Genetic variation and change

Homologous chromosomes

Pairs of chromosomes (1 each from mother and father) in diploid cells that carry the same genes coding for traits in the same order/position (loci) on the chromosome, but the alleles for the trait may not be the same.

Crossing over

The random exchange of sections of DNA between adjacent chromosomes of homologous pairs resulting in recombination. Sections break off and rejoin, swapping alleles inc. variation

Alleles located at loci further away from each other are more likely to be affected by crossing over, resulting in recombination

Independent assortment

Homologous pairs of chromosomes lining up in a random order on the equator, leading to different chromosome combos in the gametes. When the pairs segregate, only one chromosome from each pair into gametes = each gamete has diff combo of maternal and paternal chromosomes and therefore alleles, inc variation

Segregation

The chromosomes in a homologous pair separating randomly into each new cell so each gamete has only one copy of the gene. ½ no. of chromosomes as parent cell. Haploid gametes contain diff combos of alleles increasing variation.

Chromatids separate and move randomly into gametes, then gametes become diploid again at fertilisation, combination of gametes from both parents to give full no of chromosomes

Genetic drift

Random change in allele frequency due to chance events and NOT natural selection/the organism's phenotype e.g storm, fire, disaster. Results in fluctuations of certain allele frequencies, some alleles may become fixed (only one allele for a gene). Decreases g.v.

Effect is larger in small populations as larger populations are more difficult to wipe out, and in small populations each individual represents a much larger proportion of the gene pool.

Founder effect

Formation of a new population by a few founding individuals which is small in size.

Gene pool not representative of the original population, some alleles may be lost = reduced range of alleles and genetic diversity, subject to genetic drift

Bottleneck

When a population drastically reduces in size often suddenly. May be due to human action/catastrophic events.

Gene pool not representative of the original population = reduced range of alleles, some may be missing

Natural selection

Favourable alleles survive when facing selection pressures, survival of the fittest

Linked genes

Genes found on the same chromosome which are more likely to be inherited together. The closer they are the more likely they are to stay together during crossing over (chiasma not formed) and be inherited together. Do not assort independently.

Incomplete dominance

When neither allele is dominant over the other and both alleles have a partial influence on the phenotype (blended phenotype), an intermediate phenotype is produced e.g pink flower (red & white parents)

Codominance

Both alleles are expressed in the phenotype because they are equally dominant e.g black cat + orange cat = black and orange spotted cat

Complete dominance

One allele is dominant over the other and is expressed while the other is not (recessive)

Lethal alleles

An allele that produces a phenotype that reduces life expectancy, formed by mutation. The organism may die at any age but often at embryo stage. Can be dominant or recessive.

Recessive = 2 copies for death

Dominant = may die if theres 2 copies, heterozygous may survive

Multiple alleles

When a gene has more than 2 types of alleles available. Each individual can still only carry two - one on each chromosome.

Ratio for 2 heterozygous cross

9:3:3:1

Migration

The movement of individuals in and out of a population/gene pool. Increases gene flow and gains/losses of alleles

Gene pool

All available alleles in a population

Meiosis

a type of cell division that creates variation within the 4 haploid gametes produced