Patterns of inheritance

Genotype= The genetic constitution of an organism.

Phenotype= The expression of the genes and its interaction with the environment.

Homozygous= A pair of homologous chromosomes carrying the same alleles for a single gene.

Heterozygous= A pair of homologous chromosomes carrying two different alleles for a single gene.

Codominant= Both alleles are equally dominant and expressed in the phenotype.

Multiple alleles= More than two alleles for a specific gene

Sex-linkage= A gene whose locus is on the X chromosome.

Autosomal linkage= Genes that are located on the same chromosome (not the sex chromosome).

Epistasis= When one gene modifies or masks the expression of a different gene on a different locus.

The contribution of both environmental and genetic factors to phenotypic variation:

Characteristics controlled by environmental factors influence how some characteristics develop, such as cold weather may allow organisms to develop thicker fur for insulation.

Physical factors- May occur due to mutations (exposure to mutagens).

Mutations that occur in gamete formation are persistent and random, can be inherited

Chemical factors- Mustard gas.

Biological factors- Some viruses, Transposons.

Environmental factors:

Availability of water
Temperature range
Oxygen levels'
Supply of nutrients
Length of sunlight hours

We can identify if trees are different by looking at their age, colour, number of leaves, height.

These can affect which of an animal’s genes are expressed, which affects the phenotype. Can affect how animals grow and develop. These alterations caused by environmental factors cannot be inherited.

Diet in animals:

The flies that should be grey can turn yellow due to their diet.

Growing conditions for plants:

Chlorosis, occurs when the synthesis of chlorophyll is slowed down or stopped= Plants that grow in the dark and cannot access enough magnesium can turn yellow, although, genetically they should be green.

Etiolation= Plants that are grown in the dark may develop long stems with small, curled leaves.

Environmental + genetic - a combined effect

( For example, a plant may have the genes for extra height and may have enough soil nutrients, so, these factors allow it to grow taller ).

Sexual reproduction can lead to variation:

Crossing over of homologous chromosomes in PROPHASE I of meiosis to form bivalents, this forms chiasma which allows DNA to be exchanged, forms different alleles ( recombination of alleles ),
Independent assortment of chromosomes leads to variation,
Meiosis and the random fusion of gametes at fertilisation, produce many gametes.

In asexual reproduction, mutations are the only way that genetic variation can increases.

Characteristics that are purely genetic include: blood group; cystic fibrosis.

Genetic diversity= Differences in DNA between individuals and a populations, gene pool. Allows for long time survival of a species as they are able to adapt and survive changes in the environment.

Interspecific variation:

The differences between any two species.
Individuals from a different species tend to show a large amount of interspecific variation.

Intraspecific variation:

The differences between members of the same species.
There are sub species of dogs for e.g.
A population with greater genetic diversity will show greater intraspecific variation, populations with lots of alleles will have more genetic diversity.

Continuous variation: Many genes involved ( polygenic )

When there is two extremes and a full range of values inbetween.
Height.
Antigens on cells.
Number or leaves.
Most people tend to be near the mean, there is a normal distribution.
Traits that show continuous variation are often regulated by multiple genes and influenced by the environment ( e.g. Weight : how much you eat and exercise and also genetic factors ).

Discontinuous variation: One or two genes involved

Where there are distinct categories with no intermediate values.
Individuals tend to be distributed quite evenly between the different categories ( same amount of people that brown eyes = same amount of people that have blue eyes ).
Traits that show discontinuous variation are often regulated by a single gene and aren’t under environmental influence. ( e.g. a mouse born with black hair will maintain its black hair regardless of the environment ).
Blood group

Smaller standard deviation= more tightly clustered around the mean.

Greater standard deviation= less tightly clustered around the mean.

The factors that affect the evolution of a species: Natural selection

Overproduction of offspring,
Constancy of numbers,
Struggle for survival,
Variation amongst offspring,
Survival of the fittest,
Like produces like,
Formation of new species.

3) Selection pressure - climate; lack of food; lack of space; predators.

4) DNA mutations, different alleles, genetic variation

5) Those best adapted to the environment are more likely to survive due to advantageous alleles.

6) Advantageous alleles passed onto offspring, can reduce genetic biodiversity.

Allele frequency= how often a particular allele occurs within a population.

Natural selection can only occur if there is genetic diversity.

Evolution is the change in allele frequency over many generations in a population.

Artificial selection:

Humans select plants or animals with favourable characteristics and deliberately breed them together.
Manipulates the gene pool so that the favourable alleles become less common. Very small gene pool.
It is faster than natural selection.
For example, pugs were bred to have features deemed cute, leads to many ethical and medical issues.
Cows to increase milk yield
Wheat to increase yield and decrease disease susceptibility.
reduces gene pool, only animals with similar traits and similar alleles are bred together leading to a reduction in the number of alleles in the gene pool.
Can cause health problems + there are ethical issues.
Genetic biodiversity can decrease due to hunting as there is a decrease in population.

GENE BANKS are stores of biological samples ( plant seeds,, animal semen, eggs ). Selective breeding involves inbreeding, so gene banks are used to help increase genetic diversity by outbreeding. This reduces the frequency of homozygous recessive disease in the offspring.

