Gregor Mendel's pea plant experiment

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Genetics

TAQ1

What are genes and chromosomes and what do they do?

Chromosomes are thread like structures that are made up of large molecules of DNA, located in the nucleus of all human cells. DNA carries the genetic information of the cell. Most human cells consists of 46 chromosomes arranged in 23 homologous pairs, one chromosome from each pair comes from the mother and one from the father, these cells are known as diploid. One pair of the chromosomes are sex chromosomes the rest are called autosomes. Gametes however only have 23 chromosomes and are known as haploid. Located on the chromosome are short strands of DNA called genes- genes are the units of inheritance, there are around 20,000 in each chromosome, genes provide the instructions for the manufacture of proteins by the cell and are responsible for all the characteristics which make up an organism. Each pair of chromosomes contains the same genes in the same sequence but there are alternate forms of the same gene which are called alleles, allele’s code for slightly different versions of the same characteristic. It is the alleles which cause the variation between individuals. Alleles can be dominant or recessive which means people can have the genetic instruction two traits for example blonde and brown hair but one trait- the dominant will mask the other, in this case brown is dominant and blonde is recessive.

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References

S-cool,2014. Inheritance [online] Available at: http://www.s-cool.co.uk/a-level/biology/genetics/revise-it/inheritance [Accessed 11 June 2014]

Twyman, R.,2003. The Human Genome Available [online] at: http://genome.wellcome.ac.uk/doc_WTD020778.html [Accessed 20 June 2014]

TAQ 2

Discuss the work of Austrian Monk Gregor Mendel

  1. In the late eighteenth century the Austrian monk Gregor Mendel developed the early theories of inheritability that would lead to the science of genetics. He experimented with various species of plants (particularly peas) to see how certain characteristics such as colour of flower or height were passed from one generation to the next. For example, he crossed peas with red flowers with peas with white flowers. Rather than producing a mixture of the two flowers e.g. pink he found that in the first generation all the flowers were red and none white. When the offspring were crossed however, the white examples (later to be called phenotypes) reappeared but at a ratio of one white flower to 3 red. From these results he concluded that the information carried within the egg and pollen, for determining flower colour (he likened it to a particle) i.e. it was either one or the other. This was the precursor for the discovery of genes. Each gene carries an instruction for flower colour (either red or white) and the red gene is stronger –more dominant than the white gene which is weaker in influence- recessive. Therefore, if the parental combination is red and white, then the red will always predominate. In later generations, the seed offspring may (1:4 chance) have two white flower genes and the phenotype will be a white flower.
  1. When looking at a single trait of inheritance Mendel devised a monohybrid cross to compare the trait amongst two organisms. Below is a punnet diagram illustrating the offspring of a homozygous non-tongue roller and a heterozygous roller, as you can see there is 50% chance of the offspring being heterozygous tongue-rollers and 50% homozygous recessive non- tongue roller. In the case of the heterozygous genotype the allele for roller is dominant over the recessive non-tongue roller allele.

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Genotype Phenotype Ratio 1:1

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rr non-tongue roller

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  1. The Punnett diagram shows the offspring of two heterozygous PCT taster parents, the results show a 3:1 ratio of PCT tasters to non-PCT tasters. The dominant trait is the allele for tasting the PCT and the recessive trait is for a non-taster. From the punnett diagram we can see that 2 of the children would be heterozygous so would therefore possess the dominant allele which would be expressed so they would be able to taste PCT, one of the children would be homozygous dominant so would also be a PCT taster and one child would be genetically homozygous recessive and therefore would be unable to taste.

Genotype Phenotype Ratio 3:1

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Tttaster

Tttaster

ttnon-taster

  1. Mendel also devised a test to determine the inheritance of two different characteristics; he called this the dihybrid test. The punnett square below looks at the characteristics of 16 children born of mother who is a homozygous tongue roller and a non-PTC taster and a father, who is heterozygous for the both traits. As the punnett clearly illustrates all 16 children will be tongue rollers as the dominant gene is expressed in all of the genotypes, however only 25% of the children will be PTC taster as 25% have inherited the homozygous-recessive allele for PCT tasting.

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Genotype Phenotype phenotypical ratio 3:1

RRTt roller/taster

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  1. Both parents are heterozygous for both traits, the below dihybrid cross shows that the ratio for phenotype is 9:3:3:1. Nine of the children would express both the dominant genes so would therefore express both of the traits, tongue rolling and PCT tasting. Three of the children would only have the dominant gene for tongue rolling and the recessive gene for PCT tasting so would therefore only express the one trait. Another three of the children would have the dominant gene for PCT tasting but only the recessive allele for tongue rolling and one of the children would be homozygous recessive for both of the traits.

