Chapter 16 Inheritance Essay QS Flashcards
Explain similarities & differences between homologous chromosomes. (8)
Similarities:
- same genes coding for the same characteristics
- same loci/genes in same position
- same size/length
- same centromere position
Differences:
- alleles
- parent of origin
- base/nucleotide sequence
Describe the FIRST division of meiosis (meiosis I) in animal cells. (6)
- reduction division / (to) halve number of chromosomes / diploid to haploid
- homologous chromosomes pair up / bivalents form
- ref. chiasmata / ref. crossing over
- homologous chromosome pairs / bivalents, line up on equator
- INDEPENDENT ASSORTMENT
- spindle / microtubules, attached to centromeres
- chromosomes of each pair pulled to opposite poles
- by shortening of, spindle / microtubules
- nuclear envelopes re-form
- cytokinesis
Outline the behaviour of chromosomes during meiosis. (9)
DO NOT CREDIT MARKING POINTS OUT OF SEQUENCE.
Prophase 1:
- idea of condensation of chromosomes
- homologous chromosomes pair up / bivalent formed
Metaphase 1:
- homologous chromosomes / bivalents, line up on equator
- of spindle
- by centromeres
- independent assortment / described
- chiasmata / described
- crossing over / described
Anaphase 1:
- chromosomes move to poles
- homologous chromosomes / bivalents, separate
- pulled by microtubules
- reduction division
Metaphase 2:
- chromosomes line up on equator
- of spindle
Anaphase 2:
- centromeres divide
- CHROMATIDS move to poles
- pulled by microtubules
- ref. haploid number
Describe the behaviour of chromosomes during meiosis. (10)
Phases must be mentioned:
- Chromosomes, condense / thicken (prophase I)
- Homologous chromosomes pair up
- Chiasmata formation & crossing over of non-sis chromatids of homologous chromosomes
- Bivalents, lined up/arranged, on equator (metaphase)
- Independent / random, assortment (of homologous chromosomes on equator)
- No. of possible chromosome combinations = 2^n
- Chromosomes are pulled apart by microtubules
- (single) chromosomes / pairs of chromatids, line up on, equator / mid-line (metaphase II)
- at right angles to first equator
- centromeres divide (anaphase II)
- sis chromatids separate (and become chromosomes) (anaphase II)
- chromosomes uncoil & elongate (telophase I & II)
- ref. to haploid / chromosome number halved / one set of chromosomes
Describe how crossing over & independent assortment can lead to genetic variation. (9)
- occur during MEIOSIS I
Crossing over:
- between non-sister chromatids
- of, (a pair of) homologous chromosomes
- in PROPHASE I
- at chiasma(ta)
- exchange of genetic material
- LINKAGE GROUPS broken
- new combinations of ALLELES (within each chromosome)
Independent assortment:
- of homologous chromosome pairs
- each pair lines up independently of others
- line up on equator
- (during) METAPHASE I
- results in gametes that are genetically unique
Explain how meiosis & fertilisation can result in genetic variation amongst offspring. (8)
Meiosis (max 7)
- chiasma / crossing over
- between NON-sister CHROMATIDS
- of, homologous chromosomes
- in PROPHASE I
- exchange of genetic material / DNA
- linkage groups broken
- new combination of alleles
- INDEPENDENT ASSORTMENT, of homologous pairs / bivalents
- (during) METAPHASE I
- 2^n combinations
Marking points for QS without “amongst offsprings”:
- independent assortment of, sister chromatids / chromosomes, at metaphase II
- possible (chromosome) mutation
Fertilisation:
- random mating
- random fusion / fertilisation of gametes
Describe, with examples, how the alleles at one gene locus may interact with each other. [6]
- (Complete) dominance;
- Only one allele of heterozygote affects phenotype;
- Phenotype of heterozygote same as one homozygote;
- Recessive allele must be homozygous to appear in phenotype;
- Example/symbols;
- Codominance;
- Both alleles of heterozygote affect the phenotype/ functional protein;
- Example/symbols;
- Multiple alleles;
- Dominance hierarchy;
- Example/ symbols;
Explain what is meant by the terms linkage and crossing-over. [8]
Linkage
1. 2 or more genes on same chromosome;
2. do not assort independently in meiosis;
3. inherited together;
4. number of linkage groups = number of pairs of homologous
5. chromosomes/ haploid number of chromosomes;
6. genes closer together less likely to be separated by crossing-over;
Crossing-over [max 5]
7. prophase meiosis I;
8. during synapsis;
9. chromatids of a bivalent break;
10. rejoin with non-sister chromatid;
11. exchange between paternal and maternal chromatids;
12. of alleles;
13. diagram;
14. ref. chiasma;
15. ref. cross over value;
16. genes closer together less likely to be separated by crossing over;
Explain how the allele for haemophilia may be passed from a man to his grandchildren. You may use genetic diagrams to support your answer. [7]
- (haemophilia) allele on X chromosome ; A gene
- sex-linked ;
- (haemophilia) allele recessive ;
- man, homogametic / has one X chromosome ;
- Y chromosome does not have blood clotting gene ;
- only daughter(s) get his X chromosome ;
- daughter(s) carrier(s) of (haemophilia) allele ;
