Chapter 12: Genetics and Evolution Flashcards
Shorthand for nucleotide
nt
Shorthand for chromosome
Chr
Shorthand for mutation
Mutat
What are genes?
Genes are DNA sequences that code for heritable traits that can be passed from one generation to the next.
What is a chromosome?
A chromosome is a thread-like structure found inside the nucleus of a cell that carries long strands of DNA, containing the genetic information needed for an organism to function, essentially acting as the “blueprint” for an organism’s traits and development.
What are alternate forms of genes called?
Alternate forms of genes are called alleles.
What is a genotype?
A genotype is the genetic combination possessed by an individual.
What is a phenotype?
A phenotype is the observable manifestation of a given genotype.
What are homologous chromosomes? What is the exception of homologous in humans? Are homologous chromosomes identical?
Homologous chromosomes are two copies of each chromosome. The exception of homologous in humans are the sex chromosomes of genotypical males who have one X chromosome and one Y chromosome.
Remember. Homologous chromosomes are not identical to each other. They carry the same genes in the same order but contain different alleles.
What is the locus of a gene?
The locus of a gene is the location of a gene on a specific chromosome.
Explain dominant and recessive alleles.
Alleles are considered dominant if only one copy is necessary to express a given phenotype.
Alleles are considered recessive if two copies are needed to express a given phenotype.
What is a homozygous genotype? Heterozygous? Hemizygous?
A homozygous genotype is when both alleles are the same for a given gene. Simple example being RR (homozygous dominant) or rr (homozygous recessive).
Heterozygous is the alleles are different. Such as Rr.
Hemizygous are a situation in which only one allele is present for a given gene. Such as one X chromosome for genotypical males.
What is complete dominance regarding alleles? Codominance? Incomplete dominance?
Complete dominance is there is only one dominant and one recessive allele existing for a given gene.
Codominance is when more than one dominant ellese exist for a gene, for example blood: A and B are codominant in that both A and B antigens will be expressed if both dominant alleles are present on the gene.
Incomplete dominance occurs when a heterozygote expresses a phenotype that is intermediate between the two homozygous genotype. Example is certain kinds of flowers. RR is red, rr is white, Rr is pink. Memorize the F1 and F2 outcome of the following image:
What is penetrance and expressivity?
Penetrance is the proportion of a population with a given genotype who actually express the phenotype.
Expressivity is the different manifestation of the same genotype across the population.
Hypothetical mouse ear example:
List the varying degrees of penetrance.
Penetrance varies in a population from full penetrance to nonpenetrance:
Full penetrance: 100% of the individuals in a population express.
High penetrance: most express.
Reduced penetrance.
Low penetrance.
Nonpenetrance: no expression.
How does expressivity differ from penetrance?
Expressivity is the varying manifested phenotype for a given genotype. Penetrance is the percentage of the population that express a manifested phenotype for a given genotype.
What is the main takeaway from Mendel’s law of segregation?
The key takeaway from Mendel’s law of segregation is the separation of homologous chromosomes during anaphase I of meiosis segregate the chromosomes into different cells, making each gamete having only one allele for any given trait.
What is the takeaway from Mendel’s second law: independent assortment.
The takeaway from Mendel’s second law is that the inheritance is one gene does not affect the inheritance of another gene. Under genome replication during meiosis, during prophase I chromosomes will recombine allowing for inheritance that is independent of the others. Recall crossing over of homologous chromosomes effectively swapping genetic material and becoming unique among themselves.
Segregation and independent assortment of alleles allow for greater genetic diversity in the offspring.
Describe transcription and translation.
Transcription is the creation of mRNA from DNA. Translation is the creation of a protein from mRNA.
Recall transformation, transduction, and conjugation of bacteria.
Transformation is when a bacteria picks up genetic material, and thus genes, from its environment.
Transduction is when a bacteria inherits genetic information, and thus genes, from a vector such as a bacteriophage.
Conjugation is bacteria sex. lol. A sex factor must be present for bacteria to perform conjugation.
What are epigenetics? What does the word mean? What are some types and examples of epigenetics? Are there clinical impacts of epigenetics? What is imprinting?
