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A2 biology 5.1.2
OCR biology - meiosis and variation
| Question | Answer |
|---|---|
| What is meiosis? | A reduction division - 1 diploid nucleus divides twice to form 4 haploid daughter nuclei that are genetically different to each other and the parent nucleus |
| Define: homologous chromosome | Pair of chromosomes, 1 maternal and 1 paternal, same genes at same loci but alleles differ |
| Define: allele | Different versions of the same gene, occupy same locus but are expressed as slightly different polypeptides |
| Define: crossing over | Prophase 1: non-sister chromatids within a bivalent swap sections of genetic material with each other, recombination of maternal/paternal alleles, by breaking and rejoining at chiasmata |
| Define: locus | A specific position on a chromosome, occupied by a particular gene |
| Describe prophase I | Sister chromatids supercoil, homologous chromosomes form a bivalent when non-sister chromatids join at 2-3 chiasmata, crossing over, nuclear envelope disintegrates, nucleolus disappears, centriole divides in 2 = spindle (microtubules) |
| Describe metaphase I | Bivalents line up at spindle equator, attach by centromere. Random reassortment: random distribution of maternal/paternal chromosomes results in gametes having different combinations of maternal/paternal alleles, 2^n possibilities alone, chiasmata |
| Describe anaphase I | Chiasmata separate, spindles contract, chromosomes pulled to opposite poles of cell (random segregation) |
| Describe telophase I | Only in animal cells: separated chormosomes reach opposite poles of cell, nuclear envelope reofrms, nucleoli appear, cytokinesis, brief interphase |
| Where does meiosis II occur? | 90 degrees to meiosis I |
| Describe prophase II | Nuclear envelope disintegrates, nucleolus disappears, centriole divides in 2 and daughter centrioles go to opposite poles of cell to synthesis spindle |
| Describe metaphase II | Sister chromatids line up at spindle equator, centromere. Random reassortment: random distribution of sister chromatids results in gametes with different combination of non-identical sister chromatids (different alleles due to corssing over P1), 2^n |
| Describe anaphase II | Centromere divides, spindles contract, chromatids move to opposite poles of cell (random segregation) |
| Describe telophase II | Chromatids reach poles, nuclear envelopes reform, nucleoli appear, chromosomes uncoil, cytokinesis, spindle breaks down, animal: 2 cells divide to form 4 haploid cells, plant: forms a tetrad of 4 haploid cells |
| What is the difference between sister and non-sister chromatids | Sister chromatids are genetically identical - same alleles. Non-sister chromatids have same genes at same loci but alleles differ |
| What is the role of chiasmata? | Holds bivalents together, allows crossing over P1, ensures 1 chromosome in each homologous pair goes to each pole of the cell during A1 |
| What is the purpose of meiosis? | Occurs in gonads, produces haploid gametes so a dilpod zygote is formed in fertilisation, Asexual reproduction -otherwise, quantity of genetic material would double per generation. Increases genetic variation |
| Why is genetic variation so important and how is it achieved through meiosis? | Increases chance of evolution-natural selection favours organisms best adapted to changing environment. Crossing over P1, random reassortment M1/2. Fertilisation of oocyte by 1 of 300 sperm-random combination of 2 sets of genetic material. Mutations |
| Describe each type of chromosome mutation | Inversion (section of chromosome rotated 180), deletion (section of chromosome lost), translocation (section of chromosome attaches to another), non-disjunction (incorrect separation of chromosomes anaphase = gametes w. incorrect no. of chromosomes |
| What is asexual reproduction? | Mitosis in eukaroytes, binary fission in prokaryotes, daughter cells are genetically identical to each other and parent cell, variation is only introduced by chromosome mutation |
| What are the features of discontinuous variation? | Qualitative differences between phenotypes, distinct categories, can be monogenic/ controlled by several genes which interact in an epistatic way, different alleles at same gene locus have a small effect on phenotype, not influenced much by environment, h |
| What are the features of continuous variation? | Qualitative differences between phenotypes, no distinct categories, controlled by several unlinked polygenes -provide an additive component to phenotype for polygenic trait, different alleles at same locus have small effect, influenced by environment |
| Define: dominant and recessive | Dominant allele: always expressed in phenotype even if a different allele for same gene is present in genotype. Recessive allele: only expressed in phenotype in presence of identical allele/ absence of dominant allele for that gene |
