StemUp: OCR A A level Biology 6.1.2 Patterns of inheritance

How do genetic factors contribute to phenotypic variation? (3)

- Different genotypes (combinations of alleles) cause variation in phenotypes (characteristics displayed by an organism)

- Monogenic characteristics (controlled by one gene) show discontinuous variation (e.g., blood group)

- Polygenic characteristics (controlled by multiple genes) show continuous variation (e.g., height, skin color)

How do environmental factors contribute to phenotypic variation? (3)

- Environmental factors like climate, food availability, and lifestyle affect phenotype

- Examples include etiolation (long, spindly growth due to lack of light) and chlorosis (yellowing due to lack of magnesium)

- Most traits influenced by both genes and the environment show continuous variation

How does sexual reproduction lead to genetic variation within a species? (2)

- Meiosis creates unique gametes through crossing over and independent assortment of chromosomes

- Random fusion of gametes during fertilisation further increases genetic variation in offspring

What is a gene? (1)

A sequence of DNA bases coding for a polypeptide

What is an allele? (1)

Different versions of a gene

What is genotype? (1)

The genetic makeup of an organism

What is phenotype? (1)

The characteristics displayed due to the genotype and environment

What is a dominant allele? (1)

Always expressed in the phenotype

What is a recessive allele? (1)

Expressed only when homozygous

What are codominant alleles? (1)

Both alleles are expressed in the phenotype

What is a sex-linked gene? (1)

A gene located on the X or Y chromosomes

What is meant by locus? (2)

- The position of a gene on a chromosome

- Alleles of the gene are found at the same locus on each chromosome

What is meant by homozygote? (1)

An organism carrying two copies of the same allele

What is meant by heterozygote? (1)

An organism carrying two different alleles

What is a carrier? (1)

A person carrying an allele not expressed in the phenotype but can be passed on to offspring

What is monohybrid inheritance? (1)

The inheritance of a characteristic controlled by a single gene

What is dihybrid inheritance? (1)

The inheritance of two pairs of alleles

What are multiple alleles? (2)

- More than two alleles of the same gene exist in the gene pool

- E.g., the ABO blood group system

What elements should be included in a genetic diagram or Punnett square? (6)

- Parental phenotypes

- Parental genotypes

- Gamete alleles

- Offspring genotypes

- Offspring phenotypes

- Ratio of offspring phenotypes

Why are observed ratios different from expected ratios? (3)

- Random fertilisation of gametes

- Small sample size

- Offspring ratios arise by chance

How do you determine the possible gametes for a monohybrid cross? (3)

- For a parent with genotype AA → all gametes are A

- For a parent with genotype aa → all gametes are a

- For a parent with genotype Aa → the gametes are A and a

What is the Punnett square for a monohybrid cross between two heterozygous parents (Aa x Aa)? (3)

- The gametes for each parent are A and a.

- Genotypes = AA, Aa, Aa, aa

- Phenotypic ratio: 3:1 (dominant to recessive trait)

How do you determine the possible gametes for a dihybrid cross? (7)

- Use the FOIL method (First, Outside, Inside, Last) to determine gametes for a parent with two genes

- Example: AaBb

- First: AB

- Outside: Ab

- Inside: aB

- Last: ab

- Possible gametes: AB, Ab, aB, ab

How would a dihybrid cross between two heterozygous parents (AaBb x AaBb) look in a Punnett square? (4)

- Gametes for each parent: AB, Ab, aB, ab.

- 16 possible offspring genotypes (4x4 grid)

- AABB, AABb, AaBB, AaBb, etc.

