Genetics

Mendellian ratio

The phenotypic ratio for a dihybrid heterozygous cross is always 9:3:3:1

Completion and analysis of Punnett squares for dihybrid traits

-A dihybrid cross determines the genotypic and phenotypic combinations of offspring for two particular genes that are unlinked -Because there are two genes, each with two alleles, there can be up to four different gamete combinations -The easiest way to work out potential gamete combinations in a dihybrid cross is to use FOIL = First / Outside / Inside / Last

Chromosomes replicate in interphase before meiosis

-Chromosomes replicate during the S phase of the cell cycle, so that each chromosome has a copy of itself and consists of two sister chromatids -During meiosis I, chromosomes condense and synapse to form bivalents (homologous chromosomes are aligned next to each other)

Genetic modification is carried out by gene transfer between species

-A gene produces a certain polypeptide in an organism -Since the genetic code is universal, when a gene is removed from one species and transferred to another the sequence of amino acids in the polypeptide produced remains unchanged -Gene modification has been used to introduce new characteristics to certain animal species -For example goats that produce milk containing spider silk and bacteria that produce human insulin -A plant example is the production of golden rice that contains beta-carotene

Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer

-A mutation is a random change to the base sequence of a gene -Both radiation and certain chemicals can cause genetic diseases and cancer -Radiation can cause mutations if it has enough energy to chemical change one's DNA -Gamma rays and alpha particles from radioactive decay, UV radiation and x-rays are all considered to be mutagenic -Certain chemical substances can all cause chemical change in DNA and are therefore considered mutagenic

Clones are groups of genetically identical organisms, derived from a single original parent cell

-Clone: a group of genetically identical organisms or a group of cells derived from a single parent cell -Organisms that reproduce asexually, produce genetically identical offspring Identical twins in humans are also clones

Use of DNA profiling in paternity and forensic investigations

-A DNA sample is collected (blood, saliva, semen, etc.) and amplified using PCR Satellite DNA (non-coding) is cut with specific restriction enzymes to generate fragments -Individuals will have unique fragment lengths due to the variable length of their short tandem repeats (STR) -The fragments are separated with gel electrophoresis (smaller fragments move quicker through the gel) -The DNA profile can then be analysed according to need Two applications of DNA profiling are: 1. Paternity testing (comparing DNA of offspring against potential fathers) 2. Forensic investigations (identifying suspects or victims based on crime-scene DNA)

Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes

-A dihybrid cross determines the allele combinations of offspring for two particular genes that are unlinked (not on the same chromosome) -Because there are two genes with two alleles per gene (multiple alleles not required), there can be up to four different gamete combinations -To work out gamete combinations remember FOIL: • First (AaBb = AB) • Outside (AaBb = Ab) • Inside (AaBb = aB) • Last (AaBb = ab) -When calculating genotype, always pair alleles from the same gene (e.g. ABab should be AaBb) and always write capitals first

Non-disjunction can cause Down syndrome and other chromosome abnormalities

-A non-disjunction is an error in meiosis, where the chromosome pairs fail to split during cell division -Non-disjunction can occur in anaphase I where the homologous pairs fail to split, or it can occur in anaphase II, where the sister chromatids fail to split -The result of this error is too many chromosomes in a gamete cell or too few chromosomes in the final gamete cell -One of the gamete cells could have 22 chromosomes and one could have 24 chromosomes - the resulting zygote will therefore have 47 or 45 chromosomes -An example of a non-disjunction is Down syndrome - occurs when chromosome 21 fails to separate, and one of the gametes ends up with an extra chromosome 21 -Therefore, a child that receives that gamete with an extra chromosome 21 will have 47 chromosomes in every cell -Down syndrome is also called Trisomy 21 -Some Down syndrome symptoms include impairment in cognitive ability and physical growth, hearing loss, oversized tongue, shorter limbs and social difficulties -Other types of non-disjunctions are trisomy 18 (Edwards Syndrome - many of these fetuses die before birth), trisomy 13 (Patau's syndrome - causes multiple and complex organ defects and highly effects normal development)

Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases

-A pedigree is a chart of the genetic history of a family over several generations -Males are represented as squares, while females are represented as circles -Shaded symbols means an individual is affected by a condition, while an unshaded symbol means they are unaffected -A horizontal line between a man and woman represents mating and resulting children are shown as offshoots to this line Autosomal Dominance: -All affected individuals must have at least one affected parent -If two parents are unaffected, all offspring must be unaffected (homozygous recessive) -If two parents are affected, they may have offspring who are unaffected (if parents are heterozygous) Autosomal Recessive: -If two parents show a trait, all children must also show the trait (homozygous recessive) -An affected individual may have two normal parents (if parents are both heterozygous carriers) X-Linked Recessive: -If a female shows the trait, so must all sons as well as her father -The disorder is more common in males

Implications of Morgan's discovery of sex-linkage

-After breeding thousands of Drosophila, Morgan noticed 1 fruit fly with white eyes instead of normal red - he mated this with ordinary red fly -Although first generation only had very few white eyed flies, second generation the flies appeared in much larger numbers (approximately 3 red for each white) -Surprisingly all white flies in second generation were male - Mendel's principle of dominance and recessiveness would predict a 3 : 1 ratio of red to white in both females&males - but all females had red eyes

