This document discusses different modes of inheritance including autosomal dominant, autosomal recessive, X-linked recessive, and X-linked dominant inheritance. Autosomal dominant inheritance involves a 50% risk of passing on a trait with each pregnancy, affects both sexes equally, and shows no generational gaps. Autosomal recessive inheritance typically involves unaffected carrier parents who have a 25% risk of passing on the recessive trait with each pregnancy. X-linked recessive traits mainly affect males and carrier females have a 50% risk of passing the trait to sons or daughters, respectively. X-linked dominant traits affect females more than males.

1. Modes of Autosomal and Sex
Linked Inheritance
To MBBS/BDS 1st year
08-01-2017
By Dr. Laxman Khanal
Assistant Prof. (Department of Anatomy)

2. Gregor Johhan Mendel: 1822-1884

3. Blending of characters
OR
Characters are discrete
and do not blend
Law of dominance

4. An important reason for studying the pattern
of inheritance of disorders within families is
to enable advice to be given to members of a
family regarding the likelihood of their
developing it or passing it on to their children
(i.e.genetic counseling)
Why this topic is important ?

5. Some definitions
Gene: Sequence of nucleotides in DNA which forms specific RNA
followed by protein which eventually produce some observable trait.
Alleles: Different variants of gene present in specific
location(locus) of homologous chromosome.
Homologous chromosome: Similar chromosome
inherited from father and mother containing similar
genes.

6. Fibrillin
Chromosome 15
Dominant disorders
Hemoglobin
Chromosome 11
Disease expression
occurs
Disease expression
occurs
Recessive disorders
Which is more lethal condition ?

Which is manifested earlier ?


7. Mode of inheritance: Definition
• Its the manner in which a particular genetic trait or disorder is passed
from one generation to the next.
Examples:
• Autosomal dominant/ recessive
• Sex-linked dominant/recessive
• Mitochondrial inheritance.
Knowledge is important for the purpose of ‘Genetic Counselling’
22 AA + XX/ XY

8. Autosomal dominant
inheritance
• Both parent can transmit mutant gene to
both sons and daughter.
• Male = female affected
• Vertical transmission (no generation gap)
• Myotonic muscular dystrophy
• Marfan’s syndrome
• Achondroplasia
• Huntington’s disease
• Familiar hypercholesterolemia

9. Autosomal dominant
• Myotonic muscular dystrophy
• Marfan’s syndrome
• Achondroplasia
• Huntington’s disease
• Familiar hypercholesterolemia
• Both parents can transmit mutant gene
to both sons and daughter.
• Male = female affected
• Vertical transmission (no generation gap)
50% of offspring will be affected if
one of the parent has single mutant
allele.

11. Autosomal dominant inheritance
• Only one copy of a disease allele is necessary for an individual to be
susceptible to expressing the phenotype.
• With each pregnancy, there is 50% chance the offspring will inherit
the disease allele.
• All affected individuals will have at least one parent who carries the
disease allele.
• Vertical inheritance involving all generations.
• Male and females are equally affected.

12. ?
Reduced penetrance
50% chance of
being diseased.
• Pleiotropy
• Variable expressivity
• Reduced penetrance

13. Autosomal recessive inheritance
• Typically, the parents of an affected individual are not affected but are
gene carriers.
• With each pregnancy of carrier parents:
• There is a one in four (25%) chance of having the disease.
• There is a one in two (50%) chance of having carrier
• There is a one in four (25%) chance of having no disease.
• Proportion of affected male and female is equal.

14. Carrier
• 25%- affected
• 50%- carrier
• 25%- normal

15. Autosomal recessive inheritance
• Observed more frequently
in consanguineous relationship
s and in certain ethnic
backgrounds.
• Most of people have 1-5
mutant copy of gene for
autosomal recessive disorders.
• Sickle cell anemia
• Cystic fibrosis
• Tay-sachs disease

16. Sex limited disorders
• Limited in one of genders
• Mostly genes are in autosome
• Examples
Male pattern baldness
Ovarian carcinoma
Sex linked disorders
• Either X or Y linked
• Can occurs in both gender (if X linked)
• Occurs only in males (if Y linked)
• Genes are in sex chromosome

17. XY XX
Y linked inheritance
(Holandric inheritance)
Y linked disorders
• Hairy pinna
• Only occurs in male

18. X X X Y
• Father cannot transmit X linked mutant gene to sons.
• Mother can transmit X linked mutant gene to both sons and daughters.
xx XY xx XY
X linked inheritance

19. X X X Y
xx XY xx XY
X linked recessive
inheritance
• 50 % of male will be affected
• 50% of female will be carrier
XY
xx XY xx XY

20. X X X Y
xx XY xx XY
X linked recessive
inheritance
• 50 % of male will be affected
• 50% of female will be carrier
XY
xx XY xx XY

21. X X X Y
xx XY xx XY
X linked recessive
inheritance
Rare situation
Conclusion:
Males>> females affected
Mostly females act as carriers
• Duchenne muscular dystrophy
• Hemophilia A
• Color blindness
• G6PD deficiency

22. X-linked recessive inheritance
• Two copies of a disease allele on the X chromosome are required for
an individual with two X chromosomes (a female) to be affected.
• Since males are hemizygous for X-linked genes, any male with one
copy of disease allele is affected.
• Females are usually carriers because they only have one copy of the
disease allele.
• For a carrier female, with each pregnancy there is 50% chance her
sons will inherit the disease allele and 50% chance her daughters will
be carriers.

23. X X X Y
xx XY xx XY
X linked dominant
inheritance
XY
xx XY xx XY
X X
XY X XX X XY
Females >> males affected.

24. X-linked recessive inheritance X-linked dominant inheritance
Fragile X syndrome (!!)
Vitamin D resistant rickets
Charcot-Marie-Tooth disease

25. • Males and females are equally affected
• Both father and mother can transmit the
mutant gene to both sons and daughter.
YESNO Autosomal inheritanceSex linked inheritance
Vertical transmission +no generation gap
Autosomal dominant
Horizontal transmission +generation gap
Autosomal recessive
Father to sons transmission
Y-Linked
No father to sons transmission
X-Linked
Males >> females – X-recessive
Females>> males- X-dominant