Mendel’s experiments - the pea plant

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Human Biological Science 2 Genetics 

A gene is a subdivision of DNA consisting of a sequence of nucleotides which provides the codes to synthesize a protein. The four nucleotides are adenine, cytosine, guanine and thymine. A person's unique combination of genes is called a genotype. Each gene has encoded instructions enabling a cell to produce a specific product, such as an enzyme thus triggering a precise action. A persons genes decipher everything about them from their gender to their eye colour.

Chromosomes are thread-like molecules that contain hereditary information. They are an organized structures of nucleic acids and protein found in the nucleus of most living things which contain the organism’s genetic instructions, passed down through each generation. Most chromosomes are arranged in pairs inside the nucleus of a cell. Humans have 22 chromosome pairs, called autosomes and a pair of gematic cells which determines gender. Somatic cells contain 46 chromosomes but gametic cells contain only 23. Chromosomes made up of an estimate of 20,000 to 25,000 genes. They are key in cell division and ensure that DNA is accurately copied and distributed. Genetic birth defects are caused by faults in a part of or an entire chromosome, not a single gene.

Word count – 197

References

Live Science, 2013. Chromosomes: Definition & Structure. [Online] Available at: <http://www.livescience.com/27248-chromosomes.html>

MNT, 2014. What is a gene? What are genes? [Online] Available at: <http://www.medicalnewstoday.com/articles/120574.php>

TAQ2

Gregor Mendel was an Austrian monk who discovered the basic principles of heredity. He began to research the transmission of hereditary traits in plant hybrids in 1854. Mendel cross pollinated strains of peas with obvious contrasts over a long period of time to find out how genes are passed from the parent to the offspring. Mendel found that certain traits are passed down to the young without blending. For example when crossbreeding a pee plant with a purple flower and one with a white flower the offspring either flowers purple or white but never intermittent colouring. The most accepted theory at the time was that inherited traits blend. Mendel learnt that the inheritance of each characteristic is determined by genes that are passed on to the offspring without any change. An individual inherits a trait like this from each parent and a trait can still be passed on to the subsequent generation without showing in the first offspring. This is Mendel’s first law, the law of segregation. Mendel's second law, the law of independent assortment states that states that one of the inherited traits will be dominant and the other recessive. Although Mendel's research was with plants, the main principles of heredity also apply to people and other animals as they are essentially the same for all complex life forms.

Word count - 220

The ability to roll your tongue is a dominant gene therefore children with heterozygous pairs of genes can roll their tongues. The punnet diagram bellow shows that when one parent is a homozygous non-tongue roller and the other carries a heterozygous pair of genes 50% of children would be heterozygous tongue rollers and 50% would be homozygous non tongue rollers. The heterozygous rollers would still carry the non-tongue roller gene, it wouldn’t be shown but can be passed down to future generations.

A

a

a

Aa

aa

a

Aa

aa

Key

A – Gene for the ability to roll tongue (dominant)

a – Gene for inability to roll tongue (recessive)

Word count- 108

The ability to taste Phenylthiocarbamate (PTC) is a dominant gene therefore children with heterozygous pairs of genes can taste PTC. The punnet diagram bellow shows that when both parents carry a heterozygous pair of genes 75% of children would be able to taste PTC and 25% wouldn’t. 50% of the children able to taste PTC would be heterozygous gene carriers whilst 25% of would be homozygous unable to taste PTC.

B

b

B

BB

Bb

b

Bb

bb

Key

B – Gene for the ability to taste Phenylthiocarbamate (dominant)

b – Gene for inability to taste Phenylthiocarbamate (recessive)

Word count – 96

If one parent was a homozygous tongue roller and a heterozygous PTC taster and the other parent was heterozygous for both traits and they had sixteen children all offspring would be able to roll their tongues and twelve of their children would be able to taste PTC. Six of the children would be homozygous tongue rollers with the other six being heterozygous tongue rollers. Four of the children would be homozygous PTC tasters, eight of them would be heterozygous PTC tasters and four would be unable to taste PTC. Two of the children would be homozygous tongue roller and PTC tasters. Four of the children would be homozygous tongue rollers and heterozygous PTC tasters. Two of the children would be homozygous tongue roller and non PTC tasters. Two of the children would be heterozygous tongue rollers and homozygous PTC tasters. Four of the children would be heterozygous for both traits and two of the children would be heterozygous tongue rollers and non PTC tasters.

