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What is the term for genes that have small quantitative effects on the level of expression of another gene?
What is the term for a change in the location of a gene that is associated with a change in its expression?
Position effect
Epstein-Barr virus causes a translocation between chromosome 8 and chromosome 14, resulting in Burkitt's lymphoma due to the influence of strong immunoglobulin ___________.
enhancers
The probability of phenotypic expression of a gene is denoted as expressivity.
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What is a dihybrid cross viewed as in terms of monohybrid crosses?
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What is the purpose of a test cross?
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In a test cross, when the unknown parent is homozygous dominant, what percentage of offspring will express the dominant phenotype?
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What are linked genes?
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What is complete dominance in terms of allelic interaction?
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What is the basic unit of heredity?
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What are alleles?
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Genes with multiple alleles have more than two variations.
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A new allele of a gene is formed by __________ that change the nucleotide composition of the gene.
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In humans, sex cells have how many chromosomes?
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Match the following Genotype and Phenotype terms:
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What are Mendel's three laws of inheritance?
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The Law of Independent Assortment holds true for all genes.
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Study Notes
Heredity and Genetics
- Heredity is the transmission of traits from parents to their offspring.
- Genetics is the science of the heredity and variability of organisms.
Basic Concepts in Genetics
- The basic unit of heredity is the gene.
- A gene is a sequence of DNA that is transcribed, or a segment of DNA with a particular nucleotide sequence that carries information for synthesizing an RNA or polypeptide chain.
- In all organisms of the same species, genes are located in the same place (locus) on a particular chromosome.
- There are two types of genes:
- Structural genes: encode structural proteins, enzymes, tRNAs, and rRNAs.
- Regulatory genes: encode a product involved in controlling the expression of one or more structural genes.
Alleles
- Alleles are different versions of the same gene.
- They occupy the same locus in homologous chromosomes.
- Alleles define different manifestations of the same trait.
- Example: one gene determines eye color, and its alleles determine the expression of this trait - brown or blue.
Gene vs Allele
- A new allele of a gene is formed by mutations that change the nucleotide composition of the gene.
- Usually, a gene has two alleles, but some genes have more than two alleles, a condition called multiple allelism.
- Example: the gene I determining the synthesis of blood group antigens of the AB0 system has three alleles: IA, IB, and I0.
Amorph Alleles
- Recessive mutant alleles do not determine the synthesis of a particular primary product.
- These alleles are designated as amorph.
- Example: the amorph allele for albinism does not determine melanin synthesis by melanocytes.
Human Cells
- In human sex cells, the chromosome set is haploid (n), and they contain only one chromosome from each pair of homologous chromosomes.
- In human somatic (body) cells, the chromosome set is diploid (2n), they have two homologous chromosomes, and therefore two alleles of a gene.
- In diploid organisms, the gene has two alleles, which can be:
- Dominant: marked with a capital letter of the Latin alphabet (e.g. A).
- Recessive: marked with a lower case letter of the Latin alphabet (e.g. a).
Genotypes and Phenotypes
- The set of all genes in an organism is called a genotype.
- The set of all traits of an organism that are externally manifested is called a phenotype.
- Phenotype depends on both genotype and environmental factors.
Classical Genetics
- The history of genetics began in 1856 with Gregor Johann Mendel, an Austrian monk.
- Mendel conducted experiments with the garden pea (Pisum sativum).
- He crossed pea plants with different traits.
Mendel's Laws of Inheritance
- Law of dominance: some alleles are dominant while others are recessive; an organism with at least one dominant allele will mask the effect of the recessive allele.
- Law of segregation: during gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.
- Law of independent assortment: alleles of different genes can segregate independently during the formation of gametes.
Test Cross (Analyzing Cross)
- A test cross is used to determine whether a dominant phenotype is homozygous or heterozygous.
- It involves mating an unknown genotypic individual with a known homozygous recessive.
