partitioning of cytoplasmic organelles as with
chloroplasts and mitochondria during mitotic cell divisions and
results in daughter cells that contain a random sample of the
Parent cell’s organellese.g. Mitochondria of asexuallyreplicating yeast
cells

9. UNIPARENTAL INHERITANCE:
 Occurs in extra nuclear genes when only one parent contributes
organellar DNA to the offspring
 E.g. uniparental gene transmission is the maternal inheritance of
human mitochondria. at fertilization via the egg .
 The father’s mitochondrial genes are not transmitted to the offspring
via the sperm. Very rare cases which require further investigation have
been reported of paternal mitochondrial inheritance in humans, in which
the father’s mitochondrial genome is found in offspring .
 Chloroplast genes can also inherit uniparentally during sexual
reproduction. They are historically thought to inherit maternally, but
paternal inheritance in many species is increasingly being identified.
The mechanisms of uniparental inheritance from sp ecies to species
differ greatly and are quite complicated.
 For instance, chloroplasts have been found to exhibit maternal,
paternal and biparental modeseven within the same species.
 BIPARENTAL INHERITANCE:
 Occurs in extra nuclear genes when both parents contribute
organelle DNA to the offspring. It may be less common than
uniparental extranuclear inheritance, and usually occurs in a
permissible species only a fraction of the time.
 An example of parental mitochondrial inheritanceis in the
yeast, [Saccharomyces cerevisiae].
 When two haploid cells of opposite mating type
fuse theycan both contribute mitochondria to the resulting
diploid offspring.
 Perinatal Inheritance of Parasites:
 Extra nuclear transmission of viral genomes and symbiotic
bacteria is also possible.
 An example of viral genome transmission is perinatal
transmission
 This occurs from mother to fetus during the perinatal period,
which begins before birth and ends about 1 month after
birth.
 During this time viral material may be passed from mother
to child in the blood stream or breast milk .This is of
particular concern with mother scarring HIV or Hepatitis C
virus.

10. Cytoplasmic inheritance examples:
1. Shell coiling in Snail , Limnaea:-
It is the best example of
maternal effect. In this snail shell coiling is determined by a pair
of genes. The coiling may be (1) right sided is dextral and (2) left
–sided is sinisterly. The direction of coiling is genetically
controlled. The dominant allele ‘D’ controls dextral
coiling while recessive allele ‘d’ sinisterly coiling. So the
genotype of dextral is ‘DD’ and Sinistral is ‘dd’. If
these snails are mated, the coiling of the shell of offspring will be
determined by the genotype of the maternal snail. F1 progeny
produced, sinisterly coiling, though with genotype (Dd) .So a self
fertilized dextrally coiled heterozygous (Dd) will always produce
eggs that develop into dextrally coiled offspring even if the
progeny possesses the recessive alleles (dd). (F2 generation).
However, these effects last only for one generation because in
the next generation (F3 generation) sinistrally coiled offsprings
are produced by the homozygous (dd) mother even though they
themselves are dextrally coiled.
2. Kappa Particles in Paramecium :-
Kappa particles are found in
certain killer strains of Paramecium. They are responsible for
the production of a substance called Paramecin, which is toxic
to strains not possessing Kappa particles. (Sensitive strains).
The production of Kappa particles is dependent on a dominant
allele (K), so that killer strain genotype is (KK) or (Kk) and
sensitive strains is (k k). In absence of dominant allele (K),
Kappa particles cannot multiply and in absence of Kappa
particles, dominant allele (K) cannot produce them de nova. As a
result of this, sensitive strains with genotypes (KK) or (k k) can
be obtained. However, killer strains with genotype (k k) cannot
be obtained, because even if Kappa particles are present in the
cytoplasm. Would be lost in the absence of dominant allele.
If Paramecium clones with genotypes (KK) or (k k) are allowed
to multiply actually at such a fast rate , that the division of Kappa
particles cannot keep pace with the divisions of cells , Kappa
particles will be eventually lost. Consequently sensitive strains
with dominant genotype (KK), (K k) having no Kappa particles
would be obtained.

