Core inheritance pattern

• Inheritance in sexual life cycles occurs by production of haploid gametes and fusion at fertilization to form a diploid zygote.

• A diploid organism has two alleles for each autosomal gene.

• Always link inheritance to meiosis, random fusion of gametes, and allele combinations in offspring.

• In exam answers, distinguish clearly between gene, allele, genotype, and phenotype.

Genetic crosses in flowering plants

• Use P generation, F1 generation, F2 generation, and Punnett grid correctly.

• In flowering plants, pollen carries the male gametes and ovules in the ovary contain the female gametes.

• Pollination is required before fertilization can occur.

• Some plants such as peas can self-pollinate and self-fertilize, making them useful in inheritance studies.

• Genetic crosses are used in breeding crop varieties and ornamental plants.

Genotype and phenotype

• Genotype = the combination of alleles inherited by an organism.

• Phenotype = the observable traits of an organism, produced by genotype, environment, or both together.

• Homozygous = two identical alleles.

• Heterozygous = two different alleles.

• Be ready to classify traits as:

  • genotype only

  • environment only

  • genotype × environment interaction

Dominant and recessive alleles

• A dominant allele is expressed in the heterozygous condition.

• A recessive allele is expressed only in the homozygous recessive condition.

• This is why AA and Aa can have the same phenotype.

• In monohybrid crosses, keep genotype ratios separate from phenotype ratios.

• Do not say a dominant allele is “stronger”; say it is expressed in the heterozygote.

Phenotypic plasticity

• Phenotypic plasticity = ability of an organism to develop traits suited to the environment experienced by changing patterns of gene expression.

• Plasticity is not caused by a change in genotype.

• Changes may be reversible during an individual’s lifetime.

• High-scoring exam point: plasticity shows that phenotype is not determined by genotype alone.

Recessive disorders: PKU

• Phenylketonuria (PKU) is a recessive genetic disorder caused by mutation in an autosomal gene.

• The gene codes for the enzyme needed to convert phenylalanine to tyrosine.

• An affected person must inherit two recessive alleles.

• Unaffected carriers are heterozygous.

• In pedigree or cross questions, identify PKU as autosomal recessive.

Multiple alleles and SNPs

• A single-nucleotide polymorphism (SNP) is a variation at one base in the DNA sequence.

• A gene pool can contain multiple alleles of a gene.

• An individual organism still inherits only two alleles for that gene.

• Exam distinction:

  • multiple alleles = many forms of a gene in a population

  • diploid individual = only two alleles present

ABO blood groups

• ABO blood groups are an example of multiple alleles.

• Use allele notation exactly: I^A, I^B, and i.

• I^A and I^B are codominant.

• i is recessive.

• Genotypes and phenotypes:

  • I^A I^A or I^A i → blood group A

  • I^B I^B or I^B i → blood group B

  • I^A I^B → blood group AB

  • ii → blood group O

This image shows the relationship between ABO genotypes and phenotypes, including the role of I^A, I^B, and i. It is useful for checking codominance and multiple-allele inheritance at a glance. Source

Incomplete dominance vs codominance

• Codominance: both alleles are fully expressed in the heterozygote.

• Example: I^A I^B gives AB blood group with a dual phenotype.

• Incomplete dominance: the heterozygote has an intermediate phenotype.

• Example: four o’clock flower / marvel of Peru (Mirabilis jalapa).

• Exam tip: do not confuse intermediate with both shown together.

Sex determination and sex linkage

• In humans, the sex chromosome in the sperm determines whether the zygote develops male-typical or female-typical physical characteristics.

• Eggs always carry X; sperm carry X or Y.

• The X chromosome carries many more genes than the Y chromosome.

• This is why sex-linked inheritance is usually discussed as X-linked inheritance.

• Males are more likely to express X-linked recessive alleles because they have only one X chromosome.

Haemophilia as a sex-linked disorder

• Haemophilia is a classic X-linked recessive disorder.

• Write alleles as superscripts on X: for example X^H and X^h.

