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Dominant Genes and Recessive Genes

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  • Gemma Tue

TAQ 1:

Genes are sections of DNA (deoxyribose nucleic acid). They are shorter than actual DNA strands and each one has code for a specific protein. It does this in setting the order the amino acids must join together. DNA carries the genetic coding which will give living things there characteristics. DNA is unique except for that of identical twins. Chromosomes are contained in a cell’s nucleus and are made from longer DNA molecules. The genes we possess are carried in our chromosomes and we get half from the female and half from the male who conceive us. Genes are just single copies while chromosomes come in pairs. Each person has 23 pairs of chromosomes which gives a total of 46 individual ones. One set out of those 23 pairs determines your sex when you are conceived while the other 22 sets determine many of the other characteristics such as eye colour and hair colour. Not everything has the same amount of chromosome pairs. Chimpanzees have 24 pairs and fruit flies only have 4 pairs. But one certain fact anything that is living or has grown like fruit and trees have chromosomes.

TAQ 2:

1.)

Gregor Mendel was a biologist and Monk. His work on heredity became the basis of the modern theory of genetics, he developed two of the best known genetic terms; dominant and recessive. Mendel worked mostly with plants but discovered the principles applied to people and animals due to the mechanisms of heredity as they are mainly the same for all complex life forms. Mendel did all his work with peas but concluded with 3 principles of inheritance. Fundamental theory of heredity; this is the passing of genes from the parents to offspring. Mendel wasn’t aware of genes but now that we are we know that a pure pea plant line is homozygote which means it has 2 identical copies of the same allele, whereas a cross bred pea plant is heterozygote so it has 2 different alleles. His next principle was segregation which is when in reproduction the inherited factors (alleles) are separated into reproductive cells. He discovered allowing the peas to self-pollinate resulted in offspring unlike their parents. During meiosis is when the alleles segregate into individual reproductive cells like the sperm and egg. The final principle is independent assortment. Mendel discovered this with peas that had more than one trait crossed as the progeny didn’t match the parent always. This is due to some traits being inherited independently.

2.)

In a family where one parent has the dominant R gene and is homozygous – meaning both their genes are the R gene and the other parent is also homozygous even though they possess both the recessive genes r, the child should have a 100% chance of being able to roll there tongue as every one of the four options has a leading R gene.

 

R

R

r

Rr

Rr

r

Rr

Rr

In a family where one of the parents has two r recessive genes and is homozygous but the other parent has one R gene and is heterozygous the child has a 50% chance in being able to roll there tongue as shown below.

 

R

r

r

Rr

rr

r

Rr

rr

3.)

In a family where both parents are heterozygous meaning they are Rr for Phenylthiocarbamide (PTC) which is the dominant R gene, they have the potential for their children to have a 75% chance of being heterozygous and being able to taste it as they will possess Rr genes. This leaves a 25% chance of the child not being able to taste it as the child will be homozygous and have two rr recessive genes. One of the parents may be rR however it is always written as Rr. Phenylthiocarbamide is a bitter taste to people who possess the Rr genes while to the rr gene holders it is tasteless.

 

R

r

R

Rr

Rr

r

Rr

rr

4.)

A woman that is a homozygous tongue roller RR and a homozygous non-tasting PTC pp marries a man that is a heterozygous tongue roller Rr and a heterozygous tasting PTC Pp can have children that have a variety of different combinations. The Punnet table below shows the parent’s genes around the outside and on the inside the combinations it creates for the children.

 

Rp

Rp

Rp

Rp

RP

1. RRPp

2. RRPp

3. RRPp

4. RRPp

Rp

5. RRpp

6. RRpp

7. RRpp

8. RRpp

Pr

9. RPrp

10. RPrp

11. RPrp

12. RPrp

pr

13. Rppr

14. Rppr

15. Rppr

16. Rppr

The table below shows the children and what each of them will be able to do.

Child

Tongue Roller

PTC Taster

1

RR – Yes

Pp - Yes

2

RR – Yes

Pp - Yes

3

RR – Yes

Pp - Yes

4

RR – Yes

Pp - Yes

5

RR - Yes

pp - No

6

RR - Yes

pp - No

7

RR - Yes

pp - No

8

RR - Yes

pp - No

9

Rr – Yes

Pp - Yes

10

Rr – Yes

Pp - Yes

11

Rr – Yes

Pp - Yes

12

Rr – Yes

Pp - Yes

13

Rr – Yes

pp - No

14

Rr – Yes

pp - No

15

Rr – Yes

pp - No

16

Rr – Yes

pp - No

Every child out of the 16 should be able to roll there tongue as 8 of the genes are dominant RR homozygous and the other 8 are dominant Rr heterozygous. 8 of the children should be able to taste PTC as all 8 genes are heterozygous dominant Pp genes and the other 8 children won’t be able to taste PTC as the genes are recessive pp homozygous.

