B6 Inheritance, Variation and Evolution

What is Meiosis ?

Formation of four non-identical cells from one cell

What is Mitosis ?

Formation of two identical cells from one cell

What is the type of cell where the chromosomes are not paired

Gametes

In humans gametes are ?

Sperm cells and egg cells. They contain 23 single chromosomes

How are gametes formed ?

By meiosis, so they are non identical

Gametes in Flowering Plants

Pollen and egg cells

Sexual Reproduction involves

The fusion of male and female gametes. This is known as fertilisation. There is a mix of genetic information, the offspring receives the genetic information from the father and mother, every gamete is different so there is variation

Asexual Reprodcution

Involves only one parent, so there is no mixing of genetic information, it does not involve gametes so the offspring are genetically identical, these are known as clones. They do not involve meiosis as it does not involve gametes but do involve mitosis

Meiosis only takes place in ?

Reproductive organs, in humans that is the testes and ovaries

First stage of Meiosis

The chromosomes are copied, so they have double amount of genetic information. These cells then divide into two cells, each with half the amount of chromosomes, the cell divides again producing four cells, each with a quarter the amount of chromosomes. These cells are called gametes and are genetically different from each other because the chromosomes are shuffled during the process, resulting in random chromosomes ending up in each of the four cells.

Second Stage of Mitosis

These gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes. In fertilisation, the full number of chromosomes are restored.

Third Stage of Mitosis

The new cell divides by mitosis producing an embryo which is a clump of identical cells. As the embryo develops, the cells begin to take on different roles, this is known as differentiation

The genetic material in the nucleus of a cell is composed of a chemical called

DNA which is a polymer made up of two strands which wrap around each other in a structure called a double helix

The DNA in the nucleus is contained in structures called ?

Chromosomes

What is a Gene ?

A small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein

What is the Genome ?

The entire genetic material of an organism . The human genome is the entire genetic material that makes a human. Since the human genome has been fully studied, we can now understand how genes are linked to different types of diseases, the treatment of inherited disorders and tracing humans past migration patterns, helping people discover their ancestry.

What is a gamete ?

An organisms reproductive cell which has half the number of chromosomes

What is a Chromosome ?

A structure found in the nucleus which is made up of a long strand of DNA

Some characteristics are controlled by a ?

Single gene, such as fur colour in mice and red-green colour blindness in humans. Each gene may have different forms called alleles

What is an Allele

The different forms of genes- humans have two alleles for each gene as they inherit one from each parent

What is a Phenotype

The physical characteristics that are observed in the individual such as eye colour

What is a Dominant Allele

Only one out of the two alleles that is needed for it to be expressed and for the corresponding phenotype to be observed

What is a Recessive Allele

Two copies are needed are needed for it to be expressed and for the corresponding phenotype to be observed

What is Homozygous

When both inherited alleles are the same such as two dominant alleles or two recessive alleles

What is heterozygous

When one of the inherited alleles is dominant and the other is recessive

What is a Genotype

The combination of alleles an individual has eg. Aa

What is Cystic Fibrosis

A disorder of cell membranes which causes the body to produce thick mucus clogging the lungs. It is controlled by a single gene which has two alleles. In order to have cystic fibrosis, the person has to have two copies of the defective allele (recessive). If the person is hetrozygous this means they do not have cystic fibrosis but carry the CF allele so they are called ‘carriers’

A dominate allele will always ?

be expressed in the phenotype even if there is only one copy present.

However, a recessive cell is only expressed if ?

Two copies are present

What is Polydactyly

A birth defect that results in people having extra fingers or toes, it is caused by a dominant allele. A person with one copy of the polydactyly allele and one copy of the normal allele will have polydactyly. You CANNOT be a carrier of polydactyly

What is Embryo Screening

Embryos are tested to see if they have alleles for inherited disorders. Embryos which DO NOT have the defective allele are implanted into the woman, which can develop into healthy offspring.

Issues of Embryonic Screening

1.Expensive to carry out

2. A large number of embryos are created but only a small number are implanted, which means some healthy embryos are destroyed and people think this is unethical as they had the potential to become a future life.

