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CHARACTER INHERITANCE: HERITABLE AND NON-HERITABLE TRAITS
Heritable traits are defined by their ability to be passed from one generation to the next in a
predictable manner
...
The most basic
question to be asked about a trait is whether or not the observed variation in the character is influenced
genes at all
...
Gene mediated developmental process lies at the basis of every character, but
variation from individual to individual is not necessary the result of genetic variation
...

There is no environment in which cows for example, will ever speak
...
Therefore, the
question of whether or not a trait is heritable is a question about the role that differences in genes play in
the phenotypic differences between individuals or groups
...
If genes are involved, then on the average the biological
relatives should resemble each other more than the unrelated individuals do
...
Parents who are larger
than average would produce offspring that are larger than the average
...
Such correlations between relatives are however, evidence
of genetic variations only if the relatives do not share common environment more that the non-relatives
do
...
Traits are
familiar if members of the same family share them for whatever reasons
...
Some appear in principles to be relatively limited
...
Whiles some apparently are more complex
...
g
...


Traits are heritable only if the similarity arises from shared genotypes
...
The offspring of a
cow producing milk at a high rate and the offspring of a cow producing at low rate can be raised together in
the same environment to see whether despite the environmental similarity, each resembles its own
parents
...
Because of the nature of
human societies, members of the same families not only share genes but also have similar environments
...
In general people who
speak Yoruba have Yoruba parents and people who speak Ibo have Ibo parents, cross cultural activities over
the years and the movement of people doing business at different locations in the country has
demonstrated that this linguistic differences though familiar, are non genetic and non heritable
...
For example, the disease pellagra
(Vitamin deficiency disease) was ones thought to be heritable because it runs in families
...
Skin colour is clearly heritable as well as
adult height but even in these traits also we have to be very careful
...
So we
might conclude that there are some influences of genetic differences
...
Personality traits, temperaments and cognitive performance (including IQ scores)
and a whole variety of behaviours have been the subject of heritability studies in humans
...
There is indeed a correlation between parents IQ and that of their children, but the correlation
does not distinguish familiarity from heritability
...


MUTATION
A gene mutation may be defined as a change in the code of information transmitted by the DNA
molecule on the chromosome to the ribosomes in the cytoplasm of the cell by means of mRNA, which gives
it instructions to build a specific protein
...
For example, the change in the code
sent by a gene could produce an entirely different protein such as sickle cell hemoglobin which differs from
the normal adult hemoglobin in the kind and number of amino acids contained in the protein portion of the
hemoglobin molecule
...
Mutation can occur in genes carried
either on the autosomes or in those on the sex chromosomes
...
Reverse mutation may also occur
...

Gene mutations that occur in the body cells are not transmitted to the progeny of the individual
where they occur
...
The failure of a new mutation to occur in the progeny, especially if it were dominant, would
suggest it to be in the body cells
...
Black
is dominant to red so new mutation from single red gene to black would show in the individual
...
A new somatic cell mutation may occur in a cell early in embryonic life
...
An example is the appearance of brown spot within the otherwise blue eye in humans
...
A new dominant mutation in the sex cells is followed by the transmission and appearance of the
trait in the progeny of the individual where the first mutation occurred, providing the gene has a major
effect on a trait and shows complete penetrance (or always shows up in the phenotype when present in the
genotype)
...
Even then only about one of four offspring from such parents would be expected to show the
mutation
...

The new gene arising from the mutation reproduces itself exactly for succeeding generations as long as the
individual carrying it survives and reproduces until a mutation of this gene occurs to produce still another
gene in its place
...
e
...
The ABO blood type series in humans involving gene A for
blood type A, B for blood type B and A for blood type O
...
The genetic proportion of this variation is due to the
accumulation of mutations within the species
...
All variation that existed would be superficial environmental variations that could not be
transmitted from parents to offspring
...


A point mutation occurs at a certain gene locus or a part thereof
...
Individuals which are homozygote for the latter gene becomes albinos
...
In
rabbits, at least 5 different alleles are known which influences the intensity of pigmentation
...
If the segment
is large, it is more of a case of structural alteration of the chromosomes
...


Chromosome mutation: Chromosomes can change in two basic ways- by alternation in structure or in
numbers
...

