Gentics

Front 1

What is a chromosome

Back 1

It is a structure in a cell containing genetic information in the form of DNA

Front 2

Describe a prokaryotic chromsome

Back 2

1. They are circular in nature

2. There is one chromosome in a cell

3. They are 'naked' without histone protein

4. They are freely lying in the cytoplasm of the cell

Front 3

Describe a eukaryotic chromosome

Back 3

1. They are linear

2. There are many chromosomes in one cell

3. They are associated with histone proteins

4. They are contained in the nucleus

Front 4

What is a gene

Back 4

- It is a veritable factor that consists of a length/ section of DNA and influences a specific characteristic.

- Each generally occupies a parti ilar position on a specific chromosome (locus)

- Every characteristic is determined by a particular gene.

Front 5

What is an allele?

Back 5

- It is a heritable factor that consists of a length/section of DNA and influences a specific characteristic.

- Each gene occupies a particular position on a specific chromosome. The position of a gene on a chromosome is called the locus (plural: loci)Every characteristic is determined by a particular gene

Front 6

What is an allele?

Back 6

They are alternative forms of the same gene, they occupy the same position in the chromosome (the same locus). They differ from each other by a difference in the base sequence. They are the reason that there are different heritable characteristics.

Front 7

How are new alleles formed?

Back 7

New alleles are formed from other alleles by gene mutation. This could happen when a base is replaced by another base.

Front 8

Outline an example of a mutation.

Back 8

- he mutation that causes sickle cell anaemiaIn the gene that codes for the beta chain of haemoglobin at position 6, the codon present is CTC.

- This codon is transcribed into GAG, this code for amino acid glutamic acid. This makes the red blood cells normal.However a mutation that causes a substitution of base T into A in the DNA GUG in the transcribed mRNA which for valine.

- This causes haemoglobin to be less soluble at low oxygen partial pressures, thereby causing the red blood cells become sickle-shaped , which means that they don’t carry enough oxygen causing sickle cell anaemia

Front 9

Explain what is meant by a genome and state whether a genome of a particular species can entirely be known.

Back 9

- A genome is the whole of the genetic information of an organism. In other words, all the genes that an organisms has

- The genome of a particular species can entirely be known, which is proven by the human genome project whereby scientists found the entire base sequence of a human genome, therefore finding the location of many genes on the human chromosome.

Such as: - Gene for insulin is on chromosome 11 - Gene for eye colour is on chromosome 15

Front 10

What are homologous chromosomes?

Back 10

- These are chromosomes that have the same structure and carry the same genes, but different alleles

- In an organism, they are always existing in pairs. For example, in human beings, there are 46 chromosomes which exist in 23 different homologous pairs.

Front 11

Distinguish between diploid and haploid nuclei.

Back 11

Diploid nuclei have pairs of homologous chromosomes and haploid nuclei have one chromosome of each pair.

Front 12

Distinguish between autosome and sex chromosomes

Back 12

Autosomes are chromosomes that do not affect whether a fetus develops as a male or female (do not determine sex), and sex chromosomes determine the sex of a child. Two chromosomes affect the sex of a child:

- The x chromosome is relatively larger and has its centromere near the middle. It has many genes that are essential in both males and females, so all humans must at least have one x chromosome.

- The Y chromosome is much smaller and has its centromere near the end. It only has a small number of genes. One Y chromosome gene in particular causes a fetus to develop as a male.

Front 13

How do karyograms work?

Back 13

It shows the chromosomes of an organism in homologous pairs of decreasing length. Stains have to be used to make the chromosomes show up. Some stains give each chromosome type a distinctive banding pattern.

Front 14

What are karyograms used for?

Back 14

- Identify the sex of an individual; because the 23rd pair has two long x chromosomes in females while it has one x and a short Y chromosome in males.

- Detecting Down’s syndrome in which an individual has an extra chromosome 21, such that they have 3 chromosomes of number 21.