• Male genotypes:

  • X^H Y = unaffected

  • X^h Y = affected

• Female genotypes:

  • X^H X^H = unaffected

  • X^H X^h = carrier

  • X^h X^h = affected

• Exam tip: a male cannot be a carrier for an X-linked recessive trait; he is either affected or unaffected.

This image shows inheritance patterns for an X-linked recessive trait, including outcomes when the mother is a carrier or the father is affected. It helps students predict why males are more often affected and how carrier daughters arise. Source

Pedigree charts

• Pedigree charts are used to deduce patterns of inheritance of genetic disorders.

• Standard symbols:

  • square = male

  • circle = female

  • shaded = affected

  • half-shaded = carrier (when shown)

• Key clues:

  • autosomal recessive often appears in children of unaffected parents

  • autosomal dominant often appears in every generation

  • X-linked recessive affects more males and often shows no father-to-son transmission

• Be able to infer possible genotypes from the pedigree.

• The syllabus also expects awareness that marriage between close relatives increases the chance of homozygous recessive offspring.

This image shows how a pedigree chart can reveal an autosomal recessive inheritance pattern. It is especially helpful for practising how unaffected parents can produce affected offspring and how to infer carrier genotypes. Source

Continuous vs discrete variation

• Continuous variation shows a range of phenotypes with no clear categories.

• Discrete variation falls into distinct categories.

• Skin colour in humans is an example of continuous variation caused by polygenic inheritance and/or environmental factors.

• ABO blood group is an example of discrete variation.

• Exam tip: continuous variation is usually influenced by many genes and often by the environment as well.

Polygenic inheritance

• Polygenic inheritance occurs when a trait is controlled by multiple genes.

• This usually produces continuous variation.

• Environmental factors can further widen the range of phenotypes.

• Good exam phrasing: continuous variation is due to polygenic inheritance and/or environmental effects.

• Avoid oversimplifying skin colour as controlled by a single gene.

Data skills: mean, median, mode

• For continuous data, be able to apply mean, median, and mode.

• Mean = arithmetic average.

• Median = middle value when data are ordered.

• Mode = most frequent value.

• Choose measures carefully:

  • median is often more useful when outliers are present

  • mode is useful for most common value

Box-and-whisker plots

• A box-and-whisker plot displays six aspects of data:

  • outliers

  • minimum

  • first quartile (Q1)

  • median

  • third quartile (Q3)

  • maximum

• Interquartile range (IQR) = Q3 − Q1.

• A data point is an outlier if it is more than 1.5 × IQR above Q3 or below Q1 − 1.5 × IQR.

• Use box plots for continuous variables such as student height.

• In comparisons, look at median, spread, overlap, and outliers.

This image labels the main parts of a box-and-whisker plot, including the median, quartiles, and whiskers. It is ideal for revising exactly what each feature represents in IB data-analysis questions. Source

This image shows the stages of meiosis leading to four haploid cells, which helps explain segregation and independent assortment. It is useful when linking chromosome behaviour in meiosis to expected outcomes in inheritance questions. Source

Exam traps to avoid

• Do not confuse genotype with phenotype.

• Do not say codominance and incomplete dominance are the same.

• Do not forget that multiple alleles exist in a population, not all in one individual.

• Do not say males are “heterozygous” for X-linked genes; they are hemizygous because they have only one X chromosome.

• Do not assume every continuous trait is purely genetic; the syllabus allows polygenic inheritance and/or environmental factors.

• In pedigrees, do not miss the clue that close relatives are more likely to share the same recessive allele.

Fast-recall summary

• Dominant: expressed in heterozygotes.

• Recessive: expressed only in homozygotes.

• Codominance: both alleles fully expressed.

• Incomplete dominance: heterozygote is intermediate.

• Multiple alleles: more than two alleles in population.

• Sex-linked: gene on a sex chromosome, usually X-linked.

• Polygenic: many genes affect one trait.

• Linked genes: genes on same chromosome, so independent assortment may fail.

• Recombinants: new combinations caused by crossing over.