A-Level Biology OCR Notes

6.1.2 Patterns of inheritance

Download Module Cheatsheet

Google rating

4.8

Trustpilot rating

4.7

View All Resources

Genes & Alleles

The genotype is an organism’s genetic composition
The phenotype is an organism’s characteristics, often visible, which occur as a result of both its genotype and the impact of its environment
Genes are a sequence of DNA that code for a polypeptide
Genes can exist in 2 or more different forms called alleles.
In diploid cells, chromosomes occur in pairs called homologous chromosomes. This means the alleles at a specific locus can be homozygous if they are both the same type of allele or heterozygous, if both the alleles are different.
An allele is dominant if it is expressed in the phenotype of an heterozygous individual.
An allele is recessive if it is not expressed in the phenotype of an heterozygous individual.
An allele is codominant if it is expressed, along with the other allele, in the phenotype of a heterozygous individual.

Monohybrid Inheritance

Monohybrid inheritance is the inheritance of a single gene.

A test cross be used to work out the unknown genotypes of individual organisms
In the test cross the unknown genotype is crossed with a homozygous recessive individual. If all the offspring have the dominant phenotype, the unknown genotype was homozygous dominant for the trait. If half the offspring have the recessive phenotype, the unknown genotype was heterozygous.

Dihybrid Inheritance

Dihybrid inheritance involves the inheritance of two different characteristics simultaneously
During a dihybrid cross, alleles are independently assorted during gamete formation. A punnet square can show all possible genotype and phenotypes of offspring:

In a dihybrid F1 generation cross, the phenotypic ratio for the F2 generation is always 9:3:3:1

Linkage

Autosomal linkage occurs if two or more genes are located on the same autosome (non-sex chromosome). The two genes are less likely to be separated during crossing over, resulting in the alleles of the linked genes being inherited together.
For example, if GN & gn are linked in heterozygous grey bodies and normal winged individuals (GgNn), you get a 3:1 phenotypic ratio

Sex linkage occurs when there is a gene on the X chromosome, not present on the Y chromosome.
This means that males are more likely to exhibit recessive disorders like haemophilia

Epistasis

Epistasis is the interaction between two non-linked genes which causes one gene to mask the expression of the other in the phenotype
Epistatic genes can work antagonistically (against each other) or in a complementary fashion
When a gene suppresses another gene, the gene doing the suppressing is called the epistatic gene. The gene which is being suppressed is called the hypostatic gene.
Antagonistic epistasis can be either recessive or dominant.
In dominant antagonistic epistasis, the expression of the dominant allele of the epistatic gene prevents the expression of the hypostatic gene. This means that any genotypic combination with either one or two of the dominant alleles for the epistatic gene will suppress the expression of the hypostatic gene
Recessive epistasis occurs when the presence of two copies of the recessive allele at the first locus prevents the expression of another allele at a second locus.
In complementary epistasis, the two genes work together, for example, they may encode two enzymes that work in succession.

Complementary Epistasis Example

An example of complementary epistasis is in the inheritance of coat colour in mice.

A/a is the epistatic gene
AA & Aa produces coloured fur
aa produces no pigment- white fur
B/b is the hypostatic gene
BB & Bb encodes for black coloured fur
bb produces encodes for agouti coloured fur

This produces a 9:4:3 phenotypic ratio

Chi-Squared Test

If during an experiment, an unexpected result is obtained, we need to determine whether this unexpected result is due to chance or attributable to a specific cause (significant or not).
The chi-squared test is a type of statistical test that allows us to calculate whether the difference between the results we observe and the results we expected is significant
The null hypothesis assumes that any difference that occurs between the expected and observed results is due to chance.

