Inheritance

IB Biology SL· D.4 Inheritance· 15 min read

1. Key Terms and Core Mendelian Principles★★☆☆☆⏱ 5 min

📘 Definition

Gene

Alleles symbolized $A$ , $a$ for different variants

A heritable factor made of DNA that influences a specific characteristic, occupying a fixed locus on a chromosome

Example:

The gene for pea plant height is found at a fixed locus on pea chromosome 4

📘 Definition

Dominant vs Recessive Alleles

A dominant allele is expressed in both homozygous and heterozygous genotypes. A recessive allele is only expressed when no dominant allele is present (homozygous recessive).

Mendelian inheritance describes the pattern of single-gene traits first documented by Gregor Mendel in pea plants. The core law of segregation states each parent passes one randomly selected allele to their offspring.

📐 Worked Example

A pea plant heterozygous for height ( $T$ = tall, $t$ = short) is self-fertilized. What are the expected genotype and phenotype ratios of offspring?

1
Identify parental gametes: both parents are heterozygous $T t$ , so each produces 50% $T$ and 50% $t$ gametes.
2
Construct a Punnett grid to combine gametes:
3
$\begin{array}{c|c|c} & T & t \\ \hline T & TT & Tt \\ \hline t & Tt & tt \\ \end{array}$
4
Count genotypes: 1 $T T$ : 2 $T t$ : 1 $tt$ , giving a genotype ratio of 1:2:1.
5
Since $T$ is dominant, $T T$ and $T t$ are tall, only $tt$ is short. Phenotype ratio = 3 tall : 1 short.

2. Codominance and Multiple Alleles★★★☆☆⏱ 5 min

Codominance is a non-Mendelian pattern where both alleles are fully and equally expressed in the heterozygous phenotype. Human ABO blood type is the most common IB example, and it also demonstrates multiple alleles (three alleles for the same gene).

📘 Definition

Multiple Alleles

When a gene exists in more than two allelic forms in a population, leading to more than two possible phenotypes

Example:

The ABO blood group gene has three alleles: $I^{A}$ , $I^{B}$ , $i$

📐 Worked Example

A father with blood type A (genotype $I^{A} i$ ) and a mother with blood type B (genotype $I^{B} i$ ) have a child. What is the probability the child has blood type AB?

1
Identify parental gametes: father produces $I^{A}$ and $i$ , mother produces $I^{B}$ and $i$ .
2
Construct Punnett grid:
3
$\begin{array}{c|c|c} & I^A & i \\ \hline I^B & I^A I^B & I^B i \\ \hline i & I^A i & ii \\ \end{array}$
4
Only one of four possible offspring genotypes is $I^{A} I^{B}$ , which gives blood type AB.
5
Probability = 1/4 or 25%

Exam tip:

Always write superscripts for codominant alleles (e.g. $I^{A}$ not just $A$ ) to earn full marks in IB exams.

3. Sex-Linked Inheritance★★★☆☆⏱ 5 min

Sex-linked traits are controlled by genes located on the X or Y sex chromosomes. In humans, the X chromosome is much larger than the Y, so most sex-linked traits are X-linked. Males only have one X chromosome, so they express all X-linked alleles regardless of dominance, making them more likely to show recessive X-linked disorders.

📘 Definition

X-linked Recessive Inheritance

Inheritance pattern where a recessive allele on the X chromosome causes the recessive phenotype. Males are affected with one copy; females need two copies to be affected.

📐 Worked Example

A woman who is a carrier for red-green color blindness (X-linked recessive) has a child with a man with normal vision. What is the probability their son is color blind?

1
Write parental genotypes: Mother = $X^{A} X^{a}$ , Father = $X^{A} Y$ , where $X^{a}$ is the recessive color blind allele.
2
Sons inherit their Y chromosome from their father, so their X chromosome always comes from their mother.
3
The mother has one $X^{A}$ (normal) and one $X^{a}$ (color blind) allele, so there is a 50% chance the son inherits $X^{a}$ .
4
Final probability = 1/2 or 50%

4. Pedigree Chart Analysis★★★★☆⏱ 7 min

Pedigree charts show inheritance of a trait across multiple generations of a family. They are used to determine if a trait is dominant/recessive and autosomal/sex-linked, and this is a common IB SL exam question.

Squares = males, circles = females
Shaded symbols = affected individuals, unshaded = unaffected
Horizontal lines = mating pairs, vertical lines connect parents to offspring

📐 Worked Example

A pedigree shows two unaffected parents producing an affected child with an autosomal disorder. Is this possible for a dominant disorder?

1
For a dominant disorder, any affected individual must have at least one affected parent, because the dominant allele causes the trait.
2
If two parents are unaffected, they both have two copies of the recessive non-disease allele, so cannot pass on a dominant disease allele.
3
Therefore, this pattern is only possible for an autosomal recessive disorder, and cannot be dominant.

Exam tip:

If most affected individuals in the pedigree are male, the trait is almost certainly X-linked recessive, the most common sex-linked pattern tested.

5. Common Pitfalls

Wrong move:

Writing plain alleles for codominance (e.g. $A$ instead of $I^{A}$ )

Why:

IB exam markers require correct notation to award full marks for genetic cross questions

Correct move:

Always use superscripts to denote codominant alleles, following IB notation conventions

Wrong move:

Calculating probability over all offspring when the question specifies male/female

Why:

Questions about sex-linked traits often ask for probability for a child of a given sex, requiring conditional probability

Correct move:

If the question specifies the child's sex, only count offspring of that sex for your calculation

Wrong move:

Confusing codominance with incomplete dominance

Why:

IB exams explicitly test the distinction between these two inheritance patterns

Correct move:

Remember: codominance = both traits fully expressed; incomplete dominance = blended intermediate phenotype

Wrong move:

Assuming all rare traits in a pedigree are recessive

Why:

Dominant disorders can be rare if the disease allele has low frequency in the population

Correct move:

Use the rule: if an affected child has two unaffected parents, the trait is recessive

6. Quick Reference Cheatsheet

Inheritance Pattern	Key Rule	Example
Autosomal complete dominance	3:1 phenotype ratio from heterozygote cross	Pea plant height
Codominance	Both alleles fully expressed in heterozygotes	AB blood type
X-linked recessive	More affected males; affected daughters need affected father	Red-green color blindness
Autosomal recessive	Two unaffected parents can have affected child	Cystic fibrosis
Autosomal dominant	Every affected person has at least one affected parent	Huntington's disease

7. Frequently Asked

What is the difference between codominance and incomplete dominance?

In codominance, both alleles are fully expressed simultaneously in the heterozygote (e.g. human AB blood type). In incomplete dominance, heterozygotes show an intermediate blended phenotype (e.g. pink snapdragons). IB SL emphasizes testing codominance over incomplete dominance.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 1
Autosomal monohybrid cross prediction
2023 · 2
Pedigree analysis for cystic fibrosis
2021 · 1
Sex-linked inheritance of hemophilia

Going deeper

official syllabusIB Biology SL Theme D GuideFull D.4 learning objectives

What's Next

Understanding inheritance is the foundation for understanding genetic variation, biotechnology, and the genetic basis of disease, all core topics in IB Biology SL. This sub-topic regularly makes up 10-15% of total exam marks for IB Biology SL, so mastering genetic crosses, notation conventions, and pedigree analysis is critical for achieving a high score. The principles you learned here apply to all subsequent genetics topics, from biotechnology to evolution, so it is important to solidify your understanding before moving on to more advanced concepts.