5.1.2 Meiosis And Variation

1. Chromosomes are present as granular material calledchromatin

2. DNA replicates

3. New macromolecules and organelles aremanufactured in preparation for cell division

Chromosomes now consist of two identical sister chromatids,joined at the centromere. Cell contains four, copies of each chromosome.

1. Chromatin condenses and undergoes supercoilingso chromosomes shorten and thicken.

2. Homologous chromosomes come to lie closetogether in a process called synapsis; the paired chromosomes are calledbivalents (one maternal; one paternal).

3. The bivalents become shorter and thicker byfurther spiralisation and each chromosome can now be seen to consist of twochromatids held together at the centromere.

4. The pairs of identical sister chromatids are theresult of DNA replication that occurred during interphase; each group of fourchromatids is termed a tetrad.

5. Non-sister chromatids wrap around each other andattach at points called chiasmata, here they swap sections of chromatids incrossing over

6. The nucleolus disappears and the nuclearenvelope disintegrates

7. A spindle forms, made of protein microtubules

2. Bivalents are arranged randomly (randomassortment) with each member of a homologous pair facing opposite poles

3. This allows the chromosomes to independentlysegregate when they are pulled apart in anaphase 1

2. The homologous pairs of chromosomes migrate tothe opposite poles of the cell

There is a brief interphase and the chromosomes uncoil

Plant cells: the cell goes straight from anaphase I intomeiosis II

2. The nucleolus disappears, chromosomes condenseand spindles form.

2. The chromatids of each chromosome are randomlyassorted (arranged)

2. The chromatids randomly segregate

Animals: the two cells now divide to give four haploid cells

Plants: a tetrad of four haploid cells isformed

1. Crossing over

2. Independent random assortment of Bivalents

3. Independent random assortment of (recombinant) chromatids

4. Random mating - choices of mates is random

5. Random fusion of gametes - chances of gamete fertilising another specific gamete

6. Chromosome mutations - any changes to DNA further increases the variation

Occurs during prophase 1:

- Homologous pairs of chromosomes (bivalents) form

- Chiasmata form between homologous chromosomes

- Alleles swap between paternal and maternal chromatids

(Maybe 3/4 crossing over events per bivalent)

Occurs during metaphase 1:

- Chromatids line up along the cell equator randomly and independent of each other

An alternative version of a gene

Specific position on a chromosome, occupied by a specific gene

Combination of alleles in cells of an organism for a particular trait/characteristic

Physical characteristic expressed due to the influence of the genotype and the environment

If a dominant allele is present then the trait is expressed in the phenotype

A characteristic where both alleles contribute to the phenotype

The phenotype from the heterozygous individual expresses the characteristic of both alleles in the genotype

Characteristic in which the allele responsible is only expressed in the phenotype if there is no dominant allele present

Eukaryotic cell or organism that has two identical alleles for a specific gene

Eukaryotic cell or organism that has two different alleles for a specific gene

Genes for different characteristics that are present at different loci on the same chromosome are linked and inherited together.

When non-sister chromatids exchange alleles during prophase of meiosis 1.

A testcross or backcross is always used to see if genes are linked or unlinked.

- A test cross is always a cross with a homozygous recessive genotype for both genes

- This will allow all alleles to be expressed in the phenotypes

- It will allow homozygous recessive genotypes to be expressed for both genes in the offspring if the parent is heterozygous for the two genes

Gene with its locus on one of the sex chromosomes, X or Y.

As there are few genes on the Y chromosome, in humans, most sex-linked genes are on the X chromosome.

Where one gene may mask the expression of another

If it’s the dominant allele causing the ‘hiding’, we call itdominant epistasis and if it’s the recessive allele causing the ‘hiding’ wecall it recessive epistasis.

1. A gene may code for an enzyme with a complementary site for another protein

2. A gene may code for a transcription factor that regulates the expression of another gene

3. A gene may code for a competitive of non-competitive inhibitor for an enzyme synthesised from another gene

As epistasis involves two genes, the epistatic crosses between two heterozygotic individuals for both genes results in a 9:3:3:1 ratio.

All the epistatic phenotypic ratios will be variants of the 9:3:3:1 ratio.

