Genetics and Inheritance, Grade 12: Monohybrid, Dihybrid and Sex Linkage
This topic draws two diagrams that quietly refute something the meiosis topic stated earlier in the same course.
Earlier, homologous chromosomes were defined as a pair "which carry the same genetic information". Here, one diagram draws a homologous pair with a black hair allele on one chromosome and a blonde hair allele on the other, and a second does it again with eye colour. The diagrams are right. The definition was wrong, and the whole of genetics depends on which one you believe.
This page gives monohybrid and dihybrid crosses with the working set out the way the marks are awarded, sex linkage, and the three places this topic is worth checking.
Where this fits in the curriculum
| Subject | Life Sciences |
|---|---|
| Grade | 12 |
| Term | 2 |
| Topic | Topic 5, Genetics and inheritance |
| Status | No formally assessed practical. This is a calculation topic |
| Marks | Our worksheet is 90, which is the largest in the grade because this is where the marks are |
The vocabulary, settled before anything else
| Term | What it means |
|---|---|
| Gene | A length of DNA coding for one characteristic |
| Allele | A version of a gene. B for black hair and b for blonde hair are two alleles of one gene |
| Genotype | The alleles an organism carries, written as letters: BB, Bb or bb |
| Phenotype | What you can see. Black hair |
| Homozygous | Both alleles the same. BB or bb. Also called purebred |
| Heterozygous | Two different alleles. Bb. Also called a hybrid |
Genotype is written, phenotype is seen. If the answer is letters it is a genotype. If the answer is a description it is a phenotype. That one line settles a surprising number of lost marks.
The contradiction this topic inherits
Printed in the meiosis topic: homologous pairs are chromosomes "which carry the same genetic information".
Drawn in this topic, twice: a homologous pair with different alleles on the two chromosomes. Black hair on one, blonde on the other. Blue eye on one, brown on the other.
| If homologous chromosomes carried the same information | What this topic actually teaches |
|---|---|
| Every genotype would be homozygous | Heterozygotes are the entire subject. Bb is drawn on this page |
| There would be no dominant and recessive | Dominance is what the 3:1 ratio is made of |
| Monohybrid crosses would be impossible | A monohybrid is a heterozygote |
Believe the diagrams. Homologous chromosomes carry genes for the same characteristics, at the same positions. The alleles on them are usually different, and that difference is inheritance. The meiosis page sets out where the wrong definition appears and what to replace it with.
Monohybrid crosses, set out the way marks are awarded
A monohybrid cross follows one gene. Write it out in full every time, because most of the marks are for the layout rather than the answer.
| Line | What goes there |
|---|---|
| P | The parent phenotypes and genotypes |
| Meiosis | Write the word. It is where the gametes come from and it is often a mark |
| Gametes | Each parent's gametes, circled |
| Fertilisation | Write the word, and draw the lines |
| F1 | Offspring genotypes and phenotypes |
| Ratio | Stated as a ratio, not as a sentence |
A heterozygous cross, Bb x Bb, gives 1 BB : 2 Bb : 1 bb. That is a genotypic ratio of 1:2:1 and a phenotypic ratio of 3:1. Questions ask for one and get the other more often than any other error in this topic.
A test cross is how you find out whether a dominant phenotype is BB or Bb. Cross it with the homozygous recessive, bb. If any offspring show the recessive phenotype, the unknown parent was Bb.
When dominance is not complete
| What the heterozygote looks like | Ratio in the F2 | |
|---|---|---|
| Complete dominance | Like the dominant parent | 3:1 |
| Incomplete dominance | A blend. Red and white give pink | 1:2:1 |
| Codominance | Both, at once. Red and white give red and white patches, not pink | 1:2:1 |
Incomplete dominance blends, codominance shows both separately. Roan cattle and the AB blood group are codominance. Pink snapdragons are incomplete dominance. In both cases the phenotypic ratio becomes the same as the genotypic ratio, because every genotype now looks different.
Dihybrid crosses
Two genes at once, and the F2 ratio is 9:3:3:1 when the genes are on different chromosomes.
