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Genetic Variations of Inheritance

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Published: Tue, 05 Jun 2018

  • Star Daganskaia-Knighton

Genetics

TAQ1: What are genes and chromosomes and what do they do?

A gene is a unit of DNA needed to make a protein, they range between hundreds and millions of base pairs. These pairs control development and switch on and off depending on environmental factors, an example is a gene switching on when an immune cell meets a bacterium to produce antibodies to destroy it. DNA is what we are, each person has their own set of DNA which makes up their characteristics, except for identical twins who share the same DNA. The human genome is composed of approximately 3 billion bases of DNA. DNA is formed into structures called chromosomes, each cell containing 23 pairs of chromosomes – one set from each parent. The double helix coils around histone proteins, they wind together and lock into densely coiled chromatin. Each single gene encodes a protein and each of those proteins results in a distinct or inherited trait e.g. attached (dominant) or free hanging earlobes (recessive). Allele are a similar gene for eye colour, these are also recessive or dominant. The dominant gene is an allele for brown eyes so to inherit one or two alleles for brown eyes means you will have brown eyes whereas an allele for blue eyes is recessive so a person must inherit two copies of allele for blue eyes to have blue eyes.

TAQ2: Discuss the work of the Austrian Monk Gregor Mendel

1. Mendel was a Monk with a love for science, he discovered the laws of inheritance by working with pea plants. His work on pea plants went over eight years, with this he deducted that genes come in pairs and and are then inherited – one pair from each parent. He monitored the look of the offspring from each pea plant and tracked which genes were dominant or recessive (he could track this by the look of the leaves, seeds, taste). Unfortunately his results were not understood or appreciated until 1900.

Mendel had three rules of law:

  1. Law of segregation – two copies of each heredity factors segregate during the production of gametes so that one factor from each parent can be acquired by the offspring.
  2. Law of independent assortment – this is the law of chance, it is by chance which characteristics in particular will occur in the individual offspring.
  3. Law of dominance and recessiveness – one factor in the pair of traits will dominate the gene and become inherited unless both genes are recessive.

2. Punnett diagram of the recessive homozygous (non-toungue rolling father) and dominant heterozygous (tongue rolling mother). If they were to have four children then two of them would be able to roll their tongues and two would not, with their genetics ½ or 50% of their children would be able to roll their tongues. This is the genotype – the organisms genetic make up which is a combination of alleles. The phenotype is the physical characteristic you can see e.g. eye colour, earlobes, hair type (curly straight etc.) and tongue rolling.

Mother>

Father/

T

t

t

Tt

tt

t

Tt

tt

3. Phenylthiocarbamate (PTC) is a chemical that some people find incredibly bitter to taste and others can’t taste at all. There is a difference in taste thresh hold between different populations and ages, the ability to taste the chemical comes from the dominant gene. Below is a Punnett diagram showing that if both parents are hetrozygous then there is ¼ or 25% chance of having a child who can taste PTC. This means if they had 4 children, three of them may not be able to taste PTC. Mendalian law explains monohybrid is inheritance of one through two generations.

Mother >

Father /

F

f

F

FF

Ff

f

fF

ff

4.

Mother >

Father /

TF

Tf

tF

tf

Tf

TTFf

TTff

TtFf

Ttff

This Punnett diagram shows that if the mother is a homozgous tongue roller and a non PTC taster marries a man who is a heterozygous tongue roller and is heterozygous PTC taster and then goes on to have 16 children; all of the children will be tongue rollers as it is the dominant gene show in the punnett diagram as a ‘T’. 8 of those 16 children (50%) will be PTC tasters as the dominant gene (F) is carried on to half of their children.

Mendalian law explains dihybrid as inheritance two through two generations.

5.

Mother>

Father /

TF

Tf

tF

tf

TF

TTFF

TTFf

TtFF

TtFf

Tf

TTFf

TTff

TtFf

Ttff

tF

TtFF

TtFf

TtFF

ttFf

tf

TtFf

Ttff

ttFf

ttff

This shows the dehybrid cross and the independent segregation of traits, proving Mendel’s second law. This is also known as Phenotype distribution. This punnett diagram explains that if a mother and father who are both heterozygous tongue rollers and PTC tasters have 16 children they will come out as a 9:3:3:1 ratio of traits. 9 children will be TTFF dominant, 3 children will be Tff, 3 children will be ttF and 1 child will be a double recessive ttff meaning they are unable to taste PTC and are unable to roll their tongues.