The various specific forms of a gene are alleles

-Allele: One specific form of a gene, differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the same gene -There can be two or more alleles of a specific gene depending on the gene -The gene that influences human blood type has three different alleles that code for blood types A, B and O -When there are more than two alleles, this is called multiple alleles -Since each human cell consists of 2 copies of each chromosome (except X and Y), there are two copies of each gene - sometimes a person can have two of the same allele (homozygous) or two different alleles (heterozygous)

Gene loci are said to be linked on the same chromosome

-Any genes that are found on the same chromosome and are therefore more likely to be inherited together are considered to be linked -When these genes (alleles) are inherited together as a group, they are considered to be a part of the same linkage group

The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation

-At the start of meiosis (prophase I), the replicated chromosomes begin to condense and become visible -Homologous chromosomes synapse (pair up) to form bivalents or tetrads -Crossing over occurs between non-sister chromatids -Crossing over occurs when two of the non-sister chromatids exchange a segment of their chromosome with each other -Since the genes between the two chromosomes are the same, but the alleles may differ between the maternal and paternal chromosome, a new combination of alleles will be present when the chromosomes separate -These crossover points are random and lead to genetic variation in the gametes

Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells

-At the very early embryo stage, cells are still pluripotent (meaning they can become any type of tissue) -These cells can be separated artificially in a laboratory in order to create more than one of the same organism -The separated pluripotent cells can then be inserted into the uterus of a surrogate mother or mothers in order to produce genetically identical offspring -The separation of cells has to happen early in development, preferably the 8 cell stage

Analysis of data on risks to monarch butterflies of Bt crops

-Bt corn is a genetically modified maize that incorporates an insecticide producing gene from the bacterium Bacillus thuringiensis -This insecticide is lethal to certain types of larvae, particularly the European corn borer which would otherwise eat the crop -Concerns have been raised that the spread of Bt corn may also be impacting the survival rates of monarch butterflies -While monarch butterfly larva feed exclusively on milkweed, wind-borne pollen from Bt corn may dust nearby milkweeds -In 1999, a preliminary study was conducted investigating the association between exposure to Bt corn pollen and survivor rates among monarch caterpillars: -Monarch caterpillars were fed milkweed leaves that had been dusted with pollen from Bt corn (to simulate spread via wind) -Growth and mortality rates were compared against caterpillars fed on non-dusted leaves or leaves dusted with non-GM pollen -Caterpillars exposed to Bt pollen were found to have eaten less, grew more slowly and exhibited higher mortality rates

DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids

-Chromosomes are replicated in the synthesis (S) phase during interphase -This means that each chromosome will have an attached identical copy before meiosis occurs -These are called sister chromatids

Karyograms shows the chromosomes of an organism in homologous pairs of decreasing length

-Chromosomes of org. are visible in cells that are in mitosis, with cells in metaphase giving the clearest view - stains have to be used to make the chromosomes show up - some stains give each chromosome type a distinctive banding pattern -Chromosomes are arranged according to their size and structure - position of centromere and pattern of banding allow chromosomes that are of a different type of similar size to be distinguished -As most cells are diploid, the chromosomes are usually in homologous pairs - arranged by size, starting with longest pair and endin with smallest

Red-green colour blindness and hemophilia as examples of sex-linked inheritance

-Colour blindness and haemophilia are both examples of X-linked recessive conditions -The gene loci for these conditions are found on the non-homologous region of the X chromosome (they are not present of the Y chromosome) -As males only have one allele for this gene they cannot be a carrier for the condition -This means they have a higher frequency of being recessive and expressing the trait -Males will always inherit an X-linked recessive condition from their mother -Females will only inherit an X-linked recessive condition if they receive a recessive allele from both parents -When assigning alleles for sex-linked traits the convention is to write the allele as a superscript to the sex chomosome (usually X) -Haemophilia: XH = unaffected ; Xh = affected -Colour Blindness: XA = unaffected ; Xa = affected

Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl

-Common feature of nuclear bombing of Hiroshima and Nagasaki and nuclear accidents at Three Mile Island and Chernobyl is that radioactive isotopes were released into environment and as a result people were exposed to potentially dangerous levels of radiation -When atomic bombs were detonated over Hiroshima and Nagasaki 150,000-250,000 people either died directly or within a few monts -Survivors had developed 17,000 tumours, to which only 853 of these could be attributed to effects of radiation from atomic bombs -Apart from cancer other main effect of radiation that was predicted was mutations - leading to stillbirths, malformation or death -Health of 10,000 children that were foetuses when the atomic bombs were detonated and 77,000 children that were born later in Hiroshima and Nagasaki has been monitored -Accident at Chernobyl, Ukraine in 1986 involved explosions and a fire in core of nuclear reactor - workers at plant quickly received fatal doses of radiation

Chiasmata formation between non-sister chromatids can result in an exchange of alleles

-Crossing over involves the exchange of segments of DNA between homologous chromosomes during Prophase I of meiosis The process of crossing over occurs as follows: -Homologous chromosomes become connected in a process called synapsis, forming a bivalent (or tetrad) -Non-sister chromatids break and recombine with their homologous partner, effectively exchanging genetic material (crossing over) -The non-sister chromatids remain connected in an X-shaped structure and the positions of attachment are called chiasmata -Chiasma hold homologous chromosomes together as a bivalent until anaphase I -As a result of crossing over, chromatids may consist of a combination of DNA derived from both homologues - these are called recombinants

