Database results:
examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Phenotype and Genotype
Have you ever wondered why you have your mum's eyes but your dad's hair colour? Or why some traits seem to skip generations? The answers lie in understanding phenotype and genotype - two fundamental concepts in genetics that help us understand how traits are passed from parents to offspring.
Key Definitions:
Think of your genotype as the blueprint (your DNA) and your phenotype as the finished building. Your genotype is like the instructions that determine what you could look like, while your phenotype is what you actually look like after those instructions are followed.
Understanding the relationship between genotype and phenotype helps us predict inheritance patterns, understand genetic disorders and even develop treatments for genetic conditions. It's also crucial for plant and animal breeding.
Genes often come in different versions called alleles. For many traits, one allele is dominant over the other, which is recessive. This means that if you have at least one dominant allele, you'll show that trait in your phenotype.
Let's use a simple example: the gene for earlobe attachment in humans. Free earlobes (E) are dominant over attached earlobes (e).
Both alleles are dominant. Phenotype: Free earlobes.
One dominant, one recessive allele. Phenotype: Free earlobes (dominant allele masks recessive).
Both alleles are recessive. Phenotype: Attached earlobes.
Monohybrid inheritance refers to the inheritance of a single gene with two alleles. We can predict the outcomes of genetic crosses using Punnett squares - a handy grid that helps us work out all possible combinations of alleles.
Let's look at an example using pea plants. In peas, tall (T) is dominant over short (t).
Step 1: Write the possible gametes from each parent along the top and side of the grid.
Step 2: Fill in each box with the combined alleles.
Step 3: Count up the different genotypes and phenotypes.
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
Results:
Not all inheritance follows the simple dominant-recessive pattern. Sometimes, both alleles can influence the phenotype.
Both alleles are fully expressed in the phenotype. For example, in roan cattle, the allele for red coat (R) and the allele for white coat (W) are co-dominant. Heterozygous cattle (RW) have a roan coat - a mixture of red and white hairs.
The heterozygous phenotype is a blend or intermediate between the two homozygous phenotypes. For example, in snapdragon flowers, red (R) and white (r) alleles show incomplete dominance. Heterozygous plants (Rr) have pink flowers.
Some genes are located on the sex chromosomes (X and Y). In humans, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). Genes on the X chromosome are called X-linked.
Many X-linked genes have no equivalent on the Y chromosome. This means males only need one recessive allele to show the trait, while females need two.
Red-green colour blindness is an X-linked recessive condition. The normal allele (C) is dominant over the colour-blind allele (c).
For females to be colour blind, they need two copies of the recessive allele (XᶜXᶜ). But males only need one copy (XᶜY) because their Y chromosome doesn't have this gene at all.
This explains why colour blindness is much more common in males (about 8%) than females (about 0.5%).
Geneticists use special diagrams to track inheritance patterns across generations. These help us predict the likelihood of certain traits appearing in offspring.
A family pedigree is a diagram showing the occurrence of a genetic trait across multiple generations. They're particularly useful for tracking inherited disorders.
In a pedigree diagram:
By analyzing patterns in pedigrees, geneticists can determine if a trait is dominant or recessive and whether it's autosomal (on non-sex chromosomes) or sex-linked.
Understanding genotype and phenotype isn't just theoretical - it has real-world applications in medicine, agriculture and conservation.
Genetic counsellors use inheritance patterns to help families understand the risk of passing on genetic disorders. Knowing whether a condition is dominant, recessive, or sex-linked helps determine the probability of children inheriting the condition.
Plant and animal breeders use their understanding of inheritance to develop varieties with desirable traits, such as disease resistance in crops or specific coat colours in pets.
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