5 Heredity
- Syllabus
- 2025
- Section
- 5
- Level
- —
Exam analysis
5.1 Meiosis
- Syllabus
- 2025
- Topic
- 5.1
- Level
- —
How Meiosis Produces Haploid Cells
Meiosis forms haploid cells from a diploid starting cell through two divisions. Meiosis I separates homologous chromosome pairs, reducing the chromosome set; meiosis II separates sister chromatids, producing four haploid cells that can transmit chromosomes to the next generation.
| Meiosis I phase | Defining event | Chromosome consequence |
|---|---|---|
| Prophase I | Homologous chromosomes pair by synapsis; chiasmata may form; chromosomes condense and the spindle forms | Each homologous pair is organized for separation |
| Metaphase I | Homologous pairs align at the metaphase plate | Each homolog faces a different pole |
| Anaphase I | Homologous chromosomes move to opposite poles while sister chromatids remain joined | Each pole receives one homolog from every pair |
| Telophase I and cytokinesis | Nuclei may reform and the cell divides | Two haploid cells form, but each chromosome still has two sister chromatids |
| Meiosis II phase | Defining event | Chromosome consequence |
|---|---|---|
| Prophase II | A spindle forms and sister chromatids attach to spindle microtubules | Chromatids are prepared for separation |
| Metaphase II | Chromosomes align at the metaphase plate | Sister chromatids face opposite poles |
| Anaphase II | Centromere proteins break down and sister chromatids separate | Each pole receives one chromatid from each chromosome |
| Telophase II and cytokinesis | Nuclei reform, chromosomes decondense, and cells divide | Four haploid cells form with unduplicated chromosomes |
Sister chromatids do not separate in anaphase I. The reduction to haploid chromosome sets occurs because homologous chromosomes separate first; chromatids separate only in anaphase II.
Mitosis and Meiosis: Same Machinery, Different Outcomes
Mitosis and meiosis both use spindle fibers to align and move chromosomes, followed by nuclear division and usually cytokinesis. Their chromosome behavior and outcomes differ because mitosis has one division, whereas meiosis has two.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Homolog pairing | Homologous chromosomes do not pair for separation | Homologs pair during prophase I |
| What separates first | Sister chromatids | Homologous chromosomes in meiosis I |
| Number of divisions | One | Two |
| Final products | Two daughter cells | Four daughter cells |
| Chromosome set | Preserves the parent cell's chromosome number | Produces haploid cells from a diploid starting cell |
| Genetic content | Daughter cells are genetically identical to each other, barring mutation | Products are not generally genetically identical |
| Main biological role | Growth, tissue repair, and asexual reproduction | Formation of cells used in sexual reproduction |
In both processes, accurate transmission depends on spindle attachment, alignment at a metaphase plate, movement toward opposite poles, and division of the cell. The key comparison is therefore not whether chromosomes move, but which chromosome structures separate and what products result.
Meiosis II resembles mitosis because sister chromatids separate, but the overall outcomes are not the same: meiosis began with homolog pairing and a reduction division in meiosis I.
5.2 Meiosis and Genetic Diversity
- Syllabus
- 2025
- Topic
- 5.2
- Level
- —
How Meiosis and Fertilization Generate Genetic Diversity
Sexual reproduction generates genetic diversity by reshuffling existing alleles into new chromosome and gamete combinations. Crossing over acts within homologous pairs, random assortment distributes whole maternal and paternal chromosomes, and fertilization combines two independently produced gametes.
| Mechanism | When it acts | How it changes genetic combinations |
|---|---|---|
| Crossing over (recombination) | Prophase I | Non-sister chromatids of homologous chromosomes exchange corresponding DNA segments, producing recombinant chromatids |
| Random assortment | Homologous pairs align and separate in meiosis I | Each gamete receives a random mixture of maternal and paternal homologs |
| Fertilization | After meiosis | One gamete combines with another, joining two independently assembled haploid chromosome sets |
Correct separation of homologs in meiosis I and sister chromatids in meiosis II gives each gamete one haploid (1n) set. The alleles and maternal-paternal chromosome combination can differ among gametes even though each normal gamete has the same number of chromosome types.
Nondisjunction is failed chromosome separation. It produces gametes with an abnormal chromosome number, so it is not simply another normal outcome of independent assortment. Details of plant and animal sexual life cycles are outside the required AP scope.
