🔬 Welcome to B16: Inheritance – The Science of Genetics!
Have you ever wondered why you have your mum's curly hair or your dad's eye colour? This chapter, Inheritance, is all about the amazing biological instructions that are passed down from one generation to the next. We will dive into the tiny structures inside your cells that hold your genetic blueprint and learn how we can predict the traits that offspring will inherit. Don't worry if the vocabulary seems challenging at first; we will break down the complex terms into simple, understandable concepts!
This is one of the most fascinating topics in Biology, and mastering it will help you understand everything from human health to selective breeding in agriculture. Let's get started!
B16.1 Chromosomes and Genes (Core & Supplement)
The instructions for building and running an organism are stored in the nucleus of almost every cell. This instruction manual is made of chemical material called DNA.
What are Chromosomes, Genes, and Alleles?
1. Chromosomes (Core)
Chromosomes are thread-like structures found inside the nucleus. They are made of a chemical called DNA (Deoxyribonucleic acid), which contains all the genetic information. Think of a chromosome like a massive, tightly wound spool of thread (DNA).
2. Gene (Core)
A gene is a specific, short length of DNA on a chromosome. Its function is to code for a specific protein. These proteins then determine your traits (characteristics).
Analogy: If the whole chromosome is a cookbook, a gene is a single, specific recipe (e.g., the recipe for "eye-colour protein").
3. Allele (Core)
An allele is an alternative form of a gene. Because you inherit one set of chromosomes from your mother and one from your father, you have two alleles for every gene.
Analogy: The eye-colour recipe (gene) can have different versions (alleles), such as the allele for 'brown eyes' or the allele for 'blue eyes'.
🔑 Quick Review:
DNA makes up the chromosome.
A Gene is a segment of DNA that codes for a protein.
An Allele is a version of that gene.
Understanding Sets of Chromosomes (Supplement Content)
All cells in your body can be categorised based on how many sets of chromosomes they contain.
4. Diploid Nucleus (Supplement)
A diploid nucleus contains two sets of chromosomes (often symbolised as 2n). In a diploid cell, chromosomes exist as matched pairs (one from each parent).
In humans, diploid cells (like skin, muscle, or nerve cells) contain 23 pairs of chromosomes, totalling 46 chromosomes.
5. Haploid Nucleus (Supplement)
A haploid nucleus contains a single set of chromosomes (symbolised as n). These are found only in gametes (sex cells).
In humans, haploid gametes (sperm and egg cells) contain 23 chromosomes in total (no pairs).
Did you know?
When a haploid sperm (n=23) fuses with a haploid egg (n=23) during fertilisation, they form a diploid zygote (2n=46). This restores the full, paired chromosome number needed for a new organism!
Sex Determination in Humans (Core)
The inheritance of sex in humans is determined by a single pair of sex chromosomes, which are always the 23rd pair.
- Females have two X chromosomes (XX).
- Males have one X chromosome and one Y chromosome (XY).
Since females only produce eggs carrying an X chromosome, the sperm determines the sex of the offspring:
- If the egg (X) is fertilised by an X-carrying sperm, the result is XX (Female).
- If the egg (X) is fertilised by a Y-carrying sperm, the result is XY (Male).
This means there is generally a 50% chance of having a male or female child.
B16.2 Cell Division (Supplement Focus)
Cell division is crucial for growth, repair, and reproduction. There are two main types: Mitosis and Meiosis.
1. Mitosis: The Copying Machine (Supplement)
Mitosis is a type of nuclear division that produces two daughter cells that are genetically identical to the parent cell. This is often called 'equational division' because the chromosome number stays the same (diploid cells make more diploid cells).
The Role of Mitosis:
- Growth: Adding more cells to increase the size of an organism.
- Repair: Replacing damaged or worn-out cells (e.g., healing a cut).
- Asexual Reproduction: Used by organisms like yeast or bacteria to produce identical offspring.
Important Step: Before mitosis begins, the chromosomes undergo exact replication (copying themselves) to ensure that each new daughter cell receives a full, identical set. During mitosis, these copies then separate, maintaining the original chromosome number.
2. Meiosis: The Reduction Process (Supplement)
Meiosis is a specialised type of nuclear division known as a reduction division. It is involved solely in the production of gametes (sex cells).
Meiosis achieves two vital things:
- It halves the chromosome number (Diploid $\rightarrow$ Haploid). If gametes weren't haploid, the resulting zygote would have double the normal chromosome number!