- Phenotypic ratio for a dihybrid cross is typically 9:3:3:1

What is autosomal linkage? (3)

- Refers to genes located on the same autosome (non-sex chromosome)

- These genes are likely to be inherited together during meiosis because they are physically close on the same chromosome

- Are less likely to be separated by crossing over

How does autosomal linkage affect independent segregation of chromosomes? (3)

- Autosome-linked tend to stay together during independent segregation of chromosomes in meiosis I

- Meaning their alleles are often inherited together

- This reduces the genetic variation that would typically occur with independent assortment

How can crossing over affect autosomal linkage? (2)

- Crossing over during meiosis can separate linked genes if it occurs between the two loci

- This can result in new combinations of alleles being passed to offspring

What is an autosome? (3)

- Any chromosome that is not a sex chromosome

- Humans have 22 pairs of autosomes

What is sex linkage? (1)

When the locus of the allele that codes for it is on a sex chromosome (X or Y

What sex chromosomes do males and females have in mammals? (2)

- Males = XY

- Females = XX

Why are most sex-linked genes X-linked? (2)

- The Y chromosome is smaller than the X chromosome

- So most sex-linked genes are located only on the X chromosome

Why are males more likely to express recessive X-linked phenotypes? (2)

- Males have only one X chromosome, so they only have one allele for X-linked genes

- Will express the trait even if the allele is recessive

What are examples of genetic disorders caused by faulty alleles on the X chromosome? (2)

- Red-green colour blindness

- Haemophilia

What is epistasis? (2)

- The interaction of genes at different loci

- Where the allele of one gene masks the expression of the alleles of another gene

What are the two types of epistasis? (2)

- Dominant epistasis

- Recessive epistasis

What is dominant epistasis? (1)

A dominant genotype on one gene inhibits the expression of the other gene

What is the expected phenotypic ratio for dominant epistasis? (3)

12 Epistasis (inhibited): 3 Expressed (dominant): 1 Expressed (recessive)

What is recessive epistasis? (1)

A recessive genotype on one gene inhibits the expression of the other gene

What is the expected phenotypic ratio for recessive epistasis? (1)

9 Expressed (dominant): 3 Expressed (recessive): 4 Epistasis (inhibited)

What is the purpose of the Chi-Squared test? (1)

Used to determine whether the difference between observed and expected frequencies is significant

In what situations is the Chi-Squared test used? (2)

- It is used when dealing with counts of things or frequencies that can be placed into categories

- Like flower colour or health conditions

What is the formula for the Chi-Squared statistic? (3)

- O represents the observed results

- E represents the expected results

What does the Chi-Squared test compare? (2)

- Compares the observed frequencies with the expected frequencies

- To determine if the difference is significant

What is continuous variation and how is it determined genetically? (3)

- When individuals vary within a range with no distinct categories

- It is controlled by many genes (polygenic)

- And often influenced by environmental factors (e.g. height, weight)

What is discontinuous variation and how is it determined genetically? (2)

- When there are distinct categories for a trait, and individuals fall into one of them

- It is controlled by a single gene (monogenic) and is not influenced by the environment (e.g. blood group)

What is the definition of evolution? (1)

The change in allele frequency in a population over generations

How does natural selection lead to evolution? (3)

- Occurs when selection pressures (e.g. predation, disease) act on individuals with favorable alleles

- Allowing them to survive and reproduce, passing these alleles to the next generation

- Leading to evolution

Why do some species have high reproductive rates? (3)

- Ensure a sufficient population survives to reproduce

- Compensating for losses

- Due to predation, competition, and environmental factors

What are the steps in the process of natural selection? (5)

1. Mutation creates new alleles

2. Individuals with different alleles show variation

3. Selection pressures act on the population

4. Individuals with favorable traits survive and reproduce

5. Over time, favorable alleles increase in frequency

How does overproduction of offspring relate to natural selection? (4)

- When there are too many offspring for the available resources

- There is intraspecific competition

- The best-adapted individuals are more likely to survive and pass on their favorable alleles

- Driving evolution

What is stabilising selection? (2)

- Favours the average phenotype

- Reducing variation and maintaining the status quo in a population

What is directional selection? (2)

- Favours individuals with extreme phenotypes

- Leading to a shift in the population's traits over time in response to environmental changes

What is stabilising selection? (4)