11. If killer (KK) and sensitive (k k) strains are allowed to conjugate,
all exconjugants (the cells separating after conjugation) will have
the same genotype, i.e. (K k). But phenotypes of these
exconjugants will depend upon the duration for which
conjugation is allowed. If conjugation does not persists long
enough for the exchange of cytoplasm, then heterozygote (K k)
exconjugants will only have parental phenotypes. It means that
killer will remain killer and sensitive will remain sensitive even
after conjugation. But if conjugation persists, sensitive strains
will receive Kappa particles and will become killer, so that
exconjugants will be killer having genotype (K k).
Thus, it is an example of Cytoplasmic inheritance.

12. 1. Milk factor in mice:

Structure of mammary tumors in mice;
the development of genetically homozygous strains of mice combined
with the study of the influence of genetic factors on the origin of
these tumors; the investigations of the role played by hormonal
factors in the development of breast cancer in mice; and the
discovery of an extra chromosomal factor and the study of its
properties have led to a better understanding of the origin of
mammary tumors in mice.
The milk agent, because of its similarity in behavior and properties, is
considered a true virus. In its latent form, it can be activated by
various hormonal factors or by genetic selection toward the
development of breast cancer in mice.
The milk agent has been described as a “cytoplasmic constituent”
that may have arisen by the transformation of a normal cell
constituent.
The milk agent with many properties common with viruses may also
be considered a cytoplasmic agent dependent on the action of genes,
the tumor development being controlled by the variations in the
genotype of both mice— that is, of the mouse that receives and of the
mouse that supplies the milk agent.
It is now known from the studies on mouse breast cancer that cancer
as a multiple factor character may fluctuate about a physiological
level;
the closer these factors are to the point above which cancer appears,
the more likely are the chances that the individual may collect
monogenetic factors that will give rise to an early appearance of
cancer.



2. cytoplasmic male sterility in maize :
In several crops like maize
(zea mayz) and many other plants, cytoplasmic control of male
fertility in which a plant is unable to produce functional pollen, is
widespread among higher plants, if the female parent is male sterile
i.e. having plasma gene for male sterility ,the F1 progeny would
always be male sterile,

13. Cytoplasmic male sterility is passed down maternally.
3. Transmissible spongiform enephalopathy:

Prions are small, disease-causing agents that consist entirely of
proteins, they have no nucleic acid genome, and are thought tube
normal proteins that an adopt different configurations, they are found
in brains of all animal species, and they cause fatal brain
degeneration disease’s known as Transmissible spongiform
encephalopathy.
Prions are inherited maternally in the cytoplasm; they can
also be infectious in cases where animals consume meat
from infected animal’s e.g. mad cow disease.
Significance of Cytoplasmic Inheritance
1. Development of cytoplasmic male sterility several crop
plants like maize. Pearl millet, sorghum, cotton etc.
2. Role of mitochondria in the manifestation of heterosis.
3. Mutation of chloroplast DNA and mitochondrial DNA leads
to generation of new variation.



 Quantitative inheritance:
 All of the traits that we have studied to date fall into a few
distinct classes.
 These classes can be used to predict the genotypes of the
individuals.
 For example, if we cross a tall and short pea plant and look
at F2 plants, we know the genotype of s hort plants, and we
can give a generalized genotype for the tall plant phenotype.
Furthermore, if we know the genotype we could predict the
phenotype of the plant. These types of phenotypes are
called discontinuous traits.
 Other traits do not fall into discrete classes. Rather, when a
segregating population is analyzed, a continuous distribution
of phenotypes is found.

14.  An example is ear length in corn. Black Mexican Sweet corn
has short ears, whereas Tom Thumb popcorn has long ears.
 When these two inbred lines are crossed, the lengths of the
F1 ears are intermediate to the two parents.
 Furthermore, when the F1 plants are intimated, the
distribution of ear length in the F2 ranges from the short ear
Black Mexican Sweet size to the Tom Thumb popcorn size.
 The distribution resembles the bell-shaped curve for a
normal distribution.
 These types of traits are called continuous traits and
cannot be analyzed in the same manner as discontinuous
traits. Because continuous traits are often measured and
given a quantitative value, they are often referred to as
quantitative traits, and the area of genetics that studies
their mode of inheritance is called quantitative genetics
 Quantitative genetics is the study of continuous or
quantitative traits and their underlying mechanisms.
 I The main principals of quantitative genetics developed in
the20th century was largely in response to the rediscovery
of Mendelian genetics.
 Mendelian genetics in 1900 centered attention on the
inheritance of discrete characters, e.g., smooth vs. wrinkled
peas, purple vs. white flowers.
 This focus was in stark contrast to the branch of
genetic analysis by Sir Francis Galton in the 1870’s
and 1880’s who focused on characteristics that were
continuously variable and thus, not clearly separable
into discrete classes.
Definition:
Genetic inheritance of a character (as human skin color)
controlled by polygenes with each allelic pair of genes at a
given locus having a specific quantitative effect.
OR
The process in which the additive action of numerous gene
s results in
a trait, as height, showing continuous variability.
Some Puzzling Aspects of Quantitative Traits

