variation & selection

Question 1

define

mutation

Answer 1

a spontaneous change in the nucleotide sequence of DNA, chromosome number or structure of chromosomes in an organism

Question 2

explain

what genetic mutation is using the example of sickle-cell anaemia

Answer 2

genetic mutation occurs in the nucleotide sequence of DNA coding for ß-globin chain which changes sequence of amino acids in polypeptide chain
hence, mutated gene produce haemoglobin S instead of haemoglobin A

Question 3

briefly explain

why sickle-cell anaemia is deadly

Answer 3

• haemoglobin S (HbS) is insoluble in water and less efficient at carrying oxygen, resulting in anaemia
• in low oxygen concentration, the HbS molecules interact with one another via hydrophobic interactions, to crystallise into rod-like structures
• resultantly, the RBC becomes deformed into a sickle shape that block thin blood vessels, which interferes with blood circulation and deprives organs of oxygen
• thus organs may become damaged which may lead to death.
• spleen may become enlarged due to collection of sickle-shaped RBCs

Question 4

explain

what chromosomal mutation is using the example of Down’s syndrome

Answer 4

non-disjunction of chromosome 21 results in an offspring with 47 chromosomes / extra chromosome 21 when gametes fuse during fertilisation

Question 5

state

2 ways non-disjuction occurs, and what this causes

Answer 5

1. failure of a pair of homologous chromosomes to separate during anaphase I of meiosis
2. failure of chromatids to separate during anaphase II of meiosis

→ this change the number of chromosomes

Question 6

define

variation

Answer 6

differences in traits between individuals of the same species

Question 7

contrast

continuous and discontinuous variation in 4 ways

Answer 7

answer

Question 8

define

natural selection

Answer 8

the tendency of organisms that possess favourable adaptations to their environment to survive and become parents of the next generation.
evolution occurs when natural selections results in changes in allele frequencies in the population

Question 9

describe

Darwin’s observations of adaptive radiation

Answer 9

1. ancestral seed-eating finch reached Galapagos islands
2. these finches spread out to occupy the habitats in the different islands
3. in the absence of much competition, finches flourished over time, gradually resulting in competition for food while occupying different habitats
4. mutation resulted in finches with different beak size and shape
5. finches with beak size and shape suited to a particular diet on the islands survived to reproductive age to produce viable offspring
6. eventually 13 major types of finches evolved that looked similar but are different species, each of which is adapted to a particular food source
   → this process is adaptive radiation

Question 10

define

adaptive radiation

Answer 10

the process by which one species evolves into two or more species as a result of different populations becoming reproductively isolated from each other when they are adapting to different environments

Question 11

outline

how to generally analyse examples of natural selection

Answer 11

[1]
when parental (animal) mate to produce the next generation, random/spontaneous mutation occurs among offspring, causing variation in the population, forming offspring with ..(identify phenotypes)..

[2]
the selection pressure is …

[3]
in this environment, XX are of selective advantage, because….
these individuals are more likely to survive till reproductive age and produce viable offspring hence passing on favourable alleles that code for XX to the next generation

however, YY are of selective disadvantage, because….
they woud likely succumb to selection pressure and die before reaching reproductive age

[4]
natural selection operates, leading to an increase in frequency of XX and an increase in frequency of alleles coding for XX

[5] - if applicable
new species form over time since one population cannot interbreed with another, hence adaptive radiation occurs

steps would be references like this for named examples

Question 12

explain

color variation in peppered moths

just explain industrial areas

Answer 12

[1]
spontaneous mutation resulted in melanic form of peppered moth which appears black, alongside the typical white peppered moth

[2]
the selection pressure is predator birds which peck them off trees

[3]
melanic form of peppered moths are of selective advantage, because they are well-camouflaged due to pollution that blackened tree trunks, whereas the white form are of selective disadvantage because they stand out
a lot more white moths were picked off trees by birds

[4]
frequency of melanic moths became a lot higher

and vice versa in rural areas, as the background is “unsooted tree barks”

Question 13

explain

why places with high malaria incidence have high frequency of Hb^s allele

Answer 13

1. heterozygotes are at selective advantage as the parasite spends part of its life cycle in the RBC and is less able to survive in in RBCs with haemoglobin S, making them less susceptible to malaria than those who lack the Hb^s allele
2. yet they are also phenotypically similar to individuals homozygous for normal allele, they are said to exhibit the sickle cell trait without showing severe symptons of sickle-cell anaemia
3. this condition is heterozygote advantage, where the heterozygote is FITTER than either of the 2 homozygotes in the presence of malaria
   → therefore both alleles persist today

Question 14

contrast

cloning and breeding

Answer 14

cloning: asexual reproduction that produces genetically identical offspring from one parent → conserves a useful combination of genes

breeding: sexual reproduction that produces genetically dissimilar offspring by fertilisation → can result in a better combination of genes

Question 15

explain

with a named example, how to perform selective breeding in animals

Answer 15

• The Jersey cow produces plenty of good milk but it does not thrive well in warm climates,
  while the Brahman bull thrives well in warm climates but are not good milk producers.
• Breeders allow the Jersey cow to mate with the Brahman bull, and the genes with good qualities from the two parents may be passed down to the hybrids
• Breeders continue to select and hybridise their cattle until they produce a new variety of cattle that contains the good qualities of both the Jersey and Brahman breeds.
• Once this is achieved, the improved breed of cattle is maintained by inbreeding. Many matings may be required to obtain a pure breed.

Question 16

state

a disadvantage of inbreeding

Answer 16

accumulation of recessive alleles in the population → recessive alleles that were not expressed in heterozygous parents are more likely to be passed to offspring → if the recessive allele codes for a genetic disease, the offspring that is homozygous recessive for that gene will suffer from the disease.

Question 17

explain

how to perform selective breeding on plants generally (single trait)

Answer 17

1. analyse seeds and select seeds/plants that are XX
2. plant seeds from such plants and allow them to grow into new plants. carry out self-fertilisation/cross-fertilisation with plants showing the desired characteristics
3. select the seeds from offspring of the selfing/cross to be grown again and used again as parents for the next generation.
4. repeat this self-pollination/cross-pollination process to ensure the desirable genes are inherited by future generations of plants and a stable line of XX is produced

Question 18

explain

how to perform selective breeding on plants generally (hybrid trait)

based on tutorial

Answer 18

1. select a parental plant with XX trait and parental plant with YY trait
2. carry out crossing of the 2 parental plants
3. select offspring plants with combination of desired traits,
4. allow them to self-fertilise and continue this process using later generations of offsprings that are both XX and YY
5. repeat this self-pollination/cross-pollination process to ensure the desirable genes are inherited by future generations of plants and a stable line of XX is produced
   OR
   propagate successful offspring that are both XX and YY by vegetative means

Question 19

define

cross

Answer 19

deliberate mating of two organisms

Question 20

define

hybrid

Answer 20

heterozygote at gene loci in question

Question 21

define

hybridisation

Answer 21

crossing of different varieties of organisms to produce a hybrid