Mendelian Genetics: Key Concepts, Terms and Mechanisms
In genetics, Mendelian concepts and mechanisms are fundamental to understanding inheritance patterns. The following are key concepts, terms, and mechanisms related to Mendelian genetics:
Phenotype and Genotype
- Phenotype is the observable physical or biochemical characteristics of an organism, such as height or color.
- Genotype is the genetic makeup of an organism, which determines its phenotype.
- A gene is a stretch of DNA that codes for a trait, and a locus is the location of a gene on a chromosome.
- An allele is a variant of a gene. All alleles of the same gene exist at the same locus.
- A cell holds two alleles of each gene, one from each parent.
- When a gene has only two alleles, it is called a simple case. When a gene has more than two alleles, it is called multiple alleles.
Homo- and Heterozygosity, Wild Type, and Recessiveness
- Homozygous refers to when the two alleles an individual carries are the same, while heterozygous refers to when the two alleles are different.
- Wild type refers to the “normal” allele or phenotype for an organism.
- Recessive alleles are only expressed if both copies are present. Only one copy is needed for the dominant allele.
Complete Dominance and Co-dominance
- Complete dominance occurs when one allele completely masks the other in the phenotype.
- Co-dominance occurs when both alleles contribute to the phenotype, as in the case of the A and B blood type alleles.
Incomplete Dominance, Leakage, Penetrance, and Expressivity
- Incomplete dominance occurs when the phenotype of the heterozygote is intermediate between the two homozygotes.
- Leakage refers to gene flow from one species to another.
- Penetrance is the frequency that a genotype will show up in the phenotype.
- Expressivity is to what degree a penetrant gene is expressed.
Hybridization and Viability
- Hybrid vigor refers to increased viability for offspring of parents who are genetically more different.
- The mechanism is less chance to receive 2 copies of the same detrimental recessive gene.
Gene Pool, Meiosis, and Genetic Variability
- The gene pool is all of the alleles in a population.
- Meiosis introduces genetic variability by genetic recombination through independent assortment and crossing-over.
- Important differences between meiosis and mitosis include tetrad formation and haploid daughter cells in meiosis.
Segregation of Genes and Independent Assortment
- Segregation of genes occurs during meiosis when homologous chromosomes separate into different gametes.
- Independent assortment occurs during meiosis when homologous chromosomes line up randomly and separate independently.
Linkage
- Linkage occurs when genes on the same chromosome are not randomized by independent assortment.
Practice Questions – Variation and Genetics [Easy]
Improve your understanding of Mendelian genetics by testing your knowledge with these easy multiple choice questions. After reading the content, challenge yourself to solve these questions and solidify your grasp on the fundamental concepts of genotype, phenotype, alleles, homozygosity, heterozygosity, complete dominance, co-dominance, incomplete dominance, penetrance, expressivity, meiosis, independent assortment, and linkage.
1) Which of the following best defines a genotype?
- a) An organism’s observable characteristics
- b) An organism’s genetic makeup
- c) An organism’s environmental factors
- d) An organism’s behavior
2) What is the difference between homozygous and heterozygous genotypes?
- a) Homozygous genotypes have two different alleles for a given gene, while heterozygous genotypes have two identical alleles.
- b) Homozygous genotypes have two identical alleles for a given gene, while heterozygous genotypes have two different alleles.
- c) Homozygous genotypes have no alleles for a given gene, while heterozygous genotypes have two alleles.
- d) Homozygous genotypes have one allele for a given gene, while heterozygous genotypes have two alleles.
3) What is complete dominance?
- a) The phenomenon where the expression of a recessive allele is masked by a dominant allele in a heterozygous genotype.
- b) The phenomenon where the expression of a dominant allele is masked by a recessive allele in a heterozygous genotype.
- c) The phenomenon where the expression of both alleles is seen in a heterozygous genotype.
- d) The phenomenon where there is no expression of either allele in a heterozygous genotype.
4) What is co-dominance?
- a) The phenomenon where the expression of a recessive allele is masked by a dominant allele in a heterozygous genotype.
- b) The phenomenon where the expression of a dominant allele is masked by a recessive allele in a heterozygous genotype.
- c) The phenomenon where the expression of both alleles is seen in a heterozygous genotype.
- d) The phenomenon where there is no expression of either allele in a heterozygous genotype.
5) What is incomplete dominance?
- a) The phenomenon where the expression of a recessive allele is masked by a dominant allele in a heterozygous genotype.
- b) The phenomenon where the expression of a dominant allele is masked by a recessive allele in a heterozygous genotype.
- c) The phenomenon where the expression of both alleles is seen in a heterozygous genotype.
- d) The phenomenon where there is no expression of either allele in a heterozygous genotype.
6) What is the term used to describe an individual with two different alleles for a particular gene?
- a) Homozygous
- b) Heterozygous
- c) Dominant
- d) Recessive
7) What is the term used to describe the physical appearance of an organism based on its genetic makeup?
- a) Genotype
- b) Phenotype
- c) Allele
- d) Heterozygosity
8) What is the term used to describe a gene that is always expressed in an individual regardless of its pairing with another allele?
- a) Recessive
- b) Dominant
- c) Co-dominant
- d) Incomplete dominant
9) In incomplete dominance, what is the result of a heterozygous genotype?
- a) The dominant allele is fully expressed.
- b) The recessive allele is fully expressed.
- c) An intermediate phenotype is expressed.
- d) Both alleles are expressed equally.
10) What is the term used to describe the likelihood of an individual with a particular genotype expressing the associated phenotype?
- a) Penetrance
- b) Expressivity
- c) Homozygosity
- d) Heterozygosity
11) In what process does genetic recombination occur during the formation of gametes?
- a) Mitosis
- b) Meiosis
- c) DNA replication
- d) Transcription
12) What is the term used to describe a pair of genes that are located on the same chromosome and tend to be inherited together?
- a) Independent genes
- b) Linked genes
- c) Alleles
- d) Homozygous genes
13) What is the term used to describe a gene that has multiple effects on the phenotype of an individual?
- a) Recessive gene
- b) Dominant gene
- c) Pleiotropic gene
- d) Epistatic gene
14) What is the term used to describe the frequency of occurrence of a particular allele in a population?
- a) Allelic frequency
- b) Phenotypic frequency
- c) Genotypic frequency
- d) Mutation rate
15) What is the probability of having a homozygous recessive offspring if both parents are carriers of the recessive allele?
- A) 0%
- B) 25%
- C) 50%
- D) 75%
16) What is the probability of having a heterozygous offspring if one parent is homozygous dominant and the other is homozygous recessive?
- A) 0%
- B) 25%
- C) 50%
- D) 100%
17) In a dihybrid cross between two heterozygous individuals, what is the expected phenotypic ratio in the F2 generation according to the law of independent assortment?
- A) 1:1:1:1
- B) 3:1
- C) 9:3:3:1
- D) 1:2:1
18) What is the term used to describe a situation in which the expression of one gene is affected by the expression of another gene?
- A) Dominance
- B) Incomplete dominance
- C) Codominance
- D) Epistasis
19) What is the likelihood of having a child with a dominant phenotype if both parents are heterozygous for the dominant allele?
- A) 0%
- B) 25%
- C) 50%
- D) 75%
20) Which of the following describes the relationship between the terms “gene” and “allele”?
- A) Genes are alternate forms of alleles
- B) Alleles are alternate forms of genes
- C) Genes and alleles have no relationship to each other
- D) Genes and alleles are the same thing
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