AP Syllabus focus:
‘Mendel’s laws describe how alleles segregate and assort independently when genes are on different chromosomes.’
Mendel’s laws explain predictable inheritance patterns in sexually reproducing organisms. They connect observable trait ratios to the behavior of alleles during gamete formation, and they set the baseline expectations for unlinked genes.
Core ideas and vocabulary
Mendelian genetics links inheritance to paired hereditary units carried in gametes.
Allele: an alternative version of a gene found at the same locus on homologous chromosomes.
Alleles come in pairs in diploid organisms, but gametes carry one allele per gene, which is central to Mendel’s first law.
Mendel’s Law of Segregation
Law of segregation: for any gene, an organism’s two alleles separate during gamete formation, so each gamete receives only one allele.
Key implications for AP Biology:
In a heterozygote (two different alleles), the two alleles do not blend; they remain distinct units.
Gametes from a heterozygote contain each allele with equal probability (typically 1/2 and 1/2).
Fertilisation restores allele pairs by combining alleles from two gametes.
Mechanistic basis (conceptual link to cell division):
This overview diagram summarizes the major stages of meiosis, situating where key Mendelian mechanisms occur within the full cell-division sequence. It helps distinguish meiosis I (homologous chromosome separation, tied to segregation and independent assortment) from meiosis II (sister chromatid separation). Source
Allele pairs are carried on homologous chromosomes.
Separation of homologous chromosomes into different gametes provides a physical explanation for segregation.
Meiosis I provides the cellular mechanism behind segregation: homologous chromosomes pair, align at the metaphase plate, and then separate into different daughter cells. Because alleles reside on homologous chromosomes, their separation during anaphase I explains why each gamete receives only one allele per gene. Source
What segregation does and does not claim:
It predicts how alleles enter gametes, not which allele is “better” or more common in a population.
It assumes typical meiotic separation; errors can disrupt expected outcomes.
Mendel’s Law of Independent Assortment
Law of independent assortment: alleles of different genes segregate independently into gametes when the genes are on different chromosomes (i.e., unlinked).
How to interpret “independent”:
The allele a gamete receives for one gene does not influence which allele it receives for another unlinked gene.
Independent assortment creates new combinations of alleles across genes, increasing variation among gametes.
Mechanistic basis (conceptual link):
Different homologous chromosome pairs can orient randomly relative to each other during meiosis, producing many possible chromosome combinations in gametes.
This diagram shows two alternative ways homologous chromosome pairs can separate during meiosis I, producing different combinations of alleles in gametes. The key point is that the orientation of one homologous pair is independent of the orientation of another pair, so unlinked genes generate multiple equally probable allele combinations. Source
= number of homologous chromosome pairs (or independently assorting units), unitless
This relationship captures how rapidly gamete diversity increases as the number of independently assorting chromosome pairs increases.
Limits and conditions of Mendel’s laws
Mendel’s laws are models with defined conditions, so interpreting data requires checking those conditions.
Independent assortment requires unlinked genes:
If two genes are on different chromosomes, they are expected to assort independently.
If two genes are close together on the same chromosome, they may be inherited together more often than expected by independent assortment (linkage), producing deviations from Mendelian expectations.
Segregation assumes normal gamete formation:
If allele separation into gametes is unequal due to abnormal chromosome separation, segregation-based predictions can fail.
What these laws let you predict (conceptually)
From segregation and independent assortment, you can predict:
The types of gametes an individual can produce based on its allele pairs.
Whether two traits should show combined inheritance patterns consistent with independent assortment (for genes on different chromosomes).
Expected inheritance patterns in offspring when allele transmission follows these laws.
FAQ
Segregation is about allele separation into gametes, whereas dominance is about expression in a heterozygote.
Mendel inferred segregation by showing that a “hidden” allele could reappear in later generations, indicating it was transmitted intact rather than blended.
An independently assorting unit is a chromosome pair whose orientation does not constrain another pair’s orientation.
In many contexts, $n$ is the haploid number of chromosome pairs, but structural changes (e.g., translocations) can reduce independence.
Usually they assort independently, but rare exceptions can occur if chromosome behaviour is non-random.
For example, meiotic drive mechanisms can bias transmission of particular chromosomes, altering expected gamete frequencies.
“Random” refers to the orientation of chromosome pairs being equally likely among possibilities.
Predictability comes from large numbers: across many meioses, frequencies approach expected probabilities even though each meiosis is uncertain.
You look for combinations of traits appearing in proportions consistent with independent transmission.
Evidence is strongest when recombinant combinations occur as frequently as parental combinations, rather than being underrepresented (which would suggest linkage).
Practice Questions
State Mendel’s law of segregation and explain what it means for the alleles found in a single gamete. (2 marks)
States that the two alleles for a gene separate during gamete formation (1)
States that each gamete receives only one allele for that gene (1)
A student claims that two traits always assort independently. Using Mendel’s laws, explain when independent assortment is expected and why it increases the variety of gametes produced. (5 marks)
States that independent assortment applies when genes are on different chromosomes / are unlinked (1)
Explains that allele segregation for one gene does not affect allele segregation for another unlinked gene (1)
Links independent assortment to random orientation/behaviour of different homologous chromosome pairs during meiosis (1)
Explains that this produces new combinations of alleles in gametes (1)
States that increased gamete variety increases genetic variation among potential offspring (1)