5.)

If child 10 who is heterozygous for both tongue rolling Rr and PTC tasting Pp marries someone who is also heterozygous for both tongue rolling Rr and PTC tasting Pp they have the potential to end up with the following;

 

Rp

Rp

Pr

rp

Rp

1.

RRpp

2. RRpp

3. RPrp

4. Rrpp

Rp

5. RRpp

6. RRpp

7. RPrp

8. Rrpp

Pr

9. RPrp

10. RPrp

11. PPrr

12. Prrp

rp

13. Rprp

14. Rprp

15. Prrp

16. rrpp

The above punnet table shows the combinations of the children these 2 people would have. Below shows what each child is likely to inherit.

Child

Tongue Roller

PTC Taster

1

RR – Yes

pp - No

2

RR – Yes

pp - No

3

Rr – Yes

Pp - Yes

4

Rr – Yes

pp - No

5

RR - Yes

pp - No

6

RR - Yes

pp - No

7

Rr - Yes

Pp - Yes

8

Rr - Yes

pp - No

9

Rr – Yes

Pp - Yes

10

Rr – Yes

Pp - Yes

11

rr – No

PP - Yes

12

rr – No

Pp - Yes

13

Rr – Yes

pp - No

14

Rr – Yes

pp - No

15

rr – No

Pp - Yes

16

rr – No

pp - No

As can be seen from the table 12 out of the 16 children become tongue rollers with 8 being heterozygous Rr and 4 being homozygous RR gene holders. The 4 that cannot roll their tongues are rr homozygous non tongue rollers. With the PTC tasting out of the 16 children 7 will be able to taste the PTC with 9 not being able to taste it. All 7 that can taste it are heterozygous as the got Pp genes. This is because both parents were Pp heterozygous as well.

TAQ 3:

1.)

Genetic linkage is when two genes which are located closely together on a chromosome become inherited together. This was proven by Thomas Hunt Morgan when he realised that a fruit flies eye colour and sex were related. The closer the genes are together on a chromosome the more frequently they will be inherited together are. As expected the genes that are further apart on the chromosome as less likely to be inherited together. They are prone to being separated during recombination where the DNA is recombined during meiosis. These genes are physically close on the chromosome so they are physically linked. They are inherited together not independently so also they are genetically linked. However sometimes these genes can become unlinked, during meiosis which is the gamete formation process homologous chromosomes swap parts. This is recombination. This generates new combinations of the previously linked alleles. So the further away they are on the chromosome the more likely recombination will happen. Homologous chromosomes are chromosome pairs. One of these chromosomes come from each parent. They are similar in length, centromere location and gene positioning.

2.)

Sex chromosomes appear as XX or YY. When the sex cells form these pairs separate. Females carry XX and male carry XY. The normal egg cells produced by a human ovary have X cells while sperm carry X in half and Y in the other half. It’s the sperm which decides what the babies’ sex is. If the X sperm makes the egg the baby will be a girl and if the Y sperm meets the egg it will be a boy. It is important to state that we are discussing the conception of a child through the natural process of a male sperm meeting a female egg. In the age we live in it is now possible to choose or alter the sex of a child through various processes. The Y gene doesn’t contain many other things other than the sex gene however the X gene contains thousands of different genes for various inheritance features. It has been indicated that the X chromosome have a longer lifespan than the male chromosome Y. however the Y sperm have higher motility and use energy faster than the X.

3.)

The process of crossing over ends up forming new linkage groups. In the early stages of meiosis the homologous pairs of chromosomes line up along side one another so parts of the corresponding chromosomes break off and re-join, it’s that part that is the process of crossing over. Crossing over happens when the genetic linkage chromosomes are far apart from each other. If they genes on the same chromosome were close together it would be genetic linkage however when they are further apart and split off and then re-join it becomes crossing over. Crossing over occurs frequently during propase I of meiosis. This is when homologous chromosomes associate with each other in a tetrad structure. Meiosis is the process of nuclear division in a living cell. The number of chromosomes is reduced to half the original number and it will only occur in the process of gametogenesis – which is when the sex cells are being formed. Meiosis is necessary to prevent the doubling of the chromosome numbers in each generation. A normal body cell is a diploid meaning it has two of each type of chromosome, these are the homologous chromosomes. However the sperm or ovum cells are haploid which means it only contains a single chromosome of each type.

TAQ 4:

The height of a person is an example of continuous variation. There is one shortest person in the world and one tallest person in the world. Between these two any variation on height is possible which makes this continuous variation. Essentially it is a characteristic that can change in a person; height, weight, foot size, waist size etc. discontinuous variation is something that will not change in a person but also that the amount or type of the characteristic will not change. Gender is an example. Naturally (surgery exempt) a person’s gender will not change just like there are only two possibilities for gender male and female. Blood types are also included in this. A blood type will never change in a person it will always remain the same. This is discontinuous variation, just as there will only ever be four blood types; A, B, AB or O.