- It results in daughter cells (gametes) that are genetically different from each other and the parent cell, increasing variation.
Note: Details of the stages of mitosis and meiosis are not required for this syllabus. You only need to know their roles and end products.
🔑 Key Takeaway for Cell Division:
Mitosis: Identical cells, 2n $\rightarrow$ 2n, for growth/repair/asexuality.
Meiosis: Different cells, 2n $\rightarrow$ n, for gamete production (sexual reproduction).
B16.3 Monohybrid Inheritance (Core Content)
Inheritance is simply the transmission of genetic information from one generation (parents) to the next (offspring). Monohybrid inheritance focuses on tracking just one characteristic (like height or flower colour) at a time.
Key Terms in Genetics
1. Genotype (Core)
The genotype is the genetic make-up of an organism, described by the combination of alleles present. We represent the genotype using letters (e.g., TT, Tt, tt).
2. Phenotype (Core)
The phenotype is the observable features or characteristics of an organism (what you actually see). The phenotype is determined by the genotype (and sometimes the environment).
Example: Tall or Short, Red flower or White flower.
3. Homozygous (Core)
An organism is homozygous for a trait if it has two identical alleles for a particular gene (e.g., TT or tt).
Two identical homozygous individuals that breed together will be pure-breeding (all offspring will show that trait if self-crossed).
4. Heterozygous (Core)
An organism is heterozygous if it has two different alleles for a particular gene (e.g., Tt).
A heterozygous individual will not be pure-breeding, as their offspring can show different phenotypes.
5. Dominant Allele (Core)
A dominant allele is always expressed in the phenotype, even if only one copy is present (i.e., in both homozygous dominant (TT) and heterozygous (Tt) genotypes). We use a capital letter (T) to denote a dominant allele.
6. Recessive Allele (Core)
A recessive allele is only expressed in the phenotype when no dominant allele is present. This means it is only seen in the homozygous recessive genotype (tt). We use a lowercase letter (t) to denote a recessive allele.
Using Punnett Squares and Genetic Diagrams (Core)
Genetic diagrams and Punnett squares help us predict the possible genotypes and phenotypes of offspring resulting from a cross between two parents.
Step-by-Step Monohybrid Cross Example (Tt x Tt)
Let's cross two heterozygous tall pea plants (T = Tall, t = short).
- State Parents' Phenotypes: Tall $\times$ Tall
- State Parents' Genotypes: Tt $\times$ Tt
- Determine Gametes: T and t $\times$ T and t (Remember: Gametes are haploid, so they only get one allele)
-
Use a Punnett Square to combine gametes:
| | T | t |
| :---: | :---: | :---: |
| T | TT | Tt |
| t | Tt | tt | - Identify Offspring Genotypes: TT, Tt, Tt, tt
-
Calculate Ratios:
- Genotypic Ratio: 1 TT : 2 Tt : 1 tt
- Phenotypic Ratio: 3 Tall (TT, Tt) : 1 Short (tt)
The syllabus requires you to be able to calculate and use the 3:1 phenotypic ratio, which often occurs when crossing two heterozygotes for a dominant/recessive trait.
Common Mistake Alert! Always remember that when determining the gametes, you must separate the alleles. A parent with genotype Tt produces two types of gametes: T and t.
Interpreting Pedigree Diagrams (Core)
Pedigree diagrams are family trees used to track the inheritance of a particular trait (often a genetic disease) through several generations.
- Squares usually represent males.
- Circles usually represent females.
- Shaded shapes usually represent individuals who express the trait (the phenotype being studied).
- An unshaded shape means the individual does not express the trait.
By looking at who has the trait and who doesn't, especially when two unaffected parents have an affected child, you can often deduce if the trait is dominant or recessive.
Tip for Recessive Traits:
If two unshaded (unaffected) parents have a shaded (affected) child, the trait must be recessive. Why? Because the parents must have been carriers (heterozygous) hiding the recessive allele, which then appeared in the child (homozygous recessive).
Chapter Conclusion
You have successfully tackled the fundamentals of inheritance! We've established that DNA, organised into chromosomes, carries genes, which come in alternative forms called alleles. We've seen how cells divide to either multiply (Mitosis for growth) or create sex cells (Meiosis for reproduction). Finally, you now know how to use key vocabulary (genotype, phenotype, dominant, recessive) and genetic tools (Punnett squares) to predict how traits are passed down. Keep practicing those crosses—they are essential for your exam success!