15. Legendary debate in the early 1900’s on the genetic
basis of quantitative traits
A contentious debate ensued between Mundelein’s and
Biometricians regarding whether discrete characteristics
have the same hereditary and evoluti onary properties as
continuously varying characteristics.
 The Mendelians (led by William Bateson) believed that
variation in discrete characters drove evolution
through mutations with large e ffects
 The Biometricians (led by Karl Pearson and W.F.R.
Weldon)
Viewed evolution to be the result of natural selection
acting on continuously distributed characteristics.
This eventually led to a fusion of genetics and Charles
Darwin’s theory of evolution by natural selection: main
principals of
Quantitative genetics developed independently by Ronald
Fisher (1918) and Sewall Wright (1921), arguable the two
most prominent evolutionary biologists. Interestingly,
Galton’s methodological approaches to continuously
distributed traits
Marked the founding of the Biometrical school, which is
what many consider to be the birth of modern statistics.
Quantitative Inheritance

History:
 Work by Edward East (1916) on inheritance of corolla
height in long flower tobacco, and theoretical work
by R.A. Fisher reconciled the Mendelians and the
biometricians by showing that quantitative inheritance
could be explained on the assumption of Mendelian
genetics, and with the additional assumptions that
quantitative phenotype and that the phenotype was
also affected by environment several to many genes
controlled the variation in the quantitative phenotype

16. and that the phenotype was also effected by the
environment

 Inheritance of
corolla height in
long flower
tobacco under
the assumption
of a single
controlling gene
and incomplete
dominance





 Inheritance of corolla height in long flower tobacco
under the assumption of two controlling, independently
assorting, incompletely dominant genes, with equal and
additive effects on the phenotype.

17. Sir Ronald Fisher To the Rescue
1918 paper “The Correlation between Relatives
On the Supposition of Mendelian Inheritance”
Reconciled this conflict Showed that inherently discontinuous
variation caused by genetic segegation is translated into the
continuous variation of quantitative characters.
Introduction to Quantitative Genetics
 One of the central goals of quantitative genetics is the
 Quantification of the correspondence between phenotypic
and genotypic values.
 It is well accepted that variation in quantitative traits can be
attributable to many, possibly interacting; genes whose
expression may be sensitive to the environment Quantitative
geneticists are often focused on partitioning the phenotypic
variance into genetic and non-genetic components.

18.  Classical quantitative genetics started with a simple model:
Phenotype = Genetic Value + Environmental E ffects
Partitioning of Phenotypic and Genotypic Variance
Phenotype:
 Phenotype is defined as an organism's expressed physical
traits. Phenotype is determined by an individual's genotype
and expressed genes, random genetic variation, and
environmental influences.
Examples:
Examples of an organism's phenotype include traits such as color,
height, size, shape and behavior. Phenotypes indicated in the pea
pod images to the right include pod color, pod shape, pod size,
seed color, seed shape, and seed size
Genotype:
The genetic makeup, as distinguished from the physical appearance, of an
organism or a group of organisms.
Example:
 The gene responsible for eye color
 The gene responsible for hair color
 The gene responsible for height
 Multiple factor hypothesis:
 Two or more allele’s different pairs of alleles, with presumed
cumulative effects, govern the quantative traits. Those
alleles which contribute to the trait involved are called
contributing, effective or active alleles; those alleles which
do not appear to do so are referred to as non-contributing,
on-effective, null alleles. A gene, individually exerting a
slight effect on the phenotype but along with a few or many
genes, ontrols a quantative trait is alledpolygene (a term
by k.mather).sine there are usually many genes of this
kind forgone quantative trait is called multiple factor
hypothesis
 I.e. large number of genes each with a small effect to be
traed, are segregating o produce quantative variation, has
long been the basic model of quantative genetics.