TAQ3:

1. Explain genetic linkage and why it is important in the transmission of genetic characteristics.

Genetic linkage was discovered when Thomas Hunt Morgan did experiments of fruit flies and notices a difference in eye colour dependant on the sex of the fruit fly. Genetic linkage is the process in which if two genes are close to each other on a chromosome they are frequently inherited together during meiosis. In many cases two alleles inherited from one parent tend to stay together and is the same for the other parent; this is called linkage. Two alleles make up an autosome, so two associated genes on a chromosome would be hair and eye colour. These two genes are close to each other on the genetic strand (locus) so are unlikely to cross over and mix, the most likely outcome is that one parent will have the dominant gene and give child X brown eyes and blond hair instead of red hair (recessive gene) and blue eyes (recessive). This does now contradict Mendel’s law of ‘independent assortment’ and begs the question ‘how did Mendel not encounter linkage?’.

2. Explain how sex or gender is determined.

Autosomes are the first 22 chromosomes that decide everything but your gender, the 23rd chromosome pair will either decide you to be female (XX) or male (XY). The mother will always pass on the X chromosome and the father will either pass on a X chromosome in the sperm creating a female baby or will pass on a Y chromosome creating a male baby.

Mother /

Father >

X

Y

X

XX

XX

X

XY

XY

The Punnett diagram above shows that there is a 50% chance that the couple will have a boy or a girl, this is the same theory as to when you flip a coin over and over again you will eventually have even results. Of course some families will have more boys than girls or more girls than boys but when the numbers are counted world wide the male:female ratio is fairly even.

  1. Explain the crossing over of chromosomes and the role it plays in transmission of genetic characteristics.

Chromosomal crossover is the process of two chromosomes that have paired up during prophase 1 of meiosis exchange DNA. This happens when two homologous chromosomes break and reconnect at the same place and reconnecting with the other chromosome, if they break at the same base pair they exchange alleles; this is called genetic recombination. If too few chromosomal crossovers are formed gametes end up with the wrong number of chromosomes, this can cause infertility, pregnancy miscarriage and chromosomal disease (Downs Syndrome). Shuffling the DNA is the best thing for the next generation as it accounts for genetic variation so the ‘offspring’ will have a different set of alleles and genes to their parents. Chromosomal crossover means that people will look different to each other (apart form identical twins), diseases can be eradicated, evolution, being able to adapt and natural selection. Hemophilia is a blood clotting disease that is inherited, this means that if the carrier of the disease has a child then their child may inherit the gene. Hemophilia usually occurs in males but there are the rare exceptions. A Punnett diagram below shows how the disease can be inherited. Females are carriers with rare exceptions of getting the disease. Males are effected from mildly to severely. The red chromosome is the one effected, spreading.

Mother>

Father /

X

X

X

XX

XX

Y

XY

XY

TAQ 4: Using examples, explain the difference between continuous and discontinuous variations.

Depending on the features of an organism they will show continuous and discontinuous variation. A continuous variation is something that can change gradually over a period of time such as height, weight and foot length. These goes from one extreme to the other, if you plotted the height of new born babies the variation could be from 30cm to 55cm depending a variation of factors from genes, environmental factors during gestation and whether or not they were born premature or full-term. Continuous variation is the combined effect of many genes know as polygenic inheritance, this is also effected by the environment.

Discontinuous variation isn’t effected much by environmental factors, it is when a person is or isn’t part of a group, there is no in-between. They are either one blood type of the other, they are either male of female, they have either blue, green or brown eyes. Of course some people are born with indeterminate sex and others have have heterochromia (different eye colours, often inherited from parent or from injury to the eye).

TAQ 5:

Define the term mutation and explain how it is caused.

The body constantly needs new cells; a mutation occurs when stands of DNA are separated and replicated, each strand becomes a double strand, eventually a mistake occurs during the copying process (1 in 100,000,000). When damage does occur the bodies cells will repair the damage, however if DNA breaks the body is more likely to make mistakes trying to fix the problem , these mistakes can shorten lifespan. Environmental mutations are caused by chemicals, radiation and ultraviolet light, they are all enemies of DNA. They attack and damage by swapping parts of the DNA which becomes a problem when they start to replicate meaning more harmful DNA is replicating with chemicals that don’t behave.

Describe de novo mutations and provide one example.

A de novo mutation is a new mutation of a cell. Gene mutations are either inherited or acquired during a lifetime. Mutations passed from parent to child are hereditary or germline mutations as they are in the sperm or egg, this mutation will be in in every cell in the persons body and lasting a lifetime. De novo mutations that occur after fertilization explain genetic disorders which effect the child’s every cell but has no family history of the disorder. Somatic (acquired) mutations in DNA that occur in individual cell some point in a persons life are environmentally caused by radiation, chemical or by mistakes being made during cell division. Acquired mutations cannot be passed on to the next generation. De novo mutations can cause Autism when there is no parental link to the disorder. Schizophrenia has been linked to de novo mutation in the paternal germ line from older fathers. Hemophilia is also an inherited disease but a third of cases are caused by gene mutation, this is when the blood doesn’t clot and lowers the blood clotting factor levels for coagulation meaning someone will bleed for longer.

