IB Biology Topic D3.2.8: Single-nucleotide Polymorphisms and Multiple Alleles in Gene Pools Notes

A Gene Pool Holds Every Allele in a Population

1. A gene pool is the sum of all alleles present in every member of a population.
2. Each individual carries only two alleles per gene, but the population as a whole can hold many different alleles, which are the different versions of that gene.

SNPs Are Single-Base Differences That Create New Alleles

1. A single-nucleotide polymorphism (SNP) is a position in the genome where a single base pair differs between individuals in a population.
2. A SNP can change the amino acid a codon specifies, which produces a new allele of the gene.
3. Millions of SNPs are scattered across the human genome, and together they make each person genetically unique.

A Population Can Hold Many More Than Two Alleles

1. An individual inherits only two alleles for a gene, one from each parent.
2. A gene with more than two alleles across the population is said to have multiple alleles.
3. The ABO blood group is a familiar example, covered in detail in D3.2.9.

Apple Trees Use Multiple S-Alleles to Avoid Self-Pollination

1. The S-gene in apple trees blocks self-pollination, which keeps the population genetically diverse.
   1. The apple gene pool holds over 30 different S-alleles.
   2. Each tree inherits two S-alleles and rejects any pollen carrying an S-allele that matches one of its own.
2. A Jazz apple (S2S3) rejects pollen from a tree that shares an S-allele with it.
3. A Gala apple (S5S9) can pollinate the Jazz tree because none of its alleles match.

Allele Diversity Is a Population's Toolkit for Survival

Why Diversity Matters

1. SNPs and multiple alleles are the raw source of genetic variation in a gene pool.
2. This variation lets a population adapt to changing environments and resist disease, and it is the material natural selection acts on.