Front 15

Explain Cairns's technique for measuring the length of DNA

Back 15

Cairns used autoradiography to measure the lengths of DNA molecules using this technique:

- Cells were grown for two generations in a culture medium containing titrated thymidine, which is used by E. coli to make nucleotides that it uses in DNA replication, causing radioactively labeled DNA (due to tritium which is a radioactive isotope of hydrogen) to be produced by replication in the E. Coli cells

- The cells were then placed onto a dialysis membrane and their cell walls were digested using lysosome causing the cells to release their DNA onto the surface of the dialysis membrane. A thin film of photographic emulsion was applied to the surface of the membrane and left in darkness for two months, during that time some of the atoms of tritium decayed and emitted high-energy electrons, which reacted with this film.

At the end of the two months, the film was developed and examined with a microscope. At each point where a tritium atom decayed, there is a dark grain which indicates the position of DNA. It showed that E. coli is a single circular DNA molecule with a length of 1, 100 micrometres, which is remarkably long.

Front 16

What is meiosis?

Back 16

This is a process by which a diploid nucleus divides to form haploid nuclei.

Front 17

What happens in prophase 1?

Back 17

- Spindle fibres start forming.

- The nuclear membrane breaks down.

- Chromosomes condense and become visible, whereby each chromosome consists of 2 chromatids.

- Homologous chromosomes pair up. This pair is called bivalent.

- Non-sister chromatids in the homologous pairs come into contact at points known as chiasma ( singular: chiasmata).

- Crossing over occurs between non-sister chromatids of the homologous pairs. This crossing over causes variations.

Front 18

What happens in Metaphase 1?

Back 18

- The chromosomes align themselves along the equator of the spindle.

- Homologous chromosomes associate (stand side by side).

- The order in which they stand side by side is random.

Front 19

What happens in anaphase 1

Back 19

Homologous chromosomes separate and move to opposite ends.

Front 20

What happens in telophase 1

Back 20

- Chromosomes reach the opposite ends

- The nuclear membrane starts to form again

- The cell constricts across the middle and starts to divide into 2

Front 21

What happens in prophase 2

Back 21

Chromosomes condense and each chromosome has 2 chromatids.

Front 22

What happens in Metaphase 2

Back 22

The chromosomes align themselves along the cell’s equator.

Front 23

What happens in anaphase 2

Back 23

The sister chromatids of each chromosome break up at the centromere and start to move to opposite ends.

Front 24

What happens in telophase 2

Back 24

- The chromosomes reach the opposite ends.

- A nuclear membrane forms around each group of chromosomes.

- The cell then starts to divide into 2.

Front 25

Describe the role of meiosis in causing variation

Back 25

- Variations are differences among organisms of the same species

- Meiosis causes variations in two ways:

-Crossing over: This leads to the exchange of alleles between non-sister chromatids. As a result, all gametes formed are different from each other because of new gene combinations. It occurs in prophase I

-Random assortment of homologous chromosomes: Homologous chromosomes arrange themselves randomly at the spindle equator during metaphase I, leading to new chromosome combinations that end up in different gametes, at the end of meiosis.

Front 26

State one other cause of variation in sexually reproducing organisms.

Back 26

Random fusion of gametes during fertilization. Each gamete carries a unique set of genes causing variation in the offspring produced.

Front 27

Describe non-disjunction and its effects

Back 27

This is a situation in which homologous chromosomes fail to separate in meiosis I. As a result, a pair of homologous chromosomes end up in the same gamete. This causes the gamete to have an extra chromosome. For example, in Down syndrome there is a non-disjunction in the homologous pair 21, leading to an individual having an extra chromosome 21.

Front 28

Define a gene

Back 28

This is an inheritable factor consisting of DNA that determines a particular characteristic. Every characteristic is determined by a gene.

Front 29

Define an allele

Back 29

These are alternative forms of a gene. Each gene has 2 or more alleles.

