ANSC327 exam 1

Genetics Study Guide

Key Concepts Review

Mendelian Genetics:

• Understand the laws of segregation and independent assortment.

• Be able to predict genotype and phenotype ratios in monohybrid and dihybrid crosses, including F1 and F2 generations.

• Know how to perform and interpret test crosses to determine an unknown genotype.

• Understand the concepts of dominance, recessiveness, incomplete dominance, and codominance (though codominance is not explicitly in the provided text).

DNA Structure and Manipulation:

• Know the components of a nucleotide (base, sugar, phosphate) and a nucleoside (base, sugar).

• Understand the structure of DNA, including the sugar-phosphate backbone, nitrogenous bases (purines and pyrimidines), base pairing rules (Chargaff's rule), and the antiparallel nature of the double helix.

• Understand the different forms of DNA (A, B, Z) and their key characteristics.

• Know the function and properties of restriction enzymes, including recognition sites and different types of cuts (blunt and sticky ends).

• Understand the principles and applications of gel electrophoresis for DNA fragment separation.

• Explain the process and purpose of Southern blotting.

• Understand the basic principles of Polymerase Chain Reaction (PCR), including denaturation, annealing (primer hybridization), and extension.

Genes and Chromosomes:

• Understand the relationship between genes, alleles, and homologous chromosomes.

• Know the difference between autosomes and sex chromosomes (X and Y).

• Understand the process of meiosis and how it leads to haploid gametes.

• Explain X-linked inheritance patterns and how they differ from autosomal inheritance.

• Understand the concept of nondisjunction and its potential consequences.

• Know the definitions and examples of sex-linked, sex-limited, and sex-influenced traits.

Genetic Linkage and Chromosome Mapping:

• Understand the concept of genetic linkage and how it deviates from independent assortment.

• Explain how recombination (crossing over) between linked genes leads to recombinant gametes.

• Know how to calculate recombination frequency and its relationship to map distance (centimorgans).

• Understand the use of test crosses in determining linkage and mapping genes.

• Be familiar with the concept of three-point crosses and how they can be used to determine gene order and map distances.

• Understand the concepts of coefficient of coincidence and interference in relation to double crossovers.

• Know the difference between genetic maps and physical maps.

Molecular Biology of DNA Replication and Recombination:

• Understand the basic models of homologous recombination, including the formation and resolution of Holliday junctions.

• Explain how unequal crossing over can lead to gene duplication or deletion (though not explicitly detailed in the text, the concept of recombination is).

• Understand how errors in recombination between X and Y chromosomes can lead to XX males or XY females, particularly focusing on the role of the SRY gene.

Introduction to Molecular Genetics:

• Understand the central dogma of molecular biology (DNA -> RNA -> Protein).

• Be familiar with the historical context of genetics, including Mendel's contributions and the rediscovery of his work.

• Understand the role of DNA as the carrier of genetic information (Griffith's experiment and Hershey-Chase experiment are implied but not detailed).

• Know the definition of key genetic terms like genotype, phenotype, locus, and allele.

• Understand the concept of DNA polymorphisms, including Restriction Fragment Length Polymorphisms (RFLPs) and Single Nucleotide Polymorphisms (SNPs), and their applications.

Short-Answer Quiz

1. State the law of segregation and explain its significance in the context of allele inheritance.

2. Describe how a test cross can be used to determine if an organism with a dominant phenotype is homozygous or heterozygous for a particular gene.

3. Explain the difference between a nucleoside and a nucleotide, highlighting the key structural component that distinguishes them.

4. State Chargaff's rules regarding DNA base composition and explain the basis for these rules in the structure of DNA.

5. Describe the function of restriction enzymes and explain why they are valuable tools in molecular biology.

6. Outline the three main steps of a PCR cycle and briefly describe what occurs during each step.

7. Explain why males are considered hemizygous for most genes on the X chromosome and how this affects the expression of X-linked recessive traits.

