Science (Double Award) | Reproduction and inheritance

👋 Welcome to Reproduction and Inheritance!

Hello future biologists! This chapter is all about how life continues, how living things produce new individuals, and why you share certain traits with your family members.

Don't worry if terms like 'allele' or 'Punnett square' sound complicated right now. We will break down these powerful concepts into simple, easy-to-understand steps. By the end, you'll be able to predict how characteristics are passed from one generation to the next!

1. Modes of Reproduction: Keeping Life Going

All living things must reproduce to ensure their species doesn't die out. There are two main ways organisms do this: Asexual and Sexual reproduction.

1.1. Asexual Reproduction (The Copy Machine)

In Asexual Reproduction, only one parent is needed. The offspring produced are genetically identical to the parent.

• The Process: This usually involves a type of cell division called mitosis. Mitosis makes exact copies of the nucleus and then the whole cell.
• Example: Bacteria dividing, yeasts budding, or a potato sprouting new plants from its eye.

Advantages and Disadvantages of Asexual Reproduction

Think of it like photocopying a document. It's fast, but if the original has a mistake, every copy has that same mistake!

Advantages:

• Fast: It happens very quickly.
• Efficient: Only one parent is required (no need to find a mate).
• Reliable: If the parent is well-suited to the environment, all offspring will be too.

Disadvantages:

• No Variation: All offspring are identical. If the environment changes (e.g., a new disease appears), all organisms will be vulnerable.

1.2. Sexual Reproduction (Mixing and Matching)

In Sexual Reproduction, two parents (male and female) are usually involved. This involves the fusion of specialised sex cells called gametes (sperm and egg/ova).

• The Process: Gametes are produced by a type of cell division called meiosis (which halves the chromosome number). When they fuse (fertilisation), the chromosome number is restored.
• The Result: The offspring inherit a mix of genes from both parents, leading to variation.

Advantages and Disadvantages of Sexual Reproduction

Advantages:

• Genetic Variation: Offspring are different from their parents and each other.
• Adaptation: This variation means some individuals might be better suited to survive environmental changes, making the species more resilient.

Disadvantages:

• Slow: Takes time to grow, find a mate, and reproduce.
• Inefficient: Requires two individuals (and often complex courtship rituals).

2. The Human Reproductive System

Humans reproduce sexually. The reproductive systems are designed to produce gametes and ensure internal fertilisation and development.

2.1. Key Structures and Functions

The gametes (sex cells) are specialised for carrying half the genetic information (23 chromosomes).

• Male Gamete: Sperm (small, motile, with a tail for movement).
• Female Gamete: Egg (ovum) (large, non-motile, contains stored food reserves).

The Male System

• Testes (Testis, singular): Produce sperm and the male hormone testosterone.
• Sperm ducts: Tubes that carry sperm from the testes.
• Glands (e.g., Prostate): Produce fluid that mixes with sperm to form semen.

The Female System

• Ovaries (Ovary, singular): Produce eggs (ova) and the female hormones oestrogen and progesterone.
• Oviducts (Fallopian tubes): Tubes down which the egg travels. This is the site where fertilisation usually occurs.
• Uterus (Womb): A muscular organ where a fertilised egg implants and develops into a foetus.
• Cervix: The opening between the uterus and the vagina.

2.2. Puberty and the Menstrual Cycle

Puberty is the stage when reproductive organs mature and secondary sexual characteristics develop, triggered by sex hormones (testosterone in males, oestrogen in females).

The Menstrual Cycle (A 28-day routine)

The menstrual cycle prepares the female body for potential pregnancy. It is controlled by hormones released from the pituitary gland and the ovaries.

Step-by-Step Cycle:

1. Days 1–5 (Menstruation): If the egg is not fertilised, the thick lining of the uterus breaks down and is shed (a period).
2. Days 6–14 (Lining Builds): The hormone oestrogen causes the uterus lining to start thickening again, preparing a soft, bloody bed for a fertilised egg.
3. Day 14 (Ovulation): An egg is released from the ovary into the oviduct.
4. Days 14–28 (Waiting): The hormone progesterone maintains the thick uterine lining. If no fertilisation occurs, the progesterone level drops, and the cycle starts again.

2.3. Fertilisation and Development

Fertilisation is the fusion of the sperm nucleus and the egg nucleus, usually occurring in the oviduct.

This fusion produces a single cell called a zygote, which contains the full set of chromosomes (46).

Development:

1. The zygote undergoes cell division to form an embryo.
2. The embryo travels to the uterus and implants itself in the thick uterine wall.
3. A structure called the placenta develops.

The Placenta: The Life Support System

The placenta is vital. It acts as an exchange surface between the mother’s blood and the foetus’s blood.

• It transfers essential substances from mother to foetus: oxygen, glucose (food), and antibodies (protection).
• It removes waste substances from the foetus: carbon dioxide and urea.

