7 public questions tagged with this topic.
In codominance, both alleles contribute equally to the phenotype, as seen in the AB blood group.
Ref: NCERT Class 12 Biology Chapter 4: Principles of Inheritance and Variation Non-Mendelian Inheritance - Incomplete Dominance Co-dominance Multiple Alleles
Mitochondrial cells contain many copies of mtDNA molecules that can be heterogeneous within same cell, condition termed heteroplasmy. During mitotic segregation, mutant and wild-type mitochondria distribute randomly to daughter cells, generating offspring cells with varying proportions of mutant mtDNA. Phenotype severity often correlates with percentage mutant load and tissue energy demand, leading to variable expressivity even within same family sharing same mutation. Autosomal, X-linked, and Y-linked traits lack heteroplasmy because nuclear genome exists as diploid pair. Hence variable expression due to heteroplasmy characteristically describes mitochondrial inheritance.
Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 18: Heteroplasmy and Variable Expressivity
Hemophilia A and B result from loss-of-function mutations in F8 and F9 genes encoding coagulation factors VIII and IX, located on long arm of X chromosome at Xq28 and Xq27. Because males possess single X, hemizygous deficiency causes severe bleeding disorder, while heterozygous females are usually asymptomatic carriers due to random X-inactivation providing sufficient clotting factor from normal allele. Pedigrees show affected males connected through carrier females, classic example described historically in Queen Victoria family. Inheritance follows X-linked recessive pattern with characteristic absence of father-to-son transmission.
Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 4: Hemophilia as X-linked Recessive Disorder Model
Unaffected parents lacking visible dominant trait cannot harbor dominant allele under full penetrance model, so they cannot produce affected child via dominant inheritance without new mutation. In recessive mode, unaffected parents can be heterozygous carriers harboring mutant allele silently without clinical signs. By Mendelian segregation, carrier by carrier cross yields twenty-five percent homozygous recessive affected child. This pattern of normal parents with one or more affected offspring within sibship is classic for autosomal recessive inheritance and for X-linked recessive when mother is carrier transmitting to sons.
Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 2: Recessive Pedigree Criteria with Carrier Parents
If trait were recessive, two affected parents homozygous recessive aa would transmit only recessive a allele to every gamete, making all children homozygous recessive affected without exception. Observation of phenotypically unaffected child from two affected parents directly contradicts recessive model expectation. For dominant inheritance, affected individuals may be heterozygous Aa, carrying one normal allele that can be passed to child yielding normal aa phenotype. Thus presence of unaffected offspring in cross of two affected parents indicates dominant allele where parents are heterozygous carriers of normal allele.
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 2: Distinguishing Dominant vs Recessive
Vertical gene transfer conveys genetic information from parent cell to daughter cells through genome replication and division. In bacteria, this occurs during binary fission where chromosome replication followed by septation yields two genetically identical clones, except for rare mutations. All chromosomal genes and stable plasmids transmit vertically, defining lineage inheritance. This contrasts with horizontal gene transfer which moves genes between contemporaneous cells irrespective of lineage. Vertical transfer underlies phylogenetic trees, clonal expansion, and inheritance of essential housekeeping functions across generations.
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Vertical vs Horizontal Transmission
Mendelian inheritance applies principally to nuclear genes located on chromosomes within nucleus that undergo mitotic replication, meiotic segregation and fertilisation restoration. These genes exhibit predictable segregation 1:1 gametic and 3:1 phenotypic ratios, linkage and recombination. Cytoplasmic, mitochondrial and plastid genomes replicate autonomously, segregate through cytoplasmic division, typically maternal inheritance and vegetative segregation lacking Mendelian ratios. Thus Mendelian principles govern majority of trait inheritance in eukaryotes, distinguishing nuclear from organellar genetic systems. Conceptual clarity supports solving numerical problems involving segregation ratios, recombination frequencies and probability calculations in crosses.
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 3: Nuclear vs Cytoplasmic Inheritance