Word count – 164

If two parents where heterozygous of both traits then there are 9 distinct possible genotypes. 75% of their offspring would be able to roll their tongue and 75% of their offspring would be able to taste PTC. 6.3% of their children wouldn’t be able to roll their tongue or be able to taste PTC and 62.5% would be able to do both.

AB

Ab

aB

ab

AB

AA BB

AA Bb

Aa BB

Aa Bb

Ab

AA Bb

AA bb

Aa Bb

Aa bb

aB

Aa Bb

Aa Bb

aa BB

aa Bb

ab

Aa Bb

Aa bb

aa Bb

aa bb

Key

A – Gene for the ability to roll tongue (dominant)

a – Gene for inability to roll tongue (recessive)

B – Gene for the ability to taste Phenylthiocarbamate (dominant)

b – Gene for inability to taste Phenylthiocarbamate (recessive)

Word count – 137

References

Palomar Education, 2013. Mendel’s Genetics. [Online] Available at: <http://anthro.palomar.edu/mendel/mendel_1.htm>

The free dictionary, 2010. Mendel’s Law. [Online] Available at: <http://www.thefreedictionary.com/Mendel's+law>

TAQ3

Genetic linkage is the tendency of alleles that are close proximity on one chromosome to be inherited together during meiosis. For example if the genes for blond hair and blue eyes are close together on the genetic strand they will be more likely to be inherited together. However this does contradict Mendel’s laws of genetics as he stated that assortment was independent many scientists have questioned how Mendel did not come across genetic linkage in his studies. When genes are closely linked, the alleles often stay together separated only by breakage or cross-over during meiosis. The linkage breaks down when cross over occurs in meiosis. The further apart the alleles are on the chromosome the more likely a cross over is to separate them. Genes other recombine less frequently the closer they are together. Therefore the frequency in which genes recombine is used to determine their positions on the chromosome enabling the construction of genetic maps. Linkage is important as the presence of linkage can complicate the evolutionary behaviour of genes in populations. Genetic linkage is important in the transmission of characteristics as it ensures certain characteristics remain together during cross over and assortment.

Word count – 194

Sex is determined by the X and Y chromosomes called heterosomes due to them being non-homologous unlike autosomes. Female carry (XX) non-homologous whilst males carry (XY) in humans whilst in some animals this can be the other way round. Female gametes contain an X chromosome, while the male gametes can contain either a single X or a single Y chromosome. Therefore the Sex is determined by the sperm.

Male Gametes

Female Gametes

X

X

X

XX

XX

Y

XY

XY

Hesteromes carry gene for other characteristics as well as gender this is known as being sex linked.

Men and women can inherit X-linked genes however Y-linked genes can only be inherited by men. X-linked recessive traits are mainly expressed as phenotype unless they are related to feminine body characteristics. Genes within the chromosome that aren’t gender coding are normally expressed as a male phenotype weather they are recessive or dominant as there aren’t normally any alleles on the Y chromosome. A recessive allele on the X chromosome can be masked in their phenotype by a dominant allele on the other in a female. Which is why women are sometimes carriers of X-linked traits that aren’t expressed in their phenotypes. For example Retinitis Pigmentosa is a disease lined to the Y chromosome and can only be inherited by males.

Word count – 218

The combination of genes we inherit and our physical traits are dictated by genetic recombination.

