chapter 20 Flashcards
The haploid chromosome number in the koala, Phascolarctos cinereus, is 8.
Independent assortment of chromosomes in meiosis contributes to genetic variation in the gametes of the koala.
How many genetically different versions of koala gamete would it be possible for one individual to produce if independent assortment were the only source of genetic variation?
256
a haploid number of 8 would give 28 different gametic possibilities.
Which of the rows, A to D, correctly describes how genetic variation is achieved during meiosis?
Row Prophase 1 Metaphase 1 Metaphase 2 Anaphase 2
crossing over of non- sister chromatids
independent assortment of homologous chromosomes
independent assortment of chromatids
independent segregation of chromatids
Banana plants, Musa spp., first underwent artificial selection thousands of years ago. Early human populations discovered mutant banana plants that produced seedless, soft fruit. This mutation prevented pollen and seeds from developing.
Early human populations planted cuttings of these mutant plants. The bananas that are eaten today are descended from these cultivations.
Some scientists claim that banana crops will be extinct within a few years. Use the information above to justify the scientists’ claim.
no sexual reproduction (1)
no / little, genetic variation (1)
idea of susceptible to new diseases (1)
idea of susceptible to changing environment (1
Explain how sexual reproduction in Hydra leads to genetic variation in the offspring.
- genetic variation is the variety of alleles
- offspring have alleles from more than one parent
- random fertilisation
- meiosis produces genetically
unique gametes - crossing over in prophase 1
- alleles swapped between
non-sister chromatids - base sequence of
chromosomes altered - independent assortment / random segregation
- in metaphase 1
- also relevant in metaphase 2
if crossing over has occurred
Suggest why sexual reproduction in Hydra usually occurs in the winter.
(some offspring) might survive unfavourable conditions ✓
(some) offspring have useful alleles ✓
(named) unfavourable conditions mean (all) offspring might die (if asexual) ✓
Describe in words how this phenotypic ratio might be different if the two genes were autosomally linked.
higher proportion, heterozygous / like parents
OR
alleles not completely re-mixed / AW
define autosomal linkage
When two or more genes are positioned on the same autosome.
They are unlikely to be separated by crossing over during meiosis so are often inherited together.
Agammaglobulinemia and Vici syndrome are both genetic diseases. Agammaglobulinemia results in a lack of mature B lymphocytes in a person’s blood.
i. Suggest and explain one symptom of agammaglobulinemia.
greater susceptibility to, infection / pathogens ✓
no / fewer, plasma cells / effector cells / antibodies ✓
. Suggest how allele B inhibits the expression of allele A.
B, produces / codes for, repressor protein / repressor polypeptide / enzyme / transcription factor ✓
(protein / polypeptide / product of B) binds to, promoter (of A) / mRNA / ribosome ✓ (product of allele B) stops, transcription / translation (of allele A) / protein synthesis / described✓
product of B inhibits the enzyme (encoded by A) ✓
The chromosomes used to determine sex inheritance are given different letters in birds and mammals.
Identify one other way in which the sex determination in birds is different from sex determination in mammals.
(bird) females have two different (sex) chromosomes / AW ora
Suggest and explain one reason, other than epistasis, why the phenotype ratio might not be 9 : 3 : 3 : 1.
1 (autosomal) linkage
2 (both) genes / alleles, occur on same, chromosome / autosome / chromatid
3 no independent assortment
4 (so) alleles, inherited together / end up in same gamete
5 (unless) crossing over occurs / chiasma forms between gene loci
It takes time for an effective vaccine to be prepared in quantity for a new strain of bacterium. List two vulnerable groups of people for whom you would advise doctors to prescribe antibiotics
although they are not yet showing symptoms of the new disease.
babies / infants (1)
elderly / infirm (1)
immuno-compromised / on immunosuppressant drugs / HIV positive (1)
known to have been exposed (to the infection) (1)
Discuss the implications of the over-use of antibiotics when people do not show symptoms.
two from
(antibiotic is) selective pressure (1) (bacterial) gene pool / AW, has variation (1)
(only) some bacteria have resistance / some bacteria are more resistant than others (1)
two from
when exposed (to antibiotic) most-resistant survive (1)
surviving bacteria continue to reproduce to make a resistant population (1)
idea that over many generations there is an increase in proportion of resistant bacteria (under continued antibiotic pressure) (1) antibiotic becomes ineffective / new antibiotic needed (1)
Read the following five statements.
Mutations preventing lactose intolerance have occurred in humans at various times in the prehistoric past,
and in all human races.
The domestication of large lactating mammals like goats and cattle arose in Europe and parts of Africa 5 000 to 10 000 years ago.
The lowest levels of lactose intolerance are found in areas that European populations colonised, like North America.
The ability of agricultural populations to digest the milk, as well as the meat, of animals, is advantageous. It adds to their general nutrition.
Until recent times the Australian aborigines had been isolated on their island continent for around 50 000 years.
Suggest how the lactose intolerance phenotype came to be present in only 5% of a population like the Europeans, but came to be present in 97% of the Australian aborigines. Use the information given above and knowledge of Darwin’s theory of evolution by natural selection.
Europeans:
* (pre-agricultural) gene pool / genetic variation, included mutant / non-intolerance, allele
* availability of milk acted as (positive) selection pressure
* individuals / groups, with mutant / non-intolerance, allele had better, chance of survival / success in reproduction
* directional selection
* mutant / non-intolerance, allele accumulated (in gene pool)
* genetic drift (in small prehistoric population)
* mutant / non-intolerance, allele is dominant
* so expressed in heterozygotic individuals (increasing phenotype frequency).
Australian aborigines:
* ancestral population pre- agricultural
* so no selection for mutant / non-intolerance, allele
* no suitable mammals to domesticate / milk
* island, so no borders for suitable mammals to come in
* no contact / breeding, with non-Aboriginal peoples
* no gene flow (from other human populations)
* no selection pressure
* to increase mutant / non-
intolerance, allele / phenotype, frequency.
State one way in which genetic diversity can be measured.
genetic polymorphism / proportion of heterozygotes / proportion of gene variants ✓