Front 30

Differentiate between a dominant and recessive allele

Back 30

- This is an allele that expresses itself even in the presence of an alternative allele. Dominant alleles are represented as capital letters, such as: G, A, B, etc

- Alleles that only express themselves in the absence of a dominant allele. Recessive alleles are represented as small letters, such as g, a, b, etc

Front 31

What is a gene loci

Back 31

This is the exact position on a chromosome where a particular gene is located

Front 32

Differentiate between homozygous and heterozygous

Back 32

- Homozygous: A situation in which the alleles of a gene are the same. For a gene with two alleles, examples of homozygous situations: AA, aa, BB etc

- Heterozygous: A situation in which the alleles of a particular gene are different. Example: Aa, Bb

Front 33

Differentiate between genotype and phenotype

Back 33

- Genotype: The genetic makeup of an organism based on the gene that it has. A genotype can either be homozygous or heterozygous.

- Phenotype: This is the physical or observable features of an organism based on its genes: Tall, short, the seeds are round, colour of flowers, etc.

Front 34

Differentiate between F1 and F2 generation

Back 34

- F1 generation: These are offspring produced from two parents

- F2 generation: These are offspring produced by crossing the F1 individuals

Front 35

What is monohybrid inheritance

Back 35

This is the inheritance of one gene determining one characteristic.

Front 36

Describe how a punnet square is used

Back 36

Look at genetics worksheet

Front 37

What is meant by autosomal conditions, and give two examples.

Back 37

These are genetic diseases that are caused by defects in the genes located on autosomal chromosomes.

Examples:

- Huntington’s disease

- Cystic fibrosis

Front 38

Describe the cause and transmission of cystic fibrosis.

Back 38

- Cystic fibrosis is caused by a recessive allele in the gene that codes for chloride channels in plasma membranes of human cells.

- This means that the allele for the normal chloride channels is dominant.

- The alleles present:

- C = normal chloride channels

- C = cystic fibrosis

This means that:

CC=Normal

Cc=Normal but is a carrier

cc=Cystic fibrosis

Front 39

Describe the cause and transmission of Huntington's disease

Back 39

This is caused by a dominant allele for the gene that codes for the protein Huntingtin.The disease causes brain degeneration as people grow older.

The alleles present are:

- H= Huntington’s disease

- h= normal

This means that

HH=Hunington’s disease

Hh=Hunington’s disease

hh=Normal

Front 40

Outline how sex is determined in human beings

Back 40

In human beings sex is determined by the 23rd pair of chromosomes known as the sex chromosomes; the X and Y chromosomes. Females have two X chromosomes: Their genotype is XX. Males have one X and one Y chromosome. Their genotype is XY

Front 41

What is meant by the term sex-linked genes and sex-linked traits?

Back 41

- Sex-linked genes are genes that do not determine sex but are located on the sex, X chromosome and therefore are transmitted along with sex.

- Sex-linked traits are characteristics that are determined by sex-linked genes. For example:

- Haemophilia

- Colour blindness

Front 42

Describe the cause, effect and transmission of haemophilia.

Back 42

Haemophilia is caused by a recessive sex-linked allele for a gene that codes for clotting factor VIII (This is a protein that enables blood clotting to occur). A person with hemophilia will bleed excessively even to minor cuts, and may even die because they lack the clotting factor VIII. The alleles involved:

- XH = Allele for the normal clotting factor VIII

- Xh = Allele for hemophilia (No clotting factor)

Front 43

Describe the cause of color blindness and explain why there are more colour-blind individuals in the world than hemophiliacs despite a similar way of transmission.

Back 43

- Colour blindness is caused by a recessive sex-linked allele. This means that it is more common in males than in females.

- The alleles involved are:

- XC = Allele for normal color vision

- Xc = Allele for color blindeness

- The reason as to why there are more colour blind people in the world than haemophiliacs is because colorblindness is not a fatal condition; whereas haemophilia is.

Front 44

With examples, explain what is meant by codominance.

Back 44

- This is a situation in which the 2 alleles of a gene are equally dominant. They are usually represented by capital letters.

- In plants for example, flower colour is controlled by a gene with two codominant alleles, R is the allele for red flowers while W is the allele for white flowers. For flowers that have both R and W, they appear pink.

Front 45

Define multiple allele inheritance.

