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Unlock This CourseImagine you're trying to predict what colour eyes a baby might have based on their parents' eye colours. This is exactly what monohybrid cross diagrams help us do! They're like genetic fortune tellers that show us the possible outcomes when organisms reproduce.
A monohybrid cross looks at how just one characteristic (like eye colour, height, or flower colour) is passed from parents to offspring. It's called "mono" because we're only focusing on one trait at a time, making it much easier to understand inheritance patterns.
Key Definitions:
Think of dominant alleles as bossy older siblings - they always get their way! If you have one dominant allele (B) for brown eyes and one recessive allele (b) for blue eyes, you'll have brown eyes because the dominant allele "dominates" the recessive one.
Let's work through a classic example using flower colour in pea plants. We'll cross a plant with purple flowers (PP) with a plant with white flowers (pp). Purple is dominant (P) and white is recessive (p).
Follow these steps every time you draw a monohybrid cross diagram:
Write down the genotypes of both parents. In our example: Parent 1 = PP (purple), Parent 2 = pp (white)
Work out what alleles each parent can pass on. PP can only give P, pp can only give p
Create a Punnett square (a 2x2 grid) and fill in all possible offspring combinations
The Punnett square is like a genetic calculator. It shows us every possible way the parents' alleles can combine in their offspring.
Parents: PP (purple) × pp (white)
Gametes: P from first parent, p from second parent
All offspring: Pp (100% purple flowers)
Ratio: 4:0 purple to white
When we cross the original parents, we get the F1 generation (first filial generation). If we then cross two F1 individuals together, we get the F2 generation. This is where things get really interesting!
Let's cross two F1 plants (both Pp) and see what happens:
Parents: Pp × Pp
Offspring genotypes:
Phenotype ratio: 3 purple : 1 white
Monohybrid crosses aren't just about pea plants - they help us understand inheritance in all living things, including humans!
Many human characteristics follow simple dominant-recessive patterns:
Brown eyes (B) are dominant over blue eyes (b). Two brown-eyed parents (Bb × Bb) can have blue-eyed children!
Straight thumbs (S) are dominant over hitchhiker's thumbs (s). Check your own thumbs!
Free earlobes (F) are dominant over attached earlobes (f). Look in the mirror to see which you have!
This serious genetic condition is caused by a dominant allele (H). This means that if someone inherits just one copy of the allele from either parent, they will develop the disease. Using monohybrid crosses, genetic counsellors can calculate the probability of parents passing this condition to their children. If one parent has Huntington's (Hh) and the other doesn't (hh), there's a 50% chance each child will inherit the condition.
Even experienced students make mistakes with monohybrid crosses. Here are the most common ones:
Always use the same letter for the same gene. If you use B for brown eyes, don't suddenly switch to E for eyes. Stick with B and b throughout.
Always express your final answer as a ratio. Don't just say "some will be tall, some short" - be specific! Say "3 tall : 1 short".
The best way to master monohybrid crosses is through practice. Try working through different examples, starting with simple dominant-recessive patterns before moving on to more complex inheritance.
Try this one: In cats, short hair (S) is dominant over long hair (s). What would happen if you crossed a heterozygous short-haired cat (Ss) with a long-haired cat (ss)?
Cross: Ss × ss
Offspring: 50% Ss (short hair), 50% ss (long hair)
Ratio: 1 short : 1 long
Understanding monohybrid crosses is crucial for several reasons. They form the foundation for more complex genetic studies, help us predict inheritance patterns in breeding programmes, assist medical professionals in genetic counselling and explain why children sometimes look different from their parents.
From agriculture to medicine, from conservation to understanding human diversity, monohybrid crosses give us the tools to predict and understand how characteristics pass from one generation to the next. Master these basics and you'll have a solid foundation for understanding all of genetics!