IMAT Biology · Free notes

Genetics and Inheritance: Crosses, Sex Linkage and Pedigrees

Genetics and Inheritance has 30 past questions since 2011, and the ministry papers from 2023 on have asked 9 of them. The ministry papers mostly ask for meanings and facts (what an allele is, what Mendel's laws say, how X-linked recessive conditions behave), while the older papers set multi-step reasoning on crosses, gene maps and family trees. The hard part is probability: knowing when to multiply, when to add, and which children a question is really counting. Sex linkage is the most tested idea across all the years, so learn its rules until you can apply them without a Punnett square.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

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Subtopic notes

Formula & revision sheet

13 formulas · 7 reference tables · 22 gotchas across all subtopics — the exam-eve cheat-sheet

Genes, Alleles, Mendel's Laws and Mutations

Reference tables (3)

Gene, allele, genotype and phenotype: the core vocabulary8 rows

Term	Meaning	Example
Gene	A DNA sequence coding for a product, at a fixed locus	The gene for pea plant height
Allele	One alternative form of a gene	$T$ (tall) and $t$ (dwarf)
Genotype	The two alleles an individual carries	$TT$ , $Tt$ or $tt$
Phenotype	The trait that is seen or measured	Tall or dwarf
Homozygous	Both alleles the same	$TT$ or $tt$
Heterozygous	Two different alleles	$Tt$ , which is tall
Dominant allele	Expressed even when only one copy is present	$T$ in a $Tt$ plant In a heterozygote, only the dominant allele is certain to show in the phenotype.
Recessive allele	Expressed only when two copies are present	$t$ , seen only in $tt$

By convention the dominant allele takes a capital letter and the recessive allele the same letter in lower case.

Mendel's laws and the chromosomal theory of inheritance4 rows

Law or idea	What it says	Explained in meiosis by
Segregation	Each gamete gets one allele of each gene	Homologous chromosomes separating in anaphase I
Independent assortment	Alleles of different genes are sorted into gametes independently	Random orientation of homologous pairs at the equator in metaphase I
Dominance	The dominant allele masks the recessive one in a heterozygote	Not a meiosis event: it is about gene expression
Chromosomal theory	Genes are carried on chromosomes	Chromosomes and alleles both come in pairs and both segregate in meiosis

Independent assortment fails for genes close together on the same chromosome: they are linked.

Mutations: changes in the genetic information, and sickle-cell anaemia5 rows

Type	What changes	Typical effect
Silent substitution	One base, but the codon still codes the same amino acid	No change in the protein
Missense substitution	One base, giving a different amino acid	One amino acid changed (sickle-cell haemoglobin)
Nonsense substitution	One base, giving a stop codon	A shortened, usually useless protein
Frameshift	One or two bases inserted or deleted	Every amino acid after the change can be different
Chromosome mutation	Chromosome structure or number	Many genes affected (e.g. trisomy 21)

Watch out for (4)

Genes are shared; alleles differ
→ Gene, allele, genotype and phenotype: the core vocabulary
Dominant does not mean common or stronger
→ Gene, allele, genotype and phenotype: the core vocabulary
Natural selection is not one of Mendel's laws
→ Mendel's laws and the chromosomal theory of inheritance
Sickle-cell haemoglobin still carries oxygen
→ Mutations: changes in the genetic information, and sickle-cell anaemia

Punnett Squares, Probability, Test Crosses and Dihybrid Crosses

Formulas (4)

Monohybrid crosses and the Punnett square · Cross of two heterozygotes $Aa \times Aa \rightarrow \tfrac{1}{4}\,AA + \tfrac{1}{2}\,Aa + \tfrac{1}{4}\,aa$
Probability rules for crosses: multiply for AND, add for OR · Product and sum rules $P(A \cap B) = P(A)\,P(B) \qquad P(A \cup B) = P(A) + P(B)$
The test cross: revealing an unknown genotype · Test cross outcomes $AA \times aa \rightarrow \text{all } Aa \qquad Aa \times aa \rightarrow \tfrac12\,Aa + \tfrac12\,aa$
Dihybrid crosses: the 9 : 3 : 3 : 1 ratio and the branch method · Branch method for independent genes $P(\text{combined}) = P(\text{gene 1 outcome}) \times P(\text{gene 2 outcome}) \times \dots$

Watch out for (5)

