Sex-linked traits, and lethal alleles

Two further named mechanisms, each producing a ratio or pattern that looks "wrong" until the mechanism is understood.

Sex-linked inheritance

A gene carried on the X CHROMOSOME is inherited differently in males (XY, one copy only) than in females (XX, two copies) — males express whatever allele they carry, with no second copy to mask a recessive one.
A co-dominant, X-linked trait (like orange/black fur in cats) can only show BOTH colours together (tortoiseshell) in a FEMALE, since she needs two different X chromosomes (one orange allele, one black allele) to express co-dominance — a male has only one X, so he can only ever be fully orange or fully black.
Pedigree evidence for X-linked recessive: affected individuals are mostly male, and an affected son usually has an unaffected CARRIER mother.

Lethal alleles

A lethal allele causes death, usually when present in the HOMOZYGOUS state — so a cross between two heterozygotes (aurea × aurea) does NOT give the expected 3:1 phenotype ratio, because the homozygous dominant genotype dies before it can be counted.
This is why an aurea (golden-leaf) × aurea cross gives a 2:1 ratio, not 3:1 — the "1" homozygous dominant class is missing, since those individuals die.
A lethal allele must be studied alongside its DOMINANCE for colour separately from its dominance for LETHALITY — the colour can be dominant (visible in heterozygotes) while the lethal effect only appears in homozygotes, which is why the trait can persist in a population at all.

Worked example Explaining why only female cats can be tortoiseshell Orange and black fur colour in cats is co-dominant and X-linked. Explain why only female cats can be tortoiseshell. THE GENE IS ON THE X CHROMOSOME: fur colour alleles (orange/black) are carried on the X chromosome, not on an autosome. FEMALES (XX) can be HETEROZYGOUS: a female with one X carrying orange and one X carrying black expresses CO-DOMINANCE — both alleles show, producing patches of both colours (tortoiseshell). MALES (XY) CANNOT be heterozygous for an X-linked gene: a male has only ONE X chromosome, so he carries only ONE fur-colour allele and will always be fully orange OR fully black, never both. CONCLUSION: tortoiseshell requires two DIFFERENT X-linked alleles at once, which only a female's two X chromosomes can provide. ✓ The whole explanation rests on chromosome COUNT (XX vs XY), not on any special property of the colour alleles themselves — the same logic applies to any co-dominant X-linked trait. ✕

Explaining a lethal-allele ratio without checking WHICH genotype is missing

A 2:1 ratio instead of 3:1 specifically means the HOMOZYGOUS DOMINANT class died — always identify which of the three expected genotype classes is missing before concluding a lethal allele is at work, since the missing class tells you whether the lethal effect pairs with the dominant or recessive allele.

Pedigree pattern observed	Likely inheritance mode
Affected individuals mostly male; carrier mothers unaffected	X-linked recessive
Affected father → all daughters affected, no sons affected	X-linked dominant
Trait skips a generation between two unaffected parents	Autosomal recessive
Every affected person has an affected parent	Autosomal (or X-linked) dominant

Practice for this standard covers the whole topic — flashcards and quiz.