Crossing over produces new combinations of alleles on the chromosomes of the haploid cells

-Crossing over occurs during prophase I of meiosis -Crossing over only takes place some of the time between linked genes, therefore the parental combination in the gametes Ab and Ab will show up more often in the offspring, while ab and AB (recombinants) will show up with less frequency -Start: AaBb and AAbb Result after crossing over: Aabb and AABb

Prokaryotes have one chromosome consisting of a circular DNA molecule

-DNA above called the nucleoid region is circular DNA which, unlike eukaryotes, is not associated with any histone proteins -There is one copy of each gene except when the cell and its DNA are replicating

DNA profiling involves comparison of DNA

-DNA profiling is a technique by which individuals are identified on the basis of their respective DNA profiles -Within the non-coding region of an individual's genome, there exists satellite DNA - long stretches of DNA made up of repeating elements called short tandem repeats (STRs) -These repeating sequences can be excised to form fragments, by cutting with a variety of restriction endonucleases (which cut DNA at specific sites) -As individuals all have a different number of repeats in a given sequence of satellite DNA, they will all generate unique fragment profiles -These different profiles can be compared using gel electrophoresis

Diploid nuclei have pairs of homologous chromosomes

-Diploid nuclei have two copies of each type of chromosome - one chromosome comes from the mother and one from the father -Haploid gametes (sperm and egg) fuse during sexual reproduction which produces zygote with a diploid nucleus -This cell will then divide by mitosis to produce numerous cells, all with a diploid nucleus -Each nucleus has two copies of each gene, except the sex chromosomes

Dominant, recessive and co-dominant alleles

-Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects -Dominant alleles mask the effects of recessive alleles and are expressed in the phenotype -For example, if B is dominant for brown hair color and little b is recessive for blonde hair colour, an individual that is BB (homozygous dominant) will have brown hair -If the individual has the genotype Bb (heterozygous), they will also have brown hair, as the dominant B is masking the expression of b -If the individual has the genotype bb (homozygous recessive), that person will have blonde hair

Homologous chromosomes separate in meiosis I

-During meiosis I, unlike mitosis homologous chromosomes separate to opposite poles; however, their sister chromatids remain attached to each other -Homologous chromosomes can exchange material in a process called crossing over -Meiosis I is considered reduction division because the chromosome number is reduced by half (2n -> n in humans)

Sister chromatids separate in meiosis II

-During meiosis II sister chromatids separate (some are non-identical sister chromatids due to crossing over -This type of separation is very similar to mitosis as the chromatids are separated from each other

The two alleles of each gene separate into different haploid daughter nuclei during meiosis

-During meiosis a diploid nuclei in a germ cell divides to produce 4 haploid nuclei -If an individual has two of the same allele AA for a particular gene, all 4 haploid cells will contain the allele A -This is the same if the alleles for the gene are aa -If an individual has two different alleles for a particular gene such as Aa, the haploid gametes will contain 50% A and 50% a for that specific gene -The separation of the alleles into different nuclei is called segregation

Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes

-Homologous chromosomes are chromosomes within each cell that carry the same genes -One chromosome came from an individual's mother and one from the father -They have the same shape and size -These chromosomes pair up during meiosis -Even though these chromosomes carry the same genes, they could have different alleles

The halving of the chromosome number allows a sexual life cycle with fusion of gametes

-During sexual reproduction there is the fusion of two gametes to form a new cell with double the number of chromosomes - fusion of the gametes takes place during fertilization -If an organism did not reduce or half the number of chromosomes during meiosis before fertilisation took place, the new cell would contain double the number of chromosomes in comparison to the original cell -This means there would be a doubling of chromosomes with each new generation or sexual life cycle -This is why reduction division during meiosis is essential for the sexual life cycle to occur in eukaryotes -This also creates genetic diversity as the alleles on the chromosomes from each parent might be different -In prokaryotes asexual division occurs given rise to offspring that are genetically identical to their parents

A gene occupies a specific position on a chromosome

-Each gene occupies a specific location or position on a chromosome called a locus (plural loci) -There are only 46 chromosomes in a human diploid cell (23 pairs in females including two X chromosomes and 22 pairs plus and X and a Y chromosome in males) -Each chromosome contains many different genes often linked in groups

In a eukaryote species there are different chromosomes that carry different genes

-Eukaryotic chromosomes are linear chromosomes that vary in length and in position of the centromere that holds the sister chromatids together -In humans there are 23 types of chromosomes -There are 22 pairs of autosomes -The 23rd pair are the sex chromosomes -Males have XY chromosome, females have XX chromosomes -Each chromosome carries a specific sequence of genes along the linear DNA molecule -The position where the gene is located is called the locus -All eukaryotic species contain at least two different chromosomes, but most contain more than only two

Use of a chi-squared test on data from dihybrid crosses

-For example let's use Mendel's results from his pea plant crosses -When he did a dihybrid cross between two heterozygotes RrYy x RrYy, the expected phenotypic ratio due to independent assortment would be 9:3:3:1 -P-value (in graph) of 0.05 or 5% indicates is the probability of getting the results you did (or more extreme results) given that the null hypothesis is true -Results that Mendel saw were due to independently assortment of the alleles