Back 45

A genetic situation in which one gene has more than two alleles. For example; the gene that determines human blood groups

Front 46

Describe inheritance of the human blood group.

Back 46

In humans, blood groups are determined by one gene that has 3 alleles; IA, IB, IO.

- Allele IA and IB are codominant.

- Allele IA and IB are dominant over IO; so IO is a recessive allele.

Front 47

What is dihybrid inheritance?

Back 47

This is the transmission of 2 different genes coding for 2 different characteristics from parents to offspring

Front 48

Describe how a punnet square is used for dihybrid inheritance.

Back 48

Look at notes

Front 49

What is autosomal linkage?

Back 49

Autosomal linkage is a genetic situation in which two genes that code for different characteristics are located on the same autosomal chromosome.

Front 50

Describe how a cross is done for dihybrid inheritance with autosomal linkage

Back 50

Look at notes

Front 51

What is crossing over?

Back 51

This a situation in which non-sister chromatids of homologous pairs exchange parts and hence genes/alleles.

Front 52

Describe circumstances under which crossing over occurs.

Back 52

- Crossing over is only possible when dealing with dihybrid situations.

- It is only considered when the genes are linked. (Genes that are found on the same chromosome)

- To deal effectively with a genetic cross that demonstrates crossing over only two genotypes of the parent organisms should be considered

Front 53

When doing dihybrid crosses that involve crossing over how are the majority and minority of gamates produced?

Back 53

In the two parents mentioned above, if crossing over occurs during gamete formation, majority of the gametes are formed normally without crossing over, while a small percentage is formed with crossing over

Front 54

What is a recombinant offspring?

Back 54

The offspring produced as a result of crossing over are known as recombinant.

Front 55

Explain how a dihybrid cross can be done that involves crossing over.

Back 55

Look at notes

Front 56

Describe the use of a chi-squared test in genetics.

Back 56

The chi-squared test is used to determine if there is a statistically significant difference between the observed results and the expected results from a genetic cross. The formula is:

x2=(O-E)2E where O is the observed value and E is the expected value.

It is basically chi-squared GOF test

NB: If the calculated value is greater than the critical value then H0 is rejected which means that there is a significant difference, and this difference could be due to autosomal linkage and crossing over. If the calculated value is lower than the critical value then H0 is accepted which means that there is no significant difference.

Front 57

What is a gene pool?

Back 57

This is the total number of genes and their alleles in an interbreeding population.

Front 58

What is variation?

Back 58

These are differences that exist between members of the same species

Front 59

Distinguish the two types of variation

Back 59

Discrete (discontinuous) variation

- These are variations in which the population can be grouped into clear distinguishable categories. For example: Sex, blood groups.

- There are no intermediates.

- These characteristics are usually determined by 1 gene.

- They are not affected or influenced by the environment.

- When plotted, these variations produce a bar chart.

Continuous variation

- These are variations in which there are no clear cut differences in the characteristics of a population, but instead organisms range from one extreme to the other with intermediates. For example; height, weight

- They are usually determined by more than one gene, and so are called polygenic characteristics. They are influenced by the environment

- When plotted, these variations produce a normal distribution curve.

Front 60

What is natural selection

Back 60

It is a process by which organisms with better adaptations survive, reproduce and pass on their genes to future generations. On the other hand, those that are not adapted fail to survive or reproduce.

Front 61

What is directional selection?

Back 61

This is a type of selection in which a selection pressure in the environment acts against individuals of one extreme eliminating them but favouring the ones of the other extreme. As a result the mean shifts in the direction of the favoured extreme.

Look at notes for graph

Front 62

What is stabilising selection?

Back 62

This is a type of selection in which a selection pressure in the environment acts against individuals on both extremes and therefore favouring those in the middle. For example, the human birth weight.

Look at notes for graphs

Front 63

What is disruptive selection?

Back 63

This is a type of selection in which a selection pressure in the environment acts against individuals in the middle and therefore favours the two extremes.

Look at notes for graph

Front 64

What is speciation?