19.  Difference between Quantitative and
Qualitative genetics
Qualitative genetics Quantative genetics
characters of kind Characters of degree
Discontinuous variation;
distinct
Continuous variation
Single gene effects Polygenic control; effects of
single genes too slight to be
detected
Concerned with individuals
matings & progeny
Oncerned with population of
organisms consisting all
kind of matings
Analyzed by making counts
and ratios
Statistical analyses give
estimates of population
parameters such as mean
&standard deviation.
Example: round or wrinkled
seed of peas
Example: skin color in man

 Polygenic inheritance:
 A polygenic trait is one whose phenotype is influenced by more
than one gene.
 Traits that display a continuous distribution, such as height or
skin color, are polygenic.
 The inheritance of polygenic traits does not show the phenotypic
ratios characteristic of Mendelian inheritance, though each of
the genes contributing to the trait is inherited as described by
Grego r Mendel. Many polygenic traits are also influenced by the
environment and are called multifactorial.
Example sof quantative inheritance:
1. Kernel colour in wheat:
 Kernel color in wheat is a quantitative character and was
studied by

20.  H. Nilsson-Ehle for the first time in 1908. It was argued that if
one gene was considered or in other words, if the two parents
differed due to one gene only, a 3: 1 ratio for red and white
kernels was obtained in F 2 generation. However, out of three
red, one was as red as one of the parents and t wo were lighter
and were comparable to F 1 individuals. This indicated that the
dominant alleles had a cumulative effect.
 If 'K' stands for red cooler and V for white, the two parents
could be designated as RR and r r, the F1 could be designated
as Rr and F2 would be obtained in 1RR : 2Rr : 1rr ratio. In these
three classes, RR should be red, Rr should be intermediate in
color and r r should be white.
 In case there were two genes involved, a 15: 1 ratio (15
coloured: 1 white) would be obtained (Table 4.1). If different
shades are taken into account, 1:4:6:4:1 ratio will be obtained,
provided Rx and R2 contribute equally to the colour (Fig. 4.1).
However, it is known now are three genes involved in kernel
color in wheat. Obviously if the two parents differ in all three
genes, in F2 63: 1 or 1: 6: 15: 20: 15: 6: 1 ratio will be obtained
(Fig. 4.2). By the study of kernel colour in wheat, Nilsson-
Ehle reached the conclusion that the effect of each dominant
allele was cumulative and hence forwarded his multiple factor
hypotheses.
 The hypothesis states that for a given quantitative trait there
could be several genes, which were independent in their
segregation, but had cumulative effect on. Phenotype.

21. Skin colour in human:
C.B. Davenport in 1913 reported results of studies regarding the
inheritance of skin colour in Negro and white populations in United States
of America. In U.S.A. the populations derived from marriages between
Negro and white individuals are known as mulattoes. The offspring’s from
negro-white marriages give intermediate skin colour in the first
generation. When such individuals intermarry among themselves, all
shades of skin colour are obtained. If twp loci A and B are responsible for
the skin colour, Negroes can be represented by the genotype AABB and
whites as aabb. Mulattoes will be AaBb with intermediate skin colour.
White / Albino
aabbcc
(Very light)

x

Negro / Black
AABBCC
(Very dark)

Parents


AaBbCc
Intermediate
(Mulatto)

Gametes

Selfing of F1
F1 Generation

22. Subsequently, it could be shown that skin colour in humans cannot be
sharply placed in five categories. Although, this absence of sharp
categories may sometimes be due to environmental effect, it was later
shown that for skin colour more than two gene pairs may be involved .
Expected distributions were derived assuming involvement of 2, 4, 6 and
20 gene pairs and taking 70% and 30% as gene frequencies for colour
genes and their recessive alleles. These theoretical distributions when
compared with observed results, it could be shown that at least four or five
gene pairs may actually be involved in the control of skin colour This may
be further modified due to some modifying genes commonly associated
with quantitative traits.

