VARIATION AND INHERITANCE.

All human beings have some general outlines. However human beings difference from one another in some things such as for example, height, weight, eye colour… In what we know as individual variation.

This variation is determined by the inherited characteristics that we obtain from our parents through inheritance (the transmission of genetic information from generation to generation) but also it is determined by acquired characteristics that can be changed such as for example a scar. To continue we are going to see inherited characteristics.

The study of how inherited characteristics are passed from one generation to another is called genetics and the study of the effect of the environment on genes is called Epigenetics.

These inherited characteristics depend on proteins that the cell manufactures and the instructions of how to manufacture these proteins is in the form of genes in the chromosome. So we can say that genes are sections of DNA that code for a protein.

DNA is formed by Deoxyribonucleotides which are the base of DNA and which are made up of nitrogen+ deoxiribose+ fosforicacid and a phosphate backbone. A molecule of DNA can have millions of nucleotides although there are only 4 different ones because of the base they have:

A (adenine)- T (Thymine)

G (Guanine)- C (cytosine)

These nucleotides join together forming pairs that are always of A with T or G and C. Like this they form a ladder that later on twists forming a double helix.

In the synthesis of proteins, after this takes place replication occurs. For this the double helix opens and each template is to produce a new one. So each template produces its complementary so at the end you have 2 identical molecules of DNA. This occurs during the S period in Interphase.

After this transcription occurs and it consists to pass DNA to mRNA as the DNA cannot leave the nucleus but we need DNA to create proteins in the Ribosomes.

mRNA is different from DNA as it is made of Ribonucleotides instead of Deoxyribonucleotides. I mean mRNA has a base of nitrogen+ ribose+ phosphoric acid. Another difference is the base as here the bases are A, G, C and U (uracil). Moreover RNA is not a double helix but just one thread.

So for transcribing DNA into mRNA we just need to take one of threads of the double helix and turn it into RNA. Like this:

-AATTGGCCGA- DNA

-UUAACCGGCU- mRNA

There are 3 types of RNA:

mRNA (RNA messenger) tRNA (transfer RNA) It carries Amino acids to the right position rRNA (ribosomic RNA) It forms part of the structure of the ribosome.

Once this is done the mRNA leaves the nucleus through the pores of the nuclear membrane and goes to the Ribosomes were mRNA is translated into amino acids, the basic unit of protein. For this the ribosome divides RNA into triplets and each of these triplets’ codes for an amino acid. Like these proteins are formed, all proteins start with metamine –AUG- and ends with AAA meaning the end of the protein. This process only takes place during interphase as in mitosis DNA is super coiled.

So the formation of proteins occurs thanks to these processes:

DNA-mRNA-Protein-Characteristic.

Replication-Transcription-Translation

How I said genetics studied how these characteristics are passed. The father of genetics was Mendel. He was an Austrian monk who lived in XIX century. He crossed garden peas so he collected lots of data and created a study using percentages.

With this he created patterns of inheritance so his work was very important although it was unknown. In the 1900 3 researchers met the same conclusions as Mendel but they discovered that this work had already been done. They said that the father of genetics was Mendel and so Mendel’s work became worldly known.

Although Mendel and Darwin lived in the same age they did not know each other. If they have met the history of science would have improved a lot. Mendel did not know about genes as they were discovered in the year 1953 so he talked about hereditary factors. However we are going to see his work by using genes.

There are 3 types of inheritance:

Dominant-recessive.

In this type of inheritance always one character dominates over the other. So that when an individual has heterozygous homologous chromosomes (each chromosome contains different information for the same trait) only the dominant trait is shown. So only the dominant phenotype is shown meanwhile the recessive information remains hidden and only is shown in reproduction. For this type of inheritance, Mendel created 3 laws:

1. Principle of uniformity or Dominance.

It states that when 2 homocygotic organisms (that have the same information in the homologous chromosomes) for 2 opposing trades are crossed the offspring will be hybrid but will only exhibit the dominant allele (the different alternatives for a gene)

So the 100% of the filial generation will have Dd so all the individuals of the filial generation will be heterozygotic. Although they will only show the phenotype D. This principle is used for breading animals so that when we want to know if an animal has a determinate allele we cross it with a homocygotic recessive. If the animal does not have it (because it is homozygous) all the phenotypes will be the same meanwhile if the animal has the allele (because it is heterozygous) the phenotypes will be different.

2. Principle of segregation.

It states that during the formation of gametes, the 2 alleles of the hybrid for the same characters do not fuse but separate independently.

So you obtain 25% RR, Rr 50% and rr 25%. So the proportion of the genotypes is 1:2:1. However when we talk about the phenotype it is 75% red and 25% white. So the phenotype that was not shown in the previous generation is now shown.

3. Principle of independent assortment or combination.

We cross 2 individuals that are hybrid for 2 traits on separate chromosomes. This law states that during gamete formation, the alleles for one trait (eg: height ) are not inherited along with the alleles for the other trait. (eg color).

