3 — genetics Flashcards
explain the need for halving the chromosome number during a sexual life cycle and how this is done (7)
- chromosome number is halved so the offspring has the same number as the parent
- process is meiosis
- DNA replicate
- homologous chromosomes pair in prophase I
- separated in anaphase I into two cells
- after meiosis I cells are haploid
- in meiosis II, chromatids are separated
- result is 4 haploid cells
- each gamete is genetically unique
- uniqueness is due to crossing over/ independent assortment/ random alignment of chromosomes
- fertilisation results in the formation of a diploid zygote
- fertilisation results in variation in a population
explain the two types of inheritance, using the example of parents that are heterozygous for two genes A and B (7)
- unlinked genes are on different chromosomes
- unlinked alleles are inherited independently during meiosis
- in unlinked inheritance, there is an equal chance for all 4 options to occur
- dihybrid crosses involving linked genes do not produce mendelian ratios
- excluding recombinant, there is a 1:1 chance of inheriting the different options
- in linked characteristics, alleles might not migrate together if there is crossing over
- crossing over occurs in prophase I of meiosis
- when the sister chromatids migrate in meiosis II, the characteristics forming gametes are different
- formation of recombinant causes changes in ratio of inheritance
- genes which are linked but are far apart on the chromosome can display independent assortment
outline the inheritance of blood types in the ABO blood system in humans (4)
- i, IA and IB are alleles
- IA and IB are dominant and i is recessive
- group O with ii, group A with IAi/ IAIA, group B with IBi/ IBIB
- IA and IB are codominant -> group AB
- one allele of the gene inherited from each parent
outline what occurs in cells in the first division of meiosis (5)
- halves the chromosome number
- at the start of meiosis, each chromosome, consisting of two sister chromatids, are attached by a centromere
- prophase — pairing of homologous chromosomes
- crossing over occurs
- chromosomes condense by supercoiling
- metaphase — pairs of homologous chromosomes move to the equator
- anaphase — spindle fibre pulls chromosomes to opposite poles
- telophase — arrival of centromere with sister chromatids at opposite poles
outline how two parents could have a child with any of the 4 ABO blood groups (4)
- heterozygous for blood group A
- heterozygous for blood group B
- genotypes of offspring are — IAIB, IBi, IAi, ii
- phenotypes of offspring are — AB, B, A, O
outline, with examples, different types of inheritance that produce non-mendelian ratios (4)
- some traits may involve many genes/ is polygenic e.g. height, skin colour
- linked genes/ alleles of different genes on the same chromosome
- small numbers of recombinant phenotypes due to crossing over
- co-dominance of specific alleles e.g. pink flowers from red and white flowers
- sex linked effects e.g. colour blindness
environmental influence on inheritance
describe the process of crossing over (2)
- occurs during prophase I/ during meiosis
- homologous chromosomes form bivalents
- breakage and rejoining of chromatids
- exchange of DNA between non sister chromatids
outline natural methods of cloning in some eukaryotes (3)
- clones are genetically identical organisms
- asexual reproduction in plants
- common in non vertebrates e.g. budding in hydra
budding in yeast/ fungi - identical twins in humans
draw a labelled diagram of the formation of a chiasma by crossing over (3)
- crossing over between homologous chromosomes
- centromere drawn and labelled
- single strand break/ DNA cut between homologous chromosomes
- non sister chromatids labelled
outline how translation depends on complementary base pairing (3)
- translation converts a sequence of mRNA codons to a sequence of amino acids
- nucleotides on tRNA pair with complementary nucleotides on mRNA
- base pairing occurs when adenine pairs with uracil and guanine pairs with cytosine
- mRNA has codons and tRNA has anticodons
describe the polymerase chain reaction, including the role of Taq DNA polymerase
- PCR is a process by which a small sample of DNA can be amplified
- involves repeated cycling through high and lower temperatures to promote melting and annealing of DNA strands
- mixture is heated to high temperatures to break hydrogen bonds between strands of DNA
- Taq DNA polymerase can withstand high temperatures without denaturing
- primers bind to targeted DNA sequences at lower temperatures
- Taq DNA polymerase forms new DNA by adding complementary nucleotides
explain benefits and risks of using genetically modified crops for the environment and for human health (8)
environment benefits
- pest resistant crops can be made
- less spraying of pesticides
- less fuel burned in management of crops
- longer shelf life for fruits and vegetables -> less spoilage
- higher crop yield
- increase variety of growing locations
environment risks
- non target organisms can be affected
- genes transferred to crop plants to make them herbicide resistant could spread to wild plants making superweeds
- GMOs reduces biodiversity
- GM crops encourage overuse of herbicides
health benefits
- nutritional value of food improved by increasing nutrient content
- crops could be produced that lack toxins or allergens
- crops could be produced to contain edible vaccines to provide natural disease resistance
health risks
- proteins from transferred genes could be toxic or cause allergic reactions
- antibiotic resistance genes used as markets during gene transfer could spread to pathogenic bacteria
- health effects of exposure to GMO unclear
outline how proteins can be separated by gel electrophoresis (3)
- gel electrophoresis involves separating molecules according to their size and charge
- proteins differ in size
- place protein sample in a well in a block of gel
- place the gel in an electrical field
- proteins move through the gel
- separated according to size
- size markers used
explain the process of genetically modifying bacteria (8)
- genetic modification carried out by gene transfer between species
- genes transferred from one organism to another produce the same protein
- due to universality of genetic code
- mRNA for required gene transfer extracted
- DNA copies of mRNA made using reverse transcriptase
- PCR used to amplify DNA to be transferred
genres transferred from one species to another using a vector - plasmid acts as vector to transfer genes to bacteria
plasmid cut open at specific base sequences using restriction endonuclease - cut plasmids mixed with DNA copies stick together due to complementary base pairing
- DNA ligase makes sugar-phosphate bonds to link nucleotides of gene with those of plasmid
- bacteria that take up plasmid are identified
genetically modified bacteria will reproduce carrying the transferred gene
discuss briefly the potential risks and benefits of the genetic modification of crop plants (3)
risks:
- harm to wild species because GMO might compete with the wild species, removing it from the ecosystem
- spread of the transferred gene has resulted in farmed being sued for growing GMOs without permission
- consumer resistance to genetically modified crop plants can result in countries experiencing famine not accepting seeds from aid organisations
- some nutritional improvements may result in allergens being introduced
- resistance to herbicides is used to kill weeds around crops and this herbicide can cause damage from overspray
benefits:
- higher yields can feed more people
- resistance to pests because less pesticide use
- resistance to herbicides results in farmers being able to use herbicides to kill weeds without causing damage to the crops
- improved nutritional quality
- increased tolerance to drought
- reducing land use for crops makes the land available for other uses
