Oral Exam 1 Questions (Transmission Genetics) PDF

Oral Exam 1 Questions (Transmission Genetics) 1. Distinguish the following terms: chromosome, sister chromatid, and homologous chromosome. Chromosomes: A chromosome is a long DNA molecule with part or all of the genetic material of an organism Each chromosome is made of prote...

Oral Exam 1 Questions (Transmission Genetics) 1. Distinguish the following terms: chromosome, sister chromatid, and homologous chromosome. Chromosomes: A chromosome is a long DNA molecule with part or all of the genetic material of an organism Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA) chromosomes are located in the nucleus and are visible during cell division (prophase) Humans have 46 chromosomes, arranged in 23 pairs Sister Chromatid: Sister chromatids are two identical copies of a single chromosome that are connected by a centromere They are formed during the DNA replication phase (S phase) of the cell cycle During cell division (mitosis or meiosis), sister chromatids are separated into new cells to ensure each new cell receives an identical set of chromosomes Homologous Chromosomes: Homologous chromosomes are pairs of chromosomes that have the same structure and carry the same genes, but may have different alleles One chromosome of each pair is inherited from the mother, and the other from the father They pair up during meiosis l and can exchange genetic material through recombination or crossing over, which increases genetic diversity How do homologous chromosomes differ from sister chromatids Homologous chromosomes play a crucial role in genetic diversity during reproduction. They ensure that offspring inherit a mix of traits from both parents. In contrast, sister chromatids are identical copies formed during DNA replication,  crucial for accurate chromosome segregation during cell division. This distinction is vital for understanding genetics. 2. What is the relationship between a locus, a gene, and an allele? Locus: A locus is the specific location of a gene It's like “address” where a gene resides on the chromosome Gene: A gene is a segment of DNA that codes for a specific protein or trait. Each gene occupies a specific locus on a chromosome Allele: An allele is a variant form of a gene. Different alleles can produce variations in the trait that the gene controls ○ For example: a gene for eye color might have an allele for blue eyes and another allele for brown eyes Alleles are found at the same locus on homologous chromosomes Relationship between: A locus is the specific location on a chromosome where a gene is found A gene is a segment of DNA located at a particular locus that codes for a specific trait An allele is a different version of a gene that can exist at the same locus on homologous chromosomes ‘ 3. Can every trait be represented by a Punnett square? Why or why not? Provide examples to support your answer. No, not every trait can be represented by a Punnett square ○ They are not effective in estimating the distribution of genotypes and  phenotypes when there is linkage between two genes (ADD MORE EXAMPLES…) 4. What is a lethal allele? How do lethal alleles impact the results of a genetic cross? Lethal allele: A lethal allele is a variant of a gene that can cause the death of an organism that carries it These alleles usually result from mutations in essential genes required for growth or development Lethal alleles can be recessive, dominant, or conditional When lethal alleles are involved in genetic crosses, they can significantly alter the expected ratios of offspring Recessive Lethal Alleles: In a typical monohybrid cross (Aa x Aa), where ‘a’ is a recessive lethal allele, the expected 3:1 phenotypic ratio changes The homozygous recessive genotype (aa) is lethal, so these individuals do not survive ○ This results in a modified ratio of 2:1 for the surviving offspring (AA and Aa) Dominant Lethal Alleles: Dominant lethal alleles are lethal in both homozygous (AA) and heterozygous (Aa) forms. These alleles can only be passed on if the lethal effect occurs after the organism has reproduced ○ An example: Huntington’s disease in humans, where symptoms typically appear after reproductive age Conditional Lethal Alleles: These alleles are lethal only under certain environmental conditions ○ An example: A temperature sensitive lethal allele might cause death only at high temperatures  5. How is sex determined in animals? What factors can determine the sex of an individual? XY system: Common in mammals, including humans ○ Males: XY chromosomes ○ Females: XX chromosomes ZW system: Found in birds, some reptiles, and some fish ○ Females: ZW chromosomes ○ Males: ZZ chromosomes XO system: Seen in some insects like grasshoppers ○ Females: XX chromosomes ○ Males: XO chromosomes Haplodiploidy: Found in bees, ants, wasps Males develop from unfertilized eggs and are haploid (having one set of chromosomes) Females develop from fertilized eggs and are diploid (having two sets of chromosomes) Factors that can determine the sex of an individual: Hormonal influences: Hormones can play a big role in sex determination and differentiation The presence or absence of certain hormones during critical periods of development can influence the development of male or female characteristics Genetic Mutations: Sometimes mutations in specific genes can lead to variations in sex determination ○ For example, mutations in the SRY gene on the Y chromosome can result in individuals with XY chromosome developing as females Temperature: In alligators, turtles, and other reptiles temperature can determine whether an individual develops into a female or male  ○ Warmer temps: female ○ Colder temps: male 6. What is X-linked inheritance? How does the inheritance of X-linked genes differ from genes located on autosomes? X-linked inheritance: refers to the pattern of inheritance for genes located on the X chromosome X-linked recessive: Males: ○ Since males have only one x chromosome (XY), a single recessive allele on the X chromosome will result in the expression of the trait or disorder Females: ○ Females have two X chromosomes (XX), so they need TWO copies of the recessive allele to express the trait ○ If they have only one copy, they are carriers and can pass the allele to their offspring X-linked dominant: Males: ○ A single dominant allele on the X chromosome will cause the trait or disorder to be expressed ○ This can be more severe in males due to the lack of a second X chromosome to potentially counteract the effect Females: ○ Females with one dominant allele will express the trait ○ The severity can vary depending on whether the allele is present on one or both X chromosomes Differences from autosomal inheritance: Chromosome location:  ○ X-linked genes: located on the x