O is the observed numbers (no units)
E is the expected numbers (no units)

The Χ2 value is then compared to a critical value, found from a chi-squared table by looking at the p-value and degrees of freedom
- The degrees of freedom is the number of categories (or classes) minus one
- The p-value is normally taken as 0.05, meaning that there is a 5% probability that the result is due to chance only

If Χ2 < critical value, then the results are not significant (are due to chance). The null hypothesis is accepted
If Χ2 > critical value, then the results are significant (are attributable to a specific cause). The null hypothesis is rejected

Factors Affecting Evolution

Genetic drift describes change in allele frequencies in the gene pool of a population (evolution) due purely to chance events and not selection pressures
Due to the random nature of gamete production and fertilisation, certain alleles may increase in the population due to chance.
The effect of genetic drift is more prominent within small populations because chance has a greater influence, whereas in larger populations the random fluctuations even out across the whole population.
A genetic bottleneck is when an event causes a big reduction in a population’s size and gene pool. Certain alleles may be due to the event and the population will also be subject to genetic drift.
When a new population is established by a small number of individuals, the founding population will have low genetic diversity and be heavily influenced by genetic drift. This is the founder effect.
Directional selection results in the increase of a favoured allele over time

Stabilising selection maintains genetic polymorphisms in the population

Disruptive selection also maintains genetic polymorphisms in the population

Speciation

Speciation is the evolution of new species from existing ones.
Reproductive isolation followed by accumulation of genetic changes through natural selection can result in the formation of a new species. This is because the populations become genetically distinct with different allele combinations, making them unable to breed to produce fertile offspring.

Allopatric speciation is the formation of two species from an original one due to geographical isolation.
Sympatric speciation is the formation of two species from one original species due to reproductive isolation whilst occupying the same geographical location. This can be by:
- Temporal variation- breeding seasons at different times.
- Behavioural variation- mutations affecting courtship.
- Mechanical variation- anatomical differences preventing mating.
- Gametic variation- results in genetic or biochemical incompatibility.
- Hybrid sterility- cannot produce viable gametes.

Populations

A species is a is a group of individuals that have common ancestry and are capable of breeding with each other and producing fertile offspring.
Species exist as one or more populations
A population is a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.
A gene pool is all of the alleles of all the genes of all the individuals of a population
Allele frequency is the proportion of the individuals that have one copy of an allele
Allele frequencies change in response to selection pressures by natural selection between and within populations.

Population Genetics

Populations can be imagined as gene pools consisting of all the alleles of all the genes of all the individuals in the population
Populations change and evolve as allele frequencies change across generations
The frequency of alleles of a particular gene in a population can be determined using the equation encompassed by the Hardy-Weinberg Principle
Hardy-Weinberg equations:

p + q = 1
p^2 + 2pq + q^2 = 1

Where:
p is the frequency of dominant allele
q is the frequency of recessive allele
p^2 is the proportion of individuals that are homozygous dominant (AA)
q^2 is the proportion of individuals that are homozygous recessive (aa)
2pq is the proportion of individuals that are heterozygous (Aa)

Using the equations, the allele frequencies of a specific gene, genotypes & phenotypes in a population can be estimate.
The Hardy-Weinberg Principle assumes that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next. The conditions for this are that:
- The population is large
- There are no mutations
- There is no selection
- Mating is random within the population
- The population is isolated

Artificial Selection

Artificial selection is selective breeding of organisms which involves humans selecting desired characteristics (phenotypes), interbreeding those phenotypes, and therefore selecting the genotypes which contribute to the gene pool of the next generation.
In a plant selective breeding programme, plants may be grown under certain conditions e.g. low light. Those which grow best and cross-pollinated, and the seeds are grown again under the unfavourable conditions. This is repeated over many generations to produce offspring with advantageous characteristics
Artificial selection reduces the diversity of the gene pool, resulting in inbreeding depression, this increases the chance of inheriting a recessive disorder, and reduces the ability of the species to adapt to environmental changes.
Hybrid vigour is crossing individuals from two separate, inbred gene pools created healthy offspring, heterozygous at many gene loci

Download as a full cheatsheet for free!

Download Now

Google rating

4.8

Trustpilot rating

4.7