Why Colour Blindness Is Far More Common in Boys Than in Girls

Mendel's 3:1 and 9:3:3:1 ratios work beautifully for pea plants, but many traits break the pattern. Some offspring look like a blend of both parents, some traits depend on many genes, and some appear far more often in boys than in girls.

This lesson covers incomplete dominance and co-dominance, polygenic inheritance, pleiotropy and pedigrees, the chromosomal theory and sex determination, and sex-linked inheritance with linkage.

In incomplete dominance the heterozygote shows an intermediate phenotype, in co-dominance both alleles are fully expressed together, and multiple alleles means a gene has more than two alleles in a population, as with the human ABO blood groups.

Incomplete dominance — snapdragon flower colour:

- Red (RR) crossed with white (rr) gives all pink (Rr) F1 flowers
- F2 phenotypes: 1 red : 2 pink : 1 white — the same as the genotypic ratio

Co-dominance and multiple alleles — ABO blood groups:

- Controlled by gene I with three alleles: $I^A$, $I^B$ and $i$
- $I^A$ and $I^B$ are co-dominant, both expressed in group AB, and both are dominant over $i$
- Group A: $I^A{I}^A$ or $I^{A}i$; group B: $I^B{I}^B$ or $I^{B}i$
- Group AB: $I^A{I}^B$; group O: $ii$

An everyday example. Parents with blood groups A and B can have a child of any of the four groups if both are heterozygous, as in $I^{A}i\times I^{B}i$.

The substance. Multiple alleles exist only in a population — any one person carries at most two of the three ABO alleles.

Polygenic inheritance is when one trait is controlled by several genes, each adding a small effect, pleiotropy is when one gene affects several traits, and a pedigree chart traces a trait through generations of a family to reveal how it is inherited.

Polygenic inheritance:

- Traits such as human skin colour and height show continuous variation rather than clear-cut classes
- Each dominant allele adds a little to the phenotype, and the environment also plays a part

Pleiotropy:

- A single gene produces more than one effect
- In phenylketonuria, a mutation in one gene causes intellectual disability and reduced hair and skin pigmentation

Pedigree charts:

- Squares are males and circles females; shaded symbols show affected individuals
- Autosomal dominant traits appear in every generation and affect both sexes equally
- Autosomal recessive traits can skip generations and appear in children of unaffected carriers
- X-linked recessive traits affect mostly males and pass through carrier females

An everyday example. A counsellor at a genetic clinic draws a family pedigree to estimate the chance that a couple's child will inherit thalassaemia.

The substance. Polygenic inheritance and pleiotropy are opposites — many genes shaping one trait versus one gene shaping many traits.

The chromosomal theory states that genes are carried on chromosomes, whose pairing, segregation and independent assortment during meiosis explain Mendel's laws, and sex is decided by sex chromosomes in humans, birds and grasshoppers, and by the number of chromosome sets in honeybees.

Chromosomal theory of inheritance:

- Chromosomes, like Mendel's factors, occur in pairs
- Homologous chromosomes separate during meiosis, just as alleles segregate
- Different chromosome pairs assort independently, just as different genes do

Sex determination:

- Humans (XX-XY) — females XX, males XY; males are heterogametic, making X-carrying and Y-carrying sperm, so the sperm decides the sex of the child
- Grasshoppers (XX-XO) — females XX, while males have a single X, so males have one chromosome fewer
- Birds (ZW-ZZ) — females ZW and males ZZ, so the female is heterogametic and decides the sex of the offspring
- Honeybees (haplodiploidy) — females develop from fertilised eggs and are diploid, while males develop from unfertilised eggs and are haploid

An everyday example. Blaming a mother for the birth of a daughter has no scientific basis — in humans the sperm, carrying X or Y, decides the child's sex.

The substance. A male honeybee has no father and cannot have sons — it develops from an unfertilised egg, and its sperm produce only daughters.

Sex-linked genes lie on the sex chromosomes, so X-linked recessive traits such as colour blindness and haemophilia show up far more often in males, while linkage is the tendency of genes on the same chromosome to be inherited together and crossing over is the exchange of segments that breaks linkage and creates recombinants.

X-linked inheritance:

- Males have one X chromosome, so a single recessive allele on it shows its effect
- Females need the allele on both X chromosomes; with one copy they are carriers
- An affected father passes the allele to all daughters, who become carriers, but to none of the sons

Worked example. A carrier woman ($X^C{X}^c$) and a man with normal vision ($X^{C}Y$) can have daughters $X^C{X}^C$ or $X^C{X}^c$, all with normal vision, and sons $X^{C}Y$ or $X^{c}Y$ — so each son has a one in two chance of being colour blind.

Linkage and crossing over:

- Linkage — genes close together on one chromosome tend to stay together in gametes, reducing new combinations
- Crossing over — during prophase I of meiosis, non-sister chromatids exchange segments, producing recombinant gametes

An everyday example. Colour vision tests for a driving licence in India screen for red-green colour blindness, which is found mostly in men.

The substance. Linkage and crossing over pull in opposite directions — linkage keeps parental combinations together, while crossing over creates new ones.