Genetics Flashcards
Define the terms gene and allele and explain how they differ.
4 marks
- gene is a heritable factor / unit of inheritance
- gene is composed of DNA
- gene controls a specific characteristic / **codes for a polypeptide **/ protein
- allele is a ** form of a gene**
- alleles of a gene occupy the same gene locus / same position on chromosome
- alleles differ (from each other) by one / a small number of bases(s)/ base pair(s)
Describe the consequences of a base substitution mutation with regards to sickle cell anemia.
7 marks
- the sequence of nucleotide bases in DNA codes for the sequence of amino acids in proteins
- DNA is transcribed into mRNA, which is translated into amino acids of protein
- normal (ß chain) hemoglobin gene / DNA produces normal (ß chain) hemoglobin protein / amino acids
- substitution= the replacement of one (or more) nucleotide base with another
- caused by a copying mistake during DNA replication
as a result of a mutagen / X-rays / chemical / UV radiation / other mutagen - mutation in normal (ß chain) hemoglobin gene alters the sequence of nucleotide bases
- normal nucleotide sequence = CTC altered to CAC
resulting in altered mRNA (GAG to GUG) during transcription
resulting in altered sequence of amino acids in (ß chain) hemoglobin protein (glutamic acid to valine) during translation - causing red blood cells to change shape / sickle under low oxygen conditions
- causing sickle cells anemia when two copies of the mutated gene are inherited
- producing a sickle cell carrier when one copy of the mutated gene is inherited
- a sickle cell carrier has malarial protection
- sickle cells anemia reduces oxygen flow to organs, leading to their deterioration
Outline the formation of chiasmata during crossing over.
5 marks
- crossing over/chiasmata formed during prophase I of meiosis;
- pairing of homologous chromosomes/synapsis;
- chromatids break (at same point); (do not accept chromatids overlap)
- non-sister chromatids join up/swap/exchange alleles/parts;
X-shaped structure formed / chiasmata are X-shaped structures;
chiasma formed at position where crossing over occurred;
chiasmata become visible when homologous chromosomes unpair;
chiasma holds homologous chromosomes together (until anaphase)
Explain how an error in meiosis can lead to Down syndrome.
8 marks
- non-disjunction;
- chromosomes/chromatids do not separate / go to same pole;
- non-separation of (homologous) chromosomes during anaphase I;
- due to incorrect spindle attachment;
- non-separation of chromatids during anaphase II;
due to centromeres not dividing;
occurs during gamete/sperm/egg formation;
less common in sperm than egg formation / function of parents’ age;
Down syndrome due to extra chromosome 21;
sperm/egg/gamete receives two chromosomes of same type;
zygote/offspring with three chromosomes of same type / trisomy / total 47 chromosomes;
Karyotyping involves arranging the chromosomes of an individual into pairs. Describe one application of this process, including the way in which the chromosomes are obtained.
5 marks
Application of karyotyping {2 max}
- find gender
- test for Down’s syndrome / other chromosome abnormality
- identify sex chromosomes / numbers of chromosome 21 / other chromosomes counted
XX = female and XY = male / third chromosome 21 indicates Down’s syndrome / other chromosome abnormality (e.g. Klinefelter’s syndrome)
Obtaining chromosomes {3 max}
- fetal cells obtained from amniotic fluid / amniocentesis / other named source
white blood cells obtained
- chorionic sampling
- cells encouraged to divide
- cells accumulated / blocked in metaphase
prepare slide / chromosomes examined
Compare the processes of mitosis and meiosis.
6 marks
- Mitosis: one cell division & Meiosis: two divisions / reduction division
- Mitosis: chromosome number does not change & Meiosis: converts diploid to haploid cells
- Mitosis: products genetically identical & Meiosis: products genetically diverse
- Mitosis: separation of sister chromatids in anaphase & Meiosis: separation of homologous chromosomes in anaphase I and sister chromatids in anaphase II
- Mitosis: no crossing over & Meiosis: crossing over in prophase I
- Mitosis: no formation of tetrads / no synapsis & Meiosis: formation of tetrads / synapsis
- Mitosis: produce cells for growth/repair/asexual reproduction & Meiosis: produce sexual cells / gametes for sexual reproduction
- Mitosis: two cells produced & Meiosis: four cells produced
- Mitosis: daughter cells with both copies of chromosomes/random assortment does not occur & Meiosis: random assortment of maternal/ paternal chromosomes
- Mitosis: replication of DNA in interphase & Meiosis: replication of DNA in interphase I
- Mitosis: four phases: prophase, metaphase, anaphase, telophase & Meiosis: same four phases twice
Outline one example of inheritance involving multiple alleles.
5 marks
- multiple alleles means a gene has three or more alleles / more than two alleles
- ABO blood groups / other named example of multiple alleles
- ABO gene has** three alleles** / equivalent for other example
- AB, IA, IB, and i shown (at some point in the answer) / equivalent for other example
any two of these alleles are present in an individual
homozygous and heterozygous genotye with phenotypes (shown somewhere)
all six genotypes with phenotypes given (shown somewhere)
example / diagram of a cross involving all three alleles
Describe the inheritance of ABO blood groups including an example of the possible outcomes of a homozygous blood group A mother having a child with a blood group O father.
