16.1.6Inheritance
Monohybrid Inheritance — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 9 parts Lesson 7 of 7
Part 1 of 7Inheritance terms
Inheritance terms
Inheritance
Inheritance the transmission of genetic information from one generation to the next.
Gene
Gene a short length of DNA on a chromosome that codes for a characteristic.
Allele
Allele a version of a gene; one allele is inherited from each parent.
Genotype
Genotype the combination of alleles that controls a characteristic.
Phenotype
Phenotype the observable characteristics of an organism.
Part 2 of 7Dominant and recessive alleles
Dominant allele
- Dominant allele expressed if present in the genotype; given a capital letter.
Recessive allele
- Recessive allele expressed only when no dominant allele is present;
- given the lower case version of the same letter.
Hidden allele
- Hidden allele a single recessive allele remains hidden and the dominant characteristic shows.
Part 3 of 7Homozygous and heterozygous
Homozygous and heterozygous
Homozygous
Homozygous two identical alleles of a gene, either homozygous dominant or homozygous recessive.
Heterozygous
Heterozygous two different alleles of a gene.
Pure-breeding
- Pure-breeding two identical homozygous individuals breeding together always produce offspring like themselves.
Heterozygous individuals
- Heterozygous individuals are not pure-breeding, because they can pass on different alleles.
Genotype from phenotype
- Genotype from phenotype a dominant phenotype may be homozygous or heterozygous, so the genotype cannot be told by looking.
Part 4 of 7Constructing a genetic diagram
1
Choose a letter for the alleles, with clearly different capital and lower case forms.
2
Write the parental phenotypes and genotypes.
3
Write the gametes each parent can produce, one allele per gamete.
4
Draw a Punnett square with the gametes along the sides.
5
Fill in the offspring genotypes and label their phenotypes.
6
State the ratio or probability.
Part 5 of 7Common monohybrid crosses
| Cross | Offspring genotypes | Phenotypes |
|---|---|---|
| Homozygous dominant × homozygous recessive | All heterozygous | All dominant |
| Heterozygous × heterozygous | 1 : 2 : 1 homozygous dominant : heterozygous : homozygous recessive | 3 dominant : 1 recessive |
| Heterozygous × homozygous recessive | 1 heterozygous : 1 homozygous recessive | 1 dominant : 1 recessive |
| Homozygous × homozygous of the same type | All the same as the parents | Pure-breeding |
- Ratios are probabilities, not exact results, and small families may not match them.
Part 6 of 7Test cross
Purpose
- Purpose identifies the genotype of an organism showing the dominant phenotype.
Method
- Method cross it with a homozygous recessive individual.
All offspring dominant
- All offspring dominant the unknown parent is homozygous dominant.
Offspring about 1 : 1
- Offspring about 1 : 1 the unknown parent is heterozygous.
Part 7 of 7More from the specification
1
Mendelian inheritance of dominant genes
- Mendelian inheritance of dominant genes heterozygous individuals carry contrasting alleles of specific genes, expressing dominant phenotypic traits according to standard ratios.
2
Monohybrid Genetic Crosses and Punnett Squares
- A monohybrid cross investigates the inheritance of single-gene traits;
- Punnett squares systematically display maternal and paternal gametic alleles to predict offspring genotypic and phenotypic ratios.
3
Homozygous versus Heterozygous States
- Homozygous individuals carry identical alleles,
- (either homozygous dominant, e.g., BB, or homozygous recessive, e.g., bb)
- whereas heterozygous individuals carry distinct alleles (e.g., Bb).
4
Codominance in ABO Blood Group Inheritance
- Alleles I^A and I^B are codominant because both glycoproteins are simultaneously expressed on erythrocyte cell surfaces in genotype I^A I^B,
- whereas allele I^O is recessive.
5
Daughter cell
- Cell produced by cell division.
6
Blood group
- A phenotype set by alleles I-A, I-B and I-O.
7
Protein synthesis
- Making proteins from amino acids using the DNA code.
8
Mitosis
- Cell division producing genetically identical cells.
9
Protein
- A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
10
Meiosis
- Reduction division giving four genetically different haploid cells.
11
Sequence
- The order of bases in DNA or amino acids in a protein.
12
Specific
- Able to act on only one type of substrate.
Key terms
| Term | Meaning |
|---|---|
| Allele | A version of a gene. |
| Dominant | Expressed when present in the genotype. |
| Recessive | Expressed only when no dominant allele is present. |
| Homozygous | Having two identical alleles. |
| Heterozygous | Having two different alleles. |
| Genotype | The alleles an organism has. |
| Phenotype | The observable characteristic. |
| Punnett square | A grid predicting offspring genotypes. |
Avoid these mistakes
1
Saying a recessive allele cannot be passed on — heterozygotes pass it on.
2
Mixing up genotype and phenotype — genotype is the alleles and phenotype is the observable feature.
3
Writing gametes with two alleles — each gamete carries one allele.
4
Saying a 3 : 1 ratio is guaranteed in a small family — it is only a probability.
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