| Define: heterozygous, homozygous, hemizygous | Heterozygous: 2 different alleles for a particular gene. Homozygous: 2 identical alleles for a particular gene. Hemizygous: only 1 copy of a gene. |
| Define: genotype, phenotype | Genotype: alleles in cells of individual for a particular trait. Phenotype: observable characteristics of an individual, controlled by genotype + environment |
| Dihybrid inheritance: 1 parent is pure breed recessive, 1 is heterozygous for both traits, how many outcomes? | 4 |
| Dihybrid inheritance: both parents are heterozygous, ratio of outcomes? | 9:3:3:1 |
| Describe codominance | Both alleles are expressed in the phenotype of a heterozygote. Blood group (A and B are codominant, O is recessive), cattle coat colour (red and white are codominant CrCw is roan), sickle cell anaemia (HaHi is symptomless) |
| Describe sex-linkage | Gene present on sex chromosome, usually X-linked as unpaired segment of X-chromosome is larger. Red-green colour blindness, DMD, haemophilia B |
| Why are human males more likely to express recessive X-linked traits than human females? | Human males are heterogametic + hemizygous (can't be carriers). More likely to express recessive trait as father's genes have no effect, only condition is that mother must be carrier/ sufferer. Females are homogametic, father must be sufferer as well |
| In which species are the females heterogametic and males homogametic? | Birds, butterflies, moths |
| What is autosomal linkage? | Several genes on same autosome are linked so are usually inherited together- offspring phenotypes same as parental phenotypes. If chiasmata form between linked genes + crossing over during prophase I, recombinants form. |
| What reduces the chance of recombinants forming? | If linked genes are close together on the chromosome there is a lower chance of chiasmata forming between them |
| What are the expected ratios for autosomal linkage with a) 2 heterozygotes b) 1 heterozygote, 1 pure breed dominant c) 1 heterozygote, 1 pure breed recessive? | 2 heterozygotes = 3:1 + recombinants. 1 heterozygote, 1 pure breed dominant = 100% dominant. 1 heterozygote, 1 pure breed recessive = 1:1 + recombinants. |
| What is epistasis? | Interaction between unlinked genes at different loci concerned with the expression of a single characteristic, epistatic gene masks/influences expression of hypostatic gene (codes for repressor protein/ transcription factor), antagonistic/ complementary |
| What are the expected ratios for dominant epistasis? | 2 heterozygotes = 12:3:1/ 13:3. 1 heterozygote, 1 pure breed dominant = 100% dominant. 1 heterozygote, 1 pure breed recessive = 2:1:1 |
| What are the expected ratios for recessive epistasis? | 2 heterozygotes = 9:3:4. 1 heterozygote, 1 pure breed dominant = 100% dominant. 1 heterozygote, 1 pure breed recessive = 1:1:2 |
| Why does epistasis reduce phenotypic variation? | |
| Define: polygenic inheritance | The inheritance of a trait that is controlled by more than 2 genes |
| What is the function of the chi-squared test? | Finds whether the difference between observed and expected outcomes for categorical data (e.g. numbers of offspring phenotypes) is statistically significant or due to chance alone |
| How do you work out the degrees of freedom? | Number of classes - 1 |
| What does it mean if your chi-squared value is greater than the critical value? | p< 0.05 so null hypothesis/ genetic ratio is rejected, differences between the observed/ expected outcomes for offspring phenotypes are not just due to chance, mutations/ crossing over/ particular genotype is harmful |
| What criteria must be fulfilled for the chi-squared test to work? | Large population, no 0 values, raw counts only |
| Define: gene pool | The set of genetic information carried by a population |
| What affects the gene pool? | Population size, migration, mutation, selection, randomness of mating, genetic drift, competition |
| Define genetic drift | Chance changes in allele frequency between generations, can eliminate an allele from a population, can cause extinction/ speciation. Largest effect in small populations, can cause evolution e.g. if certain individuals have better breeding success |
| What is the purpose of the hardy-Weinberg principle? | Allows you to approximate allele/genotype frequencies in a population for dominant/ recessive alleles, can't be determined directly form phenotype frequency because homozygous dominant + heterozygous genotypes = dominant phenotype |
| What has to be assumed to use Hardy-Weinberg equations? | Stable allele frequencies (hardy-Weinberg equilibrium): large population, no migration, no mutation, no selection, random mating. Not achieved in reality e.g. mutations, mating isn't at random, migration, termination of pregancies |