A 3 : 1 ratio is a probability, not a rule for each family
→ Monohybrid crosses and the Punnett square
Earlier children do not change the odds
→ Probability rules for crosses: multiply for AND, add for OR
"An affected boy" is not "a boy who is affected"
→ Probability rules for crosses: multiply for AND, add for OR
Sixteen boxes are not sixteen genotypes
→ Dihybrid crosses: the 9 : 3 : 3 : 1 ratio and the branch method
9 : 3 : 3 : 1 needs both conditions
→ Dihybrid crosses: the 9 : 3 : 3 : 1 ratio and the branch method

Incomplete Dominance, Codominance, Blood Groups and Polygenes

Formulas (2)

Incomplete dominance and codominance · Heterozygote cross without dominance $C^R C^W \times C^R C^W \rightarrow 1\ \text{red} : 2\ \text{intermediate} : 1\ \text{white}$
Polygenic inheritance and continuous variation · Phenotype classes for n additive genes $\text{classes} = 2n + 1$

Reference tables (1)

Multiple alleles: the ABO and Rh blood groups5 rows

Blood group	Genotypes	Antigen on red cells	Antibodies in plasma
A	$I^A I^A$ or $I^A i$	A	Anti-B
B	$I^B I^B$ or $I^B i$	B	Anti-A
AB	$I^A I^B$	A and B	Neither anti-A nor anti-B
O	$i\,i$	Neither A nor B	Anti-A and anti-B
Rh negative	$dd$	No D antigen	Anti-D only after exposure to D

Rh positive is

DD

Dd

, with the D antigen and no anti-D.

Watch out for (3)

Incomplete dominance is not blending inheritance
→ Incomplete dominance and codominance
An AB parent cannot have a group O child
→ Multiple alleles: the ABO and Rh blood groups
Antibodies are against the antigens you lack
→ Multiple alleles: the ABO and Rh blood groups

Sex Chromosomes, the SRY Gene and X-linked Inheritance

Formulas (2)

X-linked recessive inheritance: haemophilia, colour blindness, Duchenne · Carrier mother and unaffected father $X^H X^h \times X^H Y \rightarrow \tfrac14 X^H X^H + \tfrac14 X^H X^h + \tfrac14 X^H Y + \tfrac14 X^h Y$
X-linked dominant and Y-linked inheritance · Affected father, X-linked dominant $X^R Y \times X^r X^r \rightarrow \text{daughters all } X^R X^r,\ \text{sons all } X^r Y$

Reference tables (1)

Sex determination in humans: XX, XY and the SRY gene5 rows

Situation	Sex that develops	Reason
XX	Female	No SRY gene
XY	Male	SRY on the Y starts testis development
XX with SRY moved onto one X	Male	SRY is present, even without a Y
XY with SRY lost or not working	Female	No working SRY
XXY	Male	One Y with SRY is enough

SRY can move onto an X by an abnormal crossover in the father's meiosis, giving an X-bearing sperm that makes a male.

Watch out for (4)

It is SRY, not the Y chromosome as a whole, that makes a male
→ Sex determination in humans: XX, XY and the SRY gene
A father never passes an X-linked allele to his son
→ X-linked recessive inheritance: haemophilia, colour blindness, Duchenne
"Half the sons" is not "a quarter of the children"
→ X-linked recessive inheritance: haemophilia, colour blindness, Duchenne
An X-linked allele in a boy came from his mother
→ X-linked dominant and Y-linked inheritance

Linkage, Gene Maps, Variation and Non-disjunction

Formulas (3)

Linked genes and recombination frequency · Recombination frequency $RF = \frac{\text{number of recombinant offspring}}{\text{total offspring}} \times 100\%$
Building a gene map from crossover values · Distances add along a chromosome $d(A,B) = d(A,C) + d(C,B) \quad \text{when C lies between A and B}$
Meiosis and fertilisation as sources of genetic variation · Chromosome combinations from independent assortment $\text{gametes} = 2^{n} \qquad \text{zygotes} = 2^{n} \times 2^{n}$

Reference tables (1)

Non-disjunction and aneuploidy: Down, Turner and Klinefelter syndromes5 rows

Condition	Karyotype	Sex	Main features
Down syndrome (trisomy 21)	47, +21	Either	Learning disability, typical facial features, heart defects
Turner syndrome	45, X	Female	Short stature, ovaries do not work, usually infertile
Klinefelter syndrome	47, XXY	Male	Small testes, usually infertile, sometimes breast growth
Triple X	47, XXX	Female	Often no obvious signs; may be tall
XYY	47, XYY	Male	Often no obvious signs; may be tall

An embryo missing a whole autosome almost never survives to birth; 45, X is the only monosomy commonly seen in live births.