- Environment favours individuals with the most common characteristic

- Individuals with extreme traits die out

- The common characteristic becomes more frequent

- The range (standard deviation) of traits decreases

What is directional selection? (3)

- Environment favours individuals with traits at one extreme

- Extreme trait becomes more common over time

- Normal distribution of traits shifts in that direction

What is genetic drift? (4)

- Random changes in allele frequencies

- Occurs without selective forces

- Chance dictates which alleles are passed on

- Greater effect in small populations

Why is genetic drift more important in small populations? (3)

- Random changes in allele frequencies cause larger percentage changes in small populations

- In large populations, chance variations tend to even out

- Small populations are more influenced by random effects

What is a genetic bottleneck? (4)

- Event causes a big reduction in population size

- Leads to a smaller gene pool

- Chance determines which alleles survive

- Surviving alleles become more frequent

What is the founder effect? (4)

1. A small group from a population starts a new population

2. Initial gene pool is limited

3. Reduced genetic variation in new population

4. Greater effect of genetic drift in the new population

What is a species and what is a population? (2)

- Species = A group of similar organisms that can reproduce to give fertile offspring

- Population = A group of organisms of the same species living in the same habitat, with the potential to interbreed

What is the gene pool and allele frequency? (2)

- Gene Pool: The complete range of alleles present in a population

- Allele Frequency: How often an allele occurs in a population

What does the Hardy-Weinberg principle predict? (2)

- The frequency of alleles of a particular gene

- Will stay constant from one generation to the next

What are the requirements for the Hardy- Weinberg principle to apply? (5)

1. No mutations.

2. No natural selection (all alleles equally advantageous)

3. The population is large

4. The population is genetically isolated

5. Random mating

What are the Hardy-Weinberg equations? (2)

What factors affect allele frequency? (3)

- Migration

- Mutations

- Selection pressures

What is speciation? (2)

- The process by which new species arise from existing species

- Two main types: Allopatric and Sympatric speciation

What is allopatric speciation driven by? (1)

Geographical isolation

What are the steps of allopatric speciation? (5)

1. Start with a population with variation

2. Population gets geographically isolated into groups

3. Each group faces different environments or selection pressures

4. Each group undergoes different directional selection

5. Groups change so much genetically that they cannot interbreed to produce fertile offspring = new species

What is sympatric speciation? (2)

- Speciation occurring in the same geographical area

- Driven by random mutation

How does reproductive isolation occur in sympatric speciation? (4)

- Temporal mechanism: Different breeding seasons.

- Behavioural mechanism: Different courtship behaviours

- Mechanical mechanism: Mismatch of reproductive parts

- Hybrid sterility: Individuals with different numbers of chromosomes can't reproduce sexually to produce fertile offspring

What happens to the gene pool and selection pressures in sympatric speciation? (3)

1. As the two populations are reproductively isolated, there is no gene flow between them

2. Each population experiences different selection pressures

3. The gene pools diverge due to the different environments

What is artificial selection? (1)

The selection of individuals for breeding with desirable characteristics

How is artificial selection used in modern dairy cattle? (4)

- Farmers select a female with a very high milk yield and a male whose mother had a very high milk yield

- They breed the two together

- Offspring with the highest milk yields are bred over several generations

- Produces cattle with very high milk yields

How is artificial selection used in bread wheat? (3)

- Wheat plants with a high wheat yield are bred together

- Offspring with the highest yields are bred over several generations

- Produces plants with very high yield

What are the ethical considerations of artificial selection in terms of the gene pool? (3)

- Artificial selection reduces the gene pool

- Only organisms with similar traits and similar alleles are bred together

- Leads to a reduction in the number of alleles in the gene pool

What are the future problems of a reduced gene pool due to artificial selection? (3)

- A reduced gene pool can lead to future problems

- If a new disease appears, there is less chance that any of the alleles will offer resistance

- Potentially useful alleles may be lost when other alleles are selected for