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Prokaryotes vs Eukaryotes DNA is packaged in to structures called chromosomes Eukaryotic chromosomes are linear strands 1 set = haploid 2 sets = diploid >2 sets = polyploid Prokaryotic chromosomes are circular Prokaryotic DNA considered to be “naked” as there is very little packaging
A Karyotype is the full complement of chromosomes in a particular species.
Humans have 22 pairs of homologous (matched) autosomes
We also have one pair of non-homologous (unmatched) sex chromosomes
Chromosomes differ with regard to their size, banding pattern and location of centromere.
The position of a particular gene on one of these chromosomes is referred to as its locus (plural: loci)
Other species Chromosome number is not an indication of the complexity of a species.
Sex Chromosomes The X chromosome may carry up to 1500 genes, the Y, only 300 and may be degenerating In humans the default sex is female Sex is determined by the male, via expression of the SRY gene located on the Y chromosome
Kleinfelter’s Syndrome Due to a mistake in meiosis, an individual inherits an extra X chromosome (47 XXY) Occurs in 1/650 males but is asymptomatic in 500 of these Extremely variable symptoms Small testes and reduced fertility Lanky build / youthful features Reading / language impairment Motor impairment Rounded body type Gynecomastia (increased breast tissue)
Turner’s Syndrome Due to a mistake in meiosis, an individual inherits an only one sex chromosome (45 X) Occurs in 1 in 2000-5000 Phenotypically female Short stature, broad chest Low hairline, low-set ears Webbed neck Undeveloped ovaries Diabetes, heart disease Vision problems Learning difficulties
Genes The number of genes varies between species, and as can be seen is not indicative of species complexity Other than humans, the above species are very useful for genetic studies, WHY? Answer = very short generational life cycles
There can be many more than 2 alleles for a particular gene, but an individual can only possess 2 of these.
Genotype Your genotype is a way of expressing the two alleles that you hold for a particular gene Human eye colour is controlled by one gene in particular, for which there are only 2 available alleles B – codes for phenotypically blue eyes (dominant) b – codes for phenotypically brown eyes (recessive) You need only 1 copy of a dominant allele for it to be expressed You need 2 copies of a recessive allele for it to be expressed BB = bb = Bb = Brown eyes blue eyes Brown eyes
Genotype - vocabulary When one possesses identical alleles on the maternal and paternal chromosome, this is referred to as a homozygous genotype. eg BB = homozygous dominant eg bb = homozygous recessive Having two different alleles is a heterozygous genotype. If you only have one copy of the chromosome in question (eg X or Y in males) and therefore only one copy of the gene, this is a hemizygous genotype eg SRY gene / male pattern baldness Some alleles can be co-dominant or display incomplete dominance
Carriers An individual with an unexpressed recessive allele is said to be a carrier for a trait. Even though they do not express this trait, if they have offspring with another individual who is also recessive for the trait, they may produces homozygous recessive offspring. eg. Albanism is a recessive trait, two parents who are carriers for the trait (Aa x Aa) can have an albino child (aa)
Multiple alleles & dominance There can be many more than 2 alleles for a particular gene, but an individual can only possess 2 of these. A good example is the ABO blood type system in humans. If blood type is represented as the letter i, capitalisation will represent dominance and lower case represents recessiveness. IA = type A blood IB = type B blood i = type O blood The following represents all combinations and results: Blood type A = genotypes IAIA & IAi Blood type B = genotypes IBIB & IBi Blood type AB = genotype IAIB Blood type O = genotype ii ? ? ? ?
Phenotype The expression of a gene is determined by the combination of dominant and recessive alleles possessed by the individual The ABO blood group system represents not only a gene with multiple alleles, but also a system of codominance (IA & IB are codominant and i is recessive) Phenotypic expression is not always visible, it can be physical, biochemical or physiological
Incomplete dominance Incomplete dominance is when two alleles are neither dominant nor recessive to each other The result is a phenotype that is a blend of the parents. Red flowers (RR) and white flowers (WW) will produce pink flowers (RW) RR RW WW
Co-dominance Co-dominance is when two equally dominant alleles are both expressed in the phenotype. eg. Some cattle can have red hair (RR) or white hair (WW), if these are crossed, the produce roan (RW) offspring. Roan is not a blend, these cattle have both completely red and completely white hairs
One or many genes Not all traits are controlled by a single gene, many are controlled by multiple genes. B O AB A Height in cm ABO Blood groups are controlled by a single gene and show discontinuous variation Height is controlled by multiple genes and shows continuous variation
Sex-linked traits Traits governed by genes that are located on a sex chromosome are said to be sex-linked, or more specifically x-linked or y-linked. An example is male pattern baldness (x-linked, recessive) This trait is quite uncommon in females as two copies of the allele are required (XbXb) With only one copy, they are an asymptomatic carrier (XbX) As males are hemizygous for genes on the X chromosome, a single copy results in expression of the trait (XbY)
Relationship between genotype & phenotype Blue and pink hydrangeas are genetically identical. In acidic soils (low pH) the flowers are blue In alkaline soils (high pH) the flowers are pink
PKU & Phenotype Phenylketonuria is a serious genetic disorder in humans resulting in very low production of the enzyme phe hydroxylase. Phenylketonurics cannot metabolise large quantities of the amino acid phenylalanine (phe). In their developmental years, if fed food containing large amounts of phe, this will result in mental impairment. If put on a low phe diet, development will be normal So same genotype, different phenotype Genotype: pp, Phenotype: PKU with mental impairment Genotype: pp, Phenotype: PKU with normal development
Siamese cats and temperature Precursor -> pigment Tyrosinase will only catalyse this reaction when temperature is lower than body temperature These cats are born white Pigmentation occurs in extremities due to lower temperature tyrosinase
Phenotypic Complexity Female cats and other species will sometimes display evidence of X-inactivation This is where phenotype is determined by the allele on one X in some cells and the other X in other cells. The tortoiseshell colouring is a product of the different expression within the follicle-producing cells in a XoX+ cat (Xo = orange, X+ = black). Male tortoiseshell cats (rare) will usually be XXY.
A new layer to phenotypic complexity The internal structure and organisation of DNA can also effect phenotypic expression. Eg. methyl groups or chromatin remodelling An identical mutation can have very different effects dependent on whether is on the maternal or paternal copy of the chromosome. Eg. Mutation on chromosome 15 at the q11.2-13 locus
Praeder-Willi Syndrome If mutation occurs on the paternal chromosome … The result is Praeder-Willi Syndrome Learning difficulties Insatiable hunger Behavioural problems
Angelman Syndrome If mutation occurs on the maternal chromosome … The result is Angelman Syndrome Severe learning difficulties Jerky movements Behavioural problems