8. Define genetic linkage and explain how it affects the expected phenotypic ratios in a cross compared to genes that assort independently.

9. Describe the relationship between recombination frequency and genetic map distance, including the unit used to measure map distance.

10. Explain how abnormal recombination between the X and Y chromosomes can lead to a genetically XX individual with a male phenotype.

Quiz Answer Key

1. The law of segregation states that every individual possesses two alleles for a particular trait, and during gamete formation, only one of these alleles is passed on to the offspring. This principle ensures that each gamete carries a single copy of each gene, maintaining the chromosome number across generations after fertilization.

2. To determine if a yellow-seeded plant (dominant phenotype) with a potentially YY or Yy genotype is homozygous or heterozygous, perform a test cross by breeding it with a homozygous recessive green-seeded plant (yy). If all offspring are yellow, the unknown plant is likely YY. If any green-seeded offspring appear, the unknown plant must be Yy.

3. A nucleoside consists of a nitrogenous base covalently linked to a sugar molecule (ribose or deoxyribose). A nucleotide, on the other hand, is a nucleoside with one or more phosphate groups attached to the sugar. The presence of the phosphate group(s) is the crucial difference between a nucleoside and a nucleotide.

4. Chargaff's rules state that in a DNA molecule, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C). This is because A always pairs with T via two hydrogen bonds, and G always pairs with C via three hydrogen bonds in the DNA double helix.

5. Restriction enzymes are bacterial enzymes that recognize specific short sequences of DNA called recognition sites and cut the DNA at or near these sites. They are valuable tools in molecular biology because they allow scientists to precisely cut DNA molecules into fragments of predictable sizes, which is essential for techniques like cloning, Southern blotting, and genetic mapping.

6. The three main steps of a PCR cycle are denaturation, annealing, and extension. During denaturation, the double-stranded DNA template is heated to separate the strands. In annealing, short DNA sequences called primers bind to complementary sequences on the single-stranded DNA. During extension, a DNA polymerase enzyme synthesizes new DNA strands by adding nucleotides to the 3' end of each primer, resulting in two double-stranded DNA molecules.

7. Males have an XY sex chromosome composition, meaning they have only one copy of the X chromosome. Therefore, for any gene located on the X chromosome, males only possess one allele and will express that allele's phenotype regardless of whether it is dominant or recessive. This "one copy" state is referred to as hemizygous.

8. Genetic linkage occurs when genes are located close together on the same chromosome, causing them to be inherited together more often than predicted by independent assortment. This proximity reduces the frequency of recombination between these genes, leading to offspring phenotypes that resemble the parental phenotypes more frequently than expected in a typical Mendelian cross.

9. Recombination frequency is the percentage of recombinant offspring resulting from a cross involving linked genes and reflects the physical distance between those genes on a chromosome. Genetic map distance is measured in map units (mu) or centimorgans (cM), where 1 cM is defined as the distance between genes for which 1% of the products of meiosis are recombinant.

10. In normal male meiosis, the X and Y chromosomes pair and can exchange genetic material in pseudoautosomal regions. However, if abnormal crossing over occurs outside these regions and transfers the SRY gene (sex-determining region Y) from the Y chromosome to the X chromosome, a genetically XX individual who inherits this X chromosome with the SRY gene can develop a male phenotype.

Essay Format Questions

1. Discuss the significance of Mendel's laws of segregation and independent assortment as foundational principles of inheritance. How did his work revolutionize the understanding of genetics despite being initially overlooked?

2. Compare and contrast the processes of Southern blotting and PCR in the context of DNA analysis. What are the specific applications of each technique, and what fundamental principles of molecular biology do they rely upon?

3. Explain the mechanisms of X-linked inheritance and how they lead to different phenotypic patterns in males and females. Use examples to illustrate how X-linked dominant and recessive traits are transmitted through generations.

