Mendel's laws

Alleles, dominance and phenotype

Why can a child have blue eyes when both parents have brown ones? The answer lies in a distinction established by Gregor Mendel around 1865, long before anyone knew what a gene was.

Gene and allele

GENE    : a location on a chromosome, responsible for a trait
          example: the gene for pea flower colour

ALLELE  : a possible VERSION of that gene
          example: the "purple flower" allele, the "white flower" allele

Since chromosomes come in pairs, each individual carries two alleles of every gene — one from the father, one from the mother.

   HOMOZYGOUS      the two alleles are identical       A//A   or   a//a
   HETEROZYGOUS    the two alleles differ              A//a

Dominance and recessiveness

When the two alleles differ, one may mask the other:

DOMINANT allele   : expressed even in a single copy   (written in CAPITALS)
RECESSIVE allele  : expressed only when ALONE         (written in lower case)

   genotype        phenotype
   ----------      ---------------
   A//A            dominant trait          (purple)
   A//a            dominant trait          (purple)   <- allele a is masked
   a//a            recessive trait         (white)

A recessive allele does not disappear for all that: it is transmitted silently and can reappear a generation later. That is exactly what happens with blue eyes.

Genotype and phenotype

This is the central distinction of the whole chapter:

GENOTYPE   : what one HAS         the alleles, invisible
PHENOTYPE  : what one OBSERVES    the visible trait

   A//A   and   A//a     ->  SAME phenotype (purple)
                         ->  DIFFERENT genotypes

Hence a methodological consequence: observing an individual with the dominant phenotype does not reveal its genotype. You need either to know its parents or to perform a test cross with a recessive individual — whose offspring will betray the hidden allele.

Why Mendel succeeded where others had failed

His predecessors crossed plants and described the outcome vaguely. Mendel did three things differently:

1. he chose CLEAR-CUT traits (purple or white, round or wrinkled),
   with no intermediates — no shades to interpret
2. he worked on PURE lines, stabilised over two years
3. he COUNTED: several thousand plants, and he computed proportions

The third point is decisive. By bringing counting and proportions into a hitherto descriptive science, Mendel made biology quantitative. His results, published in 1866, would nonetheless be ignored for thirty-four years.

Summary