pattern of inheritance basic bsc nursing genetics subject

1. PATTERN OF INHERITANCE
Girish singh A
Assistant professor,
Nirmala College of Nursing
Bhadravathi

2. MENDELIAN THEORY OF INHERITANCE
 Mendelian inheritance is a set of primary statements about the way certain characteristics(e.g. color of hair ,
eye skin etc.)
 Mendel’s laws
A. law of segregation
B. law of independent assortment
Law of segregation
Mendel stated that the genes normally occurs in pairs in ordinary cells of the body and each one is derived
from each parent. Two copies of same gene on its partner chromosome are called as alleles. During the
formation of gametes the two coexisting copies of a gene separate(segregate) from each other so that the
resultant gamete receives only one of the two alleles present in the parent. These alleles may behave as
dominant or recessive characters.
Law of independent assortment
Mendel concluded that different traits are inherited independently .the law of independent assortment is
also known as inheritance law. It states that alleles of different genes assort independently of one another
during gamete formation. This process contributes to the increasing genetic diversity and variability.

3. PATTERN /MODE OF INHERITANCE
 Definition: mode of inheritance is defined as the manner in which a genetical trait or
disorder is passed from one generation to the next.
Classification of genetic disorders
A. Single –gene or monogenic disorders/mendelian disorder
 Autosomal dominant
 Autosomal recessive
 X- linked dominant
 X- linked recessive
B. Chromosomal disorders( aberrations/abnormalities)
 Numerical aberrations
 Structural aberrations
 Translocations
 Inversion
 Isochromosome
 Ring chromosome ,deletion, inversion.
C. Complex/multifactorial /multigenic/polygenic disorders

4. Autosomal dominant pattern of inheritance
 Location of the mutant gene: these found on autosomes
 Required number of defective genes: only one copy of mutant gene is required
 Sex affected: both males and females
 Pattern of inheritance: every affected individual has an affected parent.
 Risks of transmission to children: when only one parent affected and other is normal 50%
chance. When both the parents are affected 75% chance.
 Additional properties:
 Penetrance
 Variable expressivity
 Eg: Huntingsons disease, polycystic kidney disease

5. Autosomal recessive pattern of inheritance
 Location of the mutant gene: these found on autosomes
 Required number of defective genes: two copy of mutant gene is required
 Sex affected: both males and females
 Pattern of inheritance: for a child to be at risk both parents must be having at least one
copy of the mutant gene.
 Risks of transmission to children:
 when both the parents are heterozygous 25% of children are affected, 50 % are unaffected
and 25% will be normal.
 When one parent is affected and other is normal children will be un affected
 When the one parent is affected and other is heterozygote the chances are 50 % children
will be unaffected heterozygotes and 50% are homozygously affected.
 When one parent is normal and the other is heterozygote this may result in 50%
unaffected heterozygote carriers and 50% normal children.
 Eg: cystic fibrosis, phenylketonuria, sickle cell anemia, thalassemia

6. X- linked recessive traits
 Location of the mutant gene: mutant gene is on the x chromosome and there is no male to male
transmission.
 Required number of mutant gene: one copy of mutant gene is required for the manifestation of
disease in males but two copies of the mutant gene are needed in females.
 Sex affected: males are more prominently affected than females; daughters of affected male are all
asymptomatic carriers.
 Pattern of inheritance: transmission is through female carrier
 Risks of transmission:
when male is normal and female is carrier: 25% Normal male,25% Normal female,25% female
carrier and 25% male sufferer.
when male is affected female is normal: Male children are not affected, female children are
asymptomatic carrier.
when male is affected and female is carrier: 25% female carrier,25% affected female,25% normal
male, 25% affected male
when male is normal and female is affected: 50% of children will be female carriers and 50% may
be male sufferers.
Eg: diabetes insipidus, fragile x syndrome, hemophilia A and B

7. X- linked dominant disorders
 Location of mutant gene: it is located in the X chromosome and there is no
transmission from affected male to son.
 Required number of mutant gene: one copy of mutant gene is required for its
effect
 There is no carrier state
 These are more frequent in females than in males.
 Often lethal in males and so may be transmitted only in female line.
 Eg. Vitamin D resistance rickets

8. Complex multigenic disorder(multifactorial and
complex genetic conditions)
 Multifactorial disorders are not caused by a single gene, but rather are due to
interactions between multiple genetic factors and environmental factors. These
include common diseases like atherosclerosis, diabetes mellitus, hypertension and
auto immune diseases.

