Beyond Mendel: complex inheritance
Most traits do not follow simple dominant-recessive rules. This lesson covers the patterns that explain blood types, skin colour, and sex-linked conditions.
Incomplete dominance
In incomplete dominance, heterozygotes show a blend. Crossing red and white snapdragons gives pink offspring; crossing two pinks gives red, pink, and white in a 1 : 2 : 1 ratio. The alleles have not mixed permanently; they still separate in the next generation.
Codominance and multiple alleles
In codominance, both alleles show fully. The ABO blood group gene has three alleles: A and B are codominant, and O is recessive. So genotype AB gives type AB blood, AO gives type A, and OO gives type O. Having more than two alleles in a population is called multiple allelism.
Polygenic traits
Many traits, such as height and skin colour, are controlled by many genes, each adding a small effect. These traits vary continuously and often follow a bell-shaped distribution in a population. Environment adds further variation: nutrition affects height, sunlight affects skin colour.
Sex chromosomes
Humans have 22 pairs of autosomes and one pair of sex chromosomes: XX in typical females and XY in typical males. The father's sperm carries either X or Y, so it determines the child's chromosomal sex. The Y chromosome carries few genes; the X carries many.
Sex-linked inheritance
Genes on the X chromosome show sex-linked inheritance. Males have only one X, so a single recessive allele there shows its effect. That is why red-green colour blindness and haemophilia are much more common in males. A carrier mother passes the allele to half her sons on average.
Pedigrees
A pedigree is a family tree showing who has a trait. Squares are males, circles females, and shaded shapes are affected. A recessive trait can skip generations and appear in children of unaffected parents; a dominant trait usually appears in every generation. Genetic counsellors use pedigrees to estimate risks.
Linked genes
Genes close together on the same chromosome tend to be inherited together, breaking Mendel's independent assortment. Crossing over can separate them; the farther apart they are, the more often this happens. Measuring how often linked traits separate let early geneticists build the first gene maps.
Worked example
A colour-blind man (XcY) and a non-carrier woman (XCXC) have children. All daughters receive his Xc and her XC, so they are carriers with normal vision. All sons receive his Y and her XC, so they have normal vision. The allele passes to his grandsons only through his daughters.
Key takeaways
Incomplete dominance blends; codominance shows both alleles, as in AB blood. Polygenic traits vary continuously. X-linked recessive traits are more common in males because they have one X. Pedigrees reveal inheritance patterns. Linked genes travel together unless crossing over separates them.
Flashcard
Heterozygote shows a blend
Check your understanding
A parent with type AB blood and a parent with type O blood have a child. Which blood types are possible?