GEN 5.1 Inheritance Patterns- Pre AP Biology Study Notes - New Syllabus.
GEN 5.1 Inheritance Patterns- Pre AP Biology Study Notes
GEN 5.1 Inheritance Patterns- Pre AP Biology Study Notes – New Syllabus.
LEARNING OBJECTIVE
GEN 5.1(a) Explain the relationship between genotype and phenotype.
GEN 5.1(b) Describe the type of inheritance pattern based on data and/or use of models.
Key Concepts:
GEN 5.1.1 Investigation of Mendelian, or single-gene, traits reveals the basis for understanding patterns of inheritance.
a. Many of an organism’s traits (phenotype) are determined by the organism’s genes (genotype), which are passed from one generation to the next.
b. Somatic cells of sexually reproducing organisms have two copies of each gene (one inherited from each parent).
c. Each gene copy may have variants called alleles.
d. If present, dominant alleles are expressed, whereas recessive alleles are expressed only in the absence of a dominant allele.GEN 5.1.2 Most traits do not follow Mendelian inheritance patterns.
a. Some traits are determined by genes on sex chromosomes, and some are influenced by environmental factors.
b. Most traits involve the interactions of multiple genes.Codominance occurs when both alleles of homologous chromosomes are fully expressed.
Incomplete dominance occurs when neither of the alleles from a homologous chromosome pair are completely dominant.
Relationship Between Genotype and Phenotype
🌿 Introduction
Every organism has observable traits such as:
- Eye color
- Flower color
- Height
- Blood type
These observable characteristics are called phenotypes.
But where do these traits come from?
They come from an organism’s genotype, which is the genetic information stored in DNA.
🧠 What Is Genotype?
Genotype refers to:
- The genetic makeup of an organism
- The specific alleles it carries
In sexually reproducing organisms:
- Somatic (body) cells are diploid (2N)
- They contain two copies of each gene
- One copy is inherited from the mother
- One copy is inherited from the father
Each version of a gene is called an: Allele
So, genotype describes the combination of alleles an organism has for a trait.
Example:
AA
Aa
aa
These are different genotypes for the same gene.
🧬 What Is Phenotype?
Phenotype refers to:
- The observable expression of a trait
- Physical appearance
- Physiological characteristics
- Biochemical traits
Phenotype is the visible or measurable outcome of gene expression.
🔬 How Genotype Produces Phenotype
Genes contain instructions for making proteins.
Proteins control:
- Structure (hair, skin, cell shape)
- Enzymes (chemical reactions)
- Pigment production
- Hormone regulation
So, the pathway is: Gene (DNA) → Protein → Trait → Phenotype
If the genotype changes, the protein produced may change.
If the protein changes, the trait may change.
🧬 Role of Alleles
Each gene has variants called alleles.
Because organisms are diploid:
- Two alleles are present for each gene
- These alleles may be identical or different
There are three possible allele combinations:
Homozygous dominant → AA
Heterozygous → Aa
Homozygous recessive → aa
The interaction between alleles determines phenotype.
🧠 Dominant and Recessive Alleles
If a dominant allele is present:
- It is expressed
- It masks the recessive allele
A recessive allele is expressed only when:
- Two copies are present (aa)
Example:
If A = dominant trait
and a = recessive trait
Genotype AA → dominant phenotype
Genotype Aa → dominant phenotype
Genotype aa → recessive phenotype
So, phenotype depends on allele interaction.
🧬 Why Genotype Does Not Always Equal Phenotype Directly
Although genotype determines phenotype, expression may also be influenced by:
- Other genes
- Environmental factors
But at the core:
Phenotype results from the genetic instructions carried in genotype.
📊 Genotype vs Phenotype Table
| Term | Meaning | Example |
|---|---|---|
| Genotype | Genetic makeup (alleles) | AA, Aa, aa |
| Phenotype | Observable trait | Purple flower or white flower |
| Allele | Variant form of gene | A or a |
| Dominant allele | Expressed if present | A |
| Recessive allele | Expressed only if no dominant allele | a |
📦 Quick Recap
Genotype = genetic makeup
Phenotype = observable trait
Diploid cells have two alleles
Alleles may be dominant or recessive
Genes → Proteins → Traits
Genotype determines phenotype
Describe the Type of Inheritance Pattern Based on Data or Models
🌿 Introduction
When studying inheritance, scientists analyze:
- Offspring ratios
- Punnett squares
- Family pedigrees
- Phenotypic data
From this data, we identify the inheritance pattern.
Some traits follow simple Mendelian patterns.
Most traits follow more complex patterns.
🧠 Mendelian (Single-Gene) Inheritance
Key Features of Mendelian Inheritance
- Controlled by one gene
- Two alleles
- One dominant, one recessive
- Predictable ratios
Recognizing Mendelian Pattern From Data
If a cross between two heterozygous parents (Aa × Aa) gives:
Genotype ratio: 1 AA : 2 Aa : 1 aa
Phenotype ratio: 3 dominant : 1 recessive
That 3:1 ratio strongly suggests:
Simple dominant-recessive Mendelian inheritance
Example pattern:
If 75% of offspring show trait A and 25% show trait a
This matches 3:1 ratio → Mendelian.
🧬 Codominance
What Is Codominance?
- Both alleles are fully expressed in the heterozygote
- No allele masks the other
How to Recognize from Data
If heterozygous individuals show BOTH traits clearly and separately:
Example pattern:
Red flower × White flower → Offspring show red AND white patches
There is no blending.
Phenotypic ratio may show:
1:2:1 (distinct phenotypes)
That suggests: Codominance
🧬Incomplete Dominance
What Is Incomplete Dominance?
- Neither allele is completely dominant
- Heterozygote shows intermediate phenotype
Recognizing From Data
If red × white → pink
And offspring ratios follow:
1 red : 2 pink : 1 white
That 1:2:1 phenotype ratio indicates: Incomplete dominance
Key difference:
Incomplete dominance → blending
Codominance → both traits fully visible
🧬 Sex-Linked Inheritance
What Makes It Different?
- Gene located on sex chromosome
- Inheritance differs between sexes
Recognizing From Data
Clues:
- Trait appears more often in one sex
- Often more common in males
- Does not follow simple 3:1 ratio
Example:
Mostly males affected → likely X-linked recessive
🧬 Polygenic Inheritance
What Is Polygenic Inheritance?
- Trait controlled by multiple genes
- Each gene contributes small effect
Recognizing From Data
- Continuous variation
- No clear dominant/recessive categories
- Bell-shaped distribution
Example:
Height varies gradually across population.
No simple ratios → Polygenic inheritance.
🧬 Environmental Influence
Some traits depend on both:
- Genotype
- Environmental conditions
Clues in data:
- Same genotype, different phenotype
- Trait changes under different conditions
This suggests environmental influence.
📊 Pattern Recognition Summary Table
| Data Pattern | Likely Inheritance Type |
|---|---|
| 3:1 phenotype ratio | Mendelian dominant/recessive |
| 1:2:1 with blended phenotype | Incomplete dominance |
| 1:2:1 with both traits visible | Codominance |
| Trait more common in one sex | Sex-linked |
| Continuous range of variation | Polygenic |
📦 Quick Recap
3:1 → Mendelian
1:2:1 blended → Incomplete dominance
1:2:1 both traits → Codominance
Sex bias → Sex-linked
Continuous variation → Polygenic
Always analyze ratios first