For examples individual heterozygous for chromosomes with different structures often have lower fertility,
and individuals with altered numbers of chromosomes may be unviable or sterile
...
When breaks occur in chromosomes, any two
broken chromosomal ends may reunite
...
In some cases
rejoining takes place and if non of the gen loci is lost or damaged, the chromosome behaves normally after
rejoining
...
In the case
however, that parts of the chromosome separate from each other (fragmentation) in which case several
things can happen
...
A Deletions can occur
...
Where an internal part of
the chromosome is missing, is called an interstitial deletion
...
In this case, the tip of the
chromosome is usually lost in cell division because it does not have a centromere
...


When deletions are

homozygous, they are often lethal, because essential genes are missing
...
A well-known example in humans is the deletion of a substantial
part of the short arm of chromosome 5 (5p), which when heterozygous causes the cri du chat (cry-ofthe-cat) syndrome
...
They generally die in
infancy or early childhood
...
Because the normal chromosome does not have a homologous region to pair within
a deletion loop is formed
...
Several other characteristics are useful in
identifying deletions
...
Second, in deletion heterozygotes, recessive alleles on the normal
chromosome are expressed because the deletion chromosome is missing the homologous region
...


For example, let us assume that genes B and c were deleted on one

chromosome
...
Deletions can be used to map the sequence of
the genes on the chromosome
...
Duplication: When a chromosomal segment is represented twice, it is called a duplication
...
First, the duplication may be
adjacent to the original chromosomal region
...

Secondly, the duplicate region may not be adjacent to the original segment, resulting in a displaced
duplication
...
Chromosomal duplication can occur during crossing-over process, when a
segment lost from one chromosome is added to another chromosome
...
When an individual is heterozygous for a duplication and
a normal chromosome, the duplicated regions does not have a homologous segment to pair with a
meiosis 1
...
In some cases, part of the
chromosome may bend back and join
...
If individuals are viable, there is a potential for further evolutionary changes in
these extra genes
...
These genes may have descended from an
ancestral gene that was duplicated and then the duplicate copies diverged in their function
...
Inversions: Most of the homologous chromosomes in a population have genes in the same sequence
...
Alterations in the sequence of genes called inversions, may be of two different kinds
relative to the position of the centromere
...
Individual heterozygous for an inversion can be

recognized by the presence of inversion loops in meiotic pachytene chromosomes
...
The only was the two homogues can pair is if one
twists on itself and makes a loop, while the other makes a loop without a twist
...

4
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There are two types of
translocations, an interstitial translocation, involving the one-way movement of a segment, and the
more common reciprocal translocation, involving a two-way exchange of chromosomal segments
...
In this case, the number of chromosomes is reduced
by the chromosomal exchange
...
Even though chromosomal segments have been exchanged
between chromosomes in a reciprocal translocation, the affinity of the homologous regions results in
pairing during meiosis I
...
During
anaphase I, two major types of segregation occur: one in which adjacent centromeres goes to the same
pole (adjacent I
...


When alternate

centromeres go to the same pole, the chromosomes often form a figure eight shape in early anaphase
I
...


On the other hand, when adjacent centromeres segregate together, adjacent

segregation, the chromosomes appear as a ring at metaphase I
...
Some plants, and also a few
animals, have a series of reciprocal translocations, so that chromosomal heterozygotes also have nearly

all the chromosomes associated in a large ring (or rings) in meiosis
...
Although translocations can resulting in normal chromosomes, they can
also cause several human diseases
...
In this instance, chromosome 14 is translocation
onto chromosome 21
...
The other half of the time, unbalanced chromosomes are produced,
either a 14 without the translocated 21 segment or a translocated 14 without the translocated 21
segment or a translocated 14 with the attached 21 plus a normal 21
...


In the second instance, three 21

chromosomes are received, resulting in Down syndrome
...
In
actual fact, the proportion is less than this, primarily because some Down individuals do not survive
gestation
...
First, Down
syndrome could recur in children of a transolocation heterokaryotye, whereas normally Down
syndrome does not recur in sibs Second, half of the phenotypically normal sibs of Down individuals are
themselves translocation heterokaryotypes, and therefore could produced Down progeny
...
As one might expect, changes in chromosome number, either euplid or aneuploid, generally have a
greater effect on survival than do changes in chromosome structure
...