Zygotes

-Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles -When the gametes (n) fuse to form a zygote (2n), two copies of each gene exist in the diploid zygote -The zygote may contain two of the same allele AA or aa or two different alleles such as Aa

Gametes are haploid so contain only one allele of each gene

-Gametes are sex cells such as sperm and eggs -Gametes contain one set of chromosomes or one chromosome of each type and are therefore haploid (n) -Since they have only one chromosome of each type, gametes also only contain one allele of each gene -The specific allele depends upon if that particular chromosome came from the mother or father and if crossing over occurred during prophase 1 -Together the two gametes form a zygote

Gel electrophoresis is used to separate proteins or fragments of DNA according to size

-Gel electrophoresis is a technique which is used to separate fragments of DNA according to size -Samples of fragmented DNA are placed in the wells of an agarose gel -The gel is placed in a buffering solution and an electrical current is passed across the gel DNA, being negatively charged (due to phosphate), moves to the positive terminus (anode) -Smaller fragments are less impeded by the gel matrix and move faster through the gel -The fragments are thus separated according to size -Size can be calculated (in kilobases) by comparing against a known industry standard

Gene transfer to bacteria using plasmids makes use of restriction endonucleases and DNA ligase

-Gene transfer is taking one gene from an organism and inserting it into another organism -An example of gene transfer is for the production of human insulin produced by the pancreatic cells -First, mRNA that codes for insulin produced in the pancreatic cells is extracted -The enzyme reverse transcriptase is mixed with the mRNA - this enzyme produces a strand of coding DNA called cDNA -Plasmids are small circles of DNA found in bacteria cells - these plasmids are cut with a restriction enzyme, leaving sticky ends to which the cDNA (cDNA is cut with the same restriction enzyme) can attach -DNA ligase is used to seal the nicks between the cDNA and the plasmid -Linking sequences are added to the cDNA allowing them to be inserted into the plasmid -The bacterial plasmid carrying the insulin gene is now inserted into plasmid free bacterial cell such as E.coli bacteria (with plasmid removed) - known as the host cell -These insulin producing bacterial cells will now reproduce rapidly during fermentation, creating millions of insulin producing bacteria cells -Finally, the insulin produced is extracted from the cell and purified to be used by diabetics

A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic

-Gene: The basic unit of heredity or a heritable factor that controls a specific characteristic -DNA consists of the base pairs adenine, guanine, cytosine and thymine -Humans have between 21,000-23,000 protein coding genes -The number of genes in an organism's genome does not indicate how complicated an organism is

Alleles differ from each other by one or only a few bases

-Genes consist of a certain sequence of DNA bases which can be 100's to 1000's bases in length -Usually different alleles of the gene vary by only one to a couple of different bases -For example, the allele for Sickle Cell Anemia is created by a mutation of a single nucleotide -Adenine is switched to Thymine (GAG to GTG) which results in glutamic acid being substituted by valine at position 6 in the Haemoglobin polypeptide -This variation when one nucleotide is switched for another is called a single nucleotide polymorphism (SNPs for short)

Genome

-Genome: The whole of the genetic information of an organism -In humans, the genome consists of 46 chromosomes plus the mitochondrial DNA -In plants, the genome also consists of chloroplast DNA on top of their chromosomes and mitochondrial DNA -Prokaryotes have a circular chromosome and plasmids in their genome

Haploid nuclei have one chromosome of each pair

-Haploid nuclei have one copy of each chromosome or one full set of the chromosomes in that particular species eg. Human 23 chromosomes -These are called gametes, which are sperm and egg -Human sperm and eggs each contain 23 chromosomes

Inheritance of ABO blood groups

-Human blood types are an example of both multiple alleles (A, B, O) and co-dominance (A and B are co-dominant) -Co-dominant alleles such as A and B are written as a superscript (IA and IB) -The (I) represents immunoglobulin -Blood type O is represented by (i) -Both IA and IB are dominant over the allele (i) -A, B and O alleles all produce a basic antigen (glycoprotein) on the surface of the red blood cells -The allele for blood type B alters the basic antigen by adding a galactose to the glycoprotein -Individuals that do not have this allele and are exposed to blood type B, will produce Anti-B antibodies that will attack and destroy these red blood cells (RBC) -The allele for blood type A alters the basic antigen by adding an acetylgalactosamine -So individuals that do not have the A allele will produce Anti-A antibodies that will attack and destroy these RBC's -The allele for blood type O produces the basic antigen that will be present on the cell membrane of these RBC's -Individuals with blood type O will produce both Anti-A and Anti B antibodies if exposed to either A or B blood cells -Individuals that have both A and B alleles will have both of the antigen modifications -Hence, the alleles for A and B are co-dominant -If exposed to blood type A or B, no Anti-A or Anti-B antibodies will be produced -If individuals with blood type A, B or AB are exposed to blood type O, no immune response will occur because blood type O only contains the basic antigen

Inheritance of huntington's disease

-Huntington's disease is an autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4 -The HTT gene possesses a repeating trinucleotide sequence (CAG) that is usually present in low amounts (10 - 25 repeats) -More than 28 CAG repeats is unstable and causes the sequence to amplify (produce even more repeats) -When the number of repeats exceeds ~40, the huntingtin protein will misfold and cause neurodegeneration -This usually occurs in late adulthood and so symptoms usually develop noticeably in a person's middle age (~40 years) -Symptoms of Huntington's disease include uncontrollable, spasmodic movements (chorea) and dementia

