Lethal genes

Lethal Genes AGB 121 Dept. Of AGB, Veterinary College Hebbal, Bangalore-24

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Lethal Genes • Genes with regular fatal effects are called lethal genes • Phenomenon of action of lethal genes is called lethality.

• Lethal genes can be recessive, dominant, conditional, semi-lethal, or synthetic, depending on the gene or genes involved.

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

LETHAL GENE • Some recessive lethal genes produce no effect in heterozygous state • Some maybe semi lethal or sub-lethal in heterozygous state such genes modify the classical Mendelian Ratio • A lethal gene causes death of individual in the appropriate genotype before they reach adulthood.

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Effects of lethal genes •Induce gross abnormalities / interfere with physiological processes / kill the animal /or are not yet been identified. They produce lethal effect in homozygous state.

•May be due to single gene or at times more than single gene.

•Expressed lethal effect is due to environment, incomplete penetrance and variable expressivity and epistasis.

•In majority of the cases it is difficult to determine the type of gene action Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

LETHAL GENE(Incompletely dominant lethal, intermediate lethal)

1904 French geneticist Lucien Cuenot first reported Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Lethal Genes – Creeper Chickens Semi dominant lethal: (Incompletely dominant lethal, intermediate lethal)

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Achondroplasia in cattle

Dexter

Kerry

It is sub-lethal The heterozygous condition results in short legged Dexter cattle Semi dominant lethal: (Incompletely dominant lethal, intermediate lethal) Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Bull dog calves

Caused by single recessive gene in homozygous condition Calves are aborted at 6-8 months Abnormal head with short face and short upper jaw, short legs and cleft palate Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Other conditions in Cattle Single recessive gene • Parrot beak- In homzygous recessive state • Amputated- Condition where the head legs and lower jaw are missing • Short spine- Compacted vertebrae ,the calf dies at or before birth

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Lethal Genes in Mexican hairless dogsDominant lethal

– hh – Hh – HH

hairy hairless lethal

normal trait one mutation present two mutations = lethal

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Lethal Alleles- Dominant lethal Example: Manx cat ML = tailless, lethal in homozygote m = tail

The allele interferes with normal spinal development, in heterozygous cats this results in lack of a tail.

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Manx Cat

M

m

M

m

MM

Mm

Lethal

Tailless

Mm

mm

Tailless

Normal

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Dominant lethal

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Lethal conditions in Human • Sickle cell Anaemia • Cooley’s anaemia or Thalassemia major • Juvenile idiocy- deterioration of mental and physical powers caused by recessive lethal genes • Infantile- Death of the child at 2-3 years of age • Blindness- Colour and night blindness caused by recessive genes in homozygous recessive state • Epidermolysis- Blisters on the skin- Homozygous recessive genes Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

PLEIOTROPY

• - Impact of a single gene on more than one characteristic • - Sickle-cell disease – - Most common inherited illness among black people – - RBCs are sickle-shaped – Can cause many problems

RBCs: Sickle-cell disease

Normal RBCs

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Sickle Cell Anemia Under conditions of low oxygen tension, hemoglobin S will precipitate, causing cells to sickle Mutations in same amino acid Some individuals die in childhood; Some individuals have mild symptoms HbA: V – H – L – T- P – G –G HbS: V – H – L – T- P – V–G Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Achondroplasia • The most common form of dwarfism, with a normal length body trunk but shortened limbs.

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Detection and elimination of lethal genes By identifying the carriers (heterozygotes) and preventing them from further breeding Intermediate lethal genes are identified by their phenotypic expression Dominant lethal kill the individuals in homozygous or heterozygous conditions Recessive lethal kill the individual only when they are in homozygous stage. Heterozygous carrier parents that produce lethal effect should be used as testers to identify the lethality in the population Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Cytoplasmic Inheritance AGB121 Dept. Of AGB Veterinary College, Hebbal, Bangalore

Definition • Factors inherited from a source other than Nucleus Also called Extra nuclear inheritance Extra chromosomal inheritance Maternal Inheritance

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Types of Cytoplasmic Inheritance 1. Plastid Inheritance 2. Male sterility in plants 3. Streptomycin resistance 4. Enzyme controlled

6. CO2 sensitivity 7. Coiling in snails 8. Eye colour in beach hoppers 9. Cancer producing milk virus in mice

5. Killer particles

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Plastid Inheritance

(Carl Correns 1908)

Inheritance of Chloroplastids only through maternal egg Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Plastid Inheritance

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Male sterility in Plants Rhodes M M

Male sterility genes are inherited in cytoplasm of the female gametes.

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Coiling in Snails

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

CO2 sensitivity in Drosophila L'Heritier and Teisser(1958) True breeding strain of Drosophila which was sensitive to Co2

Sensitive flies become unconscious when exposed to CO2 and are paralysed

When reciprocal crosses were made between CO2 sensitive and normal strains, it could be shown that the trait was inherited only from female parent.

This sensitivity can be attributed to a virus like particle Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore called sigma found in cytoplasm of the cells of a sensitive fly

Killer Particle Paramaecium Certain Killer strains posses Kappa particles which synthesize paramecin

Dominant allele K is required for production of – Kappa Particles in cytoplasm KK or Kk are Killer strains if Kappa particles are present in the cytoplasm Without kappa particles they become sensitive strains Sensitive strains are kk normally Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

No Cytoplasmic Exchange

Sensitive strain KK & kk

Cytoplasmic Exchange

Killer strain is formed when there cytoplasmic exchange

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Milk factors in Mice 1936, by Dr. John Joseph Bittner

Cancerous agent, or "milk factor", could be transmitted by cancerous mothers to young mice from a virus in their mother's milk.

If the mice were fed milk from foster mother they did’nt get cancer.

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Phenocopy A phenocopy is an individual whose phenotype (generally referring to a single trait), under a particular environmental condition, is identical to the one of another individual whose phenotype is determined by the genotype.

The environmental agents producing phenocopies includes temperature, shock, radiation, and various chemical compounds

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Phenocopy In Drosophila melanogaster T.H. Morgan in 1910 The. normal body colour is brownish-gray with black margins Larva of normal flies were fed with silver salts, develop into yellow bodied flies irrespective of their genotype

The yellow bodied flies which are genetically brown is a variant of the original yellow bodied fly.

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Phenocopy in Himalayan Rabbits Himalayan rabbits are white in colour with black tail, nose, and ears when raised in moderate temperatures.

However when raised in colder climates, they become phenotypically distinguishable.

They show black colouration of their coats, resembling the genetically black rabbits.

Hence this Himalayan rabbit is a phenocopy of the genetically black rabbit Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Freemartin • Infertile female when born as a co-twin to a male calf • The calf will have masculanized behaviour and non functioning ovaries

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Karyotype of freemartin It is Chimeric with the sample of cells showing XX/XY chromosomes. and pigs Externally, the animal appears female, but various aspects of female reproductive development are altered due to acquisition of anti-M端llerian hormone from the male twin.

It also occurs occasionally in other mammals including sheep, goats and pigs Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Freemartinism

The animal originates as a female (XX), but acquires the male (XY) component in utero by exchange of some cellular material from a male twin, via vascular connections between placentas Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore

Thank you

Dr R Jayashree, Asst. Prof.,Dept. of AGB Veterinary College, Bangalore