B6 - Inheritance, Variation and Evolution

What is sexual reproduction?

Reproduction involving the fusion of male and female gametes, producing offspring with genetic variation

What is asexual reproduction?

Reproduction involving one parent only, producing genetically identical offspring

Give examples of organisms that reproduce asexually

Some plants, animals, and microorganisms

Give two advantages of sexual reproduction

1. Produces variation
   

2. Allows adaptation to changing environments

Give two disadvantages of sexual reproduction

1. Needs two parents
   

2. Slower → fewer offspring produced

Give two advantages of asexual reproduction

1. Only one parent needed
   

2. Rapid population increase

Give two disadvantages of asexual reproduction

1. No genetic variation
   

2. Vulnerable to disease or environmental change

What is meiosis?

A type of cell division which only occurs in reproductive organs to form gametes with half the normal number of chromosomes

How many times does the cell divide in meiosis?

Twice

What happens when a cell divides to form gametes?

DNA is copied, and the cell divides twice, forming four gametes, each with a single set of chromosomes; all gametes are genetically different.

Why does meiosis produce variation?

Because the gametes have different alleles

What happens after fertilisation?

The new cell divides by mitosis; the number of cells increases; cells differentiate as the embryo develops

What is the genetic material in the nucleus made of?

DNA

What is the structure of DNA?

A polymer made of two strands forming a double helix

Where is DNA found in the cell

In chromosomes

What is a gene?

A small section of DNA on a chromosome

What does a gene code for?

A particular sequence of amino acids to make a specific protein

What is a genome?

The entire genetic material of an organism

What is an allele?

A different form a gene

What is a dominant allele?

Always expressed if present (even with one copy)

What is a recessive allele?

Only expressed if two copies are present (no dominant allele present)

What is a genotype?

The alleles present

What is a phenotype?

The characteristic that is expressed

What is homozygous?

Two identical alleles for a trait

What is heterozygous?

Two different alleles for a trait

Give 2 examples of characteristics controlled by a single gene

Fur colour in mice, and red-green colour blindness in humans

Are most characteristics controlled by one gene?

No, most are a result of multiple genes interacting

What causes inherited disorders?

Inheritance of certain alleles

What is polydactyly?

When a person has extra fingers or toes

What is polydactyly caused by?

A dominant allele

What is cystic fibrosis?

A disorder of cell membranes

What is cystic fibrosis caused by?

A recessive allele

How many chromosome pairs are in ordinary human body cells?

23 pairs

What do 22 pairs of chromosomes do?

Control characteristics only

What does the remaining pair do?

Carries the gene that determines sex

What are the sex chromosomes in males?

XY

What are the sex chromosomes in females?

XX

What is variation?

Differences in characteristics of individuals in a population

What can variation be due to?

Genetic causes (genes they inherited) and the environment (conditions in which they developed), or both

How do variants within a species happen?

Mutation

What effect do most mutations have on the phenotype?

Most have no effect, some influence the phenotype and very few determine the phenotype

What can happen if a rare mutation creates a new phenotype suited to an environmental change?

It can lead to a relatively rapid change in the species

Define evolution

A change in the inherited characteristics of a population over time through natural selection, which may result in a new species

What does the theory of evolution by natural selection state about where life came from?

All species evolved from simple life forms that first developed more than three billion years ago

How does natural selection cause evolution?

Variants with phenotypes best suited to the environment survive and reproduce, passing on their alleles

When can new species form?

When the population of the same species become so different in phenotype that they can no longer interbreed to produce fertile offspring

What is selective breeding?

When humans artificially select the plant/animal they are going to breed for particular genetic characteristics

What are the main steps in selective breeding?

Choose parents with desired characteristic → breed → select offspring with desired characteristic → breed them → repeat for many generations until all offspring show the desired characteristic

Give examples of why a characteristic might be chosen

Disease resistance in crops, improve farming and meat yields, gentle nature in dogs

What is one risk of selective breeding?

One risk is inbreeding, where some breeds are prone to disease or inherited defects

What is inbreeding?

The breeding of closely related individuals, which increases the chance of harmful recessive genes being passed on and cause problems

What is genetic engineering?

Modifying the genome by introducing a gene from another organism to give a desired characteristic

Give a plant example of genetic engineering

Crops engineered to be resistant to diseases or produce bigger, better fruits

Give a bacteria example of genetic engineering

Bacteria engineered to produce human insulin to treat diabetes

What are GM crops?

Crops with genes modified by genetic engineering

Give examples of GM crop traits

Resistant to insect attack or herbicides and increased yields

Give concerns about GM crops

They effect wild flowers and insects and the effects on human health haven’t been fully explored

What are the main steps in genetic engineering?

Enzymes isolate the gene → gene inserted into a vector (bacterial plasmid or virus) → vector inserts gene into cells → genes transferred at an early stage of development so organisms develop desired characteristics

What is some evidence for evolution?

Fossils and antibiotic resistance in bacteria

Why is there now stronger evidence for Darwin’s theory?

We know that characteristics are passed onto offspring in genes and there is further evidence in fossil record and how antibiotic resistance evolves

What is a fossil?

The remains of organisms from millions of years ago found in rocks

List 3 ways fossils may form

• Preservation in places where no decay happens (glaciers, peat bogs, amber)
  

• Parts of the organism are replaced by minerals as they decay
  

• Preserved traces such as footprints, rootlet traces and burrows

Why are scientists uncertain of how life began on earth?

• Many early life forms were soft-bodied, so they left few traces
  

• Geological activity destroyed many traces

What is extinction?

When there are no remaining individuals of a species still alive

Which factors could contribute to the extinction of a species?

Loss of habitat, climate change, pollution, new diseases

Why can bacteria evolve rapidly?

They reproduce at a fast rate

How do mutations lead to antibiotic resistance?

Mutations create new strains; some are resistant, so they are not killed; they survive and reproduce, so the resistant strain increases.

Why does the resistant strain spread easily?

People are not immune and there is no effective treatment

What is MRSA

A strain of bacteria resistant to antibiotics

How can we reduce the rate of development of the antibiotic-resistant strains?

• Doctors shouldn’t prescribe it inappropriately
  

• Patients should complete the full course
  

• Agricultural use of antibiotics should be restricted

Why is it hard to “just make new antibiotics”?

Developing new antibiotics is costly and slow and is unlikely to keep up with new resistant strains as they develop and spread

What did Linnaeus base classification on?

Structure and characteristics

What are the Linnaean groups in order?

Kingdom, phylum, class, order, family, genus, species

What is binomial naming?

Organisms are named using genus and species

Why have classification systems changed over time?

Due to improvements in microscopes and the understanding of the biochemical processes progressing, new models were developed

What is the three-domain system (Woese)?

Archaea, Bacteria, Eukaryota

What are archaea?

They are primitive bacteria that usually live in extreme environments

What are eukaryota?

Protists, fungi, plants and animals

What are evolutionary trees used for?

To show how scientists think organisms are related, using classification data and fossil data