During the first stage of meiosis chromosomes pair up and exchange a distal portion of DNA this is known as homologous recombination. Crossover is when two homologous parts of the same chromosome can break and reconnect to different end piece. If they break at the same place or locus in the sequence of base pairs, the result is an exchange of genes, called genetic recombination. The gene may duplicate and replace another if they break in different places this is called unequal crossover. Chromosomes can be lost during cell division if chromosomes break on either side of a centromere and re-join excluding it. Each gamete represents a shuffled combination of DNA which is why other than identical twins no two people are not the same. A double, reciprocal cross-over is when two linear dsDNA molecules, a double, nonreciprocal cross-over occurs between two linear dsDNA molecules. The two most common types of genetic recombination are site specific and homologous recombination. Site-specific recombination is when a DNA sequences is recognized by enzymes that are able to recognize DNA sequence and catalyze combination with recipient DNA.

Homologous recombination involves DNA sequences that are nearly identical.

Word count - 207

References

Science Daily, 2009. Chromosomal crossover. [Online] Available at: <http://www.sciencedaily.com/articles/c/chromosomal_crossover.htm>

Book rags, 2014. Linkage and Meiotic Crossing Over - Research Article from World of Genetics. (Online) Available at: <http://www.bookrags.com/research/linkage-and-meiotic-crossing-over-wog/#gsc.tab=0>

NCBI, 2000. The Cell: A Molecular Approach. 2nd edition. [Online] Available at: <http://www.ncbi.nlm.nih.gov/books/NBK9944/>

Live Strong, 2013. Y linked genetic diseases. [Online] Available at: <http://www.livestrong.com/article/74388-y-linked-genetic-diseases/>

Bio Basis, 2012. Sex linked genes. [Online] Available at: <http://anthro.palomar.edu/biobasis/bio_4.htm>

Biology Mad, 2004. Genetics, Inheritance and Variation. [Online] Available at: <http://www.biologymad.com/master.html?http://www.biologymad.com/geneticsinheritance/geneticsinheritance.htm>

TAQ4

Discontinuous variation is when individuals fall into a number of distinct categories. Some examples could be; blood groups, finger prints, eye colour, hair colour, gender, shoe size, skin colour and the ability to roll your tongue. It is controlled by alleles of a single or a small number of genes and is not effected by the environment.

Continuous variation is encountered more often than discontinuous variation it entails a complete range of data from one extreme to another. This type of variation shows an unbroken range of phenotypes some examples could be; height, weight, heart rate, finger length. Continuous variation is effected by polygenic inheritance (a combination of many genes) as well as environmental factors. When phenotypic frequencies are plotted as a graph they show as a bell-shaped distribution. There isn’t a one-to-one correspondence of genotype and phenotype in continuous distributions. Which is why there is less known about the specific types of genes responsible for traits that class as continuous variation

Word count – 162

References

Biology Mad, 1999. Continuous and discontinuous variation. (Online) Available at: <http://www.biologymad.com/resources/RevisionM5Ch2.pdf>

NCBI, 2000. An Introduction to Genetic Analysis. 7th edition. [Online] Available at: <http://www.ncbi.nlm.nih.gov/books/NBK22007/>

TAQ5

A gene mutation is a permanent change to DNA sequence within a gene. Mutations can range from a single DNA base to a large segment of a chromosome. They can either be inherited or acquired. Mutations arise due to a variety of factors. Some are the result of a spontaneous events during replication, and are known as spontaneous mutations. Other external factors such as exposure to certain chemicals, or ultraviolet radiation, can cause changes to DNA. These external agents that cause mutation are called mutagens. Mutagens can cause alterations in molecular structure of nucleotides causing substitutions, insertions, and removal within the DNA sequence.

Word count – 103

A de novo mutation is an alteration in a gene that is not passed on through the parents. The gene is mutated for the first time and has not been present in a family member before. The type of mutation is a result of an alteration in a germ cell of one of the parents or in a fertilized egg. The mutation is acquired by chance whilst DNA is replicated and passed on to offspring. Most newly formed embryos acquire about 2 de novo mutations which can cause disease due to the disruption of genes coding. De novo mutations contributes to the development of autism in families with just one child on the autistic spectrum. This also explains why advanced parental age at conception can increase the risk of autism spectrum disorder to the child as studies suggest de novo mutations become more common with age.