chromosome, which is one of the sec chromosomes ○ autosomal genes: located on the autosomes, which are the non-sex chromosomes Inheritance patterns: ○ X-linked traits: Males: inherit their X chromosomes from their mother and pass it to all their daughters. Females: inherit one X chromosome from each parent ○ Autosomal traits: both males and females inherit autosomal genes equally from both parents and the inheritance pattern does not depend on the sex of the offspring Expression in males and females: ○ X-linked recessive: more commonly expressed in males due to the presence of only one X chromosome ○ Autosomal recessive: requires two copies of the recessive allele for the trait to be expressed, regardless of sex Carrier status: ○ X-linked recessive: Females: can be carriers without expressing the trait Males: cannot be carriers, they either have the trait or they don't ○ Autosomal recessive: Both Males and Females can be carriers without expressing the trait 7. Describe the difference between haploid and diploid. How do organisms produce haploid and diploid cells? Do all organisms have both haploid and diploid cells? Haploid cells: Haploid cells (n) contain a single set of chromosomes ○ In humans this means 23 chromosomes ○ These cells are typically involved in sexual reproduction and include gametes (sperm and egg cells) Diploid cells:  Diploid cells (2n) contain two sets of chromosomes ○ One from each parent, totaling 46 chromosomes in humans ○ Most of the body's cells (known as somatic cells) are diploid Production of Haploid and Diploid cells: Mitosis: ○ Diploid cells: mitosis is the process by which a diploid cell divides to produce two genetically identical diploid daughter cells ○ This is how somatic cells replicate for growth and repair Meiosis: ○ Haploid cells: meiosis is a specialized form of cell division that reduces the chromosome number by half, producing four genetically diverse haploid cells from one diploid cell. ○ This process is crucial for producing gametes in sexually reproducing organisms Do all organisms have both haploid and diploid cells?: Not all organisms have both haploid and diploid cells ○ Most animals: typically have both haploid and diploid stages. Diploid cells make up the majority of the organism, while haploid cells are limited to gametes ○ Plants: exhibit an alternation of generation, where both haploid and diploid stages are prominent in their life cycle ○ Fungi and Algae: some species spend most of their cycle in a haploid state and only become diploid briefly during sexual reproduction ○ Certain insects: like male bees, ants, and wasps are haploid throughout their lives, while females are diploid 8. What is incomplete penetrance? Why might an individual with a heterozygous dominant genotype not display the expected trait? Incomplete penetrance: Definition: Incomplete penetrance occurs when not all individuals with a  dominant allele express the associated phenotype. This means that even if an individual carries the dominant allele, they might not show the trait ○ Example: Polydactyly, a condition where individuals have extra fingers or toes, is caused by a dominant allele. However, not everyone with the allele exhibits the extra digits Incomplete penetrance occurs when individuals with a particular genotype do not always express the expected phenotype ○ In other words, even if an individual carries a dominant allele for a trait, they might not show the trait at all An individual with a heterozygous dominant genotype might not display the expected trait due to several factors that can influence gene expression. These are some reasons: Genetic background: Modifier genes: other genes in the genome can influence whether the dominant allele is expressed ○ These modifier genes can enhance or suppress the effect of the dominant allele Epistasis: this is a form of gene interaction where one gene can mask or modify the expression of another gene Epigenetic Modifications: DNA methylation and histone modification: these changes do not alter the DNA sequence but can affect gene expression. Epigenetic modification can turn genes on or off, influencing whether a trait is expressed 9. How many alleles of a single gene can occur in a population of organisms? How many alleles of a single gene does a diploid individual have? In a population there can be multiple alleles for a single gene ○ This means that while an individual organism can only carry a limited number of alleles, the population as a whole can have many different  versions of that gene For example, ABO blood group system in humans has three alleles (IA),(IB), and (i) Multiple alleles can exist for a single gene A diploid individual has two alleles for each gene, one inherited from each parent. ○ These alleles can be the same (homozygous) or different (heterozygous)2. For instance, a person might have two identical alleles for a gene (AA or aa) or two different alleles (Aa). Only two alleles for each gene 10. What is a polygenic trait? Provide an example and describe some of the challenges of studying polygenic traits. Polygenic trait: Polygenic traits are characteristics that are influenced by multiple genes, often located on different chromosomes. Unlike single-gene (Mendelian) traits, polygenic traits exhibit continuous variation and are typically represented by a range of phenotypes rather than discrete categories Example: An example of a polygenic trait is human height. Height is influenced by the combined effect of many genes, each contributing a small amount to the overall phenotype. This results in a continuous distribution of heights in the population, forming a bell-shaped curve when plotted Challenges of studying polygenic traits: Small Effect Size: ○ Each gene involved in a polygenic trait typically has a small effect on the phenotype. Detecting these small effects requires large study populations to achieve statistical significance Gene-Gene Interactions:  ○ The interactions between multiple genes (epistasis) can complicate the analysis. Understanding how these genes work together to influence the trait is challenging Environmental Influences: ○ Environmental factors can significantly impact polygenic traits. For example, nutrition and lifestyle can affect height, making it difficult to isolate the genetic contribution Complex Genetic Architecture: ○ The genetic architecture of polygenic traits is complex, involving many loci spread across the genome. This complexity makes it hard to pinpoint specific genetic variants responsible for the trait Data Interpretation: ○ Interpreting the results of studies on polygenic traits can be difficult due to the intricate interplay between genetic and environmental factors. This complexity requires sophisticated statistical methods and computational tools