Topic 4 : Genetics

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4.1.1What are eukaryotic chromosomes made up of?

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DNA and protein

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4.1.2 a) What is a gene?

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A heritable factor that controls a specific characteristic

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4.1.2 b) What is an allele?

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One specific form of a gene, differeing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of that gene.

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4.1.2 c) What is a genome?

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The whole of the genetic informaition of an organism

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4.1.3 What is gene mutation?

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A change to the base sequence of a gene

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4.1.4 What are the consequences of a base substitution mutaion in transciption and translation in sickle-cell anemia?

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Sickle Cell anaemia is a genetic diesease that affects red blood cells in the body. It is because of a mutation on the Hb gene which codes for a polypeptide of 146 amino acids which is a part of haemoglobin.

In Sickle cell- anemia, the codon GAG found in the normal gene is mutated to GTG. This is called a base substituition mutation as Adenine is replaced by Thymine. This means that when the mutated gene is transcibed, a codon in the mRNA will be diffrent. Instead of the normal GAG, it will be GUG. This will result in a mistake during translation. Hence, the base substitution mutation has caused glutamic acid to be replaced by valine.
This has an effect on phenotype as instead of a normal donut shaped red blood cell, a sickle one will be produced. As a result this sickle shaped red blood cells cannot carry oxygen as efficiently.
However, an advantage is that sickle cells give resistance to malaria so this is quite common in places where malaria is persistant.

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4.2.1 What is meiosis?

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A reduction division of a diploid nucleus to form a haploid nuclei.

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4.2.2 What is a homologous chromosome?

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Chromosomes with the same genes as each other. They are in the same sequence but do not necessarily have the same allele of those genes.

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4.2.3 What happens in meiosis?

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For dummies (shanna)
Interphase (replication)
then mitosis (involves crossing over)
PMAT
= 2 daughter cells
then meiosis starts
Prophase - chromosomes pair up and each pair are homologous. Crossing over then occurs. (he exhange of genetic material nbetween non-sister chromatids). NucleaR membrane breaks down and spindle stretch our from each pole to equator.
Metaphase - Homologous chromosomes lines up at equator. Microtubules attach to the centromere of each chromosomes.
Anaphase- SPindle microtubules pull each sister chromatid to opposite poles in both cell.
Telophase - nuclear membrane reforms around the four daughter chromosomes. Cytokinesis then follows to divide the cytoplasm of the two cells.
= 4 daughter cells with haploid set of choromosomes.

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4.3.1 a) Genotype

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The alleles of an organism

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4.3.1 b) Phenotype

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the characteristics of an organism

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4.3.1 c) dominant allele

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an allele that has the same effect on phenotype whether its present in homo/hetero zygous state

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4.3.1 d) recessive allele

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an allele that only has an effect on the phenotype when present in a homozygous state.

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4.3.1 e) Codominant allele

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a pair of allele that both affect the phenotype when present in heterozygote

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4.3.1 f) locus

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particular position on homologous chromosome of a gene

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4.3.1 g) Homozygous

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having two identical alleles of a gene

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4.3.1 h) heterozygous

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having two different alleles of a gene

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4.3.1 i) carrier

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an individual that has one copy of a recessive allel that causes a genetic disease in individuals that are homozygous for this allele.

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4.3.1 j) Test cross

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testing a suspected heterozygous by crossing it with known homozygous reccesive.

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4.3.3 Genes have..

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more than two alleles (multiple)

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4.3.4 ABO blood groups are an example of codominance and multiple alleles.

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There are three alleles that control the ABO bloog groups.
The allele A1 corresponds to blood group A ( genotype = IaIa) and allele B1 corresponds to blood group B ( IbIb).

If A1 and B1 are present together, it froms blood group AB ( IaIb). Both allele aftect the phenotype since they are codominant.

the allele i is recessive . so ii = blood group O

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4.3.5 How do sex chromosomes control gender?