Gamet
es -->
|
v
ABC aBC Abc abC Abc Abc aBc Abc
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AABBCC
Very dark
AaBBCC
Dark
AABbCC
Dark
AaBbCC
Fairly dark
AABBCc
Dark
AABbCc
Fairly
dark
AaBBCc
Fairly dark
AaBbCc
Intermedi
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AaBBCC
Dark
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Fairly light
AbC
AABbCC
Dark
AaBbCC
Fairly dark
AAbbCC
Fairly dark
AabbCC
Intermedi
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AABbCc
Fairly dark
AAbbCc
Intermedi
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AaBbCc
Intermed
late
AabbCc
Fairly light
abC
AaBbCC
Fairly dark
aaBbCC
Intermedi
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AabbCC
Intermedi
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aabbcc
Fairly light
AaBbCc
Intermedi
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AabbCc
Fairly light
aaBbCc
Fairly light
aabbCc
Light
ABc
AABBCc
Dark
AaBBCc
Fairly dark
AABbCc
Fairly dark
AaBbCc
Intermedi
ate
AABBCC
Fairly dark
AABbcc
Intermedi
ate
AaBBcc
Intermedi
ate
AaBbcc
Fairly light
Abc
AAbBCc
Fairly dark
AaBbCc
Intermedi
ate
AAbbCc
Intermedi
ate
AabbCc
Fairly light
AAbBcc
Intermedi
ate
AAbbcc
Fairly light
AaBbcc
Fairly light
Aabbcc
Light
aBc
AaBBCc
Fairly
dark
aaBBCc
Intermedi
ate
AaBbCc
Intermedi
ate
aaBbCc
Fairly
light
AaBBcc
Intermedi
ate
AaBbcc
Fairly light
aaBBcc
Fairly light
aaBbcc
Light
Abc
AaBbCc
Intermedi
ate
aaBbCc
Fairly light
AabbCc
Fairly light
aabbCc
Light
AaBbcc
Fairly light
Aabbcc
Light
aaBbcc
Light
aabbcc
Very light
Phenotypes

: 1 (Very dark) : 6 (Dark) : 15 (Fairly dark) : 20 (Intermediate) : 15 (Fairly light) :
6 (Light): 1 (Very light)

23. Eye colour in man:
In human beings, the colourofeye is
found tube determined by polygenes. These genes have been
suggested by polygenes. These genes have been suggested tobe
x-linked.at least 9 lasses of eye colour are seen in humans.in
order ofinreasing amount of melanin pigmentation, these colours
an be designated as light blue, medium blue, dark
blue,grey,hazel,green,light brown, medium brown & dark brown.
Trangressive variation:
The appearance, in a segregating
generation, of individuals showing expression of a trait outside the
extremes defined by the parent of the cross that was used to generate
the population.
Example:
Instances where such variations occur transgressing the
limits are called transgressive variations. Punnet and Bailey have
reported some instances of transgressive variations in poultry birds.
In a cross between a large golden Hamburg chicken and small
Sebright bantam variety, the F progeny was intermediate between the
two parents as could be expected in a polygene system.
But surprisingly in the F2 generation (obtained by a cross between F,
individuals) some of the birds were either larger than the golden
Hamburg parent or (some) were smaller than the Sebright bantam
parent.
This was explained as due to the fact that the either of the parents did
not have the entire dominant or all the recessive alleles with the result
they did not set the parameters. In the F2generation
There is a possibility of some of the birds getting the entire dominant
or all the recessive genes, thus causing them to transgress the so
called limits set by the parents. The genotype here is controlled by 3
pairs of genes with the golden Hamburg having AABBCC and
Sebright bantam having aabbccDD.

24. Modifying genes:

A gene that alters or influences the expres
sion function of another gene,
including the suppression or reduction of t
he usual function of the modified gene.
Also called
Modification allele.
Example:
Mice homozygous for the recessive mutation piebald (s) exhibit a
white-spotted coat caused by the defective development of neural
crest-derived melanocytes.
The severity of white spotting varies greatly, depending on the
genetic background on which s is expressed. A backcross between
two inbred strains of s/s mice that exhibit large differences in the
degree of spotting was used to identify six genetic modifiers of
piebald spotting on chromosomes 2, 5, 7, 8, 10, and 13.
The loci differed in their spatial contribution to spotting on the dorsal
versus ventral surfaces of mice; non additive interactions were
observed between loci on chromosomes 2 and 5.
This study underscores the power of using genetic analyses to
identify and analyze loci involved in modifying the severity of
phenotypic traits in mice.




Genetics