Introduction to Genetics for MBBS students

1. INTRODUCTION TO
GENETICS

2. • Genetics is the study of genes, genetic variation, and heredity in living
organisms.
• Genetics had its start at the beginning of the 20th century, with the
recognition of Mendel’s laws of inheritance.
• Chromosome and genome analysis has become an important diagnostic
procedure in clinical medicine.

3. GREGOR MENDEL
• The Father Of Modern Genetics
• Conducted experiments on garden peas

4. MENDEL’S LAWS OF INHERITANCE
• Factors (genes) are the units of inheritance..
• Genes which code for a pair of contrasting traits are known as alleles, i.e.,
they are slightly different forms of the same gene.
• A given set of alleles at a locus or cluster of loci on a chromosome is referred
to as a haplotype.
• The genotype of a person is the set of alleles that make up his or her genetic
constitution
• The phenotype is the observable expression of a genotype

5. • Homozygote: The individual has a pair of identical alleles at a locus encoded in
nuclear DNA
• Heterozygote: The individual has different alleles at a locus encoded in nuclear
DNA
• Hemizygote: The special case in which a male has an abnormal allele for a gene
located on the X chromosome and there is no other copy of the gene
• Compound heterozygotes: Individuals with two different mutant alleles and no
normal allele

6. LAW OF DOMINANCE
• Characters are controlled by discrete units called factors.
• Factors occur in pairs.
• Some alleles are dominant while others are recessive; an organism with at least one
dominant allele will display the effect of the dominant allele.

7. LAW OF SEGREGATION
• Though the parents contain two alleles, during gamete formation, the alleles of a pair
segregate from each other such that a gamete receives only one of the two alleles.
• A homozygous parent produces all gametes that are similar while a heterozygous one
produces two kinds of gametes each having one allele with equal proportion.

8. LAW OF INDEPENDENT ASSORTMENT
• Genes for different traits can segregate independently during the formation of gametes.

9. INCOMPLETE DOMINANCE

10. CO DOMINANCE

11. NON DISJUNCTION

12. GENETIC LINKAGE

13. CHROMOSOMES

14. DEOXYRIBONUCLEIC ACID

15. PATTERNS OF MENDELIAN INHERITANCE
AUTOSOMAL RECESSIVE INHERITANCE
• Autosomal recessive disease occurs only in homozygotes or compound
heterozygotes
PARENT MATING RISK OF DISEASE
R/r × R/r 3/4 unaffected
1/4 affected
R/r × r/r 1/2 unaffected
1/2 affected
r/r × r/r All affected

17. Sickle cell anaemia Phenylketonuria
Alkaptonuria

18. Sex-Influenced Disorders
• Autosomal recessive disorders generally show the same frequency and severity in males
and females.
• However, some autosomal recessive phenotypes are sex-influenced, that is, expressed in
both sexes but with different frequencies. Eg Haemochromatosis

19. AUTOSOMAL DOMINANT INHERITANCE
The risk and severity of dominantly inherited disease in the offspring depend on
a) whether one or both parents are affected
b) whether the trait is strictly dominant or incompletely dominant
Parental Mating Offspring Risk to Offspring
D/d × d/d 1/2 Dd
1/2 dd
1/2 affected
1/2 unaffected
D/d × D/d 1/4 DD
1/2 Dd
1/4 dd
If strictly dominant:
3/4 affected
1/4 unaffected
If incompletely dominant:
1/2 affected similarly to the parents
1/4 affected more severely than parents
1/4 unaffected

21. PolydactylyAchondroplasia
Neurofibromatosis

22. X LINKED RECESSIVE INHERITANCE

23. Duchenne Muscular Dystrophy Hemophilia Colour blindness

24. X LINKED DOMINANT INHERITANCE

25. Vitamin D resistant Rickets

26. Y LINKED/HOLANDRIC INHERITANCE

27. POLYGENIC AND MULTIFACTORIAL INHERITANCE
• Trait is determined by the additive effect of many genes
• The expression of gene also affected by environmental factors

28. MITOCHONDRIAL INHERITANCE
Leber hereditary optic neuropathy