Watch out for (3)

Crossing over happens inside one parent's meiosis
→ Linked genes and recombination frequency
Variation between siblings is made before fertilisation
→ Meiosis and fertilisation as sources of genetic variation
Klinefelter is male, Turner is female
→ Non-disjunction and aneuploidy: Down, Turner and Klinefelter syndromes

Pedigree Analysis and Allele Frequencies

Formulas (2)

Probabilities from a pedigree, including the two-thirds rule · Chance of an affected child from uncertain carriers $P(\text{affected child}) = P_1 \times P_2 \times \tfrac14$
Allele frequencies and the Hardy-Weinberg principle · Hardy-Weinberg equations $p + q = 1 \qquad p^2 + 2pq + q^2 = 1$

Reference tables (1)

Reading a pedigree: symbols and the five inheritance patterns5 rows

Pattern	Typical clues	Proof or key rule
Autosomal recessive	Skips generations; both sexes equally	Two unaffected parents have an affected child
Autosomal dominant	Appears in every generation; both sexes equally	Two affected parents have an unaffected child
X-linked recessive	Mostly males; passed through unaffected carrier mothers	An affected woman's father and all her sons are affected
X-linked dominant	More females than males affected	An affected man has all daughters and no sons affected
Y-linked	Males only	Passed from father to every son

Watch out for (3)

An affected boy with unaffected parents fits several patterns
→ Reading a pedigree: symbols and the five inheritance patterns
An unaffected sibling of an affected person is a carrier with 2/3, not 1/2
→ Probabilities from a pedigree, including the two-thirds rule
Count alleles, not individuals
→ Allele frequencies and the Hardy-Weinberg principle

PYQ weightage by concept

20 concepts · 0 PYQs — where the marks actually sit, so you know what to drill first

Genes, Alleles, Mendel's Laws and Mutations0 PYQs · 0%

Concept	PYQs	Share
Gene, allele, genotype and phenotype: the core vocabularyfoundation	—	—
Mendel's laws and the chromosomal theory of inheritancefoundation	—	—
Mutations: changes in the genetic information, and sickle-cell anaemiafoundation	—	—

Punnett Squares, Probability, Test Crosses and Dihybrid Crosses0 PYQs · 0%

Concept	PYQs	Share
Monohybrid crosses and the Punnett squarefoundation	—	—
Probability rules for crosses: multiply for AND, add for ORfoundation	—	—
The test cross: revealing an unknown genotypefoundation	—	—
Dihybrid crosses: the 9 : 3 : 3 : 1 ratio and the branch methodfoundation	—	—

Incomplete Dominance, Codominance, Blood Groups and Polygenes0 PYQs · 0%

Concept	PYQs	Share
Incomplete dominance and codominancefoundation	—	—
Multiple alleles: the ABO and Rh blood groupsfoundation	—	—
Polygenic inheritance and continuous variationfoundation	—	—

Sex Chromosomes, the SRY Gene and X-linked Inheritance0 PYQs · 0%

Concept	PYQs	Share
Sex determination in humans: XX, XY and the SRY genefoundation	—	—
X-linked recessive inheritance: haemophilia, colour blindness, Duchennefoundation	—	—
X-linked dominant and Y-linked inheritancefoundation	—	—

Linkage, Gene Maps, Variation and Non-disjunction0 PYQs · 0%

Concept	PYQs	Share
Linked genes and recombination frequencyfoundation	—	—
Building a gene map from crossover valuesfoundation	—	—
Meiosis and fertilisation as sources of genetic variationfoundation	—	—
Non-disjunction and aneuploidy: Down, Turner and Klinefelter syndromesfoundation	—	—

Pedigree Analysis and Allele Frequencies0 PYQs · 0%

Concept	PYQs	Share
Reading a pedigree: symbols and the five inheritance patternsfoundation	—	—
Probabilities from a pedigree, including the two-thirds rulefoundation	—	—
Allele frequencies and the Hardy-Weinberg principlefoundation	—	—