Biology - Term 1 - Genetics

Describe founder effect

The founder effect happens when a small group starts a new population. It’s gene pool is less varied and may have different allele frequencies from the original population

Describe genetic drift

Genetic drift is a random change in allele frequency, especially in small populations. Alleles may become more common or be lost by chance, not because they are advantages

Describe gene pool

A gene pool is all the alleles present in a population. Variation in the gene pool affects how a population can respond to selection pressures.

What is allele frequency?

Allele frequency is how common an allele is in a population. It can change through mutation, natural selection, genetic drift, migration, or chance events.

What is natural selection?

Natural selection is when individuals with advantageous inherited traits survive and reproduce more successfully. Over time, favourable alleles become more common.

What is Mutation?

A mutation is a permanent change in DNA. It creates new alleles, so it is the original source of genetic variation in populations.

What is gametic mutation?

A gametic mutation happens in a sex cell or in a cell that will form gametes. It can be passed to offspring, so it can change allele frequencies over generations.

Define mutation as a source of new alleles, and explain why it increases genetic variation.

A mutation is a change in the DNA base sequence. If it occurs in a gene, it can create a new allele. New alleles add extra heritable variation, which provides more possible phenotypes for selection or chance to act on

Explain why gametic mutations can change a population gene pool, but somatic mutations usually do not.

Gametic mutations occur in cells that form gametes, so the altered allele can be inherited by offspring and enter the population gene pool. Somatic mutations affect body cells only, so they usually change only the individual, not population allele frequencies.

Describe segregation in meiosis and explain how it produces genetically different gametes.

Segregation is the separation of homologous chromosomes, and therefore allele pairs, into different gametes during meiosis. Each gamete receives only one allele for each gene, so gametes differ in allele combination and variation increases.

Explain how independent assortment creates variation during meiosis.

Independent assortment is the random orientation of homologous chromosome pairs at metaphase I. Because maternal and paternal chromosomes line up independently, different combinations go into gametes, producing genetically varied offspring.

Describe crossing over and explain why it can produce new allele combinations.

Crossing over is the exchange of DNA segments between non-sister chromatids of homologous chromosomes in meiosis. This reshuffles alleles on chromosomes, creating recombinant chromosomes with new allele combinations not present in either parent chromosome.

Discuss why linked genes do not usually give a 9:3:3:1 dihybrid ratio.

Linked genes are close together on the same chromosome, so they tend to be inherited together. Because they do not assort independently, the gametes are not produced in equal combinations, so the phenotype ratio departs from 9:3:3:1.

Explain how genes can behave as unlinked even when they are on the same chromosome.

If two genes are far apart on the same chromosome, crossing over is more likely to occur between them. This separates the alleles often enough that they assort almost as if they were on different chromosomes, so they appear unlinked.

Define sex linkage and explain why some phenotypes are more common in one sex.

Sex-linked genes are carried on a sex chromosome, usually the X chromosome. Males have only one X, so a recessive allele on that X is expressed because there is no second allele to mask it, making some traits more common in males. 

Describe how multiple alleles increase variation in a population.

Multiple alleles means a gene has more than two allele forms in the population, even though each individual still has only two. This increases the number of possible genotypes and phenotypes, so variation in the population is greater.

Explain co-dominance and why both alleles are expressed in the phenotype.

In co-dominance, both alleles in a heterozygote are fully expressed because neither allele masks the other. As a result, the phenotype shows both traits at the same time, rather than blending into one intermediate form.

Explain incomplete dominance and why the heterozygote has an intermediate phenotype.

In incomplete dominance, neither allele is completely dominant, so the heterozygote produces a phenotype between the two homozygotes. This happens because the dominant allele does not make enough product to give the full dominant phenotype.

Describe how a lethal allele alters expected inheritance ratios.

A lethal allele causes death in a particular genotype, so some offspring die before they can be counted in the phenotype ratio. This removes one genotype class and changes the expected mendelian ratio among surviving offspring.

Explain why a monohybrid cross with complete dominance often gives a 3:1 phenotype ratio.

In a cross Aa × Aa, segregation produces gametes A and a in equal proportions. Random fertilisation gives genotypes 1 AA : 2 Aa : 1 aa, but because A is dominant, AA and Aa have the same phenotype, giving a 3:1 phenotype ratio.

Explain why a dihybrid cross with unlinked genes can produce a 9:3:3:1 phenotype ratio.

For two heterozygous genes on different chromosomes, independent assortment produces four gamete types in equal probability. Random fertilisation combines these into sixteen outcomes, which group into the 9:3:3:1 phenotype ratio under complete dominance.

Discuss how natural selection changes allele frequency in a population.

Natural selection occurs when individuals with advantageous heritable phenotypes survive and reproduce more successfully. Because those phenotypes are caused by certain alleles, those alleles are passed on more often and increase in frequency over generations.

Define selection pressure and explain how it affects survival and reproduction.

A selection pressure is an environmental factor that affects survival or reproductive success, such as predation, disease, or drought. It favours some phenotypes over others, so the alleles underlying favourable traits become more common.

Describe migration (gene flow) and explain how it can change a population gene pool.

Migration, or gene flow, is the movement of alleles into or out of a population when individuals migrate and breed. This changes the gene pool by introducing new alleles or altering existing allele frequencies, which can increase or decrease variation.

Explain genetic drift and why its effect is stronger in small populations.

Genetic drift is a random change in allele frequency caused by chance rather than selection. In small populations, chance events affect a larger proportion of the gene pool, so alleles can be lost or become common very quickly.

Describe the founder effect and explain why it can reduce variation in a new population.

The founder effect happens when a small group starts a new population. Because the founders carry only part of the original gene pool, the new population has reduced variation and allele frequencies that may differ greatly from the source population.

Explain how a bottleneck effect changes allele frequency and reduces genetic variation.

A bottleneck effect occurs when population size is drastically reduced by chance events such as fire or disease. The survivors are not genetically representative of the original population, so allele frequencies shift randomly and overall variation falls.