Inheritance, variation and evolution

Welcome to Inheritance, Variation, and Evolution!

Ever wondered why you have your dad’s eyes but your mom’s hair? Or how bacteria can suddenly become "superbugs"? This chapter is all about the blueprint of life. We will explore how traits are passed down, how living things change over millions of years, and how humans are now "hacking" genetics to create new medicines and crops. Don't worry if it sounds complex—we’ll break it down bit by bit!

4.6.1 Reproduction

Sexual vs. Asexual Reproduction

There are two ways living things make more of themselves:

1. Sexual Reproduction involves the fusion (joining) of male and female gametes (sex cells). In animals, these are sperm and egg cells. In plants, they are pollen and egg cells. Because you get half your DNA from each parent, this leads to variation—no two offspring are exactly the same (except identical twins!).
2. Asexual Reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information. This creates clones—offspring that are genetically identical to the parent. This uses a process called mitosis.

Meiosis: Making Sex Cells

To make gametes, cells divide by a process called meiosis.
Analogy: Imagine a recipe book. If you just copied two books together, you'd have double the pages. Meiosis makes sure you only copy half the pages so when they join with another half, you get a full book again.

Step-by-Step Meiosis:
1. The cell copies its genetic information.
2. The cell divides twice to form four gametes.
3. Each gamete has only a single set of chromosomes.
4. All four gametes are genetically different from each other.

DNA and the Genome

DNA is a chemical that makes up your genetic material. It is a polymer (a long chain) made of two strands twisted into a double helix.
- Chromosomes: Long coils of DNA found in the nucleus.
- Gene: A small section of DNA on a chromosome. Each gene "codes" for a specific sequence of amino acids to make a specific protein.
- Genome: The entire genetic material of an organism. Scientists have mapped the whole human genome, which helps us search for genes linked to disease and trace human migration from the past.

Key Takeaway: Sexual reproduction creates variety using meiosis; asexual reproduction creates identical clones using mitosis.

4.6.1.4 Genetic Inheritance

The Language of Genetics

To understand how traits appear, you need to know these key terms:
- Allele: Different versions of the same gene (e.g., the gene for eye color could have a "blue" allele or a "brown" allele).
- Dominant: An allele that is always expressed, even if only one copy is present (represented by a Capital Letter, like B).
- Recessive: An allele that is only expressed if two copies are present (represented by a lowercase letter, like b).
- Homozygous: Having two of the same alleles (e.g., BB or bb).
- Heterozygous: Having two different alleles (e.g., Bb).
- Genotype: The collection of alleles you have (the "code").
- Phenotype: The physical characteristic that shows up (what you see).

Punnett Squares

We use Punnett squares to predict the probability of traits in offspring.
Example: Crossing two heterozygous brown-eyed parents (Bb).
\(25\%\) will be BB (Homozygous Dominant)
\(50\%\) will be Bb (Heterozygous)
\(25\%\) will be bb (Homozygous Recessive)

Inherited Disorders

Some diseases are passed down in our genes:
- Polydactyly: Having extra fingers or toes. It is caused by a dominant allele. If one parent has it, there's a high chance the child will too.
- Cystic Fibrosis: A disorder of cell membranes. It is caused by a recessive allele. Both parents must carry the allele for a child to have the disorder.

Sex Determination

Humans have 23 pairs of chromosomes. One pair determines your sex:
- Females: Have two of the same chromosomes (XX).
- Males: Have different chromosomes (XY).
Did you know? Because the mother always gives an X, it is the father's sperm (which can be X or Y) that determines the sex of the baby!

Quick Review: Dominant alleles win if they are present. Recessive alleles only show up if the dominant one is missing.

4.6.2 Variation and Evolution

Variation

Differences in a population are called variation. It is caused by:
1. Genetic causes: The genes you inherit.
2. Environmental causes: The conditions you live in (e.g., a plant being yellow because it has no light).
3. Both: Like your height (genes give you the potential, but diet helps you grow).

Natural Selection (The Theory of Evolution)

Charles Darwin proposed that all species evolved from simple life forms over 3 billion years ago.
How it works:
1. Individual organisms in a species show variation because of different genes.
2. Those with traits best suited to their environment are more likely to survive and breed.
3. The genes that allowed them to survive are passed on to the next generation.
Memory Aid: V.S.S.I. (Variation -> Selection -> Survival -> Inheritance).

Selective Breeding

This is when humans choose which animals or plants to breed to get specific traits.
- Examples: Disease resistance in crops, more meat or milk in cows, gentle dogs.
- The Danger: It leads to inbreeding. This makes the population more likely to suffer from inherited diseases or be wiped out by a single new disease.

Genetic Engineering

This is a modern process where scientists modify the genome of an organism by introducing a gene from another organism.
- Example: Bacterial cells have been engineered to produce human insulin for diabetics.
- GM Crops: Plants can be engineered to be resistant to insects or produce bigger, better fruit.

Key Takeaway: Evolution happens naturally over millions of years; selective breeding and genetic engineering happen because humans choose the traits.

4.6.3 Understanding Genetics and Evolution

Evidence for Evolution

We know evolution happened because of:
1. Fossils: The "remains" of organisms from millions of years ago found in rocks. They can be actual parts that didn't decay, mineral-replaced parts, or traces like footprints.
2. Antibiotic Resistance: We can see bacteria evolving in "real-time."

Extinction

Extinction happens when there are no remaining individuals of a species left alive. This can be caused by new diseases, new predators, or rapid changes in the environment.

Resistant Bacteria (Superbugs)

Bacteria evolve very fast because they reproduce quickly. When we use antibiotics, some bacteria might have a mutation that makes them resistant. These survivors multiply, and the resistant strain spreads.
- MRSA is a famous "superbug" resistant to antibiotics.
- To stop this: Doctors shouldn't prescribe antibiotics for viruses, patients must finish their whole course, and we must limit antibiotics in farming.

Quick Review: Fossils show us the past, but the record is incomplete because many early life forms were soft-bodied and left no trace.

4.6.4 Classification of Living Organisms

Living things are classified based on their structure and characteristics.
- Linnaean System: Created by Carl Linnaeus. It goes: Kingdom, Phylum, Class, Order, Family, Genus, Species.
- The Binomial System: Every organism has a two-part Latin name based on its Genus and Species (e.g., humans are Homo sapiens).

The Three-Domain System

As our technology improved (better microscopes!), Carl Woese added a new level called Domains:
1. Archaea: Primitive bacteria usually living in extreme environments.
2. Bacteria: True bacteria.
3. Eukaryota: Includes protists, fungi, plants, and animals.

Key Takeaway: Classification helps scientists organize the millions of species on Earth and see how they are related through "Evolutionary Trees."