Variation can be discrete or continuous

-If variation is discrete it is controlled by alleles of a single gene or a small number of genes -The environment has little effect on this type of variation -In this case you either have the characteristic or you don't -Cystic fibrosis is a good example for this; either you have cystic fibrosis or you don't -Blood groups are another example of this type of variation - you are either blood type A, B, AB or O, there is no blending of these traits -Chi-squared calculations work well when using examples with discrete variation -In continuous variation there is a complete range of phenotypes that can exist from one extreme to the other -Height is an example of continuous variation as there is a wide assortment of heights of individuals -Continuous variation is the combined effect of many genes (known as polygenic inheritance) and is often significantly affected by environmental influences -Skin colour is another example of continuous variation

Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number

-In meiosis I, homologous chromosomes split, but the centromeres do not divide since the sister chromatids do not separate -One chromosome from each pair separate and migrate towards separate poles - this separation is called a disjunction -This halves the chromosome number of each cell and is therefore called reduction division -The two new cells formed after the first division are haploid (n)

Karyotypes and Down syndrome

-Karyogram is image of chromosomes of rog., arranged in homologous pairs of decreasing length - karyotype us property of org. - is number&type of chromosomes that organism has in its nuclei -Karyotypes are studied by looking at karyograms - can be used in 2 ways: 1. To deduce whether individual is male/female - XX=female, XY=male 2. To diagnose down syndrome and other chromosome abnormalities - usually done using fetal cells taken from uterus during pregnancy - if there are 3 copies of chromosome 21 in karyotype instead of 2, child has Down syndrome (sometimes called trisomy 21) - while individuals vary, some of component features of syndrome are hearing loss, heart and vision disorders - mental and growth retardation are also common

Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles

-Many genetic diseases are caused by recessive alleles contained on the autosomal chromosomes (chromosome 1-22) -Therefore, the disease would only be expressed if an individual has two recessive alleles (i.e. aa) -If an individual has one of the dominant alleles (i.e. Aa), they will not show symptoms of the disease - these people are known as carriers -They can pass this allele on to their offspring -If the other parent is also a carrier then their offspring have a 25% chance of getting the disease -A small number of diseases are co-dominant, such as sickle cell anaemia -An example of a recessive genetic disease is cystic fibrosis and a dominant disease is Huntington's Disease

Many genetic diseases have been identified in humans but most are very rare

-Medical research has already identified more than 4,000 genetic diseases and more no doubt remain to be found -Chance of inheriting one allele for any specific disease is small but to develop the disease two alleles must be inherited and the chance of this is extremely small -Individual can only produce child with a genetic disease due to 1 of these recessive alleles if other parent of child has same rare allele

Crossing over and random orientation promotes genetic variation

-Meiosis is the formation of gametes that produce offspring that are genetically different than their parents -The two main ways variation is created in the offspring is through crossing-over and through random orientation of the chromosomes

One diploid nucleus divides by meiosis to produce four haploid nuclei

-Meiosis is the process in which the diploid (2n) nucleus divides to form four haploid (n) nuclei -Meiosis has two divisions called Meiosis I and Meiosis II -In the first division the diploid nucleus 2n, which consists of homologous pairs of chromosomes (half maternal and half paternal chromosomes), divides to form two haploid cells (n) -These cells after the first division are considered haploid because the homologous pairs of the nucleus are separated into the two new cells -In meiosis II, the haploid chromosomes in the two cells (each have 2 chromatids because replication occurs before meiosis takes place) divide to form four haploid cells each with one set of chromosomes -This is called reduction division because the chromosome number is halved

Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed

-Mendel was known as the father of genetics - performed experiments on a variety of different pea plants, crossing these varieties by using the male pollen from one variety and transferring it to the female part of another variety -He collected the seeds and grew them to determine their characteristics -He then crossed these offspring with each other and also grew their seeds to determine their characteristics -He continued performing many crosses and recorded his results -The large number of crosses and replicates he performed were essential in ensuring reliability in his test results and determining the ratios from the crosses

Unlinked genes segregate independently as a result of meiosis

-Mendel's law of independent assortment states allele pairs separate independently from other allele pairs during gamete formation (meiosis) -Therefore, traits on different chromosomes are transmitted to the offspring independently of traits on other chromosomes -An exception to this rule is linked genes

Exceptions to Mendel's rules

-Morgan discovered non-mendellian ratios in his experiments with the fruit fly, Drosophila melanogaster -At beginning of his research, Morgan was critical of Mendel's theory of inheritance and wasn't convinced by aspects of the emerging chromosomal theory of inheritance - he believed that variation he saw in organisms was better explained by environmental influence -His own observations of pattern of inheritance of white eyes led him to reconsider his own perspective - at same time as his results reinforced aspects of Mendel's conclusions, Morgan's studies identified exceptions to Mendel's principals of inheritance

New alleles are formed by mutation

-New alleles are created by random changes in the base sequence called mutations -There are a variety of different types of mutations that can be either harmful, neutral or beneficial

Random orientation

-Occurs in metaphase I of meiosis -When homologues line up along the equatorial plate in metaphase I, the orientation of each pair is random; meaning the maternal/paternal homologue can orient toward either pole -This means the number of combinations that can occur in the gamete is 2^n (n=number of chromosome pairs) -Huge number of genetic variation/possibilities in offspring

