Extended — test cross, codominance, sex linkage
A test cross crosses an unknown dominant-phenotype individual with a homozygous recessive: any recessive offspring proves the unknown parent is heterozygous. Codominance means both alleles of a heterozygote contribute to the phenotype — the ABO gene has I^A and I^B (codominant) and I^o (recessive to both), so I^A I^B gives group AB. A sex-linked gene lies on a sex chromosome; red-green colour blindness is carried on the X only, so it is commoner in males.
Genotype and phenotype — the six definitions
Inheritance = the transmission of genetic information down the generations. Genotype = the genetic make-up, i.e. the alleles present (Tt); phenotype = the observable features (tall). Homozygous = two identical alleles (TT, tt); heterozygous = two different (Tt). A dominant allele is expressed whenever present; a recessive one only when no dominant allele is there. Two identical homozygotes are pure-breeding; a heterozygote never is.
Punnett squares — the 3:1 and 1:1 ratios
A Punnett square predicts a monohybrid cross. Write each parent's genotype, split it into the single alleles its gametes carry, combine every pairing in a 2x2 grid, then read each box's phenotype and count. Core is limited to 3:1 (Tt x Tt) and 1:1 (Tt x tt). Where a cross gives more than one genotype, the square is used to show every genotype, not just the phenotypes. A pedigree diagram traces the same alleles through generations of one family.
Drawn from real examiner reports.
Gametes carry one allele, not two
A gamete is haploid, so a Tt parent makes two kinds of gamete — one carrying T, one carrying t. Writing "Tt" into a gamete box scores nothing, and it produces impossible four-allele offspring boxes. Always list each parent's gametes as single letters before filling the 2x2 grid.
Flagged s22 P31 Q3c · w22 P31 Q3b ii
A test cross needs a tt partner
The unknown individual must be crossed with a homozygous recessive (tt). Only that partner contributes recessive alleles alone, so a single recessive-phenotype offspring proves the unknown parent is Tt. Crossing it with another dominant-phenotype or heterozygous individual cannot separate TT from Tt.
Flagged s23 P23 Q32
The Y carries no colour-vision allele
The red-green colour-blindness gene sits on the X chromosome only. A male is X^B Y or X^b Y — the Y never takes a superscript, so "X^B Y^b" is wrong. A female needs two recessive alleles (X^b X^b) to be affected; one copy (X^B X^b) makes her an unaffected carrier.
Flagged s22 P41/42 Q3c · P43 Q2c
Gamete boxes need alleles, not bare X
In a sex-linkage cross the gametes are the allele-bearing chromosomes X^B, X^b and Y. Writing a plain "X" in a gamete box throws away the allele, so the offspring boxes can no longer be read as phenotypes. Carry the superscript through every stage — parental genotype, gamete, offspring.
Flagged s22 P42 Q3c iii · w22 P31 Q2b i
Pure-breeding, not selective breeding
Pure-breeding is the mark-scheme term for homozygous individuals whose offspring all show the same phenotype. "Natural breeding" and "selective breeding" are frequently offered and earn nothing — selective breeding is a separate process in which humans choose the parents. Use the exact word.
Flagged s23 P23 Q32 · s22 P31 Q3d
A fourth child is never "due" a boy
Half a father's sperm carry an X and half carry a Y, so every conception independently has a 50% chance of a boy. Three daughters already born changes nothing — the previous outcomes carry no memory. Say "each pregnancy is independent, 50%", not "a boy is more likely now".
Flagged s23 P11 Q34
Codominance is not blending
In a codominant heterozygote both alleles are fully expressed side by side. Genotype I^A I^B gives blood group AB — both the A and the B antigen are present — not a blend or an average of A and B, and not simply group A. Keep "masked" for ordinary dominant/recessive pairs only.
SEPARATE, COMBINE, READ, RATIO
Build every genetic diagram in the same order: separate each parent's genotype into single-allele gametes; combine every pairing in a 2x2 grid; read the phenotype of each box; state the ratio. Skipping SEPARATE is the single biggest source of lost marks here.
Start a test cross by naming tt
For a test cross, add a step before SEPARATE: state that the unknown-genotype parent is crossed with a homozygous recessive individual, and write that genotype (tt) down. Then run the four steps as normal.
Keep superscripts on X only
In a sex-linked cross write X^B, X^b and a plain Y at every stage. Then label each offspring box with the sex as well as the phenotype — these questions almost always ask about sons and daughters separately, and the two probabilities differ.
Give the ratio the question asks for
Count phenotypes across the four boxes and simplify to whole numbers (3 tall : 1 short, or 1:1). Give a ratio if a ratio is asked for, a probability or percentage if that is asked for. Genotype ratio (1:2:1) and phenotype ratio (3:1) are different answers.
Inheritance: the transmission of genetic information from generation to generation.
| Term | Definition |
|---|---|
| Genotype | The genetic make-up of an organism, in terms of the alleles present |
| Phenotype | The observable features of an organism |
| Homozygous | Having two identical alleles of a particular gene |
| Heterozygous | Having two different alleles of a particular gene |
| Dominant allele | An allele that is expressed if it is present in the genotype |
| Recessive allele | An allele that is only expressed when there is no dominant allele of the gene present |
| Pure-breeding | Homozygous individuals that, bred together, produce offspring all showing the same phenotype |
A heterozygous individual is not pure-breeding, because it carries two different alleles and can pass on either one.
(Extended) Test cross: cross an individual of unknown genotype (showing the dominant phenotype) with a homozygous recessive individual. Any recessive-phenotype offspring proves the unknown parent is heterozygous; all-dominant offspring is consistent with (but does not prove beyond doubt, in a small sample) a homozygous dominant parent.
(Extended) Codominance and ABO blood groups: codominance means both alleles in a heterozygote contribute to the phenotype. The ABO blood group gene has three alleles: I^A, I^B (codominant with each other) and I^o (recessive to both).
| Genotype | Phenotype (blood group) |
|---|---|
| I^A I^A or I^A I^o | A |
| I^B I^B or I^B I^o | B |
| I^A I^B | AB |
| I^o I^o | O |
(Extended) Sex linkage: a sex-linked characteristic has its gene located on a sex chromosome, making it more common in one sex. Red-green colour blindness is carried on the X chromosome only (the Y chromosome carries no allele for it):
| Genotype | Phenotype |
|---|---|
| X^B X^B | Female, unaffected |
| X^B X^b | Female, unaffected carrier |
| X^b X^b | Female, colour blind |
| X^B Y | Male, unaffected |
| X^b Y | Male, colour blind |
Because males need only one copy of the recessive allele to be affected, while females need two, colour blindness is much more common in males.
(Extended) Define codominance.
(a) Define genotype and phenotype.
(b) Define homozygous and heterozygous.
(c) State which of homozygous and heterozygous describes a pure-breeding individual.