Cross a pure yellow round pea, YYRR, with a pure green wrinkled one, yyrr. Every F1 plant is YyRr and looks yellow and round. Self those and the F2 gives 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled.
| Where the 9:3:3:1 comes from |
|---|
| A YyRr parent makes four kinds of gamete: YR, Yr, yR and yr |
| Four by four is sixteen boxes in the Punnett square |
| Nine of the sixteen show both dominant phenotypes, one shows both recessive |
The 3 : 3 in the middle are the recombinants, the combinations neither parent had. That is independent assortment made visible, and it is worth naming when you explain the ratio.
Linked genes, and the ratio that assumes crossing over never happens
You will read that if two genes are on the same chromosome, the dihybrid cross gives a 3:1 ratio, the same as a monohybrid cross.
That is true only if no crossing over occurs between those two genes, which is the opposite of what the same course says about meiosis, where crossing over at chiasmata is given as one of the two main sources of genetic variation.
| If the linked genes never recombine | What actually happens |
|---|---|
| Only two kinds of gamete, so a 3:1 ratio | Crossing over produces a small number of recombinant gametes |
| The two characteristics are inherited as one | They are inherited together most of the time, and the exceptions are measurable |
Use 3:1 in the exam, because that is what the curriculum asks for, but know why it is an approximation. The further apart two linked genes sit on a chromosome, the more often crossing over separates them, and that fact is the basis of genetic mapping.
Sex determination and sex linkage
The mother always gives an X. The father gives an X or a Y, and that is what decides sex. The ratio is 1 female : 1 male, or a 50 per cent probability either way, at every single conception.
| Why sex-linked conditions affect males more |
|---|
| The X chromosome carries many genes. The Y carries very few, and different ones |
| A male has only one X, so a recessive allele on it has no partner to mask it and is expressed |
| A female needs the recessive allele on both X chromosomes to show the condition |
| A female with one copy is a carrier: heterozygous, unaffected, and able to pass it on |
Haemophilia and red-green colour blindness are the two the exam uses. Write the genotypes with the allele as a superscript on the X, as XHXh, so the marker can see which chromosome carries what.
One caution about the sex-linked disease diagram
The X chromosome map in this topic lists bipolar disorder and cleft palate among the conditions caused by sex-linked alleles.
Both are overwhelmingly multifactorial. Cleft palate arises from many genes together with environmental factors, and bipolar disorder is polygenic with no established single sex-linked cause. Haemophilia A and B, Duchenne muscular dystrophy and red-green colour blindness are the sound examples on that diagram, and they are the ones to use in an answer.
What you should be able to do by the end
- Lay out a genetic cross with all six lines, because the layout carries the marks
- Give a genotypic and a phenotypic ratio separately without being asked twice
- Tell incomplete dominance from codominance by whether the heterozygote blends or shows both
- Produce four gamete types from a dihybrid and fill a sixteen-box Punnett square
- Write a sex-linked genotype as XHXh, not as Hh
If the crosses keep going wrong
| What happens | What caused it |
|---|---|
| The ratio comes out 1:2:1 when 3:1 was expected | That is the genotypic ratio. The phenotypic one groups BB and Bb together |
| A dihybrid gives only two gamete types | The two genes were treated as linked. On separate chromosomes a dihybrid makes four |
| Marks lost on a correct answer | The layout was skipped. P, meiosis, gametes, fertilisation, F1, ratio |
| Sex-linked genotypes written as Hh | The allele must sit on the chromosome: XHXh, XhY |
| A carrier is described as having the disease | A carrier is heterozygous and unaffected. That distinction is usually worth two marks |
What a class needs for this topic
Nothing from us. This is paper, a pen and practice. We would rather say so than sell you a genetics kit.
If you want one thing on the wall, a chart of the human chromosomes earns its place, and it is in educational wall charts. It is optional.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 90 marks, with monohybrid and dihybrid crosses laid out in full, incomplete dominance, codominance, sex linkage and a pedigree
- Marking memorandum, with every cross worked line by line and the six answers that look right and score nothing
Related practicals
- Mitosis and meiosis, Grade 12. Where the alleles this topic shuffles are actually separated, and where the wrong definition of a homologous pair appears
- The nervous system and the senses, Grade 12. The two assessed practicals in this grade
- The endocrine system and homeostasis, Grade 12. The 50-mark project, and the vessel that cannot dilate
- Variation, Grade 7. Where variation is first counted and graphed, five years before it has a mechanism