Back 64

This is the process by which new species are formed from pre-existing species

Front 65

Describe gradualism and punctuated equilibrium in relation to speciation.

Back 65

- Gradualism is a situation in which speciation occurs slowly or gradually over a long period of time.

- Punctuated equilibrium is a situation in which speciation occurs abruptly after a long period of time without appreciable changes.

Look at notes for graph

Front 66

Briefly explain how reproductive isolation can cause speciation.

Back 66

Reproductive isolation is a situation in which populations of the same species cannot interbreed or reproduce together. With time these two populations adapt to different environmental conditions and may eventually become different species

Front 67

State and explain the three types of reproductive isolation

Back 67

- Geographical isolation: This is a type of isolation in which the populations of a species are separated by a geographical barrier such as mountains, oceans, lakes, etc. Therefore the separated populations are exposed to different environmental conditions, develop different adaptations and may become different species

- Temporal isolation: This is a type of isolation in which the populations of a species breed at different times even though they may be in the same environment.

- Behavioural isolation: This is isolation in which the reproductive behaviour (courtship behaviour) of one population is not recognised or appreciated by another species.

Front 68

Explain polymerase chain reaction (PCR)

Back 68

PCR is a technique used to amplify small amounts of DNA

Steps of PCR:

1. Denaturation: This involves heating the DNA pieces at a temperature of 950C to separate the DNA double-stranded molecule into 2 separate strands

2. Annealing: This involves inserting primers onto the DNA single strands at a low temperature of about 530C

3. Elongation: Taq polymerase is now used to make new copies of DNA using the single strands.

Front 69

Explain genetic modification

Back 69

This is the transfer of genes from one organism to another. The organism that receives the genes is known as a genetically modified organism (GMO’s) or transgenic organisms.

- The first step in gene transfer involves extracting the required gene from its source using enzymes such as restriction endonuclease.

- A vector (the one that does the transfarring) such as a plasmid is cut by the same restriction endonuclease.

- The obtained gene is inserted into the plasmid using an enzyme called DNA ligase.

- The plasmids are then known as recombinant plasmids.

- The recombinant plasmids are then transferred into the bacteria.

- The bacteria are therefore modified and start to express the gene. For example, they start producing insulin, if the gene for insulin was inserted in them.

Front 70

Explain gel electrophoresis

Back 70

This is a technique that is used to separate DNA pieces/fragments by their sizes.

- The pieces/fragments of DNA are placed in wells in a gel electrophoresis tank

- When the electric current is turned on, the DNA fragments move towards the positive terminal.

- Smaller fragments move longer distances than larger ones

Front 71

Explain DNA profiling

Back 71

This is a technique used to create a pattern of bands of DNA pieces/proteins known as a DNA profile.

- To do a DNA profile, special DNA segments called STR (short tandem repeats) are obtained from an individual's DNA, These are unique to each individual.

- The STR DNA is then amplified by PCR- They are separated by gel electrophoresis to make the profile

- These are used in paternity testing and crime investigations

Front 72

Explain cloning.

Back 72

Process by which a group of identical organisms called a clone are made from a single original plant.

- In plants cloning occurs for example in bulbs, tubers, stem cuttings, runners, etc.

- In animals, cloning is artificially done in a procedure known as somatic cell nuclear transfer. This involves 3 individuals for example:

*Individual A: The one to be cloned because of a particular characteristic. A somatic cell is obtained from her

*Individual B: Donates and egg which is denucleated. The somatic cell is fused with the denucleated cell using a packet of electric current, to form an embryo.

*Individual C: Surrogate mother. The embryo is transferred into her uterus to grow until birth. The newborn baby will have DNA and characteristics of individual A.

Front 73

What are the potential benefits and risks associated with genetic modification of crops?

Back 73

Crops are modified to be resistant to particular pests/insects, or to enhance their nutrient levels

Benefits:

- Higher yields

- Reduces the use of pesticides

Risks:

- The modified plants produce toxins to kill the pests which may also kill beneficial insects such as pollinators like bees

- The insects may become resistant