EDEN COLLEGE, DURBANEDEN COLLEGE, DURBANLIFE SCIENCES GRADE 12GRADE 12

2A INTRODUCTIONGenetics is the study of heredity and deals with the similarities and differences between parents and their offspring.D.N.A, forms the core of all chromosomes.

A number of consecutive nitrogenous base pairs of the D.N.A. forms a gene.It is the unit of heredity that usually controls one characteristic of an individual (e.g., eye colour, hair colour, skin colour, height, etc.)Meiosis occurs in the sex cells of the sex organs in the formation of male and female gametes.It occurs so that sexual reproduction can take place. Sexual reproduction involves the fusion of the haploid male and female gametes to form a new cell, the zygote (which is diploid). E.g., the fusion of human sperm and ovumWhen the fusion of male and female gametes occurs, a zygote with 23 pairs (46) of homologous chromosomes results. Meiosis ensures that each gamete has 23 chromosomes, so that a zygote will always have 23 pairs of chromosomesTherefore, for example, in the 20th chromosome pair there could be the gene for eye colour, and as you can see from the drawing, the gene of the paternal chromosome codes for brown eye colour, while the gene of the maternal chromosome codes for blue eye colour.

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FERTILISATION

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In humans each of the 23 pairs of homologous chromosomes has a specific shape. and are numbered accordingly, for example, the 20th chromosome will be recognised in each and every body cell as having the same shape, AND the same genes. Each and. every 1 of the millions of cells in our bodies contains 23 pairs of homologous chromosomes. The one of the pair of homologous chromosomes is paternal (comes from the male sperm cell or gamete) and the other one of the pair is maternal (comes from the female egg cell or gamete)This is because the diploid zygote undergoes mitosis (cell replication) many times, to produce millions of cells with identical chromosomes.

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4Although you have the genes for eye colour in the homologous pair (20th), in every body cell, this gene will only work or be “switched on” in the iris of the eye, where eye colour is needed. Depending on the situation, it may be only the one gene of the homologous pair that is switched on. If it is the gene in the paternal chromosome that is switched on, then it will make the cells of the iris produce brown eye pigment. The other gene (in the maternal chromosome) may code for blue eye colour, but it is not switched on and so the person will have brown eyes.

Let us summarise what we have learnt so far: A cell with a double number of chromosomes (23 pairs in humans) is

diploid (2n) and is called a somatic (body) cell A sex cell undergoes meiosis and becomes haploid (n), 23 chromosomes in

humans

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5 Each body cell in humans has 23 pairs of chromosomes. These pairs of

chromosomes are called homologous pairs. One chromosome of a homologous pair of chromosomes comes from your

father (paternal chromosome) and the other from your mother (maternal). Homologous chromosome pairs have the same shape

Each characteristic in our bodies is coded for by a pair of genes, one on the paternal chromosome, and the other in an identical position on the maternal chromosome

The two genes on homologous chromosomes that code for the same characteristic are found on identical locations on the pair of chromosomes, called loci (singular: locus)

Alleles are alternate forms of a gene located on the same locus of homologous chromosomes A specific characteristic is always governed by the same allelic pair on the same chromosome in every cell of that type of organism.

All organisms of the same species will have the same number of chromosomes and the same genetic makeup. We cell this the genome (or genetic map) of that species

Only one gene of the pair will “switch on” and code for the synthesis of a specific protein. For example, gene number 121 of the paternal chromosome may make brown pigment, while the maternal chromosome, coding for blue protein in the iris, will not function, so the person has brown eyes.

It seems that sometimes, one gene dominates the other of the pair. We say that the one gene is dominant, while the one that is dominated, is called the recessive gene

1 This number is for understanding and is not necessarily the correct one.

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6Other organisms have different numbers of chromosomes. Why? Well, it depends on the complexity of the organism, and how many different proteins in the organism. The more complex an organism, the more different proteins it has, and the more chromosomes: Onion cells have 8 pairs Drosophila (fruit flies) have 4 pairs Mice have 20 pairs of chromosomes Garden peas have 7 pairs Most laws and discoveries in genetics have been made using garden peas (in the earlier times) and the fruit fly. This is because they have simple nuclei and characteristics, and they reproduce very quicklyIn genetics, we use letters to represent genesWe use a capital letter to represent the dominant gene, and a small letter for the recessive gene, for example:B = the gene coding for brown eyes.b = the gene coding for blue eyesB is dominant over b

The individual above has? Yes!, Brown eyes. The letters Bb indicate to us the GENOTYPE for eye colour, that is, its genetic makeup.The characteristics that we can see in an individual, for example, brown eyes, is known as the PHENOTYPE.An individual’s genotype may be: BB, which means they still have

brown eyes, or it may be bb, which means they have blue eyes (these is nothing to dominate two recessive genes)

A genotype Bb is called HETEROZYGOUS (or hybrid). Here the paired genes (ALLELES) for a particular trait (characteristic) are different.

