Biology Dot Points

Page 1

Genetics: The Code Broken?

THE STRUCTURE OF A GENE PROVIDES THE CODE FOR A POLYPEPTIDE

PROCESS INFORMATION FROM SECONDARY DATA TO OUTLINE THE CURRENT UNDERSTANDING OF Gene Expression

- Gene expression refers to how the gene shows itself via some observable phenotype.

- Some expressions of genes cannot be observed, such as those that regulate cell activity or produce an enzyme

- At the most basic level, gene expression is when a gene works to produce a polypeptide (transcription and translation)

- Genes are not always expressed, because proteins are not always needed within a cell. Therefore some genes can be switched on or off depending on environmental pressures, usually by some means that prevents transcription onto mRNA: Gene Suppression

MULTIPLE ALLELES AND POLYGENIC INHERITANCE PROVIDE FURTHER VARIABILITY WITHIN A TRAIT

GIVE EXAMPLES OF CHARACTERISTIC DETERMINED BY MULTIPLE ALLELES IN AN ORGANISM OTHER THAN HUMANS

- Alleles are different forms of the same gene (tall or short pea plants)

- Multiple alleles are when there is 3 or more different alleles, such as human blood types (A, B and O)

- Examples include Coat colour on Mice, and eye colour in Fruit Flies (approx 12)

COMPARE THE INHERITANCE OF THE ABO AND RHESUS BLOOD GROUPS. SOLVE PROBLEMS TO PREDICT THE INHERITANCE PATTERNS OF ABO BLOOD GROUPS AND THE RHESUS FACTOR

- Blood group A (I^A) can be homozygous or heterozygous

- Blood group B (I^B) can be homozygous or heterozygous

- Blood group O (i) must be homozygous

- Blood group AB co-dominant I^A and I^B

- The Rhesus Factor gives the positive or negative parts of the blood type, therefore variation within a species is greatly increased

- We must use Dihybrid crosses

Example Question 1.

A person with blood type A+ (heterozygous for both) mates with another person of the same genotype. What possible offspring could be produced?

The genotypes would both be: I^A i Rr

Possible gametes for both are therefore: I^AR, iR, I^Ar, ir

- now create a dihybrid cross

DEFINE WHAT IS MEANT BY POLYGENIC INHERITANCE AND DESCRIBE ONE EXAMPLE OF POLYGENIC INHERITANCE ON HUMANS OR ANOTHER ORGANISM.

- Polygenic inheritance is when one characteristic (phenotypic trait) is determine by more than one pair of genes, such as human skin colour which is controlled by 4 genes

- Polygenes show a continuous variation, as opposed to two distinct phenotypes (such as dark or light)

- Some variation is due to environmental factors

- Skin colour in humans is caused by several genes. Alleles A, B and C control dark pigmentation (more melanin produced) and the alleles a, b and c control light pigmentation. If two people of 'equal' Melanin are crossed (AaBbCc x AaBbCc) we can see an enormous variety of skin pigmentations possible.

- The more genes that control a characteristic, the more possible gene combinations and phenotypes exist

OUTLINE THE USE OF HIGHLY VARIABLE GENES FOR DNA FINGERPRINTING FOR FORENSIC SAMPLES, PATERNITY TESTING AND DETERMINING THE PEDIGREE OF ANIMALS

- DNA fingerprinting: used to identify individuals based on their genetic makeup

- Sections of our DNA are known as 'short tandem repeats' segment of bases repeated over and over

- This information can be used to make up a DNA profile because what differs between individuals is the number of times these tandem sequences actually repeat

- DNA is cut up using restriction enzymes and then separated by a gel using an electric eld (electrophoresis) - this separates the different samples of DNA based on their size, then stained and turned into a 'bar code' like image.

- Because the repeat sequences are different for each person, a fingerprint unique to one individual can be made.

- Because half of our DNA comes from our parents, in a paternity case the child's DNA can be compared with the alleged father; if there is an approximately 50% match, then the alleged father must be the real father

- This same technique can be used to ensure that when buying the supposed offspring of prized animals, the buyer is not being scammed ad the animal is the true offspring of the prized parents.