3. Could encourage people to pick characteristics, leading to the creation of designer babies

4. Promotes prejudice as it suggests that those with genetic disorders will not have a fulfilling life or are unwanted.

5.Decisions about terminating pregnancy have to be made

6.Procedure can lead to miscarriage

What do Family Trees only show

Phentotypes

Sex Chromosomes

Ordinary humans have 23 pairs of chromosomes, 22 pairs control characteristics only, the 23rd pair carries the sex chromosomes. Females carry two X chromosomes (XX) which are the same

Males carry XY chromosomes which are different

The phenotype depends on two things

Genotype: Genes that are selected

-Genes are passed on from the parent in sex cells

-The combination of genes from the mother and father creates genetic variation

-Only identical twins have the same genotype

-There is lots of genetic variation in a population

Environment: The place it lives in

-The conditions the organism grows and develops in also affects its appearance

-Examples include scars in animals, or smaller and yellow leaves in plants

Sometimes characteristics can result from a ?

Combination of genetics and the environment, such as weight. Weight depends on the food you eat but also how quickly your body breaks it down and how much it stores as fat depends on your genes

How is Genetic Variation Introduced

By mutations in the sequence of DNA

-Most have no effect on the phenotype

-Some will influence phenotype but it is unlikely that it will create a new phenotype

-If the mutation does determine the phenotype and it is advantageous, natural selection will mean it becomes the common phenotype relatively quickly.

What is Evolution

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

What is the Theory of Evolution

Theory states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.

Evolution occurs due to

Natural Selection

-Mutations occur which provide variation between organisms

-If a mutation provides a survival advantage the organism is more likely to survive to breeding age

-The mutation will then be passed onto offspring

-Over many generations, the frequency of the mutation will increase in population

This may cause one population of a species to become so different that they no longer interbreed to produce fertile offspring, meaning they have become a new species

What is Selective Breeding

Selective Breeding is when humans choose which organisms to breed in order to offspring with a certain desired characteristic

Process of Selective Breeding

-Parents with desired characteristics are chosen

-They are bred together

-From the offspring, those with desired characteristics are bred together

-Process is repeated many times until all offspring have the desired characteristics

The problem with Selective Breeding

Inbreeding

-Breeding those with similar desirable characteristics means it is likely you are breeding closely related individuals

-This results in the reduction of the gene pool, as the number of alleles reduce

-This means if the environment changes or there is a new disease, the species could become extinct as they all have the same genetic makeup (so the chance of a few organisms having a survival advantage and not dying is reduced)

-Small gene pool leads to a greater chance of genetic defects being present in offspring, as recessive characteristics are more likely to be present

What is Genetic Engineering

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

-plant cells have been engineered for disease resistance or to have larger fruits

-bacterial cells have been engineered for resistance or to have larger fruits such as human insulin to treat diabetes

Steps of Genetic Engineering

-Genes from chromosomes are ‘cut out’ using restriction enzymes leaving ‘sticky ends’

-A virus or bacterial plasmid is cut using the same restriction enzyme to also create sticky ends. This also contains an antibiotic marker gene

-The loop and gene sticky ends are joined together by DNA ligase enzymes

-Combined loop is placed in a vector, such as bacterial cell, and then allowed to multiply as it will now contain the modified gene. As the bacteria grows we can see which ones are resistant to antibiotics. The colonies that are,will be the bacteria that are also producing the modified gene, as they were inserted together

Genetically modified crops

The are engineered to be resistant to herbicides and insects, resulting in increased yields as less crops die

Genetically modification in medicine

May be possible to use genetic engineering to cure inherited disorders, this is called gene therapy and involves transferring normal genes into patients so the correct proteins are produced

Advantages of Genetic Engineering

It can be very useful to medicine to mass produce certain hormones in microorganisms

What are Fossils ?

Remains of organisms from millions of years ago, which are found in rocks

Fossils are Formed From

-Parts of organisms that have not decayed because oxygen or moisture were not present, meaning that the microbes that cause decay cannot survive

-Parts of the organism such as teeth,shells and bones are replaced by minerals they decay, forming a rock structure of the original part

-Preserved Traces such as footprints, burrows and rootlet traces remain due to the ground hardening around them and forming a cast

Resistant Bacteria

Bacteria are labelled resistant when they are not killed by antibiotics

Because bacteria can reproduce at a much faster rate they can

Evolve rapidly

Who made the Classification System

Carl Linnaeus

Carl divided all living organisms into two categories

The Animal Kingdom

The Plant kingdom

He then divided the kingdoms into smaller categories

Kingdom

Phylum

Class

Order

Family

Genus species

Three Domain System

Made by Carl Woese, who added three large groups called DOMAINS above kingdoms

Archaea: Primitive Bacteria which live in extreme environments

Bacteria: True Bacteria

Eukaryota: Organisms who have a nucleus enclosed in to membranes, includes the kingdoms protists, fungi , plants and animals