Inheritance Basics

Inheritance is how characteristics are passed from one generation to the next through genes. This topic is fundamental to understanding how traits like eye colour, hair texture, and even some diseases are transmitted from parents to offspring.

These flashcards will help you learn the key terms and concepts in inheritance that you'll need for your Edexcel Biology GCSE. Each card covers an essential definition or concept that you'll be expected to know for your exams.

Remember: Learning these terms isn't just about memorisation—it's about understanding how our bodies work at a genetic level!

What is a Chromosome?

Chromosomes are long, coiled molecules of DNA that carry genetic information in the form of genes. They exist in the nucleus of almost every cell in your body.

Humans have 46 chromosomes arranged in 23 pairs. These tiny structures contain all the instructions needed to build and maintain your body.

Think of chromosomes as the storage units for your genetic information—they keep your DNA organised and make sure it's properly passed on when cells divide.

Did you know? If you stretched out all the DNA from a single human cell, it would be about 2 metres long! Chromosomes help pack all that DNA efficiently into your microscopic cells.

What is a Gene?

A gene is a section of DNA that codes for a specific sequence of amino acids, which then join together to form a protein. These proteins perform various functions in your body, from building muscle to creating eye colour.

Each gene has a specific location on a chromosome. You might have thousands of genes on a single chromosome, each controlling different traits or functions.

Genes are like recipes in a cookbook—they contain detailed instructions for making specific proteins your body needs to function properly.

Think about it: Nearly every characteristic you have—from your hair texture to how quickly you metabolise certain foods—is influenced by your genes!

What are Alleles?

Alleles are different versions of the same gene. They can create variation in traits like eye colour, hair type, or blood group.

For each gene, you inherit one allele from your mother and one from your father. These paired alleles may be identical or different from each other.

The combination of alleles you have determines what traits you display. For example, you might have one allele for brown eyes and one for blue eyes, but only one will be expressed.

Quick tip: To remember this concept, think about jeans (genes) coming in different styles (alleles). Same basic item, different variations!

Genotype and Phenotype

Your genotype is your complete genetic makeup—it describes all the alleles you have. Think of it as your genetic blueprint that remains constant throughout your life.

Your phenotype, on the other hand, is your observable characteristics—like height, eye colour, and blood type—resulting from the interaction between your genotype and the environment.

Environmental factors can modify your phenotype without changing your genotype. For example, your genes might give you potential to be tall, but poor nutrition during childhood could limit your actual height.

Important distinction: Your genotype is what's in your DNA, while your phenotype is what people can actually see or measure about you.

Homozygous vs Heterozygous

When you have two identical alleles for a gene (like FF or ff), you are homozygous for that trait. This means both parents gave you the same version of the gene.

Being heterozygous means you have two different alleles for a gene (like Ff). You received different versions from each parent.

Your combination of alleles affects how traits are expressed. For example, if you're homozygous for a trait like attached earlobes (ee), you'll definitely show that characteristic.

Make it stick: Think of "homo" meaning "same" and "hetero" meaning "different" to remember these terms easily!

Dominant and Recessive Alleles

A dominant allele is always expressed in your phenotype, even if only one copy is present. These are typically represented with capital letters, like F.

A recessive allele is only expressed when two copies are present (one from each parent) and no dominant allele exists to mask it. These are shown with lowercase letters, like f.

For example, if F represents the allele for free earlobes and f for attached earlobes, someone with genotype Ff will have free earlobes because F is dominant over f.

Visualization tip: Think of dominant alleles as "loud" (they always make themselves heard) and recessive alleles as "quiet" (only heard when there's no dominant allele to drown them out).

Monohybrid Inheritance

Monohybrid inheritance refers to the study of how a single gene is passed from parents to offspring. It's the simplest form of inheritance to track and predict.

Scientists use tools like Punnett squares to predict the possible combinations of alleles in offspring. These diagrams help calculate the probability of children inheriting specific traits.

For example, if both parents are heterozygous (Aa) for a trait, a Punnett square can show us that their children have a 25% chance of being homozygous dominant (AA), 50% chance of being heterozygous (Aa), and 25% chance of being homozygous recessive (aa).

Exam alert! Monohybrid crosses frequently appear in GCSE exam questions. Make sure you can draw and interpret Punnett squares!

Genetic Disorders: Sickle Cell and Cystic Fibrosis

Genetic disorders occur when there's an abnormality in an individual's DNA. Two common examples are sickle cell anaemia and cystic fibrosis, both of which are recessive conditions.

For recessive disorders, both parents must pass on the recessive allele for the condition to appear in their child. Carriers have one normal allele and one faulty allele, so they don't show symptoms but can pass the condition on.

Using Punnett squares, we can predict the probability of children inheriting these conditions. For example, when two carriers of cystic fibrosis (Ff) have children, there's a 25% chance their child will have the condition.

Real-world application: Genetic counselling uses these probability principles to help families understand the likelihood of passing on inherited conditions to their children.

Sex Chromosomes

Sex chromosomes determine whether you develop as male or female. Humans have two sex chromosomes in each cell:

• Females have two X chromosomes (XX)
• Males have one X and one Y chromosome (XY)

The Y chromosome contains a gene that triggers testes development in an embryo, which is why inheriting a Y chromosome leads to male development.

When parents conceive a child, the mother always contributes an X chromosome, while the father can contribute either an X (resulting in a female child) or a Y (resulting in a male child).

Fun fact: The probability of having a boy or a girl is roughly 50/50, and it's determined by which sperm cell fertilises the egg—not by anything the mother does!