For medical, nursing, and physiotherapy undergraduates, and postgraduates

1. MODES OF
INHERITANCE

2. GENETIC DISORDERS
1. Chromosomal
2. Single gene
3. Multifactorial

3. TERMINOLOGIES
1. Proband
2. Propositus
3. Dominant
4. Recessive
5. Consanguineous
6. Penetrance
- whether the gene will be expressed or not

4. TERMINOLOGIES
7. Expressivity of gene – refers to degree of
expression

5. SYMBOLS
FOR PEDIGREE
CHARTING

8. MENDELIAN INHERITANCE
(Single Gene Disorders)
1. Autosomal dominant inheritance
2. Autosomal recessive inheritance
3. Sex / X –linked dominant inheritance
4. X-linked recessive inheritance
5. Y-linked recessive inheritance

9. AUTOSOMAL DOMINANT
INHERITANCE
• Expresses in heterozygote state
• Homozygotes severely affected

10. CRITERIA
• Affected person has an affected parent
• An affected person has normal and abnormal
offsprings in equal proportion
• Both male and female are equally affected
• The trait appears in every generation without
skipping
• Normal children of an affected person do not
transmit the disease.

11. DISORDERS
1. Achondroplasia
2. Tuberous sclerosis
3. Treacher collin syndrome
4. Neurofibromatosis
5. Marfan’s syndrome
6. Myotonic dystrophy
7. Waardenburg syndrome

13. AUTOSOMAL RECESSIVE INHERITANCE
• Expresses only in homozygote state
• Homozygote receives one abnormal gene
from each parent.
• The trait typically appears only in siblings.

14. CRITERIA
• The trait appears in siblings and not in parents
or offsprings
• About 25% of siblings are affected, i.e. the risk
is 1 in 4.
• Both male and female are equally affected
• The parents of proband may be
consanguineous

16. DISORDERS
1. Phenylketonuria
2. Muco-polysaccharide disorders – Hunter’s
syndrome, Tay-sach’s disease
3. Cystic fibrosis
4. Sickle cell disease

17. X –LINKED RECESSIVE INHERITANCE
• The trait affects males (rarely females)
• The trait is transmitted from an affected male
through all his daughters to half of their sons.
• No male to male transmission occurs, since
father contributes only ‘Y’ chromosome
• Affected males are related to each other
through carrier females.

19. DISORDERS
1. Hemophilia
2. Duchenne muscular dystrophy
3. Colour blindness
4. G6PD deficiency

20. X –LINKED DOMINANT INHERITANCE
• This trait occurs more frequently in females
• It is twice as common in females than in males
• Affected male transmits the trait to all his
daughters and none to his sons.

21. FEATURES
• This trait occurs more frequently in females
• It is twice as common in females than in males
• Affected male transmits the trait to all his
daughters and none to his sons.
• Affected heterozygote female transmits the
trait to half her children of either sex.

22. FEATURES
• Affected homozygote female transmits the
trait to all her children.

24. DISORDERS
1. Hg blood group
2. Vitamin D- Resistant rickets
3. Hypo-phosphataemia

25. Y –LINKED INHERITANCE
• Y chromosome bears H-Y antigen gene
• It is a small chromosome and contains few
genes.
• Several of the genes are linked to male
infertility and spermatogenesis.

27. DISORDERS
1. Retinitis pigmentosa – RPY gene
2. Jacobs syndrome – XYY gene
3. Hypertrichosis pinnae
4. Campomelic dysplasia- Neuropathy and
skeletal abnormalities – SRY gene

28. FRAGILE X SYNDROME
• It is unique in the sense as it is caused by a
combination of a mutant gene with an
associated cytogenetic abnormality.
• The mutant gene is X linked but

29. FRAGILE X SYNDROME
• Affected male show fragile X in 35 % cases
• Carrier females may not show fragile X at all.
• Located at the end of long arm of X
chromosome (Xq -27-28)
• Observed first time by Lubs in US

30. CLINICAL PRESENTATION
• Large prominent ears
• Long face, jaw
• Stuttering
• Short stature
• Social/ emotional anxiety
• Large sized testes
• Mental retardation
• (It is one of the important causes of MR in males)

32. PLEIOTROPY
• Normally each gene has one primary effect –
directs synthesis of any polypeptide
• When a single gene pair produces multiple
phenotypic effects – pleiotropy

33. DISORDERS
1. In Pheny-Ketonuria – deficiency of
Phenyalanine – primary defect
• But it leads to secondary effects –
– Severe mental retardation
– Passing phenylketones in urine
– Hypopigmentation

34. DISORDERS
2. Galactosaemia – deficiency of Galactose 1 –
phosp –uridyl – transferase – primary defect
• Secondary effects are :
– Cirrhosis of liver
– Cataract
– MR
– Galactosuria

35. SEX – LIMITED TRAITS

36. SEX – INFLUENCED TRAITS

37. MULTIFACTORIAL INHERITANCE

38. THANK YOU