What does variation refer to?
The differences between members of a species
List the sources of Variations
- Random Fertilisation (sexual reproduction)
- Random Assortment
- Crossing over
- Non disjunction
- Mutations
What does Random Fertilisation refer to?
Means that each person will produce a huge number of different sperm or eggs, there is an infinite number of possible combinations of alleles in the offspring
What does Random Assortment refer to?
During anaphase 1 each pair of chromosomes segregates (goes either north or south of equator) independently of every other pair in a random process
What is crossing over?
- Occurs during prophase 1
- Crossing over occurs when matching regions on a homologous chromosomes break then reconnect to the other chromosome, result is called genetic recombination
What does Non Disjunction refer to?
-The failure of chromosomes to seperate during meiosis, this causes a gamete to have too many or too few chromosomes
Conditions of fault;
-Trisomy and Monosomy
-In monosomy daughter cells have one chromosome missing from one of its pairs
-Trisomy has one extra chromosome on a pair
What is a gene pool?
A population is a group of organisms of the same species living together I’m a particular place at a particular time. A gene pool is a sum of all the alleles in a given population
What is a Mutation?
When a change in a gene or chromosome takes place leading to new characteristics in an organism, which can beneficial or harmful. A mutant is something affected by a mutation
What are the two types of mutations?
- Somatic Mutations
2. Germline Mutations
What are somatic mutations?
- DNA changes that occur within body cells
- Cannot be passed onto the offspring
- Only the individual with the somatic mutation is affected -Reproductive cells are not affected and once a person dies the mutation dies with them
What are germline mutations?
- DNA changes that occur in the sperm or ova
- If the reproductive organs are affected the mutation can be passed to offspring
- The individual in whom the mutation occurs is not usually affected however the individual could produce gametes with changed DNA, usually aborted early
What are gene mutations?
- Occurs during DNA replication before cell division
- Mutation is copied each time and therefore passed on to next generation
- A change in a base (nucleotide) which is code for an amino acid to build a protein, a change in just one base (point mutation) ) could alter a protein, have no effect at all, or prevent the protein from being produced could cause major to minor abnormalities
e. g. Albinism, Duchenne form of muscular dystrophy, cystic fibrous, sickle cell anemia
What is albinism in the form of a gene mutation?
The result of one missing protein, albinism is marked by an absence of pigment from the hair, skin and eyes. The hair of a person with albinism tends to be whitish blonde, and the skin tends to be extremely pale and the eyes pinkish.
What is Duchenne form of muscular dystrophy in a gene mutation?
The disease is the wasting of leg muscles and later the arms, shoulders and chest. It usually becomes apparent around the age of 3-5 yeas old when muscle weakness becomes evident, eventually death occurs due to failure of the respirator muscles. Boys that suffer from the disease will usually wouldn’t live more than 20-25 years.
What are lethal recessive gene mutations?
- Most gene mutations have a recessive allele
- If a person reproduced with a partner who had the same recessive mutation the recessive condition could appear in their offspring
- Some recessive mutations are lethal if they are not masked by a dominant normal allele. These lethal recessives cause the death of the embryo or foetus (a miscarriage or spontaneous abortion) or the early death of the child
e. g. Tay Sachs Disease (TSD)
What are chromosomal mutations?
Occurs in part of a chromosome or all of a chromosome and therefore affect not just one but a number of genes. Often so cause severe abnormalities that miscarriage often occurs in early pregnancy. Can occur due to deletion duplication, inversion, Translocation and non disjunction e.g. Trisomy and Monosomy
What is Trisomy?
-One extra chromosome, these abnormalities can be detected before birth
-Partial Trisomy
-Trisomy X e.g
>Klinefelter syndrome (XXY) - a person is more feminine, a male have small tests and do not produce sperm, breasts are enlarged and body hair is sparse and oqccaiosnally mental retarded
>Jacobs syndrome (XYY) - a person is super male
What is Monosomy?
- One less chromosome, these abnormalities can be detected before birth
- Partial Monosomy e.g. Cri-du-chat syndrome, a rare genetic disorder due to missing proportion of chrome 5. Infants cry and sound just like a meowing kitten due to problems with the larynx and nervous system
- Monosomy X e.g. Turners syndrome - females are short, infertile and lack secondary sex characteristics
What causes changes in gene pools?
- Random Genetic Drift
- Founder effect
- Genetic diseases
- Migration
- Barriers to gene flow
- Mutations
- Natural selection
What is Random Genetic Drift? and give an example
- Can change frequencies of alleles in gene pools
- Totally random
- Non directional
- Purely chance
- An allele that is rare in a large population may by chance become frequent in a small population e.g Dunkers, Bentick Island
Explain Dunkers with reference to random genetic drift
Dunkers lived in Pennsylvania, came from the Hesse community in Germany in 18th century, there religion does not allow them to marry outside of their group and therefore they became an isolated breeding population within the US. Study’s measuring blood types etc found that Dunkers varied in allele frequency from the present day Hesse and surrounding American population. The size of Dunkers allowed certain alleles to became more common by chance
Explain Bentinck Islanders with reference to random genetic drift
Bentinck Island was originally apart of the Mainland Australia however rising sea levels cut them off and became isolated and had no contact with the mainland therefore had limited alleles. Studies show that blood group of people on Bentinck Island fall outside the Aborigines in the rest of Australia. Showed high lB and low IA opposite to mainland. Thus RGD was responsible as the size of Bentinck Island allowed certain alleles to become more common purely by chance
What is the founder effect?
- Similar to random genetic drift
- Occurs when a very small group moves away form homeland to a new area and establishes a community which expands and grows
- Small group which is relatively isolated leads to a loss os genetic variation and can lead to an evolution of a new species
e. g. Pitcairn island, Pingelup, Tristan Da Cuncha, Finland
In reference to the founders effect describe the Pingelup situation
- A Typhoon hit the island of Pingelup in Micronesia and killed all but 20 people
- 20 people formed the founding population of current inhabitants
- One person who carried the heterozygous allele for Achromatopsia (total colour blindness) and this allele became more frequent purely by chance
- In Pingelup = 5% of occurrence of Achromatopsia, In world = 0.0033& occurrence, furthermore 30% of Pingelup are carriers thy are not colour blind but they do have the affected allele
- The founder effect was due to a small group relatively isolated with little immigration, gene pool was different to the original population, this founder population is unrepresentative of the original population