Interactions of Genes
- Genes determine the primary structure of proteins, which in turn determine their properties and functions.
- Sometimes, an individual protein can independently determine the expression of a trait.
- There are traits that are formed by the interaction of two or more proteins.
Types of Gene Interactions
- Allelic interaction: between two alleles of one gene.
- Non-allelic interaction: between two alleles of different genes.
- Complete dominance: a dominant allele completely inhibits the action of the recessive allele.
- Incomplete dominance: a dominant allele does not completely mask the effect of the recessive allele.
- Codominance: two alleles of the gene are expressed equally in the heterozygous individual.
- Overdominance: a heterozygote is more vigorous than both corresponding homozygotes.
- Lethal interaction: the interaction between two alleles in a homozygous state, leading to reduced vitality and death of the individual.
Causes of Deviations from Mendel's Laws
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Linked genes
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Allelic and non-allelic interactions
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Polygenic inheritance
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Incomplete penetrance and variable expressivity
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Lethal genes
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Crossing over
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Sex-linked, sex-influenced, and sex-limited inheritance
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Cytoplasmic inheritance
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Phenocopy and genocopy### Complementary Gene Interaction
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Complementary gene interaction occurs between two different genes that work together to create a specific phenotype or visible trait.
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When dominant alleles are present together, they complement each other, resulting in a new phenotype.
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Example: Inheritance of comb morphology in chickens, where the assembly of two dominant alleles R and P results in a walnut-shaped comb.
Epistasis
- Epistasis is a non-allelic interaction between genes, where alleles of one gene suppress the phenotypic expression of alleles of another gene.
- Dominant epistasis: The dominant allele of one gene inhibits the manifestation of the allele of another gene.
- Example: Cross between dogs with brown coat (iiAA) and white coat (P iiAA x IIaa), resulting in a phenotypic ratio of 12:3:1 in F2.
- Recessive epistasis: Recessive alleles in a homozygous state suppress the phenotypic expression of alleles of the other gene.
- Example: The Bombay phenomenon in humans.
Polymeric Gene Interaction
- Polymeric gene interaction is a kind of interaction of non-allelic genes, where the dominant alleles of different genes are responsible for the manifestations of the same trait.
- Polymeric gene interaction can be quantitative (the degree of trait manifestation depends on the number of dominant alleles) or qualitative (the presence of one dominant allele leads to the formation of a trait).
- Example: The inheritance of the intensity of skin pigmentation in humans.
Pleiotropy
- Pleiotropy is the ability of one gene to influence multiple traits.
- Mutations of a given gene are usually severe and affect multiple systems.
- Example: Phenylketonuria, where a mutation in the gene encoding the enzyme that catalyzes the oxidation of phenylalanine to tyrosine leads to an accumulation of phenylalanine and a decrease in tyrosine, resulting in irreversible mental retardation, albinism, and cretinism.
Modifier Genes
- Modifier genes have small quantitative effects on the level of expression of another gene.
- Example: Variations in the intensity of coloration in animals of the same breed.
Position Effect
- Position effect is a change in the location of a gene that is associated with a change in its expression.
- Causes of position effect may include unequal crossover, translocation, inversion, and disruption of linked genes.
- Example: Burkitt's lymphoma, where a translocation between chromosome 8 and chromosome 14 leads to the activation of the c-myc protooncogene, resulting in uncontrolled cell division.
Penetrance and Expressivity
- Penetrance is the probability of phenotypic expression of a gene.
- Complete penetrance: A gene either manifests or it doesn't.
- Incomplete penetrance: A measure of the penetrance of a gene is the proportion of individuals expressing the trait in a population relative to carriers of the genotype in whom the gene could have phenotypic expression.
- Examples: Blood group alleles IA and IB have complete penetrance.
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Description
This quiz covers the basics of classical genetics, including the transmission of traits from parents to offspring, genes, and DNA sequences.