In plants : cuttings. Cuttings are taken from plants with favourable characteristics to produce offspring with those advantageous alleles to produce offspring with the same characteristics. The offspring is genetically identical to the original plant.

The importance of maintaining a resource of genetic material for use in selective breeding including wild types= Increases the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype. Leads to decreased growth and survivability.

Stabilising selection:

Favours modal trait, occurs when there is no change in the environment and the modal trait remains the same. This means that more individuals will have the modal trait and are more likely to survive, which reduces the range of phenotypes.
Standard deviation decreases.
For example, human birth weight.

Disruptive selection:

Individuals containing alleles for the extreme trait are more likely to survive and pass on their alleles.
The allele frequency changes and more individuals possess this trait.
The mean trait allele becomes less frequent.
Continued disruptive selection can lead to speciation.
This will lead to two new species.

Directional selection:

Occurs when there is change in the environment.
Only one of the extreme trait alleles has a selective advantage.
The modal trait changes.
Fore example, antibiotic resistant bacteria.

Speciation : The process which results in the creation of a new species.

Occurs when one original population of the same species becomes reproductively isolated by certain barriers. This means there are two populations of the same species, but they cant breed.

This can result in the accumulation of differences in their gene pools to the extent that the two populations would be unable to interbreed to make fertile offspring, therefore, they’re classed as two species.

Summary= A population of the same species can become reproductively isolated due to geographical, behavioural, gametic, mechanical factors. This results in them not being able to breed together. Therefore, this can result in differences in their gene pool which suggests that they are unable to interbreed to make fertile offspring, so, they are classed as two different species.

Allopatric speciation:

When populations are separated geographically leading to reproductive isolation.
Can be due to natural disasters.
This separates the original population into two, they are unable to reproduce.
These two populations will accumulate different beneficial mutations over time in order to survive in their environments, where, over time the two populations become so genetically different, they are unable to interbreed to create fertile offspring.

Sympatric speciation:

When populations become reproductively isolated due to differences in their behaviour.
Could be due to a random mutation.
May cause individuals to perform a different courtship ritual or for individuals to be fertile at different time of the year.
Due to this, individuals cannot reproduce together and there will be no gene flow between the two groups.
Overtime, they become so reproductively isolated that they cannot interbreed to create fertile offspring as their DNA is so different as they accumulate different mutations.
Mechanical changes in genitalia.

GENETIC DRIFT= The change in the allele frequency within a population between generations. Substantial genetic drift results in evolution. The smaller a population is, the bigger the impact allele frequency changes have proportionally and this is why evolution occurs more rapidly in smaller populations. Can change by chance, it is random.

Genetic bottlenecks:

Caused by events that kill almost all of the population, leaving only a few individuals,
Small gene pool,
Alleles for genes are lost,
The remaining breeding population pass on their alleles,
Lack of genetic biodiversity and genetic diseases which already exist are more likely to be passed onto offspring.

The Founder Effect:

When a few individuals from a population relocate to an isolated area.
This results in a small population breeding together,
Small gene pool,
Limited genetic diversity,
More susceptible to disease.

The ethical considerations surrounding the use of artificial selection:

Selectively breeding dogs for them to have more cute, desirable features.
Leads to health issues.
Animal welfare.
Lack of diversity.
More prone to disease.
Shorter lifespan.

Patterns of Inheritance in OCR A Level Biology

Central Idea:
- Patterns of inheritance in genetics
Main Branches:
- Mendelian Genetics:
  - Monohybrid Inheritance:
    - Punnett squares
    - Genotype and phenotype ratios
  - Dihybrid Inheritance:
    - Independent assortment
    - Dihybrid crosses
- Non-Mendelian Inheritance:
  - Codominance:
    - Incomplete dominance
    - Blood type inheritance
  - Sex-Linked Inheritance:
    - X-linked recessive
    - X-linked dominant
  - Epistasis:
    - Gene interaction
    - Complementation
  - Polygenic Inheritance:
    - Multiple genes
    - Continuous variation
Sub-branches:
- Mendelian Genetics:
  - Monohybrid Inheritance:
    - Punnett squares:
      - Predicting offspring genotypes
      - Predicting offspring phenotypes
    - Genotype and phenotype ratios:
      - 1:2:1 genotype ratio
      - 3:1 phenotype ratio
  - Dihybrid Inheritance:
    - Independent assortment:
      - Law of independent assortment
      - Gamete combinations
    - Dihybrid crosses:
      - 9:3:3:1 phenotypic ratio
      - 16-box Punnett square
- Non-Mendelian Inheritance:
  - Codominance:
    - Incomplete dominance:
      - Intermediate phenotype
      - Heterozygous expression
    - Blood type inheritance:
      - ABO blood groups
      - Multiple alleles
  - Sex-Linked Inheritance:
    - X-linked recessive:
      - Carrier females
      - Affected males
    - X-linked dominant:
      - Affected females
      - Rare in nature
  - Epistasis:
    - Gene interaction:
      - Masking of alleles
      - Phenotypic ratios
    - Complementation:
      - Multiple gene pairs
      - Complementation tests
  - Polygenic Inheritance:
    - Multiple genes:
      - Additive