Non-Mendelian Genetics
Introduced| English |
|---|
| linked genes/lɪŋkt dʒiːnz/ |
| codominance/ˈkəʊdəmɪnəns/ |
| incomplete dominance/ɪŋkəmˈpliːt ˈdɒmɪnəns/ |
| multiple alleles/ˈmʌltɪpl əˈliːlz/ |
| polygenic/ˌpɒlɪˈdʒenɪk/ |
| pleiotropy/pleɪˈɒtrəpi/ |
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.
In incomplete dominance, a red and a white flower can give offspring that are…
In incomplete dominance neither allele fully dominates, so the heterozygote is a blend — pink.
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.
In codominance, both alleles…
In codominance both alleles are fully expressed together — like the A and B in blood group AB.
Some genes, like the human blood group gene, have more than two possible alleles.
The ABO gene has three alleles (A, B, O) — an example of multiple alleles.
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.
Which inheritance pattern?
Sort each example by the non-Mendelian pattern it follows.
A trait controlled by many genes together, like skin colour, is called ____.
A polygenic trait is shaped by many genes adding together, giving a smooth range of values.
Select all true statements about non-Mendelian inheritance.
Many traits do not follow the simple 3:1 pattern. The other three are correct.
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.
The test cross above contains 160 recombinants among 1000 offspring. What is the estimated map distance in map units?
160 ÷ 1000 × 100 = 16; both recombinant classes count.
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.
For XᴬXᵃ × XᴬY, what fraction of the sons are expected to express the recessive X-linked trait?
Sons receive Y from their father and either maternal X with equal probability.
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.
One gene affects red-cell shape and oxygen transport. This is an example of…
Several effects arise from the same gene.
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.
A father normally transmits his mitochondrial DNA to all his sons.
In animals, the egg usually supplies the offspring mitochondria.
Simple dominant-and-recessive is just one pattern, not the whole story. Do not force every trait into a 3 : 1 ratio — many follow incomplete dominance, codominance, multiple alleles, or polygenic inheritance instead.
The ABO blood groups:
- The gene has three alleles: A, B, and O.
- A and B are codominant — someone with both shows blood group AB.
- O is recessive to both — so type O needs two O alleles. Several patterns in one gene.
Not all inheritance is simple dominant-and-recessive. In incomplete dominance the heterozygote is a blend; in codominance both alleles show fully; some genes have multiple alleles; and polygenic traits are shaped by many genes together. These non-Mendelian patterns add huge variety.