General overview of patterns of transmission of single gene traits
General Overview Of Patterns Of Transmission Of Single Gene Traits ADEPOJU, Paul Olusegun Cell Biology & Genetics Unit Dept of Zoology, University of Ibadan Ibadan, Nigeria
Introduction • We are all humans, but we don’t look alike – not even identical twins • The basis for the similarity and the reasons for the diversity that coexist in all species have puzzled and intrigued people for thousands of years. It actually birthed genetics • Yoruba myths, traits and transmission genetics – Babatunde, Iyabo et al
Historical Contributions• Mendel and his peas (Pisum sativum)• Bateson and Punnett – epistasis (influence of gene interaction on phenotypes)• Harris – Pearson’s goodness of fit test• Morgan et al – Drosophila melanogaster (heritable mutation in fruit flies)• Sutton – marriage of cell biology and genetics to suggest genes might actually be on chromosomes• Timofeeff-Ressovsky – penetrance and expressivity
Gregor Mendel and His Peas• The way in which traits are passed from one generation to the next, sometimes skipping some generations, was first explained by GM (1885).• Why Peas?• Good model system – he could easily control fertilization by transferring pollen with a small paintbrush – self or cross fertilization• Traits – height (tall or short), pod shape (inflated or constricted), seed shape (smooth or wrinkled), pea color (green or yellow) etc• Quality control measures – tall plants had tall children, grandchildren etc. i.e. pure breeders (homozygous)• Unlike preceding blend theory, GM’s crosses yielded offspring that resembled one of the parent plants, not a blend (examples) DOMINANT & RECESSIVE TRAITS
Law Of Segregation“Every individual possesses a pair of alleles(assuming diploidy) for any particular trait and thateach parent passes a randomly selected copy(allele) of only one of these to its offspring. Theoffspring then receives its own pair of alleles forthat trait. Whichever of the two alleles in theoffspring is dominant determines how theoffspring expresses that trait (e.g. the color andheight of a plant, or the color of an animals fur).”
Law Of Independent Assortment• Also known as "Inheritance Law", states that separate genes for separate traits are passed independently of one another from parents to offspring.• IOW, alleles of different genes assort independently of one another during gamete formation.• While Mendels experiments with mixing one trait always resulted in a 3:1 ratio between dominant and recessive phenotypes, his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1 ratios. But the 9:3:3:1 table shows that each of the two genes is independently inherited with a 3:1 phenotypic ratio.• Mendel concluded that different traits are inherited independently of each other, so that there is no relation, for example, between a cats color and tail length.• This is actually only true for genes that are not linked to each other.
Applications of Patterns ofTransmission • predicting the clinical status of individuals possessing mutations • critical for assessing risk to the family members of a patient affected with a genetic disorder. • to rule out certain genetic disorders in a differential diagnosis when there is a family history of disease.
Autosomal Sex-linked Dominant – AA or AaSingle Gene Traits Recessive – AA or aa Mitochondrial • Definition from etymology • They are controlled by a single gene with two alleles; each allele producing a distinct phenotype. • Alleles are different expressions of the same gene. • All can be used to demonstrate Mendels Law of Segregation.
• Dominant Inheritance a single copy of a mutation will result in disease (a variation on this principle is dominant disorders with reduced penetrance)• Recessive Inheritancean individual will not be affected if he/she has at least one normal allele. Individuals with one normal allele and one mutated allele are called carriers. If a patient possesses no normal allele, only genes with recessive mutations, they will be affected with the disorder.• Mitochondrial Inheritancethe clinical status of a patient is correlated to the proportion of mitochondria with mutations versus mitochondria with normal gene copies.
ADI• Mutation Location: Autosomal Chromosome• Genetic transmission: Individuals possessing one copy of a mutation will be affected.• Examples: Huntingtons Disease Charcot-Marie-Tooth Disease Type 1 Spinocerebellar Ataxia Myotonic Dystrophy• Characteristics: – The child of an affected parent has a 50% chance of inheriting the parents mutated allele and thus being affected with the disorder. – A mutation can be transmitted by either the mother or the father. All children, regardless of gender, have an equal chance of inheriting the mutation. Some autosomal dominant disorders may be characterized by reduced penetrance, i.e.,an individual may inherit a mutation and not manifest clinical symptoms. However, theseindividuals may transmit the mutation and have affected offspring.