| What are the hardy Weinberg equations? | p - frequency of dominant allele, q - frequency of recessive allele, p2 - frequency of homozygous dominant genotype, q2 - frequency of homozygous recessive genotype, 2pq - frequency of heterozygotes. p+q=1. p^2 + 2pq + q^2 = 1 |
| Why do most populations in nature grow then remain at a fairly constant size? | Population size fluctuates around carrying capacity. Not all offspring reach reproductive age, environmental resistance e.g. abiotic factors (water, light, minerals, space) and biotic factors (pathogens, predators, food, mates). Intraspecific competition |
| What is a selection pressure? | An environmental factor that grants individuals in a population best adapted to their environment an increased chance of surviving, reproducing and passing on advantageous alleles to offspring |
| What are the 2 types of natural selection? | Stabilising-allele frequency remains constant as species is adapted to environment. Directional- allele frequency changes as a change in environment creates new selection pressures that grant a selective advantage to individuals with different variations |
| What is speciation? | Formation of a new species due to a barrier (isolating mechanism) - geographical e.g. river (allopatric), temporal/ seasonal, reproductive e.g. sexual organs/ courtship dance(sympatric). Sub-populations evolve - natural selection + genetic drift |
| What is the biological species concept + issues with it? | Species= a group of organisms with similar physiology/ morphology that can interbreed to produce fertile offspring. Problems: can't classify species that reproduce asexually/ fossils/ remains, some individuals in same species look very different |
| What is the phylogenetic species concept? | Species =group of organisms with similar physiology/ morphology that have descended from a common ancestor, ignores reproduction |
| Define: natural selection | The environment provides a selection pressure which grants individuals in a population best adapted to their environment an increased chance of surviving, reproducing and passing on advantageous alleles to offspring |
| Define: artificial selection | Humans provide selection pressure, grants some individuals an increased chance of surviving. Selective breeding: select, isolate, breed individuals with advantageous alleles, breed offspring with desirable traits, frequency of desirable alleles increases |
| Compare and contrast natural and artificial selection | Natural: environment provides selection pressure, allows species to adapt to environment, slow. Artificial: humans provide selection pressure, commercial value, fast. Both: exploit genetic variation in a population, change allele frequency |
| Describe how artificial selection has been used to produce the modern dairy cow | Measure milk yield of cows, select elite cows. Test semen of bulls+ store semen of bulls that produced elite daughters. Give cows hormones, fertilise egg in vitro with semen from bull (AI), clone if necessary, implant into surrogate mother uterus |
| What are the advantages of artificial insemination? | Semen of 1 bull can be used to inseminate many cows, transported easily, stored for a long time, can take a long time to determine whether or not a bull has advantageous alleles |
| Describe how artificial selection been used to produce bread wheat with a high wheat yield | Select+breed wheat plants (triticum aestivum) with high wheat yield, prevent cross pollination (isolate, bags, transfer pollen by hand, remove stamen), breed offspring with high wheat yield, wheat undergoes polyploidy (hexaploid, 42, large nuclei, cells) |
| How is wheat being improved today? | Artificial selection programmes : improve resistance to disease/ pests/ lodging (stems bend in rain), increase protein content (better for bread) |
| Why does selective breeding for 1 trait often result in the selection of several traits? | Genes may be linked. Selective breeding involves whole genomes so it is likely that other traits will follows. |
| When do small populations occur in nature? | Natural disaster cuases a population bottleneck |
| What is the danger of inbreeding? | Increases homozygosity, can cause inbreeding depression (recessive, deleterious alleles are more likely to be expressed in offspring) which reduces biological fitness of population |
| Why may some isolated communities have much higher prevalences of genetic disorders? | Founder effect: population has a disproportionate frequency of certain allele because original few settlers had less genetic variation than wider population/ non-representative sample of alleles e.g. Africaners in SA (dutch origin, huntingdon's disease) |
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