Polyploidy (Euploidy Variation): Organisms with three or more complete sets of chromosomes are called
polyploids
...
However, for organisms that are regularly polyploidy, such as
many plants, x usually refers to the number of chromosomes in a set and n to the number in a gamete
...
Polyploidy is
relatively common in plants but rare in most animals, occurring only in certain beetles, earthworms,
salamanders, fishes, and a few other organisms
...
For example, potatoes are tetraploid (4x = 48), bread wheat is
hexaploid (6x = 42), and strawberries are octoploid (8x = 56)
...
First, sex determination is often more sensitive to polypoidy in animals than in
plants
...
) can still reproduce
...
We can distinguish two types of polyploids: those that receive all
their chromosomal sets from the same species, autopolyploids, and those that obtain their chromosomal
sets from different species, allopolyploids
...
On the other hand, if diploid pollen of
one species fertilizes a diploid egg of another, related species, the offspring are allotetraploids
...
All the chromosomal sets in an autopolyploid are

homologous, just as they are in a diploid
...

Triploid organisms are usually autopolyploids (AAA) that result from fertilization involving a haploid
and a diploid gamete
...
For example, most bananas are triploids; they produced unbalanced gametes, and as a result,
are seedless (they are propagated by cuttings)
...
For example, the bread wheat Tritium aestipum is an
allohexaploid with 42 chromosomes
...
Pairing occurs only between the homologous sets, so that meiosis is
normal and results in balanced gametes of n = 21
...
Non-disjuction in meiosis itself is thought to result from
improper pairing of homologous chromosomes on opposite sides of the metaphase plane, or from failure of
chiasma formation
...
When these gametes are
fertilized by a normal gamete, they either have an extra chromosome, 2n + 1, termed trisomy, or are
missing a chromosome, 2n – 1, termed monosomy
...
Non-disjuction can also take place in mitosis, resulting in mosaics for normal and
aneuploid cells
...
Trisomics are known in many different species
...
For example, among the aneuploids that have been most
thoroughly studies are those in the Jimson weed, or thorn apple
...
In fact, a trisomic for each of the twelve
different chromosomes was found, and each had a particular phenotype
...
Trisomics have been investigated in crop
plants such as corn, rice and wheat in an effort to identify the chromosomes carrying different genes
...
For example, if a
homozygous dominant trisomy, AAA (The A symbol again indicates a dominant allele), is crossed to a
recessive diploid, aa, half the progeny are trisomic AAa half are diploid Aa
...
If the gene had
been on a chromosome that was not trisomic, the F1 would be Aa, and one-half, not one-sixth, of the
backcross progeny would be homozygous recessive (aa)
...


In animals, trisomics and other aneuploid

From analysis of the chromosomal constitution of

spontaneous abortions in humans, it appears that nearly all monosomics and many trisomics are fetal
lethals
...
One of the most common is Down syndrome, trisomy of chromosome 21,
with a frequency of one in seven hundred live births
...

In general, mortality is higher than normal: the average life span is the middle tens to the forties, depending
upon the country, but some individuals live much longer
...
The chromosomal basis of Down
syndrome was first discovered in 1959, shortly after the correct human diploid number was determined
...
However, because Down syndrome is
known so prevalently as trisomy 21, this association was not changed, and the smallest chromosome is still

called chromosome 21
...
The other autosomal trisomies are much rarer, mostly because they are
not viable as fetuses
...
Four
common viable, but abnormal chromosomal types XO, XXX, XXY and XYY, are produced through
nondisjuction
...

Klinefelter syndrome, XXX (or 47, XXY), occurs fairly frequently and generally results in a relatively
mild abnormality
...
Individuals with
Turner syndrome (XO or 45, X) are sterile females, short in stature, with some neck webbing
...
At one point, it was suggested that XYY individuals had criminal tendencies, but
further study indicates minimal correlation with behavior, if any
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Abnormal chromosome numbers in a fetus can be

diagnosed using amniocentesis
...
The fetal cells contained in this fluid are cultured for two or three weeks
...

The X chromosome is different from the other chromosomes in that only one is active in given cell
...
In normal females, only one X is active in a given cell and
the other X is heterochromatinized, or mostly inactive
...
Therefore, normal males and XO individuals have no Barr bodies;
normal females and XXY individuals have one: XXX individuals have two: and so on
...
The incidence of Down syndrome, and to some extent, other aneuploidies, increases with
the age of the mother
...
Although the exact mechanism for this increase is unknown, it appears to be related to
the difference in gametogenesis between females and males
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In older mothers, an
oocyte may remain at this stage for over forty years, during which time it may be affected by environmental
factors that may cause a non-disjunction
...
In spontaneous mutation, mutagens are not
involved
...
g
...
In its more stable configuration, it form two hydrogen
bond with thymine in the DNa
...
If the A-C pairing occur while DNA is replicating, then at the ensuring round of replication,
one of the daughter DNA helical will have a G-C pair instead of an A-T pair at that position
...
Irradiation is an
example of physical mutagens with X-ray, gamma-ray, ultraviolet light being the most commonly use
metagens
...
On the other hand, chemical mutagens can act in a variety of ways depending on the
properties of the chemical and its reactions with the bases of the DNA
...
In its keto state, it pairs with guanine
...