Describe mosaicism and provide one example.

Mosaicism happened when the egg (zygote) starts to divide after fertilization, new cells form and duplicate so there are enough chromosomes. Errors occur and a cell ends up with a different number of chromosomes, every time that cell duplicates it will have a different chromosomal number. Having more than one type of chromosomal make-up is called mosaiciam, this means that an extra cell will have trisomy 21 (chromosome 21 dictates Downs Syndrome), others will have the right amount of chromosomes. Mosaic Downs Syndrome is detected either during pregnancy or after birth via a karyotype test (a photograph of the chromosomes from one cell – using skin, blood, bone marrow or amniotic fluid), chromosomes in 20 cells are counted and if two or more are normal (without the chromosome 21) the baby is said to have mosaic downs syndrome.

Describe polymorphism and provide one example

Polymorphism is a word that come from the Greek language meaning Many (poly) Form (Morph). Polymorphism is discontinuous genetic variations where two or more forms exist in the same species of a population such as blood type (A, B, AB, O) and sexual dismorphism (male or female), height cannot be a polymorphism as this is a continuous variation. Polymorphism is seen in every species (dogs: there are hundreds of breeds of dog that look completely different but they are still the same species) and counts towards natural selection e.g. some may be able to reproduce to a higher degree than others.

TAQ 6: Protein synthesis and the two main stages explained.

Protein synthesis is formed of two steps that use protein (amino acid). For every three base pairs of DNA codes there is one amino acid , in total the body makes 20 different amino acids, other come from the diet. This process rewrites base sections of DNA and creates proteins, it all takes place on the ribosome in the cytoplasm or the the rough Endoplasmic Reticulum.

The two main stages of protein synthesis are Transcription and Translation.

Transcription starts by the DNA double helix unwinding, this shows the single stranded DNA. One of these side of the DNA will act as a template for the formation of mRNA (messenger ribonucleic acid). In this process the matching RNA nucleotides base pairs join to form a strand of mRNA, mRNA detaches from the DNA to move out of the nucleus through openings called nucleus pores and into the cytoplasm. Translation starts when the mRNA is drawn towards the ribosome which has two parts; a small and large subunit used as different binding sites. mRNA binds to the small ribosome subunit which instructs a strand of tRNA (transfer RNA) to find a place (specific codon sequence) to to bind the mRNA, they attach together holding the amino acid. The larger subunit come to complete the structure, ribosome surrounding the strands of RNA and have another strand of tRNA carrying amino acid different to the first. Again they bind in the specific codon sequence. These to amino acids bind with help from cellular energy and the ribosome to form adenosine triphosphate (ATP). This process is repeated, the codons and anti-codons match up to form base pairs which then create a 3D shape meaning the protein (polypeptide) is complete.

Reference List

Genes (27/08/14) [Online] <http://www.bbc.co.uk/schools/gcsebitesize/science/21c/genes/genetic_diseasesrev1.shtml> (27/08/14)

Cell division (27/08/14) [Online] <http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/celldivision/celldivision1.shtml> (27/08/14)

Gregor Mendel (27/08/14) [Online] <http://www.dnalc.org/view/16151-Biography-1-Gregor-Mendel-1822-1884-.html> (27/08/14)

Genetics (27/08/14) [Online] <http://www.bbc.co.uk/bitesize/higher/biology/genetics_adaptation/dihybrid_cross/revision/1/> (27/08/14)

Hemophilia (28/08/14) [Online] <http://www.nhlbi.nih.gov/health/health-topics/topics/hemophilia/> (28/08/14)

Linkage (28/08/14) [Online] <http://www.nature.com/scitable/definition/linkage-51> (28/08/14)

Crossover (28/08/14) [Online] <http://www.sciencedaily.com/articles/c/chromosomal_crossover.htm> (28/08/14)

Variation (29/08/14) [Online] <http://www.bbc.co.uk/bitesize/ks3/science/organisms_behaviour_health/variation_classification/revision/3/> (29/08/14)

Mutation (29/08/14) [Online] <http://science.howstuffworks.com/life/genetic/dna-mutation.htm> (29/08/14)

Mosaicism (29/08/14) [Online] <http://www.theupsideofdowns.org/down-syndrome/what-is-mosaicism> (29/008/14)

Polymorphism (01/09/14) [Online] <http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Polymorphisms.html> (01/09/14)

Protein synthesis (01/09/14) [Online] <http://biology.about.com/od/cellularprocesses/ss/protein-synthesis-translation_2.htm> (01/09/14)

BBC Protein Synthesis (01/09/14) [Online] <http://www.bbc.co.uk/bitesize/higher/biology/cell_biology/rna/revision/2/> (01/09/14)


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