Word count - 146

Mosaicism is a condition where an individual’s cell is composed of two genetically different types. The condition is hereditary and affects any type of cell. It is caused by cell division in the utero and can sometimes be treated dependent on the type of genetic condition caused. The blood, skin, and reproductive cells are most likely to be affected by mosaicism. It can cause a variety of syndromes and is diagnosed by karyotyping or chromosome analysis. The pairing or number of chromosomes in a person with the condition can be abnormal. Turner syndrome is a condition caused by mosaicism that affects females as a result of the absence of the normal xx chromosome pair. A person with turner syndrome will be missing either all or some of an X chromosome causing short height, failure to menstruate, and a broad flat chest.

Word count – 141

References

Wisegeek, 2014. What Is Mosaicism? [Online] Available at: <http://www.wisegeek.com/what-is-mosaicism.htm>

A polymorphism is a variation in the DNA sequence which is common in the population. In order to be classified as a polymorphism, the least common allele must be shown in at least 1% of the population or higher. Anything lower is classified as a mutation. Some variations are more common than others because

Scitable, 2014. DNA Is Constantly Changing through the Process of Mutation. [Online] Available at: <http://www.nature.com/scitable/topicpage/dna-is-constantly-changing-through-the-process-6524898>

About Education, 2014. Gene Mutation. [Online] Available at: <http://biology.about.com/od/basicgenetics/ss/gene-mutation.htm>

Autism speaks, 2011. De Novo Genetic Changes Provide New Clues for Autism. [Online] Available at: <http://www.autismspeaks.org/science/science-news/top-ten-lists/2011/de-novo-genetic-changes-provide-new-clues-autism>

Childhood disability, 2010. DE NOVO MUTATIONS IN SYNAPSE-RELATED GENES SHOW IMPORTANCE IN CAUSING AUTISM SPECTRUM DISORDERS AND SCHIZOPHRENIA [Online] Available at: <http://www.childhooddisability.ca/articles/de-novo-mutations-in-synapse-related-genes-show-importance-in-causing-autism-spectrum-disorders-and-schizophrenia/>

PLOS genetics, 2014. De Novo Mutations in Moderate or Severe Intellectual Disability. [Online] Available at: <http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1004772>

TAQ6

Protein synthesis is the production of a protein via an individual cell. The process is split up into two stages; transcription and translation. Transcription takes place inside the nucleus and it begins before the synthesis of proteins. The RNA molecule is produced via RNA transcription. The RNA polymerase synthesizes messenger RNA using one strand of the DNA double helix as a template. The messenger RNA begins to mutate whilst traveling to the cytoplasm and the non-coding sequences are removed. After this point translation begin. Translation takes place on the ribosomes inside of the cytoplasm, or the ribosome found on the rough Endoplasmic Reticulum. During translation the ribosome binds to the messenger RNA. Then goes on to elongate when complexes made up of amino acids linked to transfer RNA which successively binds to the appropriate codon thus forming complimentary base pairs with transfer RNA anticodon. The ribosome then travels codon to codon adding amino acids and translating them into polypeptidic sequences. Finally a release factor binds to the ribosome in order to discontinue the codon, releasing a complete polypeptide from the ribosome and terminating translation. The process of protein synthesis is not a constant occurrence within the cell. It happens in intervals followed by periods of genetic stillness. The cell regulates and controls this process. Gene expression can be controlled within several levels of the cell for example there is normally a period of genetic silence whilst mitosis takes place the chromatin tightens when inactive and this compaction regulates access to the gene.

Word count - 252

References

Access Excellence, 2012. Protein Synthesis. [Online] Available at: <http://www.accessexcellence.org/RC/VL/GG/protein_synthesis.php>

Cliffs notes, 2014. Protein Synthesis. [Online] Available at: <http://www.cliffsnotes.com/sciences/biology/biology/gene-expression-molecular-genetics/protein-synthesis>

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