5.3 Mendelian Genetics
- Syllabus
- 2025
- Topic
- 5.3
- Level
- —
Using Mendel's Laws to Predict Inheritance
Mendelian inheritance links chromosome behavior to allele transmission. A genotype is the allele combination an organism carries; a phenotype is the observable expression of inherited traits. For a gene, two identical alleles are homozygous and two different alleles are heterozygous.
| Law | Chromosome basis | Inheritance consequence |
|---|---|---|
| Segregation | The two alleles for a gene separate as homologous chromosomes move into different gametes | Each gamete receives one allele; a monohybrid cross predicts one-gene outcomes |
| Independent assortment | Different homologous chromosome pairs orient and separate independently | For genes on different chromosomes, one gene's allele pair does not determine which alleles of another gene enter the same gamete; a dihybrid cross combines probabilities |
Mutually exclusive outcomes: $P(A\text{ or }B)=P(A)+P(B)$ Independent outcomes: $P(A\text{ and }B)=P(A)\times P(B)$
Example: in Aa×Aa, each parent passes allele a with probability 1/2, so P(aa)=1/2×1/2=1/4. If a second gene assorting independently also has a 1/4 chance of a recessive genotype, the chance of both recessive genotypes is 1/4×1/4=1/16.
Punnett squares organize possible gametes and offspring genotypes. A testcross pairs an individual showing a dominant phenotype with a homozygous recessive individual to reveal the unknown genotype. Pedigree and offspring data can indicate dominant or recessive, autosomal, sex-linked, or genetically linked inheritance patterns.
Independent assortment is not assumed for genes that are genetically linked on the same chromosome. Also, dominant means expressed in a heterozygote; it does not mean more common, stronger, or more beneficial.
5.4 Non-Mendelian Genetics
- Syllabus
- 2025
- Topic
- 5.4
- Level
- —
Why Inheritance Can Deviate from Mendelian Ratios
A non-Mendelian pattern occurs when observed inheritance does not fit the ratios expected from a simple model of complete dominance and independent assortment. The deviation can come from chromosome location, allele expression, gene effects, or inheritance outside the nucleus.
| Pattern | Biological basis | Evidence or consequence |
|---|---|---|
| Genetic linkage | Genes on the same chromosome tend to travel together; crossing over can produce recombinant combinations | Parental combinations exceed recombinant combinations; recombination frequency estimates map distance |
| Codominance | Both alleles are expressed in a heterozygote | The heterozygote displays both allele products and differs from either homozygote |
| Incomplete dominance | Neither allele fully masks the other | The heterozygote has an intermediate or blended phenotype |
| Sex-linked inheritance | The gene is on a sex chromosome | Transmission depends on the offspring's sex-chromosome genotype and can be inferred from pedigrees |
| Pleiotropy | One gene influences multiple traits or effects | Several phenotypic effects follow the same inherited gene |
| Non-nuclear inheritance | The trait is encoded by mitochondrial or chloroplast DNA | In animals, mitochondrial traits are typically maternal; in plants, mitochondrial and chloroplast traits are typically transmitted through the ovule |
$\text{map distance (map units)}=\dfrac{\text{recombinant offspring}}{\text{total offspring}}\times100$
Analyze a deviation in order: state the expected Mendelian ratio → compare it with observed counts → determine whether the difference is statistically meaningful → use the pattern of offspring or pedigree transmission to identify which model assumption may not hold. A significant difference shows that the simple prediction is inadequate; the detailed pattern is needed to identify the mechanism.
Non-Mendelian does not mean alleles ignore meiosis. It means that linkage, allele expression, chromosome type, multiple gene effects, or organelle transmission changes the ratios or patterns predicted by the simplest Mendelian model.
5.5 Environmental Effects on Phenotype
- Syllabus
- 2025
- Topic
- 5.5
- Level
- —
How One Genotype Can Produce Different Phenotypes
Phenotypic plasticity is the ability of one genotype to produce different phenotypes under different environmental conditions. The DNA sequence remains the same, but the environment changes how the genotype is expressed.
Causal chain: environmental condition changes a cellular or organismal signal → expression of particular genes increases or decreases → protein abundance or activity changes → the observable phenotype changes.
For example, soil pH can influence flower color in plants with the same genotype, and seasonal conditions can influence fur color in arctic animals. In each case, environmental input changes the expressed trait rather than creating a new inherited allele.
To test an environmental effect, compare individuals with the same genotype under different values of one environmental variable while holding other conditions constant. A consistent phenotype difference supports an environmental contribution; comparing different genotypes would confound inherited and environmental effects.
A phenotype change does not by itself show that the genotype changed. Phenotype reflects genotype, environment, and their interaction.