Genotype Phenotype Phenotypical ratio 9:3:3:1

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RRttroller/non-taster

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rrTTnon-roller/taster

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References

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Biology mad, 2004. Genetics, Variations & Inheritance. [online] Available at: http://www.biologymad.com/master.html?http://www.biologymad.com/geneticsinheritance/geneticsinheritance.htm [Accessed 20 June 2014]

TAQ 3:

  1. Genes located on the same chromosome are linked together, in humans there are 22 autosomal linkage groups, an X chromosome linkage group and a Y linkage group. Genetic linkage is the tendency of two alleles located in close proximity on the same chromosome to be inherited together as an intact unit during meiosis, this contradicts Mendel’s principle of independent assortment which states that alleles of different genes will segregate independently into gametes- although genes located further apart on the chromosome can sort independently due to the process of crossing over, which will be explained later. Genetic linkage was unearthed when researchers William Bateson and Reginald Punnett noticed during experiments of alleles located on the same chromosome, that the phenotypical ratio was different to the expected results of a normal dihybrid cross, according to Mendelian genetics, based on these findings scientist came up with the hypothesis that some of the alleles must somehow be coupled with one another supporting the theory that alleles on the same chromosome do not segregate independently, this theory was later proved when Thomas Hunt Morgan carried out his famous fly room experiments. An example of autosomal linked genes would be red hair and freckles as the two genes are located in a similar place on the human genome so are therefore likely to be inherited together.
  1. Humans have 22 pairs of homologous chromosomes called autosomes these are not involved in gender determination; the 23rd pair of chromosomes are the sex chromosomes which are homologous in women but not in men. Gender is determined by the sex chromosomes X and Y, females have two X chromosomes (XX) and males have an X and a Y (XY). All humans are genetically pre-programmed to be female. Female gametes are homogulous so it is the male gamete, the sperm which determines the overall gender.

The Punnett diagram below shows how sex determination works, the female produces only one type of gamete so she is homozygous, the male however has sperm for both X and Y as he determines the sex, making him a heterozygote.

Male

X

Y

Female

X

XX

XY

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Ratio 1:1

  1. Crossing over (chiasma) of chromosomes is a process that occurs in prophase 1 of meiosis where homologous chromosomes pair and form synapses- this refers to intimate pairing of the two homologous chromosomes, during this phase segments of DNA are exchanged between non-sister chromatids of homologous chromosomes, resulting in genetic recombination. It is during meiosis that chromosomal cross over occurs, by mixing genes it leads to offspring having different gene combinations to their parent’s, therefore increasing genetic variability. The closer the two genes on a chromosome the less likely that a chiasma will form between them, the location of the gene on a chromosome is called a locus or loci (pleural), Chromosomes with recombinant combinations of linked genes are formed by the occurrence of crossing-over in the region between the two loci.

References

Biology mad, 2004. Genetics, Variations & Inheritance. [online] Available at: http://www.biologymad.com/master.html?http://www.biologymad.com/geneticsinheritance/geneticsinheritance.htm [Accessed 19 July 2014]

Boyle, M. and Senior, K. (2008), Human Biology Third Edition, Harper Collins Publishers, London.

Lobo, I. and Shaw, K. (2008), Discovery and Types of Genetic Linkage [online]. Available: http://www.nature.com/scitable/topicpage/discovery-and-types-of-genetic-linkage-500 [Accessed 12 June 2014]

Nature Education (2014), Some Genes are Transmitted to Offspring in Groups via the Phenomenon of Gene Linkage. Available: http://www.nature.com/scitable/topicpage/some-genes-are-transmitted-to-offspring-in-6524945 [Accessed: 12 June 2014]

Public Health Genetic Unit (2006), Genetics: Basic genomic concepts [online]. Available: http://www.healthknowledge.org.uk/public-health-textbook/disease-causation-diagnostic/2d-genetics/basic-genomic-concepts [Accessed 19 July 2014]

The Columbia Electronic Encyclopaedia (2012), Crossing Over [online]. Available: http://www.infoplease.com/encyclopedia/science/crossing-over.html [Accessed: 12 may 2014]

Twyman, R.,2003. The Human Genome Available [online] at: http://genome.wellcome.ac.uk/doc_WTD020778.html [Accessed 20 June 2014]

TAQ 4

Discontinuous variation is variation of the phenotype into either/or as discrete categories. For example, tongue rollers and non-tongue rollers, pct tasters and non-pct tasters. There is no in-between or intermediate state in phenotype. These are under control of small pairs of genes. Larger combination of genes gives rise to an infinite scale of intermediate phenotypes for example, height and other variables such as intelligence, memory capacity. These are terminal examples of continuous variation. In mathematics, there are discrete variables (either/or) memberships of sets, groups or categories and continuous variables such as length, height, distance, time, etc. Continuous variation usually occurs following a normal distribution and in theory, no two individuals possess the exact same measurements, because of the larger number of genes involved in continuous variation it is referred to a polygenic inheritance. Skin pigmentation is one such example.