9. Chromosomal aberrations
 Numerical chromosomal aberrations
Normal cells are diploid containing 46 chromosomes , 22 pairs of autosomes and 1 pair of sex
chromosomes. The total number of chromosomes may be either increased or decreased. The
deviation from normal number of chromosomes is called as numerical chromosomal aberrations.
Classification
 Aneuploidy
 Monosomy
 Trisomy
 Tetrasomy
 Poly ploidy
 Triploidy
 Tetraploidy
 Different cell lines
 Mosaicism

10. Aneuploidy
 It is defined as a chromosome number that is not a multiple of 23.it is caused by either loss or
gain of one or more chromosomes. Aneuploidy may result from non disjunction or anaphase lag.
 Trisomy: numerical abnormalities with the presence of one extra chromosome are referred as
trisomy.it may involve either sex chromosomes or autosomes.
Eg. Downs syndrome(47XX,+21)
 Monosomy: numerical abnormalities with the absence or loss of one chromosome are referred
to as monosomy. It may involve autosomes or sex chromosomes. Monosomy with autosomes is
incompatible with survival because of loss of too much genetic information.
 Polyploidy: polyploidy is chromosome number that is a multiple greater than two of the haploid
numbers. Triploidy is three times the haploid number(69), Tetraploidy is 4 times the haploid
number(92). Polyploidy is incompatible with life and usually results in spontaneous abortion.

11. causes
 Numerical aberrations can occur during meiosis and or mitosis. These errors are
non disjunction and anaphase lag.
 Non disjunction: during normal cell division the paired chromosomes separate
and move to opposite poles of the spindle in anaphase. Non disjunction is failure
of paired chromosomes to separate during meiosis or mitosis. Meiotic
nondisjunction is the most common cause.
 Anaphase lag: anaphase lag results in the loss of a chromosome during meiosis or
mitosis.

12. Different cell lines
 Mosaicism: changes in chromosome number in an individual may not necessarily be present in
all cells but may be found in some cells.
Mosaicism is defined as the presence of two or more population of cells with different
chromosomal complement in an individual.
Mitotic errors during early development , occasionally give rise to mosaicism. It can involve sec
chromosomes or autosomes
 Mosaicism of the sex chromosomes: error in the division of the fertilized ovum may lead to
one of the daughter cells with three sex chromosomes, whereas the other with only one.eg.
Variant of turner syndrome with 45X/47XXX mosaic.
 Autosomal mosaicism: it is much less common than that involving the sex chromosomes.eg. If
the two chromatids of a number 21 chromosome fail to separate during meiosis, will result in
two types of cells, one cell with 47 chromosomes and one cell with 45 chromosomes

13. Structural chromosome aberrations
 A second type of chromosomal aberration is due to alterations in the structure of
one or more chromosomes. They may occur either during mitosis or meiosis.
 Types
 Translocations
 Balanced reciprocal translocations
 Robertsonian translocations
 Inversions
 Paracentric
 Pericentric
 Isochromosome
 Deletions
 Ring chromosomes
 insertions

14. Translocation
 It is structural alteration between two chromosomes in which segment of one
chromosome gets detached and is transferred to another chromosome. There are
two types of translocations
 Balanced reciprocal translocation: it is characterized by single breaks in each of
two chromosomes with exchange of genetic material distal to the break.
 Robertsonian translocation: it is a translocation between two acrocentric
chromosomes. The breaks occurs close to the centromeres of each chromosome.
Transfer of the segments leads to one very large chromosome and one extremely
small one.

