Core O - Inheritance Flashcards
Describe how crossing over and independent assortment can lead to genetic variation. (9)
• Occur during meiosis I
• Crossing over: ~ between non-sister chromatids ~ of a pair of homologous chromosomes (bivalents) ~ in prophase I ~ at chiasmata ~ exchange of genetic material ~ linkage groups broken ~ new combination of alleles within each chromosome
• Independent assortment:
~ of homologous chromosome pairs (bivalents)
~ each pair lines up independently of others
~ line up on equator
~ during metaphase I
• results in gametes that are genetically unique
Describe how you would carry out a test cross to determine the genotype for a particular characteristic. (2)
- Cross the characteristic with a homozygous recessive
- If all offspring share the same characteristic as parents, then parents are homozygous
- If some same some different, parents are heterozygous
Explain what is meant by a dominant allele. (2)
• Allele:
~ different / alternative form of a gene
~ one of two or more alternative nucleotide sequences at a single gene locus
• Dominant
~ Allele that always expresses itself in the phenotype when present
~ Allele which influences the phenotype even in the presence of an alternative allele
Describe the first division of meiosis (meiosis I) in animal cells. (6)
- Reduction division / to halve number of chromosomes
- Homologous chromosomes pair up / bivalents form
- Chiasmata / 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
Discuss the link between the frequency of sickle-cell anaemia and the number of cases of malaria. (9)
- Frequency of sickle-cell anaemia is highest in areas where malaria is common
- Sickle-cell anaemia red blood cells cannot carry oxygen very well
- Homozygous Hs have sickle-cell anaemia and may die
- Homozygous Hn have normal red blood cells
- Heterozygotes have sickle-cell trait (Sickle-cell trait red blood cells not severely affected)
- Malaria parasite / Plasmodium affects red blood cells
- Malaria lethal
- Heterozygotes less likely to suffer from severe effects of malaria
- Have selective advantage
- Pass on both Hn and Hs
- Malaria selects against the homozygous Hn
- Sickle-cell anaemia selects against homozygous Hs
- Sickle-cell allele is maintained within population because of sickle-cell trait individuals
Explain how changes in the nucleotide sequence of DNA may affect the amino acid sequence in a protein. (8)
- Amino acid code is three bases / nucleotides
- Gene mutation
- Base substitution / addition / deletion
- Addition / deletion has large effect on amino acid sequence
- Frameshift
- Completely new code after mutation / alters every three days sequence which follows
- Substitution may have little or no effect / silent mutation
- Different triplet but same amino acid / new amino acid in non-functional part of protein
- Substitution may have big effect on amino acid sequence
- Could produce stop codon
- Sickle-cell anaemia / PKU /cystic fibrosis
- Transcription and translation
Explain how the allele for haemophilia may be passed from a man to his grandchildren. (7)
- Haemophilia allele on X chromosome
- Sex linked
- Haemophilia allele recessive
- man homogametic (has one X chromosome)
- Y-chromosome does not have blood clotting gene
- Only daughters get his X chromosome
- Daughters carriers of haemophilia allele
- Grandson has 50% chance of having haemophilia allele
- Granddaughters 50% chance of carrying haemophilia allele
- Genetic diagram included
Explain how an addition mutation could produce a stop triplet. (2)
- Addition/insertion of a base to DNA
- Changes the sequence of three bases
- Triplet no longer codes for an amino acid
LCA is a condition caused by a recessive allele.
Suggest and explain why LCA is suitable for treatment using gene therapy. (3)
- Caused by a single gene
- Caused by a recessive allele
- Delivery of correct / dominant / normal allele (could correct the condition)
- Only need to get allele into a few cells
- Ease of access to affected area
- Serious so worth the risk
- Only targets eye / no surgery needed
Genetically modified adenoviruses (a type of virus that can cause respiratory infections) were used as vectors.
Suggest two ways in which the genome of the adenoviruses used as vectors would differ from that of normal adenoviruses. (2)
- Virus no longer able to cause infections
- Correct / dominant / normal allele of protein added
- Promoter added
Suggest why trials were designed to include such a small number of participants. (2)
- Not known if the technique might have side effects
- Rare condition (for some)
- Expense
- Trial to see if delivery method works
Explain the meaning of the terms dominant and gene. (2)
Dominant - allele that always expresses itself in the phenotype when present
Gene - length of DNA / sequence of nucleotides coding for a specific polypeptide
Suggest how a person may a develop a condition caused by a dominant allele when there is no family history of the condition. (2)
- Spontaneous / random / chance
- Mutation of gene / allele
- Carcinogens / UV light etc
Suggest how delivering normal alleles of the CFTR gene into epithelial cells in the airways could relieve the symptoms of cystic fibrosis. (4)
- Correct form of CFTR protein made
- Delivered to membrane
- Acts as chloride channel
- Chloride ions leave cell
- Water leaves cell
- Normal (less viscous) mucus formed
- No blockage of airways so less chance of infections
Explain how meiosis and fertilisation may result in genetic variation in offspring. (7)
- Chiasma / crossing over
- Between non-sister chromatids
- Of homologous chromosomes / bivalent
- In prophase I
- Exchange of genetic material
- Linkage groups broken
- New combination of alleles
- Independent assortment of homologous chromosomes
- At equator
- During metaphase I
- Possible mutation
- Random mating
- Random fusion / fertilisation of gametes