TAQ 5

  1. The term mutation refers to interference in the chain or sequence of amino acids that go to make up the protein structure that, in turn goes to make up the DNA sequence in the genes. Since genetic sequencing requires protein reproduction, any discontinuity or fault is going to be reproduced and this fault may have serious consequences although occasionally they can be beneficial. Mutation occurs when cells reproduce either through meiosis or mitosis-whenever there is replication of amino-acid sequences. These can be dislocated or disrupted by all manner of external agents such as electromagnetic radiation e.g. UV light, chemicals such as teratogens. The generic term for agents that cause mutations is mutagen.
  1. The term de novo mutation literally means new mutation or at now mutation that takes place during cell division. The result is the development of a heritable disease such as a sex linked condition that occurs in the sperm or egg cell giving rise to conditions that include sickle cell anaemia. This condition occurs when in chromosome II, there is a substitution of the amino acid (number 7) of glutamine is replaced by valine. The consequence of this disrupted amino acid sequence is the inadequacy of haemoglobin, subsequently synthesised, to function properly in the transportation of oxygen. The allele that is responsible for sickle cell anaemia is described a autosomal incomplete dominant and a person receiving the defective gene from both parent develops the condition and a person that inherits one healthy and one defective allele becomes a carrier (remaining healthy themselves)
  1. The term mosaicism refers to the existence, within one body, of two sets of cells with different genotypes. Normally all cells within the body have the same genotype. Mosaicism associated with gamete cells is known as gonadal mosaicism and mosaicism associated with tissue cells is known as tissue specific mosaicism. Gonadal mosaicism also known as germaline mosaicism is thought to be a result of a mutation in early stem cell development that results in the growth of gonadad tissue. The result is the production of some but not all mutant gametes. This unknown and variable factor can make genetic prediction very difficult as some offspring may be affected, even for an illness that is dominant. An example of a condition that falls into this category is a version of klinefelters syndrome called 46/47 XY/XXY-indication that the XXY cells have 47 chromosomes as opposed to 46.
  1. Polymorphism is the presence of two or more allelic forms in a species and describes genetic changes that affect over 1% of the population. Anything over 1% is regarded as within the range of the normal distribution (of genetic variation) such polymorphisms are the cause of things such as blood group, eye colouration, hair colour and height. Polymorphisms, although not necessarily a risk factor in themselves, can influence the probability of the onset of certain diseases and influence drug responses.

References

Twyman, R., 2003. Mutation or polymorphism [online] Available at: http://genome.wellcome.ac.uk/doc_WTD020780.html [Accessed 08 July 2014]

National Human Genome Research Institute, 2011. Learning about Klinefelters syndrome [online] Available at: http://www.genome.gov/19519068 [Accessed 08 July 2014]

Simpkins, J and J.I. Williams, 1987.Advanced Human Biology.London: Unwin Hyman Limited

TAQ 5

  1. Protein synthesis refers to the manufacture of protein- the manufacture of protein molecules in and by the body. Protein molecules are very big, but their primary structured feature consists of four amino acid bases in the case of DNA. The sequence of these amino acid bases is critical and must be maintained in order that synthesis takes place without any mistakes. The DNA molecule is large and consists of nucleotide bases and it is arranged in a double helix. The DNA is contained within the nucleus of the cell.
  2. In order for the DNA molecule to reproduce itself, the double helix unzips itself by breaking the hydrogen bond that connects nucleotide bases to the two strands. The two strands then separate, then new appropriate bases are affixed to each loose end and a new base sequence and new strand is formed-identical to its template. The sequence continuous until the molecule replicates itself. The term gene refers to a certain length of base pair sequences that form a code that enables certain specialised proteins to be synthesised.
  3. Protein synthesis involves the action of ribonucleic acid (RNA) which differs slightly from DNA in that it contains the base uracil in place of thymine. During the process, there appear three types of RNA each are crucially involved in the construction process. They are messenger RNA, Transfer RNA and Ribosomal RNA. They in turn are involved in the construction process. Transcription is where the DNA unzips and places replicas of each strand are formed. Then there is translation where the messenger RNA now formed leaves the nucleus and migrates to the part of the cell where it is required (i.e. which organelle). This is usually in the ribosome. Within the ribosome the protein is rebuilt along the tRNA template via rRNA. The result is a replica protein molecule-identical to the formula dictated by the amino acid sequence in the original DNA chain.

References

BBC,2014.Biology RNA and Protein Synthesis [online] Available at: http://www.bbc.co.uk/bitesize/higher/biology/cell_biology/rna/revision/1/ [Accessed 24 July 2014]

Boyle, M. and Senior, K. (2008), Human Biology Third Edition, Harper Collins Publishers, London.

Simpkins, J and J.I. Williams, 1987.Advanced Human Biology.London: Unwin Hyman Limited