Epigenetics is a general term for changes in DNA that do not involve an alteration to the nucleotide sequence. The prefix epi- means “over” or “above”, so genetic changes that come from sources other than changed nucleotide sequence.
Examples include covalent attachment of different chemical groups to nucleotides or histone proteins including methylation and acetylation. These modifications can increase or decrease the expression of specific genes.
Another example is X-inactivation in cells with two X chromosomes in which one of the X chromosomes is silenced via methylation. The inactive X is called a Barr body.
Clinical impacts include hypermethylation of oncogenes and tumor suppressor genes have been implicated in some forms of cancer.
Imprinting is an epigenetic process in which gene expression is determined by the contributing parent. Differentiated methylation can come from a parent.
What is imprinting? What is the book example of imprinting?
Imprinting is an epigenetics process in which gene expression is determined by the contributing parent.
Prader-Willi syndrome: development disorder caused by a deletion in chromosome 15. If the deletion of the region of the chromosome occurs on the paternal chromosome and the maternal copy is heavily methylated.
A totally different outcome happens when the maternal chromosome portion of chromosome 15 is missing and the paternal chromosome is heavily methylated: Angleman syndrome
What is a mutation? What are wild type alleles? What are mutant alleles? Mutations can happen in a number of ways, name some.
A mutation is a change in DNA sequence and it results in a mutant allele. Mutant alleles are those that have been changed via a mutation. Wild type alleles are those considered normal or natural and are ubiquitous in the study population.
Mutations can be caused by ionizing radiation and chemical exposures (mutagens). DNA polymerase can make occasional errors. Transposons can insert and remove themselves from the genome.
What is a mutagen?
A mutagen is a substance that can cause mutations.
How can transposons cause mutations?
Transposons can insert or delete the moves from the genome. If a transposons inserts in the middle of a coding sequence, the mutation could disrupt the gene.
What is a nucleotide level mutation? What are the three point mutations?
A nucleotide level mutation occur at the level of a single nucleotide. The three types of nucleotide level mutations are:
Silent mutation. Change in nucleotide has no effect on the final protein synthesized from the gene. This often happens when the changed nucleotide is transcribed is the third nucleotide in a codon because of degeneracy (wobble) in the code.
Missense mutation. Change in nucleotide results in substituting an amino acid in the final protein.
Nonsense mutation. Change in the nucleotide results in substituting a stop codon for an amino acid in the final protein.
What is a frameshift mutation? What is a codon? Reading frame?
Frameshift mutations occur when nucleotides are inserted into or deleted from the genome, causing a shift in the reading frame which usually results in either changes in amino acid sequence or premature truncation of the protein (due to generation of a nonsense mutation). Categorized in insertion or deletion mutations.
This happens because mRNA is transcribed from DNA using three letter sequences called codons and the translation of mRNA is directional.
What is a chromosomal mutation? What are the five common types of chromosomal mutations.
A chromosomal mutation is a larger scale mutation in which large segments of DNA are affected.
Deletion chromosome mutation: section of chromosome is deleted.
Duplication chromosome mutation: section of chromosome is duplicated in same chromosome.
Inversion chromosome mutation: a section is inverted in same chromosome.
Insertion chromosome mutation: segment of DNA is moved from one chromosome to another. Considered frameshift mutations.
Translocation chromosome mutation: occurs when a segment of DNA from one chromosome is swapped with a segment of DNA from another chromosome.
Give an example of advantageous and deleterious mutation. What are inborn errors of metabolism?
An example of an advantageous mutation would be sickle cell disease, a single nucleotide mutation that caused sickle shaped hemoglobin. Heterozygotes for this disease are resistant to malaria as it shortens the lifespan of red blood cells, just short enough for the parasitic Plasmodium cannot reproduce in red blood cells.
An example of deleterious mutation would be XENODERMA PIGMENTOSUM (XP) which is an inherited defect in the nucleotide excision repair mechanism. DNA damage caused by ionizing radiation (UV) cannot be repaired. Patients with XP are frequently diagnosed with malignancies, especially of the skin.
Inborn errors of metabolism are a class of deleterious mutations, specifically in genes required for metabolism. PHENYLKETONURIA as an example.
What is nucleotide excision repair? What disease impacts NER?