Ex: yellow: A Green: a A>a

Smooth: B Rough: b B>b

All this possible individuals of the second filial generation are shown in the Punnet Square. So that when this situation takes place we will always hace a proportion of 9:3:3:1; 9 yellow and smooth, 3 yellow and rough, 3 green and smooth, 1 green and rough. However this situation only occurs when the 2 characters are on different homologous chromosomes.

CODOMINANCE

In this type of inheritance we have more than 2 alleles and 2 of those alleles have the same power so that both of them can be shown. For example this is what happens whit blood groups:

Traits: IA,IB,I0 IA=IB>I0

So an individual can show an A group phenotype if it has IA and IA or if it has IA and I0; to the B group if it has IB and IB or if it has IB or I 0; to the AB group if it has IA and IB or to the 0 group if it has I0 and I0.

These blood types mean that if you have Group A blood your red blood cells create antigens against blood of B type. If you have B type blood, you have antigens for A blood. If you belong to the AB group you have both and if you have blood 0 you do not have antigens.

Theoretically this means that if you have AB group you can receive blood from anyone and that if you have 0 type blood you can give blood to anyone. However the blood apart of being AB, it can be positive or negative. It is positive if it has the Rhesus factor (a protein) if not, it is negative. Positive bloods attack negative bloods. This is a problem for women who are positive and have a negative baby in their second pregnancy.

INTERMIDIATE INHERITANCE

In this type of inheritance you have blended phenotypes. This occurs when the alleles are not very powerful. So in this type of inheritance the genotypes remain the same but the phenotypes change as the heterozygotic homologous chromosomes show a mixture of trait.

Inheritance linked to sex.

This type of inheritance affects genes that go on sexual chromosomes also known as heterochromosomes. Heterochromosomes determinate the sex of the individual and the sexual characteristics of each sex. However they are not considered homologous chromosomes as the Y chromosome does not have the upper part so that they are partially homologous. Sexual chromosomes are also responsible of colour blindness and haemophilia.

Haemophilia affects blood coagulation. So people that suffer this are known as Haemophilic. This disease is caused by the factor VIII (represented as H) which is a protein that is determinate by the X chromosome of women. So women can be normal if they have (XHXH), carrier (XHXh) or affected (XhXh) although this last situation does not occur normally as when the embryo has this configuration normally a natural abortion takes place. Men can be: affected (XhY) or Normal (XHY). So we can say that there are only haemophilic men.

Daltonism or colour blindness is also a disease that affects the X chromosomes. In it people cannot differentiate colour or are partially blind to green and red. It mainly affects men as men cannot be carrier as they only have one X chromosome.

Inherited diseases.

Albinism. People are not able to synthesis Melanine which is a pigment of the skin. So that people that suffer this do not have colour in the skin, neither on the eyes not in the hair. It is produced by a recessive allele and normally albinos have problems to see and skin cancer in the case of Africa where they are prosecuted.

Sickle cell anaemia is an endemic illness that takes place in the North of Africa and people with this disease have strange shaped red blood cells that do not carry on well oxygen. It is caused by a protozoon called Plasmodium and it is transmitted by a mosquito which is a vector. People that have SS are normal, people that have Ss have some symptoms but they can go on living and people that have ss die. The good part of this disease is that people that have Ss are

protected against malaria and survive anaemia. So Ss people formed up the majority of people that survive. However, the bad thing is that the gene is passed.

Huntington’s chorea: It is a faulty allele that appears through mutation so not everyone has this allele. It is a degenerating disease that appears in middle age people as it affects the nervous system like Parkinson. The dominant alleles is the one that causes the disease so people with the allele normally select the embryo so that it is not passed.

Cystic fibrosis: It is an alteration formed up by a recessive allele. This allele produces lots of mucus so it blocks the respiratory apparatus, the reproductive and the digestive system. You have problems for respiration and for reproduction. It has a treatment to ease the symptoms by using antibiotics or physiotherapy. However it cannot be cured.

Polydactyly: An alteration that makes you have and extra finger and toe, It is formed by a dominant allele.

Mutations.

A mutation is a change in the sequence of DNA or in the structure or number of chromosomes. If they affect DNA we call them genetic mutations. Meanwhile the mutations that affect chromosomes can affect autosomes or heterochromosomes. If they affect autosomes they can cause:

Dawn Syndrome (it is a trisomy of the 21st chromosome) It implies mental retardation, congenital diseases that affect the heart, a fold in the eyelid…

Edward’s syndrome (it is a trisomy of the 18th chromosome) It implies microcephaly and alterations in some organs.

Patau’s syndrome (it is a trisomy of the 13th chromosome) It implies polydactyly and mental retardation.

If they affect heterochromosomes it can cause for example the duplo Y syndrome which is a man that has XYY; the klinefelter’s syndrome, a man that has XXY; the turner’s syndrome, a women that only has X and the triple X syndrome, XXX.