Cytogenetics Midterms

• Alterations to Gene Expression: Lethal Alleles, Multiple Alleles, Incomplete Dominance, Codominance, Epistasis, Penetrance and Expressivity, Pleiotropy, Genetic Heterogeneity, Phenocopy.
• Introduction: Non-Mendelian Genetics refers to any inheritance pattern that does not follow one or more laws of Mendelian Genetics.
• Law of Segregation (First Law) - During gamete formation, alleles for a trait separate from each other and reunite during fertilization.
• Law of Independent Assortment (Second Law) - During gamete formation, alleles for different traits segregate independently of one another.
• Law of Dominance (Third Law) - One dominant allele "masks" the expression of a recessive allele.
• Genetic interactions that do not follow complete dominance are expressed together or influence each other's expression.
• Single genes seldom completely control a phenotype as suggested by Mendel's experiments with peas.
• Other factors such as environmental or genotypic factors can affect the phenotype.
• There may be exceptions to Mendel's Laws, but they are not common.
• Lethal Alleles: Genotype causes death before reproduction, removing an expected progeny class following a specific cross.
• Multiple Alleles: A gene can have multiple alleles due to deviations in its sequence.
• Incomplete Dominance and Codominance: Inheritance patterns where neither allele is completely dominant or recessive, resulting in intermediate or shared phenotypes.
• Epistasis: Interaction between different genes where the expression of one gene is dependent on the presence of another gene.
• Penetrance and Expressivity: Variability in the extent to which a genotype is expressed in the phenotype.
• Pleiotropy: A single gene having multiple effects on different traits.
• Genetic Heterogeneity: Different genetic mutations can result in the same phenotype.
• Phenocopies: Non-genetic factors or environmental influences that mimic a particular phenotype.
• Examples of Lethal Alleles: Achondroplastic dwarfism and Mexican hairless dogs.
• Genotypes and Phenotypes in Lethal Alleles: AA (lethal), Aa (achondroplasia), aa (normal height).
• Inheritance in Cross 1: 1/4 die as embryos (AA), 2/3 survivors have achondroplasia (Aa), 1/3 survivors have normal height (aa).
• Inheritance in Cross 2: All survive, 1/2 have achondroplasia, 1/2 have normal height.
• The PKU gene has hundreds of alleles resulting in four basic phenotypes.
• Expressivity: A genotype is associated with a phenotype of varying intensity.
• The CF gene has over 1500 alleles, some of which can cause frequent severe respiratory infections and others only male infertility.
• In incomplete dominance, one allele is not completely dominant over another, resulting in blended traits rather than distinct occurrences.
• An example of incomplete dominance is the blending of flower colors, where homozygous dominant alleles produce red flowers, homozygous recessive alleles produce white flowers, and heterozygotes produce pink flowers.
• In codominance, both alleles for a particular trait are expressed equally, with both alleles being expressed at the same time when an organism is heterozygous for that trait.
• In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.
• In humans, the ABO gene encodes a cell surface protein and the A and B alleles are codominant, while the i (IO) allele does not produce antigens.
• ABO blood types illustrate codominance, where offspring from parents with blood type A and blood type B will be heterozygous AB.
• Epistasis is another form of non-Mendelian genetics.
• One gene affects the expression of a second gene.
• Interaction of two or more gene pairs at different loci can influence the same trait, with one allele having an overriding effect on the phenotype.
• Modifier gene is a gene that affects the expression of another gene.
• Examples of this phenomenon include albinism and the Bombay phenotype.
• Hair color in Labrador dogs is determined by two sets of gene pairs: one for the quantity of pigment produced and one for pigment deposition.
• In humans, the Bombay phenotype is an example where the H gene is epistatic to the I gene, and the presence of the H protein is necessary for the attachment of A or B antigens to the surface of red blood cells.
• Penetrance is the percentage of individuals who have a certain genotype and show the expected phenotype.
• Expressivity refers to the severity or extent of the phenotype an individual shows.
• Decreased penetrance means that some people inherit a genotype but do not show the phenotype.