AP Syllabus focus:
‘Sexual reproduction increases genetic variation through crossing over, random assortment of chromosomes, and fertilization of gametes.’
Sexual reproduction reshuffles genetic information across generations. In AP Biology, the key idea is that meiosis and fertilisation create new allele combinations, producing offspring that are genetically unique and providing raw material for evolution.
Core idea: why sexual reproduction creates variation
Sexual reproduction generates genetic variation because it mixes alleles from two parents and because meiosis itself produces genetically non-identical gametes.
Genetic variation: Differences in DNA sequences (alleles and chromosomal combinations) among individuals in a population.
Variation from sexual reproduction is especially powerful because it acts at two levels:
Within chromosomes (new allele combinations along a chromosome)
Among chromosomes (new sets of whole chromosomes in a gamete)
Source 1: crossing over (recombination) during meiosis
Crossing over occurs when homologous chromosomes exchange corresponding DNA segments, producing recombinant chromatids with new allele combinations.
Homologous chromosomes are shown paired and aligned by the synaptonemal complex during prophase I. This tight alignment positions nonsister chromatids so that crossing over can exchange corresponding DNA segments, producing recombinant chromatids with new allele combinations. Source
The key outcome is that alleles previously linked on the same chromosome can be rearranged into new combinations.
What crossing over changes
Creates new haplotypes (combinations of alleles on the same chromosome)
Increases variation beyond what would occur by simply separating intact maternal vs paternal chromosomes
Ensures that even gametes receiving the same set of chromosomes can still differ genetically
Why it matters for offspring
Because each gamete can carry chromatids that are mosaics of maternal and paternal DNA, siblings can inherit different allele combinations even from the same parents.
Source 2: random assortment of chromosomes during meiosis
Random assortment (often described as independent orientation) refers to the fact that homologous chromosome pairs line up randomly relative to the poles during meiosis I.
Homologous chromosome pairs can orient in more than one equally likely way at the metaphase plate in meiosis I. Different orientations send different maternal vs. paternal homologs to each pole, generating distinct chromosome combinations in gametes even before considering crossing over. Source
As a result, each gamete receives a random mix of maternal and paternal chromosomes.
Alternative orientations of homologous chromosome pairs during meiosis I lead to different maternal/paternal chromosome combinations in gametes. The diagram links chromosome movement in anaphase I to the distinct allele combinations that appear in the resulting haploid gametes, illustrating the mechanism behind independent assortment. Source
What random assortment changes
Produces different chromosome combinations across gametes
Generates variation even if no crossing over occurred, because the whole-chromosome source (maternal vs paternal) varies by gamete
The number of possible chromosome combinations from assortment alone depends on the haploid number.
= Number of possible chromosome combinations in gametes (unitless count)
= Haploid number of chromosomes (unitless count)
Random assortment is amplified in organisms with larger , making chromosomal combinations among gametes extremely diverse.
Source 3: fertilisation of gametes is random
Fertilisation combines two independently produced gametes, restoring diploidy and creating a genotype that has not existed before.
Fertilisation: Fusion of two haploid gametes to form a diploid zygote with a new combination of alleles.
How random fertilisation increases variation
Any one egg can be fertilised by many genetically distinct sperm (or pollen nuclei in plants)
The zygote’s genotype depends on the chance pairing of gametes, multiplying diversity created by meiosis
Putting the three sources together
Sexual reproduction increases genetic variation through:
Crossing over: reshuffles alleles within homologous chromosomes
Random assortment: reshuffles which homologous chromosomes enter each gamete
Fertilisation: randomly pairs two unique gametes
These mechanisms ensure that offspring from the same parents can differ substantially in genotype, which can translate into phenotypic differences depending on gene expression and dominance relationships.
FAQ
Crossing over can change which alleles are inherited together by breaking up allele combinations along a chromosome.
It has the biggest effect when homologues carry different alleles at multiple loci.
Yes, but the amount depends on heterozygosity.
If many loci are homozygous in both parents, gametes differ less because there are fewer alternative alleles to reshuffle.
Meiosis creates a pool of distinct gametes; fertilisation determines which two meet.
The combination step multiplies diversity because each gamete can pair with many possible partners.
They primarily create new combinations of existing alleles rather than adding or deleting genes.
New combinations can still alter phenotype by changing which allele pairs occur together in offspring.
Key limits include low heterozygosity, small chromosome number ($n$), and fewer crossover events.
Biological constraints on gamete production and mating patterns can also reduce the range of gamete pairings.
Practice Questions
State two processes in sexual reproduction that increase genetic variation. (2 marks)
Crossing over/recombination during meiosis (1)
Random assortment/independent orientation of homologous chromosomes (1) (Allow random fertilisation as an alternative for either mark.)
Explain how sexual reproduction generates genetic variation, referring to meiosis and fertilisation. (5 marks)
Crossing over exchanges DNA between homologous chromosomes, forming recombinant chromatids (1)
This produces new allele combinations on a chromosome (1)
Homologous pairs assort randomly into gametes during meiosis I (1)
This produces different combinations of maternal and paternal chromosomes in gametes (1)
Random fertilisation combines two genetically different gametes, creating a unique diploid genotype (1)
