Inheritance, Variation and Evolution

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DNA

DNA - Deoxyribonucleic Acid

The chemical that all genetic material is made from

Contains coded information - instructions to put organism together and make it work

Determines what inherited characteristics you have

Found in the nucleus - in chromosomes

Chromosomes come in pairs

DNA is a polymer - two strand coiled together in double helix

Gene - small section of DNA found in a chromosome

Each gene codes for particular sequence of amino acids - make specific proteins

Tell cells in what order to put amino acids together

DNA determines what proteins the cell produces

That in turn determines what type of cell it is

Genome - entire set of genetic material in an organism

Understanding human genome is important for science and medicine:

Allows scientists to identify genes in genome that are linked to different types of disease

Knowing which genes are linked to inherited diseases could help to understand them better and develop effective treatment

Can look at genomes to trace migration of populations

Genome is mostly identical in all humans but gradually developed as people moved around the world

By investigating changes, scientists can work out when new populations developed

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Reproduction

Sexual Reproduction

Sexual reproduction - where genetic information from two organisms id combined to produce offspring

Genetically different to either parent

Mother and father produce gametes by meiosis

Humans - each gamete contains 23 chromosomes - half number of chromosomes in a normal cell

Instead of having two of each chromosome, gamete has just one of each

Egg and sperm fuse together (fertilisation) to form a cell with full number of chromosomes

Sexual reproduction - fusion of 2 male and female gametes

2 parents, offspring contain a mixture of their parents' genes

Mixture of genetic information produces variation in offspring

Flowering plants can reproduce in this way - also have egg cells

Asexual Reproduction

Only one parent

Offspring are genetically identical to parent

Happens by mitosis

Ordinary cell makes new cell by dividing in two

New cell has identical genetic information - called a clone

No fusion of gametes, no mixing of chromosomes and no genetic variation

Bacteria, some plants and some animals reproduce asexually

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Meiosis

Gametes - one copy of each chromosome, so when fusion takes place there's the right amount of chromosomes

To make gametes which only have half original number of chromosome, cells divide by meiosis

Involves two cell divisions

Humans - only happens in reproductive organs

Before cell divides, it duplicates genetic information

Forming two armed chromosomes - one is an exact copy of the other

After replication chromosomes arrange themselves into pairs

In the first division, chromosome pairs line up in the centre of the cell

Pairs are then pulled apart - each new cell only has one of each

In second division, chromosomes line up again in the centre of cell

Arms of chromosomes are pulled apart

Get four gametes, each with only a single set of chromosomes

Each of the gamete is genetically different from others

Chromosomes get all mixed up during meiosis and each gamete gains half of them

After 2 gametes have together, resulting new cell divides by mitosis to copy itself

Mitosis repeats to produce lots of new cells in an embryo

As the embryo develops, cells then start to differentiate into different types of specialised cell that make up whole organism

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X and Y chromosomes

23 pairs of chromosomes in every human body cell

Of these 22 are matched pairs of chromosomes - just control characteristics

23rd pair - labelled XY or XX - control the sex

When making sperm, X and Y chromosomes are drawn apart in first division of meiosis

50% chance of each sperm cell gets an X chromosome and 50% chance it gets a Y-chromosome

Similar thing happens when making eggs

Original cell has two X-chromosomes, so all eggs have one X chromosome

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Genetic Diagram

Inherited genes control characteristics you develop

Different genes control different characteristics

Some are controlled by single but most characteristics are controlled by several genes interacting

All genes exist in different versions called alleles

2 alleles for gene that are the same - Homozygous

2 alleles for gene that are different - Heterozygous

If 2 alleles are different, only one can determine what characteristics is present

Allele for characteristics is called the dominant allele (capital letter)

Other is recessive (shown with small letters)

To display a recessive characteristic, both alleles must be recessive

To display a dominant characteristic, organism can be either both dominant or one dominant and one recessive

Dominant allele overrules recessive one if organism is heterozygous

Genotype is combination of alleles

Alleles work at molecular level to determine characteristics you have - phenotype

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Inherited Disorders

Cystic Fibrosis

Genetic disorder of cell membranes

Results in body producing lots of thick mucus in air passages and in pancreas

Allele which causes it is recessive - carried by about 1 in 25

People with only one copy of the allele won't have the disorder - known as carriers

Both parents must be carriers or have the disorder

There's a 1 in 4 chance of a child having the disorder if both parents are carriers

Polydactyly

Genetic disorder where baby is born with extra fingers or toes

Isn't life threatening

Caused by a dominant allele - can be inherited if just one parent carries dominant allele