Crossing over

-Occurs in prophase I of meiosis -Crossing over occurs between non-sister chromatids of a particular chromosome -Chiasmata are points where two homologous non-sister chromatids exchange genetic material during crossing over in meiosis -Chromosomes intertwine and break at the exact same positions in non-sister chromatids -Segments of the adjacent homologues are exchanged during crossing over, therefore the two sister chromatids are no longer identical -Crossing over creates new combinations of linked genes (genes on the same chromosome) from the mother and the father -When the chromatids are separated into different gametes after anaphase II, the gametes produced will not contain the same combination of alleles as the parental chromosomes -This creates variation in the offspring regardless of random orientation

Methods have been developed for cloning adult animals using differentiated cells

-Once cells start to differentiate and embryos develop into a fetus and eventually an adult cloning becomes much more difficult -Therapeutic cloning is an example of cloning using differentiated cells -This type of cloning can be used to create a specific tissue or organ -Cloning using differentiated cells can also be used to reproduce organisms like dolly the sheep -This is done through somatic-cell nuclear transfer

PCR can be used to amplify small amounts of DNA

-PCR is a way of producing large quantites of a specific target sequence of DNA -It is useful when only a small amount of DNA is avaliable for testing -E.g. crime scene samples of blood, semen, tissue, hair, etc. -PCR occurs in a thermal cycler and involves a repeat procedure of 3 steps: 1. Denaturation: DNA sample is heated to separate it into two strands 2. Annealing: DNA primers attach to opposite ends of the target sequence 3. Elongation: A heat-tolerant DNA polymerase (Taq) copies the strands -One cycle of PCR yields two identical copies of the DNA sequence

Polygenic traits such as human height may also be influenced by environmental factors

-Phenotypic characteristics are not solely determined by genotype, but are also influenced by environmental factors -The added effect of environmental pressures functions to increase the variation seen for a particular trait -One example of a polygenic trait that is influenced by environmental factors is human height -Human height is controlled by multiple genes (polygenic), resulting in a bell-shaped spectrum of potential phenotypes -Environmental factors such as diet and health (disease) can further influence an individual human's height -Another example of a polygenic trait that is influenced by environmental factors is human skin colour -Skin colour is controlled by multiple melanin producing genes, but is also affected by factors such as sun exposure

Some prokaryotes have plasmids but eukaryotes do not

-Plasmids are small separate (usually circular) DNA molecules located in some prokaryotic cells -Plasmids are also naked (not associated with proteins) and are not needed for daily life processes in the cell -The genes in plasmids are often associated with antibiotic resistant and can be transferred from one bacterial cell to another -Plasmids are readily used by scientists to artificially transfer genes from one species to another (ie. Gene for human insulin)

Diagrams of stages of meiosis (meiosis I)

-Prophase - condensation of chromosomes -Metaphase - attachment of spindle microtubules -Anaphase - movement of chromosomes to poles -Telophase - decondensation of chromosomes

Identification of recombinants in crosses involving two linked genes

-Recombinants of linked genes are those combinations of genes not found in parents -For example, in a test cross of a heterozygous fruit fly (grey bodied, normal wings) with a homozygous recessive mutant (black bodied, vestigial wings), the recombinants would be the grey bodied, vestigial winged offsprings and the black bodied, normal winged offspring -Linked genes that have undergone recombination can be distinguished from unlinked genes via a test cross because the frequency of the recombinant genotypes will always be less than would occur for unlinked genes (crossing over does not happen every time) For example: Heterozygous test cross of unlinked genes = 1 : 1 : 1 : 1 phenotypic ratio Heterozygous test cross of linked genes = 1 : 1 : 0.1 : 01 phenotypic ratio (uncommon phenotypes are recombinants)

Using autodiography to measure DNA molecule

-Semi-conservative mode of replication of bacterial chromosome was also demonstrated by-J. Cairns -Using the technique of autoradiography Cairns first supplied the cells with suitable radioactive material like tritiated thymidine (H3-TdR) -H3 is heavy isotope of hydrogen and it replaces normal hydrogen in thymidine to give rise to tritiated thymidine) - used because this will selectively label only DNA and will not label RNA, since the thymine base is absent in RNA -The tritiated thymidine gets incorporated into DNA and replaces ordinary thymidine -The cellular material is then sectioned or else the cells may be broken down to release the intact bacterial chromosomes on slides -These slides are then covered by photographic emulsion and stored in dark -During this storage the particles emitted by tritiated thymidine will expose the film, which can be developed -This photograph will then show the regions of the presence of tritium and thus indirectly show the presence of labelled DNA -The results showed that autoradiographs from this replicating material prepared at regular known intervals demonstrated semi-conservative mode of replication -One of the two strands in the daughter DNA molecules is derived from the parent molecule and the other is newly synthesized

Sex determination

-Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex -The X and Y chromosome determine the sex of an individual -The X chromosome is quite large in comparison to the Y chromosome and has a centromere that is located near the centre or middle of the chromosome -The Y chromosome is relatively small with its centromere located near the end of the chromosome -If an individual has two X chromosomes they will be a female and if they have an X and a Y chromosome they will be a male -The X chromosome has many genes located on it essential to human development, while the Y chromosome has a small number of genes (some of these are shared with the X chromosome) -The rest of the genes on the Y chromosome are only necessary for male development