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There are two chromosomes that determine gender (sex chromosomes)

Femlas hae two X, males have one X and one Y.

cX is larger then cY.

Female always passes X and males passes either X or Y.

Gender depeds on whether the sperm which fertilizes the egg is carrying an X or Y chromosme.

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4.2.4 What can happen in non-disjuction illustrated by reference to Down Syndrome.

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Problems can arise during meiosis.
This is when the chromosomes dont separate properly during meiosis (anaphase)
This leads the production of gamates that either have too little(die) or too many chromosomes.
When a zygots is formed from the fertilization of these amtes with extra chromosomes, three chromosomes of one type are present instead of two.
Down syndrome is a disease in which the chromosomes fail to seperate properly leading to three chromosomes of type 21 instead of two. A person with this condition therefore has a total of 47 chromosomes instead of 26.

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4.2.5 Karyotyping

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Chromosomes are arrange in pairs according to their size and structure

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4.2.6 When is karyotyping performed?

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It is performed using cells collected by chorionic villus sampling/ amniocentesis for pre-natal diagnosis of chromosome abnormalities.

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4.2.7 How to determine gender and whether non-disjunctino has occured?

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Gender an be deduced by loking at the sex chromosomes. Females will have two X chromosomes while males have one X and one Y.
We can distinguish this as the Yc is smaller than the X.

For disjunctions, we can see if a chromosome is missing or there is an extra one

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4.3.6 Some genes are....

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present on the Xc and absent from the shorter Yc in humans.

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4.3.7 Sex linkage

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The gene controling the characteristic is located on the sex chromosome and so it is associated the characteristic with the gender.

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4.3.8 Describe the inheritance of sex linkaged disease ( haemophilia and sex linkage)

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Most of the time sex-linked genes are carried on the X chromosome. Women carry 2 and men only carry 1.

If the child is a boy then the father has passed on the Yc which does not contain the allele of the gene. So whether the child has the disease or is unaffected depends on which allel the mother had passed on. If she passed on a recessive allel (xh) then the male child will have hemophilia, however if she has passed on the dominate allele(xH) then child will be unaffected.
50% chance of the child having hemophilia if it is males as half of the eggs produced by the mother will carry the recessive allele. And 0% on the female having hemophilia since the father always passes on the dominant allele on the X chromosome.

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4.3.9 A human female can be homozygous....

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and heterozygous with respect to sex-linked genes.

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4.3.10 Why are females carriers are heterozygous for X-linked recessive alleles.

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Because they need both a dominant and recessive allele to be carriers. They inherit one recessive and one dominant from each parent.

If a carrier mother and an unaffected father have offspring then the unaffected father will always pass on his dominant allele to his female offspring. If the carrierr mother passes the recessive allele then her female offspring will be a carrier as well.

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4.3.11 Predict the genotypic and phenotypic ratios of offspring of monohyprid crosses involving any of the above patterns of inheritance.

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Crossing between two heterozygous individuals gives a 3:1 ratio if one of the alleles is dominant and other is recessive.

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4.3.2 Genotypes and phenotypes of individuals in pedigree charts

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- if more than half of the males in pedigree are affected the disorder is X-linked.
-50/50 ratio between men and women, the disorder is autsomal.
-if the disorder is dominant, one of the parents have the disorder
-if the disorder is recessive then neither of the parents has to have the disorder as they can be heterozygous.

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4.4.1 Whats the use of polymerase chain reaction

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It is used to copy and amplify minute quatities of DNA. It can be useful when only a small amount of DNA is available but large amount is required to undergo testing.
We can use DNA from blood, semen, tissues from crime scenes for example.
The PCR requires high temperature and a DNA polymerase enzyme from thermus aquaticus. ( a bacterium that lives in hot springs)

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4.4.2 Gel electrophoresis

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Fragments of DNA move in an electrical field and are separated according to their size.