- grandson(s) 50% chance of having, (haemophilia) allele / haemophilia ;
- granddaughter(s) 50% chance of carrying, (haemophilia) allele ;
Allow following marks from diagram
10. correct symbols ; e.g. XH and Xh explained
11. man’s genotype ; e.g. XhY ignore partner’s genotype
12. F1 (daughter’s) genotype ; e.g. XHXh ignore her partner’s genotype
13. F2 (grandson’s) genotypes ; e.g. XhY XHY both required
14. F2 (granddaughter’s) genotypes ; e.g. XHXH XHXh both required or XhXh XHXh
Explain how the presence of a mutant allele can result in albinism. [7]
1 TYR gene codes for enzyme tyrosinase
2 normal gene product is tyrosinase
3 tyrosine converted to, DOPA / dopaquinone
4 melanin / pigment, made ; ora
5 in melanocytes ;
6 mutant allele is recessive ;
7 tyrosinase, not produced / inactive ;
8 affects, hair / skin / irises ;
9 only in homozygous recessive people
Outline the effects of mutant alleles on the phenotype in albinism and haemophilia. [7]
Albinism (max 4):
1 caused by recessive (allele) ;
2 (mutant allele) affects production of tyrosinase / causes production of faulty tyrosinase
3 results in, absence / reduced production of, melanin ;
4 pale / white, hair or skin ;
5 pink eyes ;
6 increases susceptibility to, sunburn / skin cancer
Haemophilia (max 4):
7 caused by recessive (allele) ;
8 factor VIII / factor IX, not produced ;
9 gene / allele, is carried on X chromosome ;
10 sex-linked ;
11 prevents / reduces, clotting of blood ;
12 description of symptoms ;
e.g. excessive bleeding
bleeding into joints
large bruises
internal bleeding
Describe Huntington’s disease (HD) in humans and explain how it is inherited. [8]
Max 4:
1. involuntary muscle movement/ chorea;
2. mental deterioration;
3. brain cells lost;
4. ventricles enlarge;
5. (commonly) onsets in middle age;
Max 6:
6. dominant allele;
7. autosomal/ chromosome 4;
8. most sufferers heterozygotes;
9. 1 in 2 chance of passing on condition;
10. stutter;
11. CAG (triplet) repeat;
12. sufferers have > 37/37 - 100, repeats;
13. more repeats earlier onset;
14. increased number with each generation;
15. inheritance from male and female different;
16. not truly Mendelian
Note: ‘Option’ paper syllabus was more detailed than 2022 syllabus. You may find some points
that are not in the 2022 syllabus/ textbook.
Explain how different types of gene mutation can affect the phenotype and outline the effects of the mutant alleles that cause Huntington’s disease on the phenotype of a person. [9]
Gene mutation
1 base substitution ;
2 (often) does not have a significant effect on phenotype / silent mutation ;
3 base, insertion / deletion leads to, frame shift / described ;
4 (so) has significant effect on phenotype ;
5 change in, primary structure / amino acid sequence
6 change in, tertiary structure / 3D shape / folding ;
7 loss of function in protein or enzyme / example described ;
8 (premature) stop codon ;
Huntington’s disease:
9. (mutant allele) is dominant ;
10. HD / dominant, allele has more repeats of base triplet CAG (than normal) ;
11. heterozygote will have disease ;
12. brain cells die more rapidly (than normal) / brain degeneration ;
13. involuntary movements / mental deterioration or described / mood changes ;
14. onset in middle age / idea that no change in phenotype in earlier life ;
15. AVP ; e.g. greater number of CAG repeats affects, earlier onset / severity of disease
Describe the genetic control of protein production in a prokaryote using the lac operon. [7]
1 ref. to regulatory gene ;
2 codes for repressor protein ;
3 (repressor protein) binds to operator ;
In presence of lactose:
4 lactose binds to repressor protein ; A allolactose
5 (repressor protein) changes shape ;
6 (repressor protein) no longer binds to operator ;
In absence of lactose:
7 repressor protein blocks promoter
8 RNA polymerase cannot bind to promoter
9 (named) gene cannot be transcribed / mRNA not synthesised
10 enzymes / named enzyme, cannot be synthesised
Using named examples, describe the differences between structural and regulatory genes and the differences between repressible and inducible enzymes. [9]
Structural genes:
1 code for, non-regulatory / structural, proteins / polypeptides ;
2 named example of structural gene ; e.g. lac Z / lac Y / lac A
3 (proteins associated with) rRNA / tRNA ;
4 (proteins such as) enzyme / named (structural) protein
Regulatory genes:
5 code for, regulatory / non-structural, proteins / polypeptides ;
6 named example ; e.g. gene coding for repressor protein / lac I / PIF / correct ref. DELLA protein / gene for transcription factors
7 detail ; e.g. switches genes on or off / ref. gene expression / ref. transcription ;
Repressible enzymes:
8 (generally) produced continuously ;
9 synthesis can be prevented by binding of repressor protein to, specific site / promoter / operator ;
10 named example ; e.g. enzyme involved in tryptophan synthesis
Inducible enzymes:
11 synthesis only occurs when, substrate / inducer, is present ;
12 idea that transcription of the gene only occurs when, substrate / inducer, binds to, transcription factor / repressor
protein;
13 named example ; e.g. β galactosidase / lactose permease / transacetylase
