Lecture for B.Sc Nursing II Year

1. MRS BINCY VARGHESE
ASSOCIATE PROFESSOR
NURSING
Unit I (Genetics)
Patterns of Inheritance

2. Terminologies
 Genetics: Study of heredity.
 Genome: An organism’s complete set of DNA
including genes.
 Genomics: Study of genes, their functions and
related techniques.

3. Mendel and Mendelism
 The principles of Genetics was laid by Gregor Johann
Mendel. (Father of Genetics)
 Born on July 22, 1822 in Heinzendorf, Austria
(Czechoslovakia).
 He conducted experiments on Garden pea plant. He
selected 7 pairs of contrasting characters in the
Garden pea such as height, shape, texture of seed,
flower position and colour.
 He crossed these varieties of plants.

4.  Heterozygous/Hybrids thus obtained formed F1
generation.
 Plants in F1 generation resembled one of their
parents. (Cross between tall (T) and dwarf plants (t)
resulted in all Tall plants (T)).
 The characteristics expressed in hybrids were called
Dominant character.(T)
 The characteristics not expressed in hybrids were
called Recessive character.(t)

5.  Plants in F1 generation were allowed to self pollinate.
This led to F2 generation.
 Analysis of F2 generation revealed both types of
plants, one expressing dominant character (tall-T)
and the other expressing recessive characters (dwarf-
t).
 F2 generation plants expressing recessive characters
were self pollinated. This resulted in F3 generation
with all the plants expressing recessive character (t).

6. MENDEL’S LAW OF INHERITANCE
Law of dominance
 In heterozygous, the allele which masks the other is
referred to as Dominant while the allele that is
masked is referred to as Recessive.
 Dominant alleles are expressed exclusively in a
heterozygotes while recessive traits are expressed
only if the organism is homozygous for the recessive
allele.

7. Law of Segregation
 This law states that members of gene pair separate
and pass to different gametes. (Chromosomes
separate into different gametes during meiosis, the
two different allele for a particular gene also
segregate so that each gamete acquires one of the
two alleles.)
 A diploid organism passes a randomly selected allele
for a trait to its offspring such as that the offspring
receives one allele from each parent.

8. Law of independent assortment
 Separate genes for separate traits are passed
independently of one another from parents to
offsprings during gametogenesis.

9. Mendelian Disorder/Single Gene Disorder
They are caused by a single mutant gene. They follow
one of three patterns of inheritance:
 Autosomal Dominant Inheritance
 Autosomal Recessive Inheritance
 Sex linked inheritance (X linked dominant and
recessive inheritance & Y linked inheritance)

10. Autosomal Dominant Inheritance
 "Autosomal" means that the gene in question is
located on one of Autosomes.
 "Dominant" means that a single copy of the
disease-associated mutation is enough to cause the
disease.
 This is in contrast to a recessive disorder, where two
copies of the mutation are needed to cause the
disease.
 Autosomal dominant trait expresses in
heterozygous state

11. Characteristics :
 An affected person has an affected parent.
 There is 50% chance of dominant trait being
transmitted to offsprings from affected parent.
(normal and abnormal offsprings in equal
proportions)
 Both males and females are equally affected

12.  The trait appears in every generation without
skipping.
 Normal children of an affected person do not
transmit the disease.
 They can have variable expression. Some people
have mild or more intense characteristics than
others.
 Some people can have dominant gene copy but not
show any signs of the gene.

14.  Most common, life threatening, renal disease.
 Fluid filled cysts develop and enlarge in both kidneys
leading to Renal failure.
 Usually develops in the ages of 30-40, symptoms
tends to get worse with time.
 Treatment includes Antihypertensives and Dialysis
(in case of renal failure)
E.g. Autosomal Dominant Polycystic
Kidney Disease

15. Autosomal Recessive Inheritance
 The recessive trait only expressed in homozygote
state. (To have autosomal recessive disorder, two
mutated genes are needed, one from each parent.
These disorders are passed on by two carriers)
 Two carriers have a 25% chance of having an
unaffected child with 2 normal genes, 50 % chance of
having an unaffected child who is a carrier, 25 %
chance of having an affected child with 2 recessive
genes.

16. E.g. Sickle Cell Anemia
 Inherited red blood cell disorder which lacks healthy
RBCs to carry oxygen.
 People with this disorder have atypical Hemoglobin
called Hemoglobin-S which can distort the RBC into
the shape of sickle or crescent moon.
 Sign and symptoms: Sickling of RBC (breakdown
prematurely and leads to anemia), jaundice, organ
damage.
 No cure, only symptomatic management.