12:3:4

Grouping of the dominant phenotypes, genotypes (9:3 = 9+3 = 12)

9:3:4

Grouping of the recessive phenotypes, genotypes (3:1 = 3+1 = 4)

9:7

Grouping of the intermediate and recessive phenotypes, genotypes (3:3:1 = 3+3+1 =7)

No distinct categories (quantitative)

Polygenic - many genes contribute to the final phenotype (2 or more)

- Each gene provides an additive effect on the phenotype

Genes are not linked, they are on separate chromosomes

Influenced by the environment

Eg. Height, weight, etc.

Distinct categories (qualitative)

Monogenic - one gene contributes to the final phenotype

- Epistasis if there is more than one gene at work

Different alleles have a large effect on the phenotype

Different genes at each different loci have large effects on the phenotype

E.g. blood groups, ear lobe shape, etc.

The phenotype is influenced by a combination of both the environment and the individuals genotype

'The genes load the gun and the environment pulls the trigger'

Polygenic traits are more easily influenced by the environment that monogenic traits

If there is no variation then all individuals have an equal chance of surviving

This would mean if the environment changed then all would live and pass on their genes to the next generation or all would die and the species would become extinct

Variation ensure evolution as some individuals of the species are more suitably adapted to the environment

1. The population is small

2. Mating is not random

3. There are mutations

4. There is migration in or out of the population

5. There is genetic drift

6. There is natural selection

p + q = 1

p² + 2pq + q² = 1

The total number of different alleles within a population.

- Populations have gene pools

- Individuals have genomes

Environmental factor that confers greater chances of surviving and reproducing on some members of the population than on others.

The environmental factor could be natural or artificial.

Biotic:

- Disease

- Food

- Competition (interspecific or intraspecific)

Abiotic:

- Water availability (soil or pools)

- Mineral availability

- Space

- Light

- Temperature

- Size of yield

- Quality of yield

- Progeny

- Disease resistance

- Temperament

Stabilising selection

Favours individuals with a general phenotype

Maintains the populations mean phenotype by selecting against the phenotypic extremes

E.g. Calf birth weights

Too small = difficulty carry our thermoregulation

Too large = they cannot pass through the pelvic girdle

Directional selection

Favours individuals with phenotypes at one extreme and selects against individuals the other extreme

Allele frequency is shifting in one direction

E.g. Giraffe neck lengths

Favours individuals at both extremes and selects against those with a general phenotype

Supports speciation

E.g. Galapagos Finches beak size

- The change in allele frequency in a population, as some alleles pass to the next generation while others disappear

- Chance or random selection of individuals and hence their alleles

- Individuals are selected against by natural disasters and chance events as opposed to their suitability to the environment

Accelerates evolution

Most exaggerated in small populations

E.g. Earthquake

A mechanism that divides population of organisms into smaller sub groups.

Isolating a population will:

- Decrease the gene pool

- Increase inbreeding which could lead to an increase in homozygous recessive genotypes within a population

- Increase the effects of genetic drift

When two species or populations occupy different habitats within the same environment

Can be separated by a physical barrier, such as a mountain range, river, road systems, island etc.

Example: Galapagos finches

When the reproductive structures are no longer physically compatible

Example: Great Dane will not mate with a Chihuahua

When two species or populations live within the same area but are reproductively active at different times

Example: American frog species

- Wood frog: fertile in April

- Tree frog: fertile in June

- Bull frog: fertile in July

When two species or populations evolve different courtship displays which are essential for successful mating

Example: bird of paradise

Can interbreed to produce fertile offspring

Reproductively isolated

Have the same:

- Morphology

- Physiology

- Anatomy

- Embryology

- Behaviour

- Occupy the same ecological niche

All living things have RNA / DNA / proteins

Comparisons to relatedness of these molecules can determine relatedness of organisms

Haplotypes can also be used for comparisons:

- Holotypes are particular base sequences

- Can be 1 locus, several loci or an entire chromosome

- They are a combination of alleles at different loci transmitted together

A group of organisms with similar haplotypes

Shows the common ancestor and all descendants

A monophyletic group can be 1 or more clades

Only linear more like traditional classification

Mechanism for evolution

- Organisms that are well adapted to their environment

- More likely to survive and reproduce

- Passing on the alleles for favourable characteristics

- Humans select organisms with desirable characteristics

- They reproduce, passing on the alleles for favourable characteristics