15. Inversion
 It involves two breaks within a single chromosome the affected segment inverts
with the reattachment of the inverted segment. The genetic material is transferred
within the same chromosome. There are two types of inversion namely paracentric
and pericentric.
 Paracentric inversions result from breaks on the same arm of the chromosome.
 Pericentric inversions results from breaks on the opposite sides of the centromere
where both short and long arms are involved.

16. Isochromosome
 They are formed due to faulty centromere division. Normally centromere divide in
a plane parallel to long axis of the chromosome. If a centromere divides in a plane
transverse to the long axis it results in the pair of Isochromosome. One pair consist
of two long arms and other pair consist of two short arms.

17. Deletion
 It is the loss of a part of a chromosome.it is of two types namely: interstitial(
middle) and terminal(rare).
 Interstitial deletions: it occurs when there are two breaks within a chromosome
arm. This is followed by loss of the chromosomal material between the breaks and
fusion of the broken ends of the remaining portion of the chromosome.
 Terminal deletions: it results from a single break at the terminal part in a
chromosome arm, producing a shortened chromosome bearing a deletion and a
fragment with no centromere. The fragment is then lost in next cell division.

18. Ring chromosome
 It is a special form of deletion. Ring chromosomes are formed by a break at both
the ends of a chromosome. There is deletion of the acentric fragments formed due
to break and end to end fusion of the remaining centric portion of the
chromosome at the cut ends resulting in a ring chromosome. The consequence
depends on the amount of genetic material lost due to the break. Loss of
significant amount of genetic material will result in phenotypic abnormalities.

19. Insertions
 It is a form of non reciprocal translocation in which a fragment of chromosome is
transferred and inserted into a non homologous chromosome. Two breaks occur in
one chromosome which releases a chromosomal fragment. This fragment is
inserted into another chromosome following one break in the receiving
chromosome , to insert this fragment

20. Cytogenic disorders involving sex chromosomes
Sex linked inheritance
 Genetic diseases of the sex chromosomes are more common than those of autosomal
aberrations. The disorders due to numerical abnormalities of sex chromosomes are less severe
than autosomal abnormalities. Y chromosome carries only small amount of genetic material
compared to X chromosome. Regardless of the number of X chromosomes, the presence of
single Y determines the male sex.
 Features that are common to all chromosomal disorders are:
 Diagnosis is usually difficult at birth.
 Usually detected at the time of puberty due to minor problems relating to sexual development
and fertility.
 Higher the number of X chromosomes, in both male and female , the greater the likelihood of
mental retardation.

21. Klinefelter syndrome
 Klinefelter syndrome is an important genetic cause of male hypogonadism in which there
are two or more X chromosomes.it is the most important genetic disease involving
trisomy of sex chromosomes; it is associated with reduced spermatogenesis and male
infertility.
 Pathogenesis: 47XXY karyotype,47XXXY
 Clinical features: it is usually diagnosed after puberty, patients are tall , thin with
relatively long legs, average IQ is reduced. At puberty testes and penis remains small with
lack of secondary male characteristics. Female characteristics include a high pitched
deep voice, gynecomastia and a female pattern of pubic hair. Azoospermia results in
infertility

22. Turner syndrome
 Turner syndrome is a spectrum of abnormalities that result from complete or partial
monosomy of the X chromosome in a phenotypic female . It is characterized by
hypogonadism and is the most common sex chromosome abnormality in females.
 Karyotype: missing of an entire x chromosome 45X karyotype
 Clinical features: turner syndrome is usually not discovered before puberty. It presents with
failure to develop normal secondary sex characteristics. important diagnostic features are
 Adult women with short stature, primary amenorrhea and sterility.
 Webbed neck , low posterior hair line
 Genitalia remain infantile, breast development is inadequate and there little pubic hairs
and ovaries are converted into fibrous streaks.
 Pigmented nevi becomes prominent as age advances
 Cardiovascular anomalies like congenital heart disease particularly coarctation of the
aorta.

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