The first involves the incorporation of the normal 5-bu into DNA during replication
...

Other chemical mutagens are 2-minopurine which is also a base analogue that can bond with both
thymine and cytosine in its two forms
...

It removes the ammonia group (NH2) from the bases altering their base pairing abilities and hence inducing
mutation
...
Hydroxylamine NH2OH) induces mutation in a specific way in that it can
react with cytosine hydroxylating it so that it can only pair with adenine thereby inducting a G-C to A-T
pairing
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LETHALS AND GENETIC ABNORMALITIES
Death of an organism may occur at any stage of development – immediately following fertilization,
during embryonic differentiation, at parturition, or postnatally
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Any cause
of death is termed lethal effects
...
These genes are known as lethal (deadly) genes
...
Some other genes
which are deleterious to the organisms may not be lethal, provided that environmental factors especially
are favorable
...

These genes are called semi-lethal or sub-lethal genes
...
These genes are referred to as nonlethal or detrimental genes
...
In a strain of horse, a sex-linked recessive lethal gene has been reported that kills approximately
half of the male offspring of carrier females, so there are approximately twice as many females as males at

birth
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The dwarfs are born alive, as a general rule, but most invariably die before they are
one year of age
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In some instances, the partial dominant genes affect the
heterozygous individuals so that they are intermediate in phenotype between the normal and the
homozygous recessives
...
Examples of sex linked lethals are the
hemophilia and Duchene muscular dystrophy genes
...
Duchene
muscular dystrophy is a disease in which the affected individual though apparently normal in early
childhood exhibit progressive wasting away of the muscles resulting in confinement to wheel chair about
the age of twelve and death in the teen years
...

Detrimental recessive genes are generally present at low frequencies in a population, and in many
cases, only inbreeding, line breeding or chance will cause their occurrence in the homozygous state
...
There are no absolute rules for determining
whether the abnormality is hereditary or environmental in origin, whether it is due to some combination of
hereditary and environmental influences or whether it is merely an accident or development
...

1
...


2
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3
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Inbreeding does not create abnormalities,
but since most abnormalities are recessive, it tends to bring them to light as a result of increased
homozygosity resulting from inbreeding
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If it occurred in more than one season when rations and environment differed
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1
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2
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3
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In strict sense every abnormality are the product of heredity and environment
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Recessive Defects with Some Expression in Heterozygote
Most lethals and abnormalities are recessive in inheritance, whether due to one or several genes
...
A few cases are known in which recessive-ness is not complete, and the
heterozygotes or carriers have characteristics which make them more favored or desired by breeders than
the homozygote normal
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Cattle of this type are
always heterozygous for a semi dominant gene which when homozygous produces a lethal acondroplasia
(bulldog calves)
...
When inter-mated,
Dexter produces ¼ long legged individuals known as kerrys, ½ short legged Dexter and ¼ bulldog calves
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5
...
It would
be an easy matter for breeders to cull them of they so desired
...
Kerry X Dexter matings give 50% each of
kerry and Dexter types
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A type of infertility
characterized by gonadal hypoplasia was found to be highly hereditary but not a clear cut case of a single
gene pair of gene action
...
The gene or genes responsible for the condition attained such frequency in the
breed that bilateral gonadal hypoplasia (with resultant sterility) reach a level constituting a serious problem
to the breed
...

In the mid years of the twentieth century, a type of dwarfism characterized by small size, high
mortality, bulging foreheards, undershot jaws, difficult breathing, a tendency to bloat, and poor
coordination reached a frequency in at least two breeds of cattle in the U
...
high enough to constitute an
economic problem
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Initially the condition
appeared to be inherited as a simple recessive
...

A partial deficiency of urdine monophosphate (UMP) synthase was also discovered in Holstein
cattle
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Affected animals have half the normal activity for this enzyme when heterozygous
for the condition
...
Advances in molecular
genetics have permitted the identification of carriers through DNA probes
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Semilethal Recessive Related to Economically Desirable Traits
...
In several breeds of swine in both the U
...
and
Europe, a port quality problem became apparent in the mid fifties and sixties in type selected for
muscularity and thin back fat
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In the carcasses of the
affected individuals the lean tissue is light in color and lack firmness; fluid may seep from cut surfaces
...
In some cases susceptible animals may live to market
weight and even reproductive ages if not subjected to undue stress
...
A test
involving exposure of pigs to standard level of halothane anesthetic for a prescribed period was devised to
identify stress susceptible animals
...


PSS has been found to be inherited as a simple recessive gene trait
...
The most dramatic example of the relationship of PSS to
economic traits was reported from Switzerland, where two lines of pigs were selected from the same base
population
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After six generations, 42 percent of the superior line was halothane susceptible whereas none of
the inferior line was affected
...

Most of these are unrelated to productivity, but a few are of economic worth
...
Again, pigment in and around
the eyes of white-faced cattle reduces the incidence of eye and eyelid cancers
...

Most basic color variations are inherited in a fairly simple faction, and in many cases maintenance
of the breed trademark constitutes no special problem
...
In spite of the fact that, there is no known relationship between shade of
color and productivity, fads for certain shades have sometime developed
...
One of the first of this effects was found in a certain strain of
yellow mice
...
It was found that homozygous yellow individuals died at an early stage of
gestation, and the surviving yellow animals were heterozygous
...
Platinum foxes are also known to be heterozygous, because they produced two
platinum to one silver offspring when mated
...

Some lethal coat colors have also been reported in farm animals
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This indicates that black is

recessive
...
A recessive gene for gray coat color
in Collie dogs is accompanied by an increase susceptibility to infections and death at a young age
...


In most breed of sheep and cattle, the presence or absence of horns depends fairly upon simple
genetic patterns
...

In most fine – wool sheep, the presence or absence of horns depends upon a single pair of alleles with
heterozygote being horned in males
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The polled gene is related in some way in breeds of this type to cryptorchidism, a
defect in which testicles are retained in the abdominal cavity rather than descended in to the scrotum
...
A few normal ram have been progeny tested at high levels of probability and are
apparently homozygous for the polled gene
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Close linkage which has been
broken off
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Presences of modifying genes which prevented the expression of cryptorchidism even
though this is a normal plieotropic effect of the gene
...

The expression of abnormalities in laboratory animals and plants varies within the range of normal
environments
...
(Drosophilia melanogaster) is one of the best known
of these
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In bareye individuals the number is much reduced, but the reduction is much larger at high rearing temperatures
than low
...
In
swine, scrotal hernia has a hereditary base, but its incidence is also influenced by a maternal effect
...

Several defects in farm animals are conditioned partially by hereditary variations of quantitative
nature and partially by environmental factors
...
It occurs

more frequently in Herefords than in other breeds
...
Latitude and
altitude are also related to incidence, probably are a result of differences in the ultraviolet component of
sunlight
...
Its incidence increases, and it occur at younger sage in
cattle maintained on high level of nutrition
...
Hereford cattle with pigmented eyelids and corneoscleral areas are
less susceptible than white eyed types
...
A sire may have left many offspring before it is discovered
that his daughters have an unusually high susceptibility to a condition such as cancer eye
...


Strategy for Control of Genetic Defects
The action to be taken if a lethal or genetic abnormality is discovered in a herd depends upon the
type of herd and the seriousness of the abnormality
...
For most traits the frequency is so low that the probability of obtaining new sires which also carry
the defective gene
...
Corrective measures may need to be
more drastic in sed-stock herds since the owner has an obligation to provide stocks which will performe
well for future customers
...

1
...


2
...

3
...
They may
be placed in an auxiliary herd and used to progeny – test future herd sires to determine whether they
are heterozygous for the gene(s) responsible for the defect
...
Cull other close relatives of affected individuals including normal offspring of sires and dams which have
produced defective individuals
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If the affected individuals are viable and fertile, retain them for progeny – testing prospective breeding
animals
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Progeny – test prospective herd sires before using them extensively in the herd
...
In the examples of gene interactions, the genetic basis of the
dependence of one gene on another has been worked out from clear genetic ratios
...

Penetrance is defined as the percentage of individuals, with a given genotype which exhibit
the phenotype associated with that genotype
...
Penetrance can b used to measure such an effect when it is not known which of
these types of modification underlies the effect
...
Again, the lack of full
expression may be due to the allelic constitution of the rest of the genome or to environmental
factors
...
For example, if a disease-causing allele is not
fully penetrant (as often is the case), it is difficult to give a clean genetic bill of health to any
individual in a disease pedigree (for example, individual R in Figure below)
...