ARI• Mutation Location: Autosomal Chromosome• Genetic transmission: Individuals possessing two copies of a mutation will be affected.• Examples: Spinal Muscular Atrophy (SMA) Friedreichs Ataxia• Xtics: – An individual will be a "carrier" if they posses one mutated allele and one normal gene copy. – There is a 50% chance that a carrier will transmit a mutated gene to a child. – All children of an affected individual will be carriers of the disorder. – A mutation can be transmitted by either the mother or the father. – All children, regardless of gender, have an equal chance of inheriting mutations.
If two carrier parents have a child… • 25% chance that both will transmit the mutated gene; in this case, the child will inherit only mutated copies of the gene from both the mother and the father and thus will be affected with the disorder. • 50% chance that one carrier parent will transmit the mutated gene and the other will transmit the normal gene; in this case, the child will have one mutated gene and one normal gene and will be a carrier of the disorder. • 25% chance that both carrier parents will transmit the normal gene; in this case the child will have only normal genes and will not be affected and will not be a carrier.
XLD – pro male • Mutation Location: X-Chromosome • Genetic transmission: Individuals possessing one copy of a mutation will be affected. • Examples: Charcot-Marie-Tooth Disease Type X1 • Xtics – A male or female child of an affected mother has a 50% chance of inheriting the mutation and thus being affected with the disorder. – All female children of an affected father will be affected (daughters possess their fathers X-chromosome). – No male children of an affected father will be affected (sons do not inherit their fathers X-chromosome).
XLR – pro female• Mutation Location: X-Chromosome• Genetic transmission: Individuals possessing no normal gene copies will be affected; typically, only males are affected.• Examples: Duchenne/Becker Muscular Dystrophy; Norrie Disease; Spinal and Bulbar Muscular Atrophy (Kennedys Disease)• Xtics: – Females possessing one X-linked recessive mutation are considered carriers and will generally not manifest clinical symptoms of the disorder. – All males possessing an X-linked recessive mutation will be affected (males have a single X-chromosome and therefore have only one copy of X-linked genes). – All offspring of a carrier female have a 50% chance of inheriting the mutation. – All female children of an affected father will be carriers (daughters posses their fathers X-chromosome). – No male child of an affected father will be affected (sons do not inherit their fathers X-chromosome).
M.I. • Mutation Location: Mitochondrial DNA (brief recap) • Genetic Transmission: Dependent on proportion of normal and mutated mitochondrial DNA (mtDNA). • Examples: Kearns-Sayre Syndrome, MELAS - Mitochondrial Myopathy Encepholopathy, Lactic Acidosis, and Stroke Like Episodes, MERRF - Myoclonus with Epilepsy Ragged Red Fibers • Also implicated in DM, deafness, heart disease, Alzheimer’s disease, Parkinson disease, and Leber’s hereditary optic neuropathy
Xtics of MRI• All children of a mother with a mtDNA mutation are at risk to be either affected with the disorder or asymptomatic carriers of the disorder.• An individual will be affected with a mitochondrial disorder if the percentage of mitochondria possessing mutated mtDNA reaches a threshold value beyond which the normal mtDNA does not compensate for the mutated mtDNA.• The mixture of mitochondria possessing mutated mtDNA and mitochondria with normal DNA is referred to as heteroplasmy.
OTHERSCONDITION DETAILS PATTERNWet ear wax DPTC Tasting DDarwin tubercle little bump on the inside of the ear DS-methylthioester detection you smell asparagus odor in urine? RPigmented iris any color but blue DPolydactyl more than 5 fingers and/or toes DShort big toe The big toe is shorter than your second DLong eyelashes >1cm DWooly hair RDimples DFreckles D
Cleft Chin• Impaired fusion of left and right sides of lower jaw• Dimple Appearance• Congenital• Low incidence• Genetic (AD), but could be acquired due to facial asymmetry resulting from one jaw being slightly long.• Also known as superhero chin