STUDIES OF OFFSPRING REFLECT THE INHERITANCE OF GENES ON DIFFERENT CHROMOSOMES AND GENES ON THE SAME CHROMOSOME

USE THE TERMS 'DIPLOID' AND 'HAPLOID' TO DESCRIBE SOMATIC AND GAMETIC CELLS

- "-ploid" refers to the number of copies of each chromosome in a biological cell

- Diploid is chromosomes appear in lots of two (humans, which 23 pairs of chromosomes) and Haploid is one

- Somatic cells (body cells) are diploid, however during Meosis the diploid parent cell divides to form four haploid daughter sex cells (gametes)

- This means that all the chromosomes in human sex cels have 23 single chromosomes (not pairs); this is so that during fertilisation, the correct number of chromosomes in restored

DESCRIBE THE OUTCOMES OF DIHYBRID CROSSES INVOLVING SIMPLE DOMINANCE USING MENDEL'S EXPLANATIONS.

PREDICT THE DIFFERENCE IN INHERITANCE PATTERNS IF TWO GENES ARE LINKED

MODEL LINKAGE

- Dihybrid crosses are when we consider the inheritance of two characteristics.

- During the formation of gametes, the two alleles responsible for a trait separate (recombined at fertilisation)

- Using Mendel's peas as an example, round peas (R) are dominant over wrinkled (r) and yellow peas (Y) are dominant over green (y). When two heterozygous ROUND-YELLOW (RrYy) plants are crossed, the possible gametes are: RY, Ry, rY, ry. Putting this into a punnet square can predict the inheritance patterns.

- However when scientists began doing these crosses, they did not always get these ratios. Bateson and Punnet experiment with the genes involved in flower colour and pollen grain length (long pollen grains are dominant (L) and purple flowers are dominant (P))

- The two scientists took pure breeding lines (LLPP and llpp) and crossed them producing all LlPp as expected. They then crossed LlPp x LlPp, expecting the ration of 9:3:3:1. They found they got roughly 3 purple-long to 1 short red. This is because the genes or pollen length and flower colour were linked, THEIR GENES WERE ON THE SAME CHROMOSOME!

- because they were linked they didn't separate during meiosis. So instead of forming four gametes, the plants only produced 2: P-L and p-l. It was as if they were just crossing one characteristic, producing the 3:1 ratio

EXPLAIN HOW CROSS-BREEDING EXPERIMENT CAN IDENTIFY THE RELATIVE POSITION OF LINKED GENES

- Bateson and Punnet found roughly 3:1 ration of purple-long to red-short, however there were a small number of purple-short and red-long. This inconsistency was accounted for through crossing over

- in some cells where crossing over occurred, the gametes P-l and p-L were formed, which meant a small number of offspring were purple-short, red-long.

- If genes are far apart, it is more likely crossing over will take place than if they are close together. Using the number of unexpected results, we can calculate the relative distance of two genes on a chromosome. In order to do this we must cross a heterozygous parent with a homozygous recessive parent.

- The presence of unexpected phenotypes meant crossing over has occurred, and to calculate the distance we use the equation :

- Number of unexpected types/total offspring x 100

DISCUSS THE ROLE OF CHROMOSOMAL MAPPING IN IDENTIFYING RELATIONSHIPS BETWEEN SPECIES

- Chromosomal mapping can be used to compare different species genome in order to estimate evolutionary distance. If two species are closely related, then their chromosome maps will be very similar

- However chromosome maps produced by recombination percentages does not completely represent a chromosome. These maps only provide relative distances, not absolute ones.

THE HUAMN GENOME PROJECT IS ATTEMPTING TO IDENTIFY THE POSITION OF GENES ON CHROMOSOMES THROUGH THE WHOLE GENOME SEQUENCING

DISCUSS THE BENEFITS OF THE HUAMN GENOME PROJECT

DESCRIBE AND EXPLAIN THE LIMITATIONS OF DATA OBTAINED FROM THE PROJECT

- HGP was a collaborative 13-year effort to map the entire human genome

- Main goals of the project were to:

IDENTIFY all the genes in human DNA

DETERMINE the sequence of our approximately 3 billion base pairs

STORE all this information

IMPROVE tools for analysis

TRANSFER related technologies to the private sector

ADDRESS the ethical, legal and social issues

Benefits: