Assume that a female with Down syndrome who carries three copies of chromosome 21 could successfully reproduce with a normal 46­chromosome male. Draw the arrangement of the three chromosome 21s from the female at metaphase I of meiosis I, ignoring all the other chromosomes

 

  1. Assume that a female with Down syndrome who carries three copies of chromosome 21 could successfully reproduce with a normal 46­chromosome male.
    1. Draw the arrangement of the three chromosome 21s from the female at metaphase I of meiosis I, ignoring all the other chromosomes. (1 mark)
    2. What proportion of the offspring would be expected to have Down syndrome? Justify your answer. (2 marks)

50%. The segregation at anaphase I will result in 50% of the gametes carrying two 21 chromosomes and 50% carrying one 21 chromosome.

  1. A pure breed of rabbits with normal ears and normal teeth is introduced into a new habitat and successfully reproduces with the local pure breed which are characterized by floppy ears and overgrown teeth. All of the F1 progeny have floppy ears and normal teeth.
    1. Using ‘F’ (dominant) or ‘f’ (recessive) for floppy ears and ‘M’ (dominant) or ‘m’ (recessive) for overgrown teeth, write the genotypes of the parents of this cross and the F1 progeny. (1 mark)

P: ffMM X FFmm

F1: Ff Mm

  1. If F1 males and females were crossed, how many F2 progeny with normal ears and overgrown teeth would be expected out of a total of 64? Assume that the two genes are on different chromosomes. Show your working. (3 marks)

BIO00007C

F1 cross: Ff Mm X Ff Mm

Ears                    Teeth

¾ floppy ears             ¾ normal teeth

¼ normal ears         ¼ overgrown teeth

Probability of normal ears and overgrown teeth: 1/16, hence 64/16=4

  1. The newly introduced rabbits are black with fluffy hair and the local rabbits are albino with normal hair. All F1 rabbits are black with normal hair. Using the symbols ‘B’ (dominant) or ‘b’ (recessive) for fur colour and ‘A’ (dominant) or ‘a’ (recessive) for type hair, write the genotypes for these two genes for the parents and their F1 progeny. (1 mark)

P: BBaa X bbAA

F1: BbAa

  1. If F1 rabbits were crossed to bbAaMm rabbits, assuming all genes involved are on different chromosomes, what fraction of the offspring would be of the bbAamm genotype? (3 marks)

Cross: BbAaMm X bbAaMm

bb= 1/2 Aa=1/2 mm=1/4

Hence, 1/16

  1. Triple heterozygous female mice for the linked recessive traits y, t, and h were crossed to males that phenotypically express y, t, and h. The following phenotypic ratios were obtained in the progeny (+ denoting the dominant trait of each gene):

+ t h             385 y t h               85

y + h               18 + t +         20

+ + +               90 + + h          1 y + +           401

No other phenotypes were observed.

  1. Classify each class according to the type of crossover that originated them. (1 mark)

+ t h 385 NO crossover y + + 401   NO crossover y t h     85   Single CO + + +     90      Single CO y + h     18          Single CO + t +   20 Single CO

+ + h       1     Double CO

  1. Which gene is in the middle? (1 mark)

t

  1. Calculate the genetic distances between the genes. (2 marks)

y­t = (85+90+1)/1000=0.176=17.6cM

t­h= (18+20+1)/1000=0.039=3.9cM

BIO00007C

  1. The pedigree below shows an individual affected by a disease inherited in an autosomal recessive pattern. Calculate the probability that II.3 will be a boy heterozygous for the mutation that causes this disease. (2 marks)

p=1/2 (boy) *2/4 (het) =1/4

  • a) Provide a short definition for each of the following terms:
  1. i) Spliceosome (1 mark)

A nuclear RNA­protein complex responsible for removing introns from pre­mRNA

  1. ii) Charged tRNA (1 mark)

A tRNA attached to its amino acid

iii) Shine­Dalgarno sequence (1 mark)

Ribosome binding site in prokaryotic mRNAs

  1. Put the following events relating to the expression of a typical eukaryotic gene in the correct order:

Translation; Transcription initiation; RNA export from the nucleus; Splicing (1 mark)

Transcription initiation; Splicing; RNA export from the nucleus; Translation

  1. A frameshift mutation has occurred near the end of an exon of a eukaryotic gene. Will it affect the reading frame of the next exon in the mRNA? Explain your answer. (2 marks)

Yes. Exons do not represent individual open reading frames and so shifting the reading frame in one exon will affect everything from that point in the mature mRNA.

