Non-Mendelian Genetics

Why offspring counts can miss the expected ratio

• A cross predicted to produce four equally frequent combinations instead produces mostly two parental combinations. Linked genes 连锁基因 may be travelling together.
• Other deviations involve codominance 共显性, sex linkage or organelle DNA. Use the inheritance mechanism and the observed counts to choose a model, rather than forcing every cross into 3 : 1.

Incomplete dominance 不完全显性

• In incomplete dominance, neither allele fully wins.
• The heterozygote is a blend of the two.
• A red flower crossed with a white one gives pink offspring.
• The phenotype is intermediate; the alleles stay distinct and can separate in meiosis.

Sex linkage gives different results for sons and daughters

Codominance

• In codominance, both alleles show fully, at the same time.
• Neither blends and neither hides — you see both.
• In blood group AB, both the A and the B alleles are expressed.
• The result shows each allele clearly, side by side.

Multiple alleles 复等位基因 and polygenic 多基因 traits

• Some genes have multiple alleles — more than two versions in a population.
• The blood-group gene has three: A, B, and O.
• A polygenic trait is controlled by many genes together.
• Height and skin colour are polygenic, giving a smooth range of values.

Linked genes travel together

• Linked genes lie on the same chromosome and tend to enter the same gamete. Crossing over can separate them; genes farther apart usually show more recombination.
• For a small interval, map distance in map units is approximately the recombination percentage. This is a statistical estimate, not a physical length in millimetres; observed recombination cannot exceed 50%.

Worked example: map a short interval

• A test cross produces 420 AB, 420 ab, 80 Ab and 80 aB offspring. The common parental combinations are AB and ab; the rare recombinant combinations are Ab and aB.
• Recombination frequency = (80 + 80) ÷ 1000 × 100 = 16%, so the interval is about 16 map units. Count both recombinant classes, not just one.

Sex linkage changes probabilities

• A recessive X-linked allele can be expressed in an XY individual with just one copy, because the Y usually lacks the matching gene. In an XX individual, two recessive copies are usually needed. An XY father gives his X to daughters and his Y to sons.
• Worked cross: an unaffected father XᴬY and carrier mother XᴬXᵃ have a 1/2 chance of an affected son among sons, or 1/4 among all children if the sexes are equally likely. Other species use different systems: female birds are ZW; in many bees, males develop from unfertilized haploid eggs.

One gene can have several effects

• Pleiotropy 多效性 means one gene affects several traits. A change in a haemoglobin gene can affect red-cell shape and oxygen transport: the effects share a genetic cause.
• This differs from a polygenic trait, where many genes affect one characteristic. Multiple alleles exist across a population, but a diploid individual has at most two alleles at one autosomal locus.

Not all inherited DNA is nuclear

• Mitochondria and chloroplasts contain their own DNA. Their distribution into daughter cells can be uneven, so their traits need not follow a simple nuclear Punnett-square ratio.
• In animals, mitochondrial DNA is usually inherited through the egg. In many plants, organelle DNA comes through the ovule rather than pollen; maternal inheritance is common, but not an exception-free rule.

Write from a pink fraction of 1/2

• Flower colour is incomplete dominance. RR is red, rr is white, and Rr is pink.
• Two pink plants are crossed. The lesson map distance of 16 is a different cross.