Single Gene Disorders

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Classification of genetic disorders/diseases

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Monogenic - Mendelian disorders
Polygenic - disorders with multifactorial or multigenic inheritance
Chromosomal aberrations – due to changes in the chromosomes, thus genes located on the corresponding parts are moved and the genetic expression changes

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Monogenic disorders

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Most of genetic disorders are due to “single gene mutation” and “chromosomal aberrations”

Mendelian type of inheritance. (1 gene)

About 5000 diseases

Autosomal dominant
Autosomal recessive
X-linked

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Autosomal dominant / recessive

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"Autosomal" means that the gene in question is located on one of the numbered, or non-sex, chromosomes.

"Dominant" means that a single copy of the disease-associated mutation is enough to cause the disease.

"Recessive" disorder need two copies of the mutation are needed to cause the disease

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X-linked

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Single gene disorders that reflect the presence of defective genes on the X chromosome

Either recessive or dominant

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Simple recessive

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an individual receives defective genes from each of its parents.

aa – shows the defect (homozygoys recessive )

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various types of mating that can arise

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Aa x Aa --- ¼ AA (normal) : ½ Aa (normal) : ¼ aa (shows the defect)

(mating not possible if aa is lethal)

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carriers

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Parents with heterozygous Aa

(defective gene causes some biological process to fail)

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Metabolism

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chemical or physical changes undergone by substances in a biological system

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metabolic disease

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any disease originating in the chemical reactions / metabolism of individuality.

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Inborn errors of metabolism

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Example of IEM

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Citrullinaemia in Holstein-Friesian calves

Calf appears normal at birth - depression within a few hours - tongue protrusion, and unsteady gait next day - froth in the mouth - pressing the head against solid objects - Calf collapse and die by 3-5 days - ammonia toxicity

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Citrullinaemia

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Ammonia accumulation in the body due to lack of the enzyme, argininosuccinate synthetase (ASS) - converting citrulline to argininosuccinate.

A single base-substitution T for C in the 86th triplet code (CGA=arginine) of the gene;
Thus mutant TGA, being the stop code, causes incomplete polypetide chain

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Genotype and Phenotype

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In this example, say:
- D normal allele
- d for defective allele

There will be 3 different genotype – DD, Dd, and dd
Phenotype (observable trait)

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nature of point mutation

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Sex-limited inheritance

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Phenotype associated with the mutation (female feathering in male) can be seen only in males

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example of sex-limited inheritance

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Sex-linked inheritance

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Holandric inheritance

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Some genes carried on the Y chromosome and are passed directly from father to son

(Y-linked inheritance)

Hairy ear in man - only in man

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Difference between Sex-limited and Sex-linked

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Sex-limited or sex-influenced traits are autosomal traits that are influenced by sex.

Sex-linked traits are expressed by genes carried on the sex chromosomes.

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Management of Genetic Defects

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Identify - genetic origin or nutritional deficiencies or injury.

determine if siblings or other descendants also display the condition - to compare the observed symptoms with other genetic defects with similar physical characteristics.

Check if other producers have observed the same problem

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Management of Genetic Defects - Testing

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Genetic defects

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Syndactyly / Polydactyly

- one hoof or many hoofs or fingers

Albinos - (Albinism)

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Chromosomal aberrations

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During mitosis, meiosis or fertilization.
Changes - karyotypes and abnormalities in animals

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Types of Chromosomal aberrations

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Monosomy – lack of a chromosome
XO - a female lacking one X chromosome

Trisomy - the presence of 3 X chromosomes (XXX) - caused by Non-disjunction during miosis

XXX individuals are less abnormal than XO individuals - often fertile

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Non-disjunction

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Trisomy

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an extra copy of ONE chromosome in an egg or sperm cell. As a result, after fertilization the fetus has 47 chromosomes instead of the usual 46. An example of this is Downs Syndrome, in which there is an extra chromosome 21

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Trisomy karyotype

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Triploidy

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the fetus has an entire extra set of chromosomes -- so it has 69 chromosomes instead of the usual 46.

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Triploidy karyotype

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__somy

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all possible combinations of abnormal sperms and egg

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Abnormal Chromosome Structures

Translocation

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Abnormal Chromosome Structures

Single chromosome mutations

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Abnormal Chromosome Structures

Insertion

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Freemartin

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XX/XY chimerism (freemartins have both XX and XY leucocytes)

Major effect seen on gonads of female

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Freemartin symptoms

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Ovotestes – containing both ovarian and testicular tissue
External genitalia – same as normal female except clitoris (enlarged)
Repression of Mullerian duct derivatives (follopian tubes, uterus, cervix, vagina)
Overdevelopment of Wolfian duct derivatives (epidydimis, vas deference, seminal vesicle)
Degree of musculinization highly variable, all intermediate combinations are possible

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Intersex

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individuals having various mixtures of maleness and femaleness.

Hermaphrodite: synonymous with intersex
- presence of both ovarian and testicular tissue

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Pseudo-hermaphrodite

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Male hermaphrodism - Animal has the chromosomes of a male, but external genitals are incompletely formed, or clearly female. Internally, testes may be normal, malformed, or absent.
Female hermaphrodism - Animal has the chromosomes of a female, and have the ovaries , but external genitals that appear male.

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Intersexuality

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Anything is possible in the realm of intersexuality
Intersex rabbit
- Start life as a male
- Sired more than 250 offspring
- Lost interest in mating with females
- Become pregnant to itself
- Give birth to a litter of 3 males and 4 females raised successfully on its own milk