TAQ 5:

1.)

A mutation is a natural process changing the DNA sequence. Mutations can be one small part of DNA but others can be a large segment of chromosome. Gene mutations are either inherited or acquired. Germline or hereditary mutations are those passed from a parent through the sperm or egg. De Novo mutations are ones which only occur in egg or sperm cells or that occur just after fertilization. The acquired mutations can be caused by environmental factors like UV radiation or if DNA copies itself during cell division and a mistake is made. These types of mutation cannot be passed on unless they are in the sperm or egg cells.

2.)

De Novo mutations are ones which only occur in egg / sperm cells or occur just after fertilization. These mutations will affect the child but the parents can be unaffected. The parents can live not only totally unaffected but also totally unaware of the gene being carried by them. Sickle Cell Anaemia is one such condition. In order for the child of get sickle cell both parents need to carry the gene, both parents need to have it or one parent needs to have it. Sickle Cell is where the shape of the red blood cells instead of being flexible and smooth it’s rigid and inflexible. They are also shaped like crescents where as normal RBC’s are disk shaped. They block blood vessels due to there shape and cause blockages in tissue and organs. These episodes are called sickle cell crisis. It is not curable however is well research and treatable. The main aim is ensuring comfort to patients.

3.)

Mosaicism is a situation when cells divide during growth in the embryo, the individual will have some cells with mutation and some without genetic change. Cells affected are not always egg or sperm cells but also blood and skin cells. It can be hard to predict symptoms or if the child will have any issues, however sometimes it can be less severe if the child has both normal and abnormal cells. Down syndrome is a de novo mutation. Down syndrome children typically have shorter life expectancies, typically have other health issues, some level of learning disability and a range of physical features. Down syndrome is caused by nondisjunction which is when a pair of chromosomes fail to separate during egg/ sperm formation. Due to this extra copy of the chromosome – number 21 there are now three copies which makes it difficult for the cell to control how much protein is made.

4.)

Polymorphism is effectively how our DNA differs from person to person. One of the main differences as well as similarities in people is there hair colour. The genes that control hair colour are polymorphic. Hair colour comes about because of a pigment called melanin. There are two types of melanin; eumelanin which makes hair dark and phomelanin which makes it lighter. On top of that there are three factors; how much of the pigment there is, to what degree the eumelanin or phenomelanin is present and how close together the melanin granules are. Each parent gives the child four hair colour genes making 8 (four from each parent). Hair genes are not dominant or recessive they are either on or off. The higher amount of on genes the darker the child’s hair. If a mother has OOOO (large O being on) and a father has OOoo (o being off) the child will end up with darker hair.

TAQ 6:

1.)

Protein synthesis is the process of making proteins by using information provided in the DNA and occurs on the ribosomes in the cytoplasm. Or on the rough endoplasmic reticulum (ER).

2.)

The process of protein synthesis occurs in two steps; transcription and translation.

3.)

RNA is a type of nucleic acid called ribonucleic acid. There are two types the first messenger RNA (mRNA) is formed in the nucleus. It rewrites the sequence of bases of a DNA section which is the process of transcription. The second type of RNA is transfer RNA (tRNA) found in the cytoplasm. The tRNA picks specific amino acids from the cytoplasm and positions them on the ribosomes surface. They are then joined together in a certain order of make a specific protein. This is the translation process. The process begins in the cell’s nucleus, enzymes unwind the DNA section so that it can be read and accessed. A RNA copy is then made and it will move to the cytoplasm. Inside the cytoplasm it will bring all the components together to create the actual protein synthesis these include amino acids, transport RNAs and ribosomes. The protein polymers are synthesized in the cytoplasm. This is known as protein biosynthesis. The mRNA then waits for the tRNA to essentially feed it amino acids. Slowly more and more all slightly differing are added onto the chain. This continues until there are uncoupled codon sequences on the mRDA. Most living cells synthesize hundreds of proteins every second.

Bibliography:

BBC. (2014).Genes & Inheritance.Available: http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel/classification_inheritance/genesandinheritancerev1.shtml. Last accessed 13th August 2014.

BBC. (2014).RNA & Protein Synthesis.Available: http://www.bbc.co.uk/bitesize/higher/biology/cell_biology/rna/revision/2/. Last accessed 15th August 2014.

Scitable. (2014).Linkage.Available: http://www.nature.com/scitable/definition/linkage-51. Last accessed 31st August 2014.

Shmoop. (2014).The Theme of Genetic Linkage in Genes.Available: http://www.shmoop.com/genetics/genetic-linkages.html. Last accessed 31st August 2014.

BBC. (2014).Determination of Gender.Available: http://www.bbc.co.uk/schools/gcsebitesize/science/21c_pre_2011/genetics/genesinheritencerev3.shtml. Last accessed 31st August 2014.


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