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4.4.3 What is gel electrophoresis of DNA for?

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It is used in DNA profiling

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4.4.4 How can DNA profiling by applied?

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Organisms have short sequences of bases which are repeated many times. There are called satelilte DNA. Thes repeated sequences vary in length from person to person. The DNA is copied using PCR and then cut up into small graments using restriction enzymes.

Gel electrophoresis separates fragmented pieces of DNA accourding to their size and charge. This gives a pattern of bands on a gel which is unlikely to be the same for two individuals. This is called DNA profiling. It can be used to determine paternity and also in forensic investigations.

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4.4.5 How to draw conclusions in paternity test and investigations?

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Look for similarities between the DNA of both suspect and testee

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4.4.6 What are the three outcomes of the sequencing of the complete human genome?

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- It is now easier to study how genes influence human development
-helps identify genetic diseases.
-allows the production of new drugs on DNA base sequences of genes or the structure of proteins coded for by these genes.
-more info on the origins, evolution and migration of humans

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4.4.7 What happens when genes are transferred between species?

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When genes are transffered between species, the amino acid sequence of polypeptides translated from them is unchanged becase the genetic code is universal.

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4.4.8 What happens during a basic technique used for gene transfer?

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The human gene that codes for insulin can be inserted into a plasmid and then that plasmid can be insterted into a host cell such as a bacterium. The bacterium can then synthesis insulin which can be collected and used by diabetics.

The mRNA which codes for insulin is extracted from a human pancreatic cell which produces insulin.
DNA copies are then made by using enzyme rever transcriptase and these DNA copies are then given extra guanine nucleotides to the end of the gene to create sticky ends.

At the same time, a selected plasmid is cut using restrictionryhtru6tmjuyjghkyuh,luki.l

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4.4.9 What are two examples of the current uses of genetically modified crops or animals

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The transfer of a gene for factor IX which is a blood clotting factor, from humans to sheep so this factor is produced in the sheep's milk

The transfer of a gene that gives resistance to the herbicide glyphosphate from bacterium to crops so that the crop plants can be sprayed with the herbicide and not be affected by it.

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4.4.1 What are the potential benefits of genetic modification?

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- Since there is less damage to the crops, there is a higher crop yield which can lessen food shortage
-Since there is a higher crop yield, less land is needed to grow more crops but become an area for wild life conservation instead.
-reduction in the use of pesticides which are expensive and harmful to the environment

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4.4.1 What are the harmful effects of the benefits of genetic modification?

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- Consequences to humans and animals eating the crops are unsure of
-other organisms which are not harmful could also be killed
-cross pollination can occur which results in other/wild plants being genetically modified as well and have advantage over others as they will be resistant to certain insects and so some plants may become endangered

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4.4.11 Clone

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A group of genetically organisms or a group of genetically identified cells derived from a single parent cell

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4.4.12 What is a technique for cloning using differenciated animal cells.

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Dolly the sheep was cloned by taking udder cells from a donor sheep. These cells were then cultured in a low nutrient medium to make the genes switch off and becom dormant.
Then an unfertilized eff was taken from another sheep. The nucleus of this egg cell was removed by using a micropipette and then the egg cells were fused with the udder cells using a pulse of electricity. The fused cells developed like normal zygotes and became embryos.

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4.4.13 What are the ethical issues involving therapeutic cloning in humans?

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FOR
Embryonic stem cells can be used for therapies athe save lives and reduce pain for patients. Since a stem cell can divide and differenciate into any cell type, they can be used to replace tissues or organs required by patients
-Cells can be taken from embryos that have stopped developping and so these cells would have died anyway
-cells are taken at a stage when the embryos have no nerve cells and so they cannot feel pain

AGAINST
-every human embryo is a potential human being and should be give the chance of developing
-more embryos are generally produced than are needed and so many are killed
-there is a risk of embryonic stem cells developing into tumour cells.