  • a) In translation, which codon(s) are able to pair with a tRNA with the anticodon 5’­IAU­3′, given that I (inosine) can pair with A or U or C? (1 mark)

5’­AUA­3′

5’­AUU­3′

5’­AUC­3′

  1. Which amino acid(s) would be carried by the tRNA with an anti­codon 5’­IAU­3′? A genetic code table is provided on the next page. (1 mark)

Ile

  1. Identify which of the following amino acid changes could result from a mutation involving the substitution of one base for a different base? (3 marks)
    1. Leu­to­Gln (Yes CUN­to­CAA/G)
    2. Phe­to­Ser (Yes UUU/C­to­UCU/C)
  • Val­to­Cys (No GUN and UGU/C)

 

  1. The enz operon codes for two enzymes (ENZ1 and 2) and is regulated by compound X. Mutations in 4 regions of the operon (termed A, B, C and D) have the following effects where a plus sign (+) signifies that the indicated enzyme is synthesized and a minus sign (­) signifies that it is not.
Compound X Absent Compound X Absent Compound X Present Compound X Present
ENZ1 ENZ2 ENZ1 ENZ2
Wild­Type + + ­ ­
Mutation in A ­ + ­ ­
Mutation in B + + + +
Mutation in C + ­ ­ ­
Mutation in D ­ ­ ­ ­

 

  1. Is the enz operon induced or repressed by compound X in the wild­type? (1 mark)

Repressed.

  1. State which region (A, B, C or D) corresponds to the following components of the enz operon and provide a brief explanation for your answer. (4 marks)
    1. Regulator gene
    2. A mutation in B results in a failure to repress the operon
    3. Promoter
    4. A mutation in D results in no expression.
  • Gene encoding Enzyme 1
  1. A mutation in A results in no expression of ENZ1 but ENZ2 is expressed as wild­type.
  2. Gene encoding Enzyme 2
  3. A mutation in C results in no expression of ENZ2 but ENZ1 is expressed as wild­type.
  1. In the absence of sexual reproduction, bacteria have alternative means for transmission of genetic material.
    1. What are the four modes of transmission of genetic material in bacteria? (2 marks)

Vertical/parental, Horizontal: Transformation, Transduction, Conjugation (0.5 mark each)

  1. How can genetic material be transferred directly between two living bacteria, and what are the key features of this process? (4 marks)

Conjugation (1 mark) – F factor/plasmid in donor strain (1 mark), producing pili on cell surface (1 mark). F factor can exist as plasmid, or integrated into genome in Hfr strains (1 mark).

 

  1. How can horizontal gene transfer be exploited in molecular biology?

(2 marks)

Transformation (1 mark) – bacterial cells can be made chemically competent or electroporated to uptake recombinant plasmid DNA for gene cloning (1 mark)

  1. Recombinant DNA technology is an invaluable tool for molecular geneticists.
    1. What are the key features of a cloning vector? You may use a diagram. (3 marks)

It has an origin of replication (1 mark), a selectable marker (1 mark), multiple­cloning site with unique restriction site (MCS) (1 mark)

  1. How can a gene fragment of interest be inserted into a cloning vector? (3 marks)

Fragment amplified by PCR (with restriction sites in primers) OR cut from other plasmid with restriction enzymes (either alternative OK, 1 mark). Cloning vector is cut with restriction enzymes to produce compatible ends (1 mark). DNA ligase is used to insert gene fragment into MCS of cloning vector (1 mark).

  1. Prokaryotic and eukaryotic cells are distinguishable by many features, including their genomes. You have been provided with samples of total DNA from prokaryotic and eukaryotic cells. How could you distinguish the two samples experimentally? (3 marks)

DNA would need to be (shotgun) sequenced and annotated (1 mark). Any two of the following in the sequences: higher density of genes in prokaryote vs eukaryote; introns in coding regions in eukaryotes; large amount of repetitive DNA in eukaryotes; total genome size (small prokaryote, large eukaryote), evidence of mitochondrial or chloroplast DNA in eukaryotes (2 marks)

Could not know structural difference eg. circular or linear chromosomes, presence of nucleosomes, chromatin etc. (0 marks)

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