variation and selection

Question 1

what is mutation

Answer 1

spontaneous (natural) change in
1. sequence of nucleotides in DNA (genetic mutation, results in new alleles of genes)
2. chromosome number (chromosomal mutation)
3. structure of chromosomes

Question 2

if the rate of spontaneous mutation is very low, what can highly increase the rate of mutation

Answer 2

exposure to mutagenic agents, such as
1. UV light
2. Gamma/Beta/Alpha radiation

Question 3

what causes sickle cell anemia

Answer 3

• in a normal adult, HbA (haemoglobin A molecule) has 2 alpha-globin and 2 beta-globin chains
• Genetic Mutation : alters sequence of DNA coding for the beta-globin chain, leading to a change in the amino acid sequence in the polypeptide
• produces HbS molecule which is the same as HbA except in one amino acid
• occus during interphase when errors in DNA replication happens + mutagenic agents

Question 4

why is HbS dangerous

Answer 4

1. exposed region —> insoluble in water
2. less efficient in carrying oxygen —> anemia
3. low oxygen concentration —> HbS molecules interact with one another via hydrophobic interactions —> molecule crystallises into rod like structures —> red blood cells deform into sickle shape —> blocks thin blood vessels —> deprives organs of oxygen

Question 5

traits of the sickle cell anemia allele

Answer 5

sickle cell anemia is homozygous recessive

Normal: HbᴬHbᴬ
Sickle cell trait: HbᴬHbˢ
Diseased: HbˢHbˢ

Question 6

what is non-disjunction

Answer 6

failure to seperate :

1. pair of homologous chromosomes during anaphase I of meiosis
2. pair of chromatids during anaphase II of meiosis

Question 7

what is down syndrome

Answer 7

presence of extra chromosome 21 (3 chromosome 21 in total)

Question 8

what is variation

Answer 8

differences in traits between individuals of the same species

Question 9

what is discontinuous variation

Answer 9

• clear cut phenotypes with no intermediates (extremes)
• not affected by environment
• brought about by one or a few major genes
• represented by a bar graph (bars not touching)

etc. (ABO blood group, Sex, ability to roll tongue)

Question 10

what is continuous variation

Answer 10

• range of phenotypes from one extreme to another without any break
• brought about by combined effect of a number of genes
• represented by histogram / normal distribution curve
• affected by environment

etc (Height, Mass, Skin color)

Question 11

what is natural selection

Answer 11

tendency of organisms that posses favorable adaptations to their environment to survive to reproductive age and become parents of the next generation, resulting in changes in allele frequencies of a population

Question 12

what is adaptive radiation

Answer 12

Process where one ancestral species evolves into 2 or more species due to different populations being reproductively isolated from one another when they are adapting to different environments

Question 13

example of adaptive radiation (finches)

Answer 13

Galapagos finches

1. Finches spread out to occupy different islands
2. Absence of competition —> finches flourished over time
3. over time, food became less abundant —> increase in competition for food
4. Mutation —> finches that have different beak size and shape suitable for a particular diet on their island —> survive to reproductive age to produce viable offspring
5. 13 major types of finches evolved, all adapted to a particular food source

Question 14

example of natural selection (moth)

Answer 14

Peppered moth
two forms - white(typical) and black(melanic)

1. in 1848, majority of the moths were white but in 1900, majority of moths in industrial area were black.
2. happened due to spontaneous mutation
3. In industrial areas:
   - most trees were black as they were covered by pollutants
   - black moths camouflage more —> less likely to be eaten by birds
   - survive to reproductive age to produce viable offspring—> increase in frequency of black moth allele
4. in rural areas:
   - tree is white
   - white moths camouflage more —> less likely to be eaten by birds
   - survive to reproductive age to produce viable offspring—> increase in frequency of white moth allele

Question 15

example of heterozygote advantage (sickle cell vs malaria)

Answer 15

Places with high frequency of malaria cases also have high frequency of sickle cell anemia allele

People with sickle cell anemia (homozygous recessive) : less susceptible to malaria as malaria parasites spends most of is life cycle in the red blood cell —> less able to survive in red blood cells with HbS

Healthy people (homozygous dominant) : highly susceptible to malaria

People with sickle cell trait (heterozygous) : less likely to die from sickle cell (mild symptoms only) or malaria (less susceptible)

HETEROZYGOTE ADVANTAGE : HETEROZYGOTE IS FITTER THAN EITHER OF THE HOMOZYGOTES

Question 16

answering technique for natural selection

Answer 16

STEP 1: identify different phenotypes (note that variation is always due to spontaneous mutation)

STEP 2: identify selection pressure (dangers)

STEP 3: follow structure:

• random and spontaneous mutation has resulted in the variation of (traits)
• (organism) has different (traits)

1. in an environment with (selection pressure), individuals with (advantageous trait) have a selective advantage as they are able to ___________.
2. they are more likely to survive to reproductive age to produce viable offspring and pass on (allele that causes advantageous trait) to the offspring
3. on the other hand, individuals with (disadvantageous trait) have a selective disadvantage as ________. they are less likely to survive to reproductive age
4. natural selection operates, leading to an increase in frequency of (allele causing advantageous trait) in areas where (selection pressure)

Question 17

what is artificial selection (in the selective breeding of organims)

Answer 17

deliberate cross between two species of an organism to produce offspring with desirable traits (most of the time, a hybrid which is a heterozygote)

Question 18

example of artificial selection in animals

Answer 18

Jersey cow (good milk, cannot thrive in warm weather) is crossed with a Brahman bull (thrives well in warm weather but bad milk) to give rise to a Jersey-Brahman hybrid with the good traits of the parents (good milk, thrive in warm weather)

Question 19

pros and cons of inbreeding/sibling mating

Answer 19

pros : mantainance of an improved breed/species (all are heterozygous)

cons: recessive alleles that code for diseases (not expressed in heterozygous parents) may be accumulated in offspring

Question 20

process for selection in plants

Answer 20

1. Select plants that produce seeds with (desirable trait 1)
2. self-fertilise OR cross them with other plants showing (desirable trait 2)
3. select the produced seeds with (desirable traits) and use them as the parents for the next generation
4. after many generation, plants that produce seeds with desirable traits are obtained.
5. self pollinate these plants to ensure desirable genes are passed on to future generations.

Question 21

example of artificial selection in plants

Answer 21

cross between Java sugarcane (rich in sugar, succumbs to disease) and wildtype sugarcane (resistant to disease, low in sugar) to produce a hybrid that is rich in sugar and resistant to diseases