Parent that has the defective allele will have the condition too since allele is dominant

There's a 50% chance of a child having the disorder if one parent has one dominant allele

Embryonic Testing

During IVF, embryos are fertilised in a lab and then implanted into the mother's womb

It's possible, before implantation, to remove a cell from each embryo and analyse its genes

Many genetic disorders can be detected in this way

Possible to get DNA from an embryo in the womb and test for disorders

Lots of ethical, social and economical concerns surrounding embryo screening

For embryos produced by IVF - after screening, embryos with "bad" alleles would be destroyed

For embryos in the womb - screening could lead to decision to terminate the pregnancy

Against

Implies people with genetic problems are "undesirable" - increase prejudice

May come a point where everyone wants to screen embryos for desirable qualities

Screening is expensive

For

Help stop people from suffering

Treating disorders costs the government (& taxpayers) lots of money

Laws to stop it going too far

Parents can't select sex of their baby

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Variation

Variation - change or slight difference in something

Can be genetic - means it's caused by differences in genotype

Genotype is all of the genes and alleles that an organism has

An organism's genotype affects its phenotype - characteristics it displays

Genes are inherited from parents

Not only genotype that can affect an organism's phenotype

Interactions with environment can also

Most variation in phenotype is determined by a mixture of genetic and environmental factors

Sometimes a gene can mutate

Rare, random change in an organism's DNA that can be inherited

Occur continuously

Mutations mean gene is altered

Produces a genetic variant (different form of the gene)

As genes code the sequence of amino acids that make up a protein, gene mutations can lead to changes in the protein

Most genetic variants have very little or no effect on the protein the gene codes for

Some will change it so little that its function is unaffected - means most mutations have no effect on phenotype

Some variants have a small influence on phenotype - alter individual's characteristics but only slightly

Some characteristics are controlled by more than one gene

Mutation in one of the genes may change eye colour - difference may not be huge

Very occasionally, variants can have a dramatic effect that they determine phenotype

Cystic fibrosis is caused by a mutation that has a huge effect on phenotype

Gene codes for a protein that controls movement of salt and water into/out of cells

However, protein produced by the mutated gene doesn't work properly - leads to excess mucus production in lungs and digestive system

If environment changes, and the new phenotype makes an individual more suited to a new environment

Can become more common throughout species relatively quickly by natural selection

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Evolution

Theory of evolution - all of today's species have evolved from simple life forms that first started to develop over three billion years ago

Survival of the Fittest

Darwin came up with theory of evolution, called natural selection

Knew that organisms in a species show wide variation in their characteristics (phenotypic variation)

Also knew organisms must compete for limited resources

Concluded that organisms with most suitable characteristics for environment would survive - survival of the fittest

Successful organisms that survive are more likely to reproduce and pass on the genes for the characteristics that made them successful to their offspring

Organisms that are less well adapted would be less likely to survive and reproduce

So are less likely to pass on their genes to the next generation

Beneficial characteristics become more common in the population and species change - it evolves

Darwin's theory wasn't perfect

Relevant scientific knowledge wasn't available wasn't available and couldn't give a good explanation for why new characteristics appeared or how organisms passed on characteristics

Discovery of genetics supported Darwin's idea

Provided an explanation of how organisms born with beneficial characteristics can pass them on and shows their phenotypes are suited to the environment

Fossils also supported the idea

Allows to see how changes in organisms developed over time

Discovery of how bacteria can evolve to become resistant to antibiotics also further supports evolution by natural selection

Over a long time, phenotype of organisms can change so much because of natural selection that a new species is formed - speciation

This happens when populations of the same species change enough to become reproductively isolated - means they can't interbreed to produce fertile offspring

Species become extinct for these reasons:

Environmental changes are too quick

New predator kills them all

New disease kills them all

Can't compete with another species for food

Catastrophic event that kills them all

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Selective Breeding

When humans artificially select plants or animals that are going to breed so that genes for specific characteristics remain in the population

Organisms are selectively bred to develop features that are useful/attractive

Animals that produce more milk/meat

Crops with disease resistance

Dogs with a good, gentle temperament

Decorative plants with bug/unusual flowers

Selective Breeding process:

From existing livestock, select ones with desirable characteristics

Breed them

Select the best of the offspring, and breed them together

Continue this process over several generations, and the desirable trait gets stronger and stronger