Note: Although the placenta allows exchange, the mother's and baby's bloodstreams usually remain separate, preventing infection or mixing of blood types.

3. Inheritance: Passing on Traits

Inheritance explains how you get your traits, like eye colour or blood type, from your parents. We need to learn some key vocabulary first—these are the building blocks of genetics!

3.1. DNA, Genes, and Chromosomes

• DNA: Deoxyribonucleic acid. This is the huge molecule that contains the instructions (the 'code') for building and operating an organism. It has a famous twisted ladder shape called a double helix.
• Gene: A short section of DNA that carries the instructions for making a specific protein (which usually controls a specific characteristic or trait). E.g., A gene for eye colour.
• Chromosome: A very long, tightly coiled strand of DNA, found in the nucleus of a cell. Humans have 23 pairs of chromosomes (46 total).

3.2. Essential Genetic Terminology

Don't worry if this looks like alphabet soup! Use the analogies to help you remember.

• Allele: A different version of a gene. If the gene is "Eye Colour," the alleles are "blue," "brown," or "green."
• Dominant Allele: An allele that is always expressed (shows up in the phenotype) if it is present. Represented by a capital letter (e.g., B for brown eyes).
• Recessive Allele: An allele that is only expressed if two copies are present (i.e., there is no dominant allele to mask it). Represented by a lowercase letter (e.g., b for blue eyes).

How we describe the individual's genetic makeup:

• Genotype: The genetic makeup of an organism—the actual letters/alleles it has. Example: BB, Bb, or bb.
• Phenotype: The physical characteristics shown—what you can see. Example: Brown eyes, blue eyes, tall plant.

How we describe the pair of alleles:

• Homozygous: Having two identical alleles for a trait.
  • Homozygous Dominant: BB
  • Homozygous Recessive: bb
• Heterozygous: Having two different alleles for a trait. Bb (The dominant trait will be shown).

4. Genetic Crosses (Monohybrid Inheritance)

A monohybrid cross looks at the inheritance of just one single characteristic (like height or colour). We use a tool called a Punnett Square to predict the possible genotypes and phenotypes of the offspring.

4.1. Step-by-Step: Using the Punnett Square

Let's cross two heterozygous tall plants (Tt x Tt), where T is the dominant allele for Tallness, and t is the recessive allele for shortness.

Step 1: Determine the Parents' Genotypes.

Parent 1: Tt (Tall)
Parent 2: Tt (Tall)

Step 2: Determine the Gametes.

During reproduction, the alleles separate. Each gamete receives only one allele.

Parent 1 Gametes: T or t
Parent 2 Gametes: T or t

Step 3: Draw the Punnett Square.

(Since we cannot draw a grid here, imagine a 2x2 grid)

        |  T  |  t  |
      --|-----|-----|
      T | TT  | Tt  |
      --|-----|-----|
      t | Tt  | tt  |
      --|-----|-----|
    

Step 4: Calculate the Ratios.

• Possible Genotypes: TT, Tt, tt
• Genotype Ratio: 1 TT : 2 Tt : 1 tt
• Phenotype Ratio: 3 Tall (TT, Tt, Tt) : 1 Short (tt)

This means there is a 75% chance the offspring will be Tall and a 25% chance the offspring will be Short.

4.2. Sex Determination (A special cross)

In humans, sex is determined by the Sex Chromosomes:

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

The mother can only provide an X chromosome. The father can provide either an X or a Y.

        |  X (F)  |  X (F)  |
      --|---------|---------|
      X (M)| XX (Girl) | XX (Girl) |
      --|---------|---------|
      Y (M)| XY (Boy)  | XY (Boy)  |
      --|---------|---------|
    

The ratio of boys to girls is 50% : 50%. The father’s sperm determines the sex of the child.

5. Variation and Mutation

5.1. Sources of Variation

Variation is the differences between individuals in a species. It is crucial for evolution and survival.

Variation comes from two main sources:

1. Sexual Reproduction: The shuffling of genes during meiosis (making gametes) and the fusion of genetically different gametes (fertilisation) creates endless new combinations.
2. Mutation: A rare, random change in the DNA structure (the sequence of bases in a gene).

5.2. Understanding Mutation

A mutation is like having a typo in the instruction manual (the DNA code).

• If a mutation occurs in a body cell (non-gamete), it only affects the individual.
• If a mutation occurs in a gamete (sperm or egg), it can be passed on to the offspring and become inheritable variation.

Most mutations are harmless or slightly harmful, but very rarely, a mutation might be beneficial, helping an organism survive better in its environment.

You’ve covered a huge amount of biology! The next time you see a family resemblance, you’ll know exactly which genes and alleles are responsible. Keep practicing those Punnett squares—they are the key to mastering inheritance!