Inheritance and Natural Selection Flashcards
Why is expected phenotypic ratio different from the observed ratio?
- Small sample size -> observed numbers not the same as expected
- Expected ratio is a predicted ratio which is not exact
How do dominant and recessive alleles work?
The dominant allele will completely mask the expression of the recessive allele
- Only dominant trait is expressed in heterozygous individual
How do codominant alleles work?
None of the alleles mask the expression of the other allele -> proteins of both are expressed
Traits of both alleles appear together
How do sex linked alleles work?
Recessive and dominant alleles are linked to the X chromosome in sex
- Males are more at risk as they only have one X chromosome
What is a homozygous recessive?
The organism only has the recessive allele
What is heterozygous?
The organism has one recessive and one dominant allele
What is homozygous dominant?
The organism has two dominant alleles
What is a Test cross?
Crossing between an organism of unknown genotype and a homozygous recessive individual to determine unknown genotype
What are the two types of genetic variation?
Discontinuous and Continuous Variation
Features of discontinuous variation
- Classified into distinct categories
- Controlled by a single of few genes
- Usually not affected by environment
Features of continuous variation
- Traits vary in a broad range
- Influenced by environmental factors
- Controlled by multiple genes
How is evolution brought about?
Genetic variation through:
1. Random mutations
2. Meiosis
3. Random fertilisation
How do mutations give rise to genetic variation?
- Random mutations occur during DNA replication
- Creates alternative forms of genes
- New alleles as a result
How does meiosis give rise to genetic variation?
- Crossing over -> sister chromatids carry combinations of alleles not present in maternal or paternal chromosomes
- Independent assortment -> Each gamete receives unqiue combination of alleles/traits
How does fertilisation give rise to genetic variation?
- Random fertilisation of any sperm cell with any egg cell
- Results in different zygotes being produced
- Offspring from same set of parents are different
What causes natural selection?
- Selection pressures such as predators, limited food and water or disease
- Causes individuals to compete with each other
What is the process of natural selection?
- Individuals with favourable phenotypes are selected for
- Individuals with unfavourable phenotypes selected against
- Individuals selected for survive and reproduce more -> passes on its traits/alleles
What is artificial selection?
- Humans select the desired trait to pass to offspring
- Often done through inbreeding to maintain desirable traits
Negative impacts of artificial selection
- Accumulation of harmful recessive alleles in population
- Render offspring more susceptible to diseases or changes in environment
Cases of artificial selection
- Wild mustard plant to become crops for farming
- Wild dog breeds to become a variety of dog breeds
Features of Cloning Vectors
- 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
Features of Restriction Enzymes
- 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
Features of Sticky Ends
- 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
Features of DNA ligase
- Joins 2 DNA fragments by ligation -> formation of phosphodiester bonds
- Hydrogen bonds between sticky ends increase chance of restriction fragments being joined together
Process of formation of recombinant plasmid
- Plasmid is cut once by a restriction enzyme that produces sticky ends.
- The same restriction enzyme is used to cut the human gene.
- The sticky ends of the plasmid form hydrogen bonds between base pairs with complementary sticky ends of the human gene.
- DNA ligase then forms phosphodiester bonds between the ends of the cut plasmid and human gene.
- Results in recombinant plasmid formed
- Recombinant plasmid is introduced into target cells