Fusion of gametes from different parents promotes genetic variation

-The fusion of two gametes to form a zygote is the start of a new organism and new life -It combines genetic information from two different individuals -Immeasurable possible different combinations of alleles in the zygote, due to different combinations that could exist in each gamete -Fusion of gametes from different parents thus promotes genetic variation

Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes

-These are patterns of inheritance where the ratios are different in males and females because the gene is located on the sex chromosomes -Generally, sex-linked diseases are on the X chromosome -Sex-linked inheritance for eye colour is observed and studied in Drosophila (fruit flies)

Techniques used for genome sequencing

-To sequence a genome, its first broken up into small DNA lengths - each is sequenced separately -To find base sequence of fragment DNA, single-stranded copies of it are made using DNA polymerase, but the process is stopped before whole base sequence has been copied by putting small quantities of non-standard nucleotide into reaction mixture -Its done separately with non-standard nucleotides carrying each of the possible 4 DNA bases - 4 samples of DNA copy of varying length are produced, each with 1 of 4 DNA bases at end of each copy -These samples are separated according to length by gel electrophoresis -For each number of nucleotides in the copy there is a band in just 1 of 4 tracks in gel, from which the sequence of bases in DNA can be deduced

The entire base sequence of human genes was sequenced in the Human Genome Project

-What they found: Most of the genome does not code for proteins (originally labeled "junk DNA") -Some of these regions consist of areas that can affect gene expression or are highly repetitive sequences called satellite DNA -Scientists can now also predict which sequences do code for protein (approximately 21000-23000 sequences)

Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I

-When Mendel first did his experiments on pea plants, he looked at the traits that were passed on from generation to generation -He did not know how the traits were inherited in terms of meiosis -We now know that independent assortment is an essential component in explaining how chromosomes align themselves during meiosis -It also explains how unlinked genes are passed on from generation to generation -As explained above, when homologues line up along the equatorial plate in metaphase I, the orientation of each pair of is random; meaning the maternal or paternal homologue can orient towards either pole -Also the orientation of how one set of homologues line up has no effect on how any of the other homologues line up -For example, if chromosome pair one is heterozygous for a certain trait, there is a 50% chance that the gamete will receive the dominant trait and a 50% chance that the gamete will receive the recessive trait -Also if chromosome pair five is heterozygous for a particular trait, again there is a 50% chance that the gamete will receive the dominant allele and a 50% chance that it will receive the recessive allele -Both of these homologues line up independently during meiosis and have no effect on which gamete the other alleles will end up in

Crossing over is the exchange of DNA material between non-sister homologous chromatids

-When chiasmata form between bivalents in prophase I, DNA can be exchanged between non-sister homologous chromatids -This exchange of genetic material is called crossing over and produces new allele combinations on the chromosomes -These chromosomes that consist of genetic material from both homologues are called recombinant chromosomes -Crossing over results in new combinations of alleles in haploid cells and thus increases the genetic diversity of potential offspring

Orientation of pairs of homologous chromosomes prior to separation is random

-When homologues line up along the equatorial plate in metaphase I, the orientation of each pair is random; meaning the maternal or paternal homologue can orient towards either pole -The two homologous chromosomes in each bivalent is attached to a different spindle fibre, randomly attaching them to either pole -The orientation of how one set of chromosomes lines up has no effect on the other bivalents (i.e. The bivalent formed for chromosome 1, does not affect how the bivalent for chromosome 2 will orient) -This means the number of combinations that can occur in the gamete is 2n (n=number of chromosome pairs) -Now when you consider there is the same number of possible combinations in the other gamete that it will combine with to form a zygote (random fertilization); the genetic possibilities are staggering -If one takes into consideration crossing over, which was explained above, the genetic variation possibilities in the offspring is immeasurable

Dominant allele

An allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state - the dominant allele masks the recessive in the heterozygous state

Recessive allele

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

Carrier

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

Causes of sickle cell anaemia

Cause of Sickle Cell Anaemia: -A base substitution mutation is the change of a single base in a sequence of DNA, resulting in a change to a single mRNA codon during transcription -In the case of sickle cell anaemia, the 6th codon for the beta chain of haemoglobin is changed from GAG to GTG (on the non-coding strand) -This causes a change in the mRNA codon (GAG to GUG), resulting in a single amino acid change of glutamic acid to valine (Glu to Val) -DNA: GAG to GTG (non-coding strand) -mRNA: GAG to GUG -Amino Acid: Glu to Val -The amino acid change alters the structure of haemoglobin, causing it to form fibrous, insoluble strands -This causes the red blood cell to adopt a sickle shape

Consequences of sickle cell anaemia

Consequences of Sickle Cell Anaemia: -The insoluble haemoglobin cannot effectively carry oxygen, causing individual to feel constantly tired -The sickle cells may accumulate in the capillaries and form clots, blocking blood supply to vital organs and causing several health problems -Also causes anaemia (low RBC count), as the sickle cells are destroyed more rapidly than normal red blood cells -Sickle cell anaemia occurs in individuals who have two copies of the codominant 'sickle cell' allele (i.e. homozygotes) -Heterozygous individuals have increased resistance to malaria due to the presence of a single 'sickle cell' allele (heterozygous advantage)

