Patterns of inheritance and variation

Keys terms:

Genotype: the genetic makeup of an organism

Phenotype: observable charateristics of an organism

Dominant allele: version of a gene that will always be expressed if present

Recessive allele: version of a gene that will only be expressed if two copies of this allele are present

Homozygous: two identical alleles for a characteristic

Herterozygous: two different alleles for a characteristic

Phenotypes in plants

Chlorosis is when the plant suffers from a lack of chlorophyll, due to:

• lack of light
• mineral deficiencies
• virus infections

Phenotypes in animals

Animal body mass: is determined by environmental and genetic factors.

Genetic variation

Genetic variation is created by the versions of genes you inherit from your parents. There are a number of different variations for each gene. The combination is determined by sexual reproduction involving meiosis and the random fusing of gametes.

Key terms:

Continuous variation:

a characteristic that can take any value within a given range.

This could be genetic and environmental controlled by a number of genes.

An example includes leaf surface area, animal mass, skin colour.

Discontinuous variation:

A characteristic that can only appear in specific values, it is mostly genetic and is controlled by one or two genes.

Examples include blood group, albinism and round and wrinkled pea shape

This occurs when two different alleles occur for a gene. The result is both allelesare expressed in the phenotype of an organism.

Some characteristics are determined by genes carried on the sex chromosomes, these are sex linked.

Haemophilia is an example.

A dihybrid cross is used to show the inheritance of two different characteristics, caused by 2 genes, which may be located on different pairs of chromosomes.

Linkage

Linkage means that genes are located on the same chromosome. When the genes that are linked are found on one of the other pairs of chromosomes it is called the autosomal linkage. Linked genes are inherited as one unit and there is no independent assortment.

The recombinant frequency

number of recombinant offspring/ total number of offspring

A recombinant frequency of 50% indicates that there is no linkage and the genes are on seperate chromosomes.

If the value is higheer than the significant value we do not have strong evidence to reject our null hypothesis. Therefore we accept that there is no significant difference between what we observed and what we expected.

This is the interaction o9f genes at different loci. gene regulation is a form of epistasis.

Dominant and recessive epistasis

An epistatic genes may influence the activity of other genes as a result of the present of dominant or recessive alleles.

Key terms:

Gene pool: sum total of all the genes in a population at a given time

Allele frequency: the relative frequency of a particular allele in a population at a given time

Founder effect:when a few individuals of a species colonise a new area

Directional selection: Natural selection that favours one extreme of a phenotype

Disruptive selection: natural selection that favours both extremes of a phenotype

Stabilising selection: Natural selection tat favours average phenotypes

Population genetics investigates how allele frequencies change over time.

Model the m athematical relationship between the frequencies of alleles and genotypes in a theoretical population that is stableand not evolving. It states that:

in a stable population with no disturbing factors, the alleles frequencies will remain constant from one generation to the next and there will be no evolution.

p2= frequency of homozygous dominant

2pq= heterozygous

q2= homozygous recessive

• mutation
• sexual selection
• gene flow
• genetic drift
• natural selection

Key terms:

Speciation: the formation of a new species through the process of evolution

Allopatric speciation: happens when some members of a population are seperated by physcial barriers

Sympatric speciation: occurs within populations that share the same habitat. Two different species interbreed and create fertile offspring

Polymorphic: display more than one distinct phenotype