16. Inherited Change Flashcards
Homologous pairs of chromosomes
Pairs of chromosomes that are similar in structure and composition
- same length
- same shape
- same order of gene
- same banding pattern
Haploid
Containing a single set of unpaired chromosomes
Diploid
Containing two sets of chromosomes
COMPARE the role of meiosis in gametogenesis to produce sperm cells in humans with the role of meiosis in gametogenesis in producing pollen grains in flower plants. [4m]
Similarities
- both produce 4 cells
- both halve the chromosome number
- both produce genetically different cells
Differences
- sperm is a gamete; pollen grain is not
- sperm cell has 1 haploid nucleus; pollen grains contain 2 haploid nuclei
- sperm formed by mitosis then meiosis; pollen grain cells formed by meiosis then mitosis
Gene
A sequence of nucleotides that forms part of a DNA molecule and codes for a polypeptide
Locus
The position of a gene on a chromosome
Allele
- Different forms of a gene
- Due to different nucleotide sequence in DNA
- Resulting in different polypeptide produced
Dominant
An allele that is expressed if it is present
Recessive
An allele that is only expressed when there is no dominant allele of the gene present
Codominant
Alleles that both have an effect on the phenotype of a heterozygous organism
Test cross
A genetic cross in which an organism with a dominant phenotype is crossed with an organism that is homozygous recessive, to determine whether the organism is heterozygous or homozygous
F1 generation
The offspring of two different pure-breeding, homozygous organisms
F2 generation
The offspring of two F1 (heterozygous) organisms
Phenotype
The observable features of an organism
Genotype
The genetic make-up of an organism in terms of the alleles present
Homozygous
Having two identical alleles of a particular gene
Heterozygous
Having two different alleles of a particular gene
Sex linkage
Gene carried on the X chromosome
Autosomal linkage
2 or more genes on the same chromosome (not a sex chromosome) will be inherited together, without independent assortment
Gene mutation
Random change in nucleotide sequence in DNA, during DNA replication
Explain how the presence of galactosaemia in a foetus may be determined. [2m]
- genetic screening
- obtain fetal cells/DNA
- by amniocentesis / chorionic villus sampling
- electrophoresis + probe
A female has a family history of ocular albinism (non-progressive disorder and clarity of vision remains stable throughout life) but she does not have any symptoms.
A test to find out if she has the mutant allele is available.
2 reasons for taking this test and 3 reasons against taking this test.
FOR
- decide whether to have children
- earlier treatment if child born with ocular albinism
AGAINST
- test may be expensive
- condition is not life-threatening
- cannot be treated
Explain why some genes show constitutive expression. [1m]
The gene products / proteins are needed all the time.
Discuss the link between the frequency of sickle cell anaemia and the number of cases of malaria. [9m]
- Plasmodium (malaria parasite) cannot complete life cycle in sickle cells
- heterozygotes have resistance to malaria
- sickle cell anaemia ppl die due to SCA
- normal ppl die due to malaria
2 selection pressures are involved!
- frequency of sickle cell anaemia is highest in areas where malaria is common
- sickle cell anaemia RBCs cannot carry O2 very well
- homozygous H^S have sickle cell anaemia and may die
- homozygous H^A have normal Hb / red blood cells
- heterozygotes have sickle cell trait
- Plasmodium affects red blood cells
- malaria is lethal
- heterozygotes less likely to suffer from (severe effects of) malaria
- have selective advantage
- pass on both H^A and H^S alleles
- malaria selects against homozygous H^A
- sickle cell anaemia selects against homozygous H^S
- idea that sickle cell ALLELE is maintained within population because of sickle cell trait individuals
Explain why there are usually more than 100 copies of mtDNA in a cell, but only two copies of nuclear DNA. [2m]
- many mitochondria per cell but only 1 nucleus
- cell is diploid / has 2 copies of each chromosome in nucleus
❖ Explain the function of transcription factors in gene expression in eukaryotes. [6m]
- bind to DNA
- so RNA polymerase binds to promoter
- so transcription begins / gene expressed
1. can activate genes in correct order; for correct pattern of development
2. determine sex
3. allow responses to environmental stimuli
eg. correct genes expressed in response to very high temp. / light exposure
4. regulate cell cycle - role in cell cycle checkpoints & apoptosis
5. in cell signalling - response to hormones
Gene R/r, which codes for a protein involved in pigment production, has 2 alleles.
Suggest ways in which the expression of allele R allows pigment production. [3m]
- example of gene interaction / epistasis
- (may code for) an enzyme in pigment production pathway
- (may code for) transcription factor
- which binds to promoter
- & allows binding of RNA polymerase
- (may code for) co-factor to activate enzyme