Nucleotide excision repair (NER) is a DNA repair mechanism that removes bulky DNA caused by environmental factors such as ionizing radiation and chemicals. Lack of NER can cause xeroderma pigmentosum, among other things, which prevents repair of DNA from repairing itself causing malignancies of the skin.
What is phenylketonuria? What consequence of mutation is it an example of?
PKU is a disease where phenylalanine hydrolase is defective. This prevents complete metabolism of phenylalanine which causes toxic metabolites to build up causing seizures, learning disabilities, and impairment of cerebral function.
PKU is an example of inborn errors of metabolism, a deleterious mutation.
What is genetic leakage? Example?
Genetic leakage is a flow of genes between species. Hybrids can be formed in some cases such as mules and beefalo. Mules cannot reproduce, but beefalo (cattle x bison) can reproduce as their parents have the same number of chromosomes (60).
A) What is the genotypic and phentotypic result from the monohybrid cross of homozygous dominant (PP) and homozygous recessive (pp)?
B) Heterozygous monohybrid cross?
A) The result would be 100% heterozygous dominant and 100% phenotype expressing dominant.
B) The result would be 1:2:1 homozygous dominant: heterozygous: homozygous recessive genotype, 3:1 phenotypic dominant: phenotypic recessive.
What is the genotypic and phenotypic result of crossover of two heterozygotes for a trait with complete dominance? Definitely want to know this handily.
The genotypic result would be 1:2:1 (PP:Pp:pp)
The phenotypic result would be 3:1 (dominant:recessive).
Keep in mind that these ratios are theoretical probabilities and will not always hold true especially for small populations. More offspring means closer to theoretic ratios.
What is a test cross? What is another term for test cross and why is it called that?
A test cross is used to determine an unknown genotypes through deduction by crossing with a known genotype. ***The unknown genotype will be crossed with homozygous recessive. Test crosses are known as back crosses because the parent generation is determined by the filial generation.
Monocross example:
If the unknown was PP (homozygous dominant) then the filial generation (F1) will be 100% genotypic heterozygous dominant and 100% phenotypically dominant.
If the unknown was Pp (heterozygous), then the filial generation (F1) will be 1:1 heterozygous:homozygous recessive, and 1:1 phenotypically dominant:recessive.
What is the expected phenotypic result of a dihybrid cross of two parents that are heterozygous for both traits? Is that associated with another ratio? Definitely need to know this.
TtPp
P purple
p white
T tall
t short
This would be PpTt x PpTt
The result would be 9:3:3:1 phenotypic ratio (purpletall:purpleshort:whitetall:whiteshort)
The 3:1 phenotypic ratio holds for each individual trait:
12 tall : 4 short 12 purple : 4 white
What are sex linked crosses?
Sex linked crosses are crosses that help determine x linked traits. Slightly different system is used to symbolize but the crossing strategy is still the same.
Xh is typically used for the allele with the trait in question, usually a recessive defective allele. Heterozygote females are considered carriers.
What is gene mapping? Recombination frequency? Genetic map? Map unit (centimorgan)?
Gene mapping is a tool used to determine proximity of genes on a chromosome. Recombination frequency is the likelihood that two alleles are separated during crossover (higher frequency is higher likelihood of alleles being separated). Recombination frequency is roughly proportional to the distance between genes.
A genetic map represents the relative distance between genes on a chromosome. One map unit or centimorgan corresponds to 1% chance of recombination occurring between two genes.
Recombination frequencies can be added to approximate the order of genes in a chromosome.
Punnett square questions
Assume HWE and 9% of population is homozygous dominant. Solve for
Frequency of dominant allele.
Frequency of recessive allele.
Proportion of population that is heterozygous.
Proportion of population with a homozygous recessive genotype.
The portion of the population with a dominant phenotype.
Frequency of dominant allele = p
Frequency of recessive allele = q
Proportion of population that is heterozygous = 2pq
Proportion of population with a homozygous recessive genotype = qq
The portion of the population with a dominant phenotype = pp+2pq
What is divergent evolution?
Divergent evolution refers to the independent development of dissimilar characteristics in two or more lineages sharing a common ancestor.
Example would be seals and cats. They are both mammals in the order Carnivora, yet they differ in general appearance. These two species live in a very different environment and adapted to different selection pressures while evolving.