Figure: Lack of penetrance illustrated by a pedigree for a dominant allele
...
An
individual such as R cannot be sure that his or her genotype lacks the allele
...
In humans, the drug thalidomide taken during pregnancy caused
phenocopies of the rare genetic disease phocomelia, children were born with severe limb defects
...
From systematic pedigree analysis in the light of Mendel's laws, geneticists can tell if a trait is
determined by alternative alleles of a single gene and whether a single-gene trait is dominant or recessive
...
Pedigrees are a convention for keeping track of genetic traits used
to infer genotype
...
The information available may
be of three types: the genealogical structure (how the members of the pedigree are related to each other),
the phenotypes (the "data" collected on each pedigree member), and the mode of transmission (the
genetic-or other-mechanism underlying the distribution of phenotypes over the members of the pedigree)
...
Pedigrees are the
human equivalent of test crosses
...
A member of a family who first comes to the attention of a geneticist is
called the propositus
...
Many pairs of
contrasting human phenotypes are determined by pairs of alleles inherited in exactly the same manner
shown by Mendel's peas
...

Traits associated with dominant, recessive, sex linked, etc
...
e
...

Pedigree analysis can also allow estimation of gene penetrance and gene expressivity
...
Immediate family
members (parents, siblings, spouse, children) are added next, followed by aunts, uncles, cousins,
grandparents, and others in the proper orientation
...

The square or circle is filled in for any affected individuals to reflect their disease status
...
A vertical line descends from this
marriage line and then connects to another horizontal line, the sibship line
...
All members of one generation are shown
adjacent to one another
...


Fig
...

Generations are numbered from the top of the pedigree in uppercase Roman numerals, I, II,
III etc
...

Each generation is labelled at the left with a Roman numeral beginning with the first
generation
...
In this way, each person can be specifically
identified
...

Once the family members are properly arranged, important medical facts can be added
...
The first step in pedigree analysis is to observe the number and
relationships of all individuals who express the same or similar clinical features
...
For example, an autosomal disease can usually be
distinguished by seeing male-to-male transmission of the mutation, but since males pass only the Y
chromosome to their sons, there should never be father to son transmission of an X-linked gene
...
In general, a dominant disease will be seen in
approximately half of the individuals in each generation, but recessives occur very rarely
...


Most human genes are inherited in a Mendelian manner
...
We can follow the inheritance of the abnormal phenotype and deduce whether
the variant allele is dominant or recessive
...

Most matings will be of the form M/m x m/m, i
...
heterozygote to homozygous recessive
...
A typical pedigree might look like this:

Examples of autosomal dominant conditions include Tuberous sclerosis, neurofibromatosis and
many other cancer causing mutations such as retinoblastoma
Autosomal recessive
A recessive trait will only manifest itself when homozygous
...
An autosomal recessive condition may be
transmitted through a long line of carriers before, by ill chance two carriers mate
...
The pedigree will therefore often only have one 'sibship'
with affected members
...
The degree of risk that
both alleles of a pair in a person are descended from the same recent common ancestor is the degree
of inbreeding of the person
...
Considering any child of a first
cousin mating, we can trace through the pedigree the chance that the other allele is the same by
common descent
...
A total risk of½ x
½ x ½ x ½ = 1/16
This figure, which can be thought of as either the chance that both maternal and paternal
alleles at one locus are identical by descent or the proportion of all the individual's genes that are
homozygous because of identity by common descent, is known as the coefficient of inbreeding and
is usually given the symbol F
...
It is inherited only through the egg,
sperm mitochondria never contribute to the zygote population of
mitochondria
...


A mitochondrial inheritance pedigree

All the children of an affected female but none of the children of an affected male will inherit the
disease
...
By carefully observing the position of affected individuals, mutation
carriers may be identified
...

Pedigrees are also maintained for many animals, though the purpose of pedigree analysis is
somewhat different
...
Animals with unfavourable traits are eliminated
from consideration so that the next generation will include individuals with more of the preferable
traits
...
In the thoroughbred world, pedigree
analysis tries to combine speed with stamina and a will to win that will yield winning racehorses
...
Even some plants have pedigrees as researchers strive to find drought and
pest resistant species with high crop yields
...


The following pedigree could be the result either of the segregation of an autosomal

dominant condition or of an autosomal recessive
...
In the latter case, who in the pedigree is an
obligate carrier? And which other members of the pedigree are at risk of being carriers
...




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