17. Sex linked inheritance
 The inheritance of a trait (phenotype) that is determined by
a gene located on one of the sex chromosomes is called Sex
linked inheritance.
 In mammals, the female is homogametic, with two X
chromosomes (XX), while the male is the heterogametic
sex, with one X and one Y chromosome (XY).
 Genes on the X or Y chromosome are called sex-linked.
 So, sex-linked diseases are carried by sex chromosomes
only.
 In humans it is called X-linked or Y-linked
inheritance.

18.  The Y chromosome is much shorter and contains
many fewer genes.
 The X chromosome has about 800-900 protein-
coding genes with a wide variety of functions, while
the Y chromosome has just approx. 200 protein-
coding genes.
 The human Y chromosome plays a key role in
determining the sex of a developing embryo. This is
mostly due to a gene called SRY (“sex-
determining region of Y”). SRY is found on the Y
chromosome and encodes a protein that turns on
other genes required for male development.

19.  XX embryos don't have SRY, so they develop as
female.
 XY embryos do have SRY, so they develop as male.

21. X LINKED INHERITANCE
 When a gene is present on the X chromosome, but not on
the Y chromosome, it is said to be X-linked.
 Since a female has two X chromosomes, she will have two
copies of each X-linked gene.
 A male has different genotype possibilities than a female.
Since he has only one X chromosome (paired with a Y),
he will have only one copy of any X-linked genes.
 In humans, the alleles for certain conditions (including
some forms of color blindness, hemophilia, and muscular
dystrophy) are X-linked. These diseases are much more
common in men than they are in women due to their X-
linked inheritance pattern.

22. E.g.
 A mother is heterozygous for a disease-causing allele.
Women who are heterozygous for disease alleles are
said to be carriers, and they usually don't display any
symptoms themselves.
 Sons of these women have a 50% percent chance of
getting the disorder, but daughters have little chance
of getting the disorder (unless the father also has it),
and will instead have a 50% percent chance of being
carriers.

24.  Recessive X-linked traits appear more often in males
than females because, if a male receives a "bad" allele
from his mother, he has no chance of getting a
"good" allele from his father (who provides a Y) to
hide the bad one.
 Females, on the other hand, will often receive a
normal allele from their fathers, preventing the
disease allele from being expressed.

25. Hemophilia (X linked Disorder)
 Hemophilia, a recessive condition in which a person's blood
does not clot properly.
 Hemophilia is caused by a mutation of genes located on the X
chromosome that help in blood clot.
 Since the mother is a carrier, she will pass on the hemophilia
allele (Xh) on to half of her children, both boys and girls.
 None of the daughters will have hemophilia (zero chance of
the disorder). That's because, in order to have the disorder,
they must get a (Xh) allele from both their mother and their
father.
 The sons get a Y from their father instead of an X, so their
only copy of the blood clotting gene comes from their mother.
The mother is heterozygous, so half of the sons, on average,
will get an (Xh) allele and have hemophilia (1/ 2 chance of the
disorder).

26. X linked Dominant and Recessive
Disorder
 X linked Dominant Inheritance/ X linked
dominance is a mode of genetic inheritance by
which a dominant gene is carried on the X
chromosomes.
 Only one copy of allele is sufficient to cause the
disorder when inherited from the parent with the
disorder.
 All fathers affected by X linked Dominant Disorder
will have affected daughters but not son.
 However, if the Mother is also affected then sons will
have a chance of being affected.

27. List of X linked Dominant Diseases:
 Alport Syndrome (Characterised by
Glomerulonephritis, ESRD and hearing loss, vision
changes)
 Fragile X Syndrome (Characterised by mild-
moderate intellectual disability, long and narrow
face, large ears, autism, hyperactivity, seizures).
 Rett’s Syndrome (Brain disorder, problems with
language, coordination, repetitive movements,
seizures, scoliosis)

28.  X linked Recessive Inheritance refers to genetic
conditions associated with mutations in genes on the
X chromosomes. A male carrying such a mutation
will be affected because he carries only one X
chromosomes. A female carrying a mutation in one
gene with a normal gene on the other X
chromosomes is generally unaffected.
 E.g Red Green Colour Blindness (Person cannot
distinguish between shades of red and green. Their
visual acuity is normal)
Hemophilia

30. Y Linked Inheritance
 Describes traits that are produced by genes located
on the Y chromosome.
 For a trait to be considered Y linkage, it must exhibit
these characteristics:
- occurs only in males
- appears in all sons of males who exhibit that trait
- is absent from daughters of trait carriers
 The concept of dominant and recessive do
not apply to Y linked traits as only one allele
is present in males.

31. E.g. Y Chromosome Infertility
 Y chromosome infertility s a condition that affects
the production of sperm and causes male infertility.
 The affected man’s body may produce no mature
sperm cells (Azoospermia), fewer than the usual
number of sperms (oligospermia) or sperm cells that
are abnormally shaped