GENE VARIATION LEVEL 2 NCEA BIOLOGY

Meiosis

Meiosis

a type of cell division division that produces sex cells/gametes with half the number of chromosomes as the somatic cell. Produces genetically unique daughter cells.

3 sources of variation in gametes

1. independent assortment

2. segregation

3. Crossing over

Independent assortment

where the homologous pairs line up randomly during meiosis. 
Alleles are reshuffled and new combinations of chromosomes are created. 

Segregation

where homologous chromosomes pull apart and migrate to the cell poles during meiosis.
Only one chromosome from each homologous pair is placed into the new gametes made.

Crossing Over

 the exchange of alleles / segments of chromosomes between homologous pair chromosomes. Crossing over can create new allele combinations of unlinked genes.

Homologous chromosomes

Each carries the same genes in the same order, but the alleles for each trait may not be the same. One is inherited from each parent.

Unlinked genes

found on different chromosomes. Genes that are found further apart are more likely to be unlinked, because they will not stay together during crossing over.

Incomplete Dominance

type of relationship between alleles, with a heterozygote phenotype intermediate between the two homozygote phenotypes, is called incomplete dominance.

Meiosis steps

1. Interphase

2. Prophase 1

3. Metaphase 1

4. Anaphase 1

5. Telophase 1

6. Cytokinesis 1

7. Prophase 2

8. Metaphase 2

9. Anaphase 2

10. Telophase 2

11. Cytokenesis 2

Interphase

DNA replication

Prophase 1

Chromosomes condense, spindles form, nuclear envelope breaks down, Crossing over occurs

Metaphase 1

Pairs of homologous chromosomes move to the equator of the cell, Independent assortment occurs

Anaphase 1

Homologous chromosomes move apart to opposite poles of cell, Segregation occurs

Telophase 1

chromosomes gather at poles of the cell

Cytokinesis 1

Cytoplasm divides forming 2 daughter cells

Prophase 2

Chromosomes condense, new spindle form around chromosomes, nuclear envelope breaks down

Metaphase 2

Chromosomes move to the equator of the cell

Anaphase 2

sister chromatids are separated and pulled to opposite ends of the cell

Telophase 2

Chromosomes gather at the poles of the cell and the nuclear envelope reforms

Cytokinesis 2

Cytoplasm divides forming 4 unique daughter cells with half the number of chromosomes

Linked Genes

located on the same chromosome-inherited together

What do linked genes cause

reduce genetic diversity and produce less-diverse phenotypes

why do linked reduce diversity of genotypes

they cannot independantly assort and segreggate

Linked genes CAN NOT

independently assort and seggragate

Multiple alleles

Genes with more than two allele forms.

Locus

Specific location of a gene on a chromosome.

Lethal alleles

a condition when an allele is able to produce a phenotype that kills the organism, it results in a nonfunctional protein. Dominant lethal alleles are usually eliminated rapidly in the population as their expression is fatal.

Co-dominance

A genetic scenario where both alleles in a heterozygote are fully expressed.

Incomplete dominance

A genetic situation where neither allele is completely dominant, resulting in a blended phenotype.

Gene Pool

A gene pool refers to the combination of all the genes (including alleles) present in a reproducing population or species

Allele Frequency

the number of times that an allele occurs in a population

Population

interbreeding organisms of the same species in a particular area

4 things that change the frequencies of alleles in a gene pool

1. Mutation-the source of all new alleles

2. Gene Flow-entering or exiting the gene pool

3. Natural selection

4. Genetic drift (bottleneck, founder effect)

Migration

the movement of individuals from one population to another

When individuals migrate…….

alleles are removed from the original population and added to the new population

Gene flow (migration) DOES NOT…..

create new alleles in the species

Gene flow (migration) DOES…

change allele frequencies

Genetic Drift

the change in frequency of an existing gene variant in the population due to random chance. Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation.

Founder effect

the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population, seperate from the original population

Bottleneck effect

a phenomenon in which a population is reduced in size due to natural disasters, habitat loss, or overhunting, then recovers. Even though the population size may increase, the variation is still reduced as many alleles are lost

Consequence of bottleneck, founder effect/ Genetic drift

the loss of variation in a population caused by the reduction of allele frequencies.

Natural selection

the process where individuals that posses the ‘best’ phenotype in an environment survive and reproduce, passing their alleles which increases the frequency of this allele in population

Nature selects….

Phenotype

Organisms with less favourable phenotype…..

will not pass on their alleles as often, and therefore the frequency of their alleles in the gene pool decreases

high diversity means

more alleles in the gene pool

Types of evolutionary/selection pressures

1. Biotic

   1. predation and disease

   2. competition

2. Abiotic

   1. Availability of resources

   2. Climate

non-random events cause

Natural Selection

Random events cause

Genetic Drift

Gene fixation

the change in a gene pool from a situation where there exists at least two alleles of a gene to a given situation where only one of the alleles remains. This remaining allele is fixed and the one that does not is lost.

Gene flow

the movement of alleles from population to population

The advantages of multiple alleles

more phenotypes are expressed, and therefore more phenotypes can respond to natural selection and increase the survival chances of the species.

What do linked genes cause

reduce diversity of genotypes and produce less diverse phenotype.