4. Describe the process of genetic linkage and how recombination frequencies can be used to construct genetic maps. What factors can influence recombination frequency, and why are genetic maps important in understanding genome organization?

5. Discuss the role of meiosis in sexual reproduction and the potential consequences of errors during this process, such as nondisjunction and unequal crossing over. How can these meiotic errors lead to genetic disorders or variations in sex determination?

Glossary of Key Terms

• Allele: Different versions of the same gene, occupying the same locus on homologous chromosomes.

• Autosome: Any chromosome that is not a sex chromosome.

• Chargaff's Rules: The observation that in DNA, the amount of adenine (A) equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C).

• Dominant Allele: An allele that expresses its phenotype even when heterozygous with a recessive allele.

• Gene: A unit of heredity that carries the instructions for a specific trait; a sequence of nucleotides in DNA that typically codes for a protein or functional RNA molecule.

• Genotype: The genetic makeup of an organism, referring to the specific alleles it possesses for a particular trait or set of traits.

• Heterozygous: Having two different alleles for a particular gene at a specific locus on homologous chromosomes.

• Homologous Chromosomes: Pairs of chromosomes in a diploid organism that have the same genes in the same order, though they may carry different alleles. One chromosome of each pair is inherited from each parent.

• Homozygous: Having two identical alleles for a particular gene at a specific locus on homologous chromosomes.

• Incomplete Dominance: A pattern of inheritance where the heterozygous phenotype is a blend or intermediate between the phenotypes of the two homozygous genotypes.

• Law of Independent Assortment: Mendel's second law, stating that during gamete formation, the alleles of different genes segregate independently of one another.

• Law of Segregation: Mendel's first law, stating that each individual possesses two alleles for any particular trait and that during gamete formation, these alleles segregate or separate, with each gamete receiving only one allele.

• Linkage (Genetic): The tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction.

• Locus (pl. Loci): The specific physical location of a gene on a chromosome.

• Meiosis: A specialized type of cell division that reduces the chromosome number by half, producing four haploid gametes from a single diploid cell.

• Nondisjunction: The failure of homologous chromosomes or sister chromatids to separate properly during meiosis or mitosis.

• Nucleoside: A molecule consisting of a nitrogenous base covalently linked to a sugar (ribose or deoxyribose).

• Nucleotide: The basic structural unit of nucleic acids (DNA and RNA), consisting of a nitrogenous base, a sugar (ribose or deoxyribose), and one or more phosphate groups.

• PCR (Polymerase Chain Reaction): A laboratory technique used to amplify a single or a few copies of a segment of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence.

• Phenotype: The observable physical or biochemical characteristics of an organism as determined by its genotype and environmental influences.

• Recessive Allele: An allele that only expresses its phenotype when homozygous; its effect is masked in the presence of a dominant allele.

• Recombination (Genetic): The process by which genetic material is exchanged between homologous chromosomes during meiosis, resulting in offspring with combinations of traits that differ from those found in either parent. Also known as crossing over.

• Restriction Enzyme: A bacterial enzyme that cuts DNA at specific recognition nucleotide sequences.

• Sex Chromosomes: Chromosomes that determine the sex of an organism (e.g., X and Y chromosomes in mammals).

• Sex-influenced Trait: A trait whose expression differs in males and females, even when they have the same genotype.

• Sex-limited Trait: A trait that is expressed in only one sex, even though the genes for the trait may be present in both sexes.

• Sex-linked Trait: A trait whose gene is located on a sex chromosome (usually the X chromosome).

• Southern Blotting: A technique used to detect the presence of specific DNA sequences within a DNA sample.

• Test Cross: A cross between an individual with an unknown genotype (expressing a dominant phenotype) and a homozygous recessive individual; the phenotypes of the offspring can reveal the genotype of the unknown parent.

• X-linked: Referring to a gene located on the X chromosome.

• Y-linked: Referring to a gene located on the Y chromosome.