Inherited Change (AQA A-Level Biology): Revision Notes
Monohybrid Inheritance
What is monohybrid inheritance?
Monohybrid inheritance describes the pattern of inheritance for a single gene. This type of inheritance follows predictable patterns that can be studied and represented using genetic crosses and diagrams.
When studying inheritance, scientists focus on how characteristics pass from parents to offspring through genes. Each gene can exist in different versions called alleles, and the combination of alleles an organism possesses determines its observable characteristics.
Understanding monohybrid inheritance is fundamental to genetics because it provides the foundation for studying more complex inheritance patterns involving multiple genes.
Key terminology and concepts
Several important terms help us understand how single genes are inherited:
Pure-breeding organisms consistently produce offspring with the same characteristic when bred together repeatedly. These organisms are homozygous, meaning they carry two identical alleles for a particular gene.
Dominant alleles mask the expression of other alleles, while recessive alleles are only expressed when no dominant allele is present. The genotype refers to the genetic makeup (the alleles present), whilst the phenotype describes the observable characteristics that result.
During reproduction, organisms produce gametes (sex cells) that carry only one allele from each gene pair. This process ensures that offspring receive one allele from each parent.
Representing genetic crosses
Creating accurate genetic diagrams requires following specific conventions to avoid confusion and errors. When selecting letters to represent alleles, choose symbols where the upper and lower case forms clearly differ in shape. For example, use G and g rather than S and s, as the latter pair can be easily confused.
Critical Conventions for Genetic Diagrams:
- The dominant allele should always be represented by a capital letter
- The recessive allele uses the corresponding lowercase letter
- Never use completely different letters when one characteristic has dominant and recessive forms
- Always choose letters where uppercase and lowercase forms are clearly distinguishable
Label all components clearly in genetic diagrams. Show the parental phenotypes and genotypes, indicate the gametes produced by each parent, and present the offspring genotypes and phenotypes systematically. Using a grid format (Punnett square) helps organise the results and reduces calculation errors.
The inheritance of pod colour in peas
The classic example of monohybrid inheritance comes from Gregor Mendel's work with pea plants, specifically studying pod colour. Pea pods appear in two basic colours: green and yellow.
Pure-breeding strains
When pea plants with green pods are bred repeatedly with each other, they consistently produce plants with green pods. Similarly, pure-breeding yellow-pod plants always produce yellow-pod offspring. These pure-breeding strains are homozygous - green-pod plants possess two identical alleles for green pods, while yellow-pod plants carry two identical alleles for yellow pods.
First filial (F1) generation cross
When pure-breeding green-pod plants cross with pure-breeding yellow-pod plants, all offspring in the F₁ generation produce green pods. This result demonstrates that the allele for green pods is dominant over the allele for yellow pods, which is recessive.
Worked Example: F₁ Cross
Parents: Pure-breeding green pods × Pure-breeding yellow pods
Parental genotypes: GG × gg (where G = green, g = yellow)
Gametes: All G × All g
F₁ genotypes: All Gg
F₁ phenotypes: All green pods (100%)
The F₁ plants are heterozygous, carrying one allele for green pods and one for yellow pods. Since green is dominant, all F₁ plants display green pods despite carrying the yellow allele.
Second filial (F2) generation cross
When F₁ plants (heterozygous for pod colour) are crossed with each other, the F₂ generation shows both phenotypes in a specific ratio. Approximately three plants display green pods for every one plant showing yellow pods - a ratio.
Worked Example: F₂ Cross
Parents: F₁ heterozygous × F₁ heterozygous
Parental genotypes: Gg × Gg
Gametes: G and g × G and g
F₂ genotypes: GG, Gg, Gg, gg
F₂ phenotypes: green : yellow
This ratio appears because the heterozygous F₁ plants can produce two types of gametes. Half carry the dominant green allele, while half carry the recessive yellow allele. When these gametes combine randomly during fertilisation, they create offspring with different genotype combinations.
Law of segregation
These observations led to the formulation of a fundamental principle in genetics called the law of segregation.
Mendel's Law of Segregation states that in diploid organisms, characteristics are determined by alleles that occur in pairs. During gamete formation, only one allele from each pair can be present in a single gamete.
This principle explains why the F₂ generation shows the characteristic phenotypic ratio. The alleles separate during gamete production, then recombine in various ways during fertilisation, producing predictable patterns of inheritance.
Constructing genetic crosses
When drawing genetic crosses, start by clearly stating the parental phenotypes and genotypes. Show the types of gametes each parent can produce, remembering that each gamete receives only one allele from each gene pair.
Step-by-Step Process:
- State the parental phenotypes and genotypes clearly
- Determine the types of gametes each parent can produce
- Use a Punnett square to show all possible gamete combinations
- Label male gametes across the top and female gametes down the side
- Fill in squares to show resulting genotypes
- Determine corresponding phenotypes and calculate expected ratios
Use a Punnett square to systematically show all possible combinations of gametes. This grid method helps visualise how parental alleles can combine in the offspring. Label the male gametes across the top and female gametes down the side, then fill in the squares to show the resulting genotypes.
Finally, determine the phenotypes that correspond to each genotype, and calculate the expected ratios. In monohybrid crosses between heterozygous parents, expect to see a phenotypic ratio in the offspring.
The reliability of these ratios increases with larger sample sizes. Small numbers of offspring may not show the expected ratios due to chance variation, but larger samples will approximate the theoretical predictions more closely.
Summary
Key Points to Remember:
- Monohybrid inheritance involves the study of how a single gene passes from parents to offspring
- Pure-breeding organisms are homozygous and consistently produce offspring with the same characteristics
- The F₁ generation from crosses between different pure-breeding lines typically shows only the dominant phenotype
- The F₂ generation from F₁ crosses shows a characteristic phenotypic ratio
- The law of segregation explains that allele pairs separate during gamete formation, with only one allele from each pair entering each gamete