Achievement criteria and explanatory notes

Achievement Standard

2.5 Genetic variation and change

Key terms

Sources of variation allele codominance cross (genetic cross) crossing over dihybrid inheritance dominant allele genetic variation genotype heterozygous homozygous incomplete dominance independent assortment lethal allele linked genes locus meiosis monohybrid inheritance mutation phenotype recessive allele recombination trait

Gene pools bottleneck effect founder effect gene pool genetic drift migration (=gene flow) natural selection variation

A population's gene pool consists of all the alleles (genetic variants) present. Mutation is the source of all new alleles. Processes in gene pools, including natural selection and genetic drift, result in allele frequencies changing over time.

Achievement criteria and explanatory notes

Achievement criteria for achieved, merit, and excellence

c A Demonstrate understanding of genetic variation and change: Define and use annotated diagrams or models to describe genetic variation and change. Describe characteristics of, or provide an account of, genetic variation and change. c M Demonstrate in-depth understanding of genetic variation and change: Provide reasons as to how or why genetic variation and change occurs. c E Demonstrate comprehensive understanding of genetic variation and change: Link biological ideas about genetic variation and change. The discussion may involve justifying, relating, evaluating, comparing and contrasting, or analysing.

Kristian Peters

Explanatory notes: Genetic variation and change Activity number

Genetic variation and change involves the following concepts...

c 1 Sources of variation within a gene pool.

59 - 82

c 2 Factors that cause changes to the allele frequency in a gene pool.

87 - 95

Biological ideas and processes relating to sources of variation within a gene pool

Activity number

Select biological ideas and processes from...

c i Mutation as a source of new alleles.

60 62 - 64

c ii Independent assortment, segregation, and crossing over during meiosis. 65 - 69 c iii Monohybrid inheritance to show the effect of codominance, incomplete dominance, lethal alleles, and multiple alleles.

70 - 78

c iv Dihybrid inheritance with complete dominance. 79 82 c v The effect of crossing over and linked genes on dihybrid inheritance. 80 81 82

Biological ideas and processes relating to factors affecting allele frequencies within a gene pool

Select biological ideas and processes from...

c i Natural selection (differential survival and reproduction of individuals as a result of differences in phenotype). Activity number

87 - 92

c ii Migration (transfer of alleles or genes from one population to another). 87 c iii Genetic drift (random fluctuations in the numbers of gene variants in a population). 93 - 95

What you need to know for this Achievement Standard

Marc King

Jeff Podos

Sources of variation within a gene pool

Activities 59 - 86

By the end of this section you should be able to: c Define the terms allele, locus, heterozygous, and homozygous in relation to chromosomes.

Demonstrate your understanding of how these terms are used in inheritance. c Explain what is meant by mutation and describe mutation as the source of all new alleles. c Identify both genetic and environmental causes of variation. Describe examples of discontinuous and continuous variation in populations and their origin. c Describe the main events occurring in meiosis, including the reduction division and its significance, the segregation of alleles, crossing over and independent assortment, and second division. c Explain recombination as the exchange of alleles between homologous chromosomes as a result of crossing over. c Explain how meiosis and fertilisation contribute to variation in the offspring. c Describe Mendel’s principles of inheritance and explain their importance to our understanding of heredity and evolution. c Define the terms dominant allele, recessive allele, lethal allele, codominant alleles, incomplete dominance, and multiple alleles. c Define genotype, phenotype, monohybrid cross, dihybrid cross, F1 generation, F2 generation and demonstrate correct use of these terms in studies of inheritance. c Solve monohybrid crosses involving simple dominant-recessive inheritance, codominance, incomplete dominance, and lethal alleles. Describe the resulting phenotype and genotype ratios in each case. c Solve problems involving dihybrid crosses of unlinked, autosomal genes for two independent traits showing complete dominance. c Describe and explain the effect of crossing over and linked genes on dihybrid inheritance.

Jeff Podos

Factors causing changes to allele frequencies in gene pools

Activities 87 - 98

By the end of this section you should be able to: c Define the term gene pool and allele frequency. Understand that a change in the allele frequencies of a population over time is evolution. c Recall the sources of variation in gene pools as a result of mutation and sexual reproduction. c List factors that a cause change in allele frequencies in a gene pool, including mutation, natural selection, gene flow (migration), and genetic drift. c Describe the process of natural selection and explain its role in sorting variation and establishing adaptive genotypes. Using examples, explain how natural selection tends to reduce genetic variation within a gene pool and increase genetic differences between populations. c How gene flow (migration) may affect allele frequencies. Explain how gene flow tends to reduce the genetic differences between populations. c Define genetic drift and explain its consequences to allele frequencies in populations. Using examples, explain when genetic drift is most important.

c Define the founder effect. Using examples, describe the genetic and evolutionary consequences of the founder effect, including the greater impact of genetic drift in founder populations. c Define the bottleneck effect (also called genetic bottleneck or population bottleneck). Using examples, describe the genetic and evolutionary consequences of the bottleneck effect, including the greater impact of genetic drift in populations that have been through a bottleneck.