Inheritance of cystic fibrosis

Cystic Fibrosis: -Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7 -Individuals with cystic fibrosis produce mucus which is unusally thick and sticky -This mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts -Heterozygous carriers who possess one normal allele will not develop disease symptoms

Scientists attempt to assess the risks associated with genetically modified crops or livestock

Example: Maize introduced with a bacterial gene encoding a toxin to the European Corn Borer (i.e. Bt Corn) Potential Benefits: -Allows for the introduction of a characteristic that wasn't present within the gene pool (selective breeding could not have produced desired phenotype) -Results in increased productivity of food production (requires less land for comparable yield) -Less use of chemical pesticides, reducing the economic cost of farming -Can now grow in regions that, previously, may not have been viable (reduces need for deforestation) Potential Harmful Effects: -Could have currently unknown harmful effects (e.g. toxin may cause allergic reactions in a percentage of the population) -Accidental release of transgenic organism into the environment may result in competition with native plant species -Possibility of cross pollination (if gene crosses the species barrier and is introduced to weeds, may have a hard time controlling weed growth) -Reduces genetic variation / biodiversity (corn borer may play a crucial role in local ecosystem)

Mendel's law of independent assortment

Gregor Mendel was a 19th century Moravian monk who demonstrated that the inheritence of traits (i.e. genes) followed particular laws... Law of Segregation: -Each hereditary characteristic is controlled by two alleles, which segregate and pass into different reproductive cells (gametes) Law of Independent Assortment: -The separation of alleles for one gene will occur independently of the separation of alleles for another gene -According to the law of independent assortment, different allele combinations should always be equally possible -However this law only holds for genes that are on different chromosomes - the law of independent assortment does not apply to linked genes

Heterozygous

Having two different alleles of a gene

Homozygous

Having two identical alleles of a gene

The phenotypes of polygenic characteristics tend to show continuous variation

Human Skin Colour: -The colour of human skin is determined by the amount of dark pigment (melanin) it contains -At least four (possibly more) genes are involved in melanin production; for each gene one allele codes for melanin production, the other does not -The combination of melanin producing alleles determines the degree of pigmentation, leading to continuous variation Grain Colour in Wheat: -Wheat grains vary in colour from white to dark red, depending on the amount of red pigment they contain -Three genes control the colour and each gene has two alleles (one coding for red pigment, the other coding for no pigment) -The most frequent combinations have an equal number of 'pigment producing' and 'no pigment' alleles, whereas combinations of one extreme or the other are relatively rare -The overal pattern of inheritance shows continuous variation

Obtaining cells from a fetus

Pre-natal karyotyping is often used to: -Determine the gender of an unborn child (via identification of sex chromosomes) -Test for chromosomal abnormalities (e.g. aneuploidies resulting from non-disjunction) Amniocentesis: -A needle is inserted through the abdominal wall, into the amniotic cavity in the uterus, and a sample of amniotic fluid containing foetal cells is taken -It can be done at ~16th week of pregnancy, with a slight chance of miscarriage (~0.5%) Chorionic Villus Sampling: -A tube is inserted through the cervix and a tiny sample of the chorionic villi (contains foetal cells) from the placenta is taken -It can be done at ~ 11th week of pregnancy, with a slight risk of inducing miscarriage (~1%)

Many plant species and some animal species have natural methods of cloning

Natural cloning in named plant species: -When plants reproduce asexually by producing runners, the offspring are all natural clones of the parent plant: the genetic material comes from one place -Spider plants grow new plants, called plantlets, on their stems -Potato plants produce tubers (the part we eat), which can grow new roots and shoots -Strawberry plants grow stems called runners, which have plantlets on them Natural cloning in named animal species: -Example in homo sapiens - Monozygotic twins (identical twins) are formed when the zygote splits into two, and so such twins are natural clones -The unfertilized eggs of some animals (small invertebrates, worms, some species of fish, lizards and frogs) can develop into full-grown adults under certain environmental conditions -- usually a chemical stimulus of some kind - process is called parthenogenesis, and the offspring are clones of the females that laid the eggs

Co-dominant alleles

Pairs of alleles that both affect the phenotype when present in a heterozygote

Locus

Particular position on homologous chromosomes of a gene

Analysis of examples of DNA profiles

Paternity Testing: -Children inherit half of their alleles from each parent and thus should possess a combination of their parents alleles Forensic Investigation: -Suspect DNA should be a complete match with the sample taken from a crime scene if a conviction is to occur

Production of cloned embryos produced by somatic-cell nuclear transfer

Somatic Cell Nuclear Transfer (SCNT) is a method of reproductive cloning using differentiated animal cells: -A female animal (e.g. sheep) is treated with hormones (such as FSH) to stimulate the development of eggs -The nucleus from an egg cell is removed (enucleated), thereby removing the genetic information from the cell -The egg cell is fused with the nucleus from a somatic (body) cell of another sheep, making the egg cell diploid -An electric shock is delivered to stimulate the egg to divide, and once this process has begun the egg is implanted into the uterus of a surrogate -The developing embryo will have the same genetic material as the sheep that contributed the diploid nucleus, and thus be a clone

Genotype

Specific alleles of an organism (the gene expression)

Test cross

Testing a suspected heterozygous by crossing it with a known homozygous recessive

Phenotype

The observable characteristics or traits of an organism