Eventually all offspring will have the characteristics

In farming, selective breeding can be used to improve yields

Selective breeding isn't new

How we ended up with edible crops from wild plants

How we got domesticated animals like cows and dogs

Reduces gene pool - number of different alleles in a population

Farmers keep breeding from the "best" animals or plants - which are all closely related

Inbreeding can cause health problems because there's more chance of inheriting harmful genetic defects

Can also be problems is a new disease appears - not much variation in the population

All the stock are closely related so if one of them is killed by a disease it's likely that they will all be

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Genetic Engineering

Basic idea is to transfer a gene responsible for a desirable characteristic from one organism's genome into another organism - so that it has desired characteristics

Useful gene is isolated from one organism's genome using enzymes and is inserted into a vector

Vector's usually a virus or a bacterial plasmid depending on type of organism that the gene is being transferred to

When vector is introduced to target organism, useful gene is inserted into its cells

Bacteria have been genetically modified to produce human insulin that can be used to treat diabetes

Genetically modified crops have had their genes modified, to improve size and quality of their fruit, or make them resistant to disease, insects and herbicides

Sheep have been genetically engineered to produce substances, like drugs, in their milk that can be used to treat human diseases

Scientists are researching genetic modification treatments for inherited diseases caused by faulty genes

In some cases, the transfer of the gene is carried out when the organism receiving the gene is at an early stage of development

Means organism develops with the characteristic coded for by the gene

It's an exciting area of science - has the potential for solving many problems

Worries about long-term effects of genetic engineering

Changing an organism's genes might accidentally create unplanned problems - could get passed on to future generations

Pros

GM crops could be developed to contain nutrients that are missing in people's diets

GM crops are already grown in some places - without problem

Characteristics chosen for GM crops can increase yield, making more food

Cons

Transplanted genes could get into the natural environment

Some say that growing GM crops will affect number of wild flowers that live in and around the crops - reducing farmland biodiversity

GM crops might not be safe and people might not fully understand effects

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Fossils

Remains of plants and animals

Provide evidence that organisms lived years ago

Can tell how much or how little organisms have changed

They form in three ways:

Gradual replacement by minerals

Things like teeth, shells and bones - don't decay easily

Eventually replaced by minerals as they decay - forms a rock-like substance shaped like the original

Surrounding sediments also turn to rock - fossil stays distinct inside the rock

Casts and impressions

​​​​​​​Fossils can be formed when an organism is buried in a soft material like clay

Clay later hardens around it and organism decays, leaving a cast

Things like footprints can also be pressed into these materials when soft, leaving an impression when it hardens

Preservation in places where no decay happens

​​​​​​​In amber and tar pits - no oxygen or moisture so decay microbes can't survive

In glaciers - too cold for decay microbes to work

Peat bogs - too acidic for decay microbes

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Antibiotic Resistant Bacteria

Bacteria sometime develop random mutations

Can lead to changes in bacteria's characteristics

Can lead to resistant strains forming as gene become more common

Bacteria are rapid at reproducing - can evolve quickly

Problem has increased because of overuse and appropriate use of antibiotics

More antibiotics are used, the more they develop a resistance

In farming, antibiotics can be given to animals to prevent them becoming ill and to make them grow faster

Drug companies are trying to develop antibiotics that are effective against resistant strains

Rate of development is slow - unlikely to keep up with the demand

Very costly process

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Classification

Traditionally, organisms have been classified according to a system

Proposed by Carl Linnaeus in the 1700s

In which living things are grouped according to characteristics and structures

Living things are first divided into kingdoms

Kingdoms are then subdivided into smaller and smaller groups

Phylum, class, order, family, genus and species

In 1990, Carl Woese proposed the three domain system

Using evidence gathered from new chemical analysis techniques

Species thought to be closely related weren't actually as closely related as thought

Organisms are split into three large groups called domains:

Archaea - primitive bacteria. Often found in extreme places such as hot springs and salt lakes

Bacteria - true bacteria. Often look similar to Archaea - biochemical differences

Eukaryota - broad range of organisms including fung, plants, animals and protists

Subdivided into smaller groups - kingdom, phylum, class, order, family, genus and species

Every organism is given its own two part Latin name

First part refers to genus - information of organism's ancestry

Second part refers to species

Used worldwide - means scientists in different countries all refer to species by the same name

Evolutionary trees

Show how scientists think different species are related

Show common ancestors and relationships

More recent common ancestor, the more closely related the two species

And the more characteristics they're likely to share

Analyse lots of different types of data to work out relationships

Use current classification data - living organisms

Use information from fossil records - extinct species