5 marks
example of co-dominance
multiple alleles / 3 alleles
(phenotype) O has (genotype) ii
B can be IB IB or IB i
A can be IA IA or IA i
AB is IA IB
(P are) i i x IA IA
(gametes) i and IA
(F1 genotype) IA i
(F1 phenotype) blood group A
Outline sex linkage.
5 marks
- gene carried on sex chromosome / X chromosome / Y chromosome
- inheritance different in males than in females
- males have only one X chromosome therefore, only one copy of the gene
- mutation on Y chromosome can only be inherited by males
- women can be carriers if only one X chromosome affected
- example of sex linked characteristics (e.g. hemophilia / color blindness)
example of cross involving linkage
Explain, using a named example, why many sex-linked diseases occur more frequently in men than women.
9 marks
- named example of sex-linked disease
- caused by recessive allele
- on the X chromosome
- example of pair of alleles (e.g. X H and X h) (reject if alleles do not correspond)
- females are XX and males are XY
- females have two alleles of the gene and males have only one
- allele causing the disease is rare / uncommon
-** probability of femles inheriting rare allele twice as low**
calculation of squaring the gene frequency - female would have to inherit the allele from her father
- who would have suffered from the disease
so females can carry the gene but still be normal
but males (with the gene) will have the disease
- Describe the methods and aims of DNA profiling.
8 marks
Award 5 max for methods.
- DNA sample obtained;
from hair/blood/semen/human tissue;
DNA amplified / quantities of DNA increased by PCR/polymerase chain reaction;
- satellite DNA/highly repetitive sequences are used/amplified;
- DNA cut into fragments;
- using restriction enzymes/restriction endonucleases;
- gel electrophoresis is used to separate DNA fragments;
using electric field / fragments separated by size;
number of repeats varies between individuals / pattern of bands is unique to the
individual/unlikely to be shared;
Award 4 max for aims.
- forensic use / crime scene investigation;
example of forensic use e.g. DNA obtained from the crime scene/victim compared
to DNA of suspect / other example of forensic use;
paternity testing use e.g. DNA obtained from parents in paternity cases;
- biological father if one half of all bands in the child are found in the father;
- genetic screening;
presence of particular bands correlates with probability of certain phenotype /allele;
other example;
brief description of other example;
Outline a technique for transferring genes between species.
5 marks
- Isolation of gene and vector (by PCR)
- Digestion of gene and vector (by restriction endonuclease)
- Ligation of gene and vector (by DNA ligase)
- Selection and expression of transgenic construct
gene of interest is cut out
with restriction enzyme
RNA used to produce DNA
using reverse transcriptase
plasmid cut open with same restriction enzyme
gene inserted into plasmid
blunt ends / sticky ends
spliced together by DNA ligase
recombinant plasmids are cloned / many copies produced
recombinant plasmids are inserted into new host cells / virus / bacteriophage / yeast
inserted by shooting / spraying / microencapsulation / by heat treatment
Describe the technique for the transfer of the insulin gene using E. coli.
6 marks
- mRNA is extracted
- DNA copy of RNA is made using reverse transcriptase
- plasmids are cut open with endonucleases (at specific sequences)
- insulin gene and plasmid are mixed together
addition of ‘sticky ends’ to the DNA copy (so that it will combine with the cut plasmid) -
DNA ligase will seal the plasmid
recombinant plasmid is inserted into E. coli - E. coli is cultured
- E. coli begins to make insulin
Discuss the potential benefits and possible harmful effects of genetic modification.
7 marks
- named example of desired outcome e.g. herbicide resistance
Possible benefits: 4 max
- benefits include more specific (less random) breeding than with traditional methods
- faster than traditional methods
- some characteristics from other species are unlikely in the gene pool / selective breeding cannot produce desired phenotype
- increased productivity of food production / - less land required for production
- less use of chemical (e.g. pesticides)
- food production possible in extreme conditions
less expensive drug preparation
Possible harmful effects: 4 max
- some gene transfers are regarded as potentially harmful to organism (especially animals)
- release of genetically engineered organisms in the environment
- can spread and compete with the naturally occurring varieties
- some of the engineered genes could also cross species barriers
- technological solution when less invasive methods may bring similar benefits
- reduces genetic variation / biodiversity
Discuss the ethical arguments for and against the cloning of humans.
4 marks
arguments against cloning: 3 max
- reduces the value / dignity of the individual / causes psychological problems
- high miscarriage rates / cloned individuals are likely to have developmental disorders / health problems / cloned individuals may show premature aging
- costly process and money could be better spent on other types of healthcare
- cloning may be done for inappropriate motives / replace lost loved one / perfect race etc.
arguments for cloning: 3 max
- identical twins are formed by cloning so it is a natural process
- cloned embryos can be tested for genetic disease / genetic screening
- increased chance of children for infertile couples
- cloning research may lead to spin-offs for other research areas such as cancer / transplant research / regeneration research
