INTRODUCTION TO GENETICS

• 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.

GREGOR MENDEL

• The Father Of Modern Genetics

• Conducted experiments on garden peas

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

• 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

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.

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.

LAW OF INDEPENDENT ASSORTMENT

• Genes for different traits can segregate independently during the formation of gametes.

INCOMPLETE DOMINANCE

CO DOMINANCE

NON DISJUNCTION

GENETIC LINKAGE

CHROMOSOMES

DEOXYRIBONUCLEIC ACID

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 unaffected1/4 affected

R/r × r/r 1/2 unaffected1/2 affected

r/r × r/r All affected

Sickle cell anaemia Phenylketonuria

Alkaptonuria

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

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 Dd1/2 dd

1/2 affected1/2 unaffected

D/d × D/d 1/4 DD1/2 Dd1/4 dd

If strictly dominant:3/4 affected1/4 unaffected

If incompletely dominant:1/2 affected similarly to the parents1/4 affected more severely than parents1/4 unaffected

PolydactylyAchondroplasia

Neurofibromatosis

X LINKED RECESSIVE INHERITANCE

Duchenne Muscular Dystrophy Hemophilia Colour blindness

X LINKED DOMINANT INHERITANCE

Vitamin D resistant Rickets

Y LINKED/HOLANDRIC INHERITANCE

POLYGENIC AND MULTIFACTORIAL INHERITANCE

• Trait is determined by the additive effect of many genes

• The expression of gene also affected by environmental factors

MITOCHONDRIAL INHERITANCE

Leber hereditary optic neuropathy