Inheritance and Natural Selection

Question 1

Why is expected phenotypic ratio different from the observed ratio?

Answer 1

• Small sample size -> observed numbers not the same as expected
• Expected ratio is a predicted ratio which is not exact

Question 2

How do dominant and recessive alleles work?

Answer 2

The dominant allele will completely mask the expression of the recessive allele
- Only dominant trait is expressed in heterozygous individual

Question 3

How do codominant alleles work?

Answer 3

None of the alleles mask the expression of the other allele -> proteins of both are expressed

Traits of both alleles appear together

Question 4

How do sex linked alleles work?

Answer 4

Recessive and dominant alleles are linked to the X chromosome in sex
- Males are more at risk as they only have one X chromosome

Question 5

What is a homozygous recessive?

Answer 5

The organism only has the recessive allele

Question 6

What is heterozygous?

Answer 6

The organism has one recessive and one dominant allele

Question 7

What is homozygous dominant?

Answer 7

The organism has two dominant alleles

Question 8

What is a Test cross?

Answer 8

Crossing between an organism of unknown genotype and a homozygous recessive individual to determine unknown genotype

Question 9

What are the two types of genetic variation?

Answer 9

Discontinuous and Continuous Variation

Question 10

Features of discontinuous variation

Answer 10

• Classified into distinct categories
• Controlled by a single of few genes
• Usually not affected by environment

Question 11

Features of continuous variation

Answer 11

• Traits vary in a broad range
• Influenced by environmental factors
• Controlled by multiple genes

Question 12

How is evolution brought about?

Answer 12

Genetic variation through:
1. Random mutations
2. Meiosis
3. Random fertilisation

Question 13

How do mutations give rise to genetic variation?

Answer 13

1. Random mutations occur during DNA replication
2. Creates alternative forms of genes
3. New alleles as a result

Question 14

How does meiosis give rise to genetic variation?

Answer 14

1. Crossing over -> sister chromatids carry combinations of alleles not present in maternal or paternal chromosomes
2. Independent assortment -> Each gamete receives unqiue combination of alleles/traits

Question 15

How does fertilisation give rise to genetic variation?

Answer 15

1. Random fertilisation of any sperm cell with any egg cell
2. Results in different zygotes being produced
3. Offspring from same set of parents are different

Question 16

What causes natural selection?

Answer 16

• Selection pressures such as predators, limited food and water or disease
• Causes individuals to compete with each other

Question 17

What is the process of natural selection?

Answer 17

1. Individuals with favourable phenotypes are selected for
2. Individuals with unfavourable phenotypes selected against
3. Individuals selected for survive and reproduce more -> passes on its traits/alleles

Question 18

What is artificial selection?

Answer 18

• Humans select the desired trait to pass to offspring
• Often done through inbreeding to maintain desirable traits

Question 19

Negative impacts of artificial selection

Answer 19

• Accumulation of harmful recessive alleles in population
• Render offspring more susceptible to diseases or changes in environment

Question 20

Cases of artificial selection

Answer 20

1. Wild mustard plant to become crops for farming
2. Wild dog breeds to become a variety of dog breeds

Question 21

Features of Cloning Vectors

Answer 21

• DNA molecule that can carry foreign DNA into cell
• Example: Bacterial Plasmids that can replicate independently
• Easily manipulated to form recombinant plasmids and introduced into target cells

Question 22

Features of Restriction Enzymes

Answer 22

• Recognises specific nucleotide sequences -> active site of restriction enzyme structurally complementary to shape of specific nucleotide sequence
• Hydrolyse phosphodiester bonds in nucleotide sequence -> creates sticky ends or blunt ends

Question 23

Features of Sticky Ends

Answer 23

• Sticky ends can form hydrogen bonds which are temporary associations
• With complementary sticky ends cut from the same restriction enzyme
• Increases chance of restriction fragments joining

Question 24

Features of DNA ligase

Answer 24

• Joins 2 DNA fragments by ligation -> formation of phosphodiester bonds
• Hydrogen bonds between sticky ends increase chance of restriction fragments being joined together

Question 25

Process of formation of recombinant plasmid

Answer 25

1. Plasmid is cut once by a restriction enzyme that produces sticky ends.
2. The same restriction enzyme is used to cut the human gene.
3. The sticky ends of the plasmid form hydrogen bonds between base pairs with complementary sticky ends of the human gene.
4. DNA ligase then forms phosphodiester bonds between the ends of the cut plasmid and human gene.
5. Results in recombinant plasmid formed
6. Recombinant plasmid is introduced into target cells