A genotype BB or bb is HOMOZYGOUS. Here the paired genes (ALLELES) for a particular trait (characteristic) are identical

Genetics

Female gametes

♀

7Let us do a few interesting genetic combinations. Remember genetics involves hereditary characteristics, what you inherit from your parents, and so sexual reproduction and meiosis are involved:If for example

marries P1 (first parental generation) Bb x bb

(Brown-eyed male) (blue-eyed female)

Meiosis I Meiosis I B b b b

Meiosis II Meiosis II

B B b b b b b b

Male gametes (sperm) Female gametes (egg cells) Two types: B and b One type b b Fertilisation Possibilities

Zygotes Bb bb bb Bb

We would like to find out what ratio of brown-eyed children and blue-eyed children could occur. This is called the phenotypic ratio (we can see above that 2 will have brown eyes, 2 blue eyes)The way we did the fertilisation above can be complicated, with many arrows, so instead we use an easier way:

We use what is called a PUNNET SQUARE Male gametes ♂ (symbol for male)

From the punnet square, we can calculate the genotypic ratios in four squares, TWO have the genotype Bb, and TWO have the genotype bb Therefore we would expect half to be Bb and half bb (2Bb:2bb), or the

ratio could be stated as 1Bb:1bb Phenotype : Half are Brown-eyed, half are blue-eyed (1:1)

Genetics

GAMETES B b

b Bb bb

b Bb bb

8 Genotype : half are heterozygous brown (Bb) and half are

homozygous blue (bb): 1Bb:1Bb Here is an interesting problem:

Why can two brown-eyed parents have a blue-eyed child?

P1 Bb x Bb P1

Meiosis

B or b B or b ratio of gametesFertilisationF1

1 homozygous brown (BB)2 heterozygous brown (Bb) 1 homozygous blue (bb)

(F1 = first filial generation, in other words, the possible types of eye-colour of children the parents may have)The PHENOTYPIC RATIO: 3 brown-eyed child:1 blue-eyed childThe GENOTYPIC RATIO: 1 homozygous brown (BB): 2 heterozygous brown (Bb): 1 homozygous

blue (bb)

1BB:2Bb:1bb or 25%BB:50%Bb:25%bbB MENDEL’S EXPERIMENTS

Gregor Mendel, an Austrian monk, in the 17th Century (1600’s), performed breeding experiments on garden peas. He obtained pure-breeding plant seeds, e.g., pure-breeding tall, short plants, pea plants which bear red flowers, pure-breeding plants breeding plants bearing white flowers, etc.

By crossbreeding, he could follow particular characteristics from one generation to the next. He formulated a number of laws because Of this:

MENDEL’S FIRST LAW – THE LAW OF DOMINANCE AND SEGREGATIONMendel found that by crossing two pure-breeding plants of contrasting characteristics (e.g., tallness and shortness) the first hybrid (heterozygous) generation resembled the parent generation with the dominant gene

e.g., Tall Plants x Short PlantsGenetics

GAMETES B b

B

b

9Let T = gene for tallnessLet t = gene for shortness (note: you must use the same letter for a

characteristic)P1 TT x tt P1 (crossed 2 homozygous plants)

T T t t (Gametes)FertilisationF1 (Punnet square)

All the offspring of F1 will be Tt (heterozygous tall)

Thus, Mendel said that when two characteristics meet in an individual, one dominates over the other, called the recessive

(LAW OF DOMINANCE AND RECESSIVENESS).

MENDEL’S FIRST LAW – THE LAW OF SEGREGATIONMendel took the offspring from F1 (Tt) and crossed them

P2 Tt x Tt P2 (2nd Parental generation) Meiosis

T t T t gametesFertilisation

F2

1 homozygous tall (TT)

2 heterozygous Tall (Tt)1 homozygous short (tt)

When the hybrid (or heterozygous) plants produce gametes, the characteristics for tall and short segregate (during meiosis), so that

Genetics

GAMETES t t

T

T

GAMETES T t

T

t

10half the gametes have the factor for tallness, and the other half of the gametes have the factor for shortness. Remember, we only represent 2 types of gamete, although 4 are formed, 2 for l’s) each characteristic (this is Mendel’s LAW OF SEGREGATION).

MENDEL’S SECOND LAW – THE LAW OF INDEPENDENT ASSORTMENT

Mendel stated that different pairs of genes segregate independently of the members of other pairs, when two or more characteristics (traits) are involved in a dihybrid cross. This means that each characteristic separates on its own during meiosis.

That is, the biological selection of a particular gene in the gene pair for one trait to be passed to the offspring has nothing to do with the selection of the gene for any other trait. More precisely the law states that alleles of different genes assort independently of one another during gamete formation.

Mendel performed similar experiments using other contrasting pairs of characteristics, e.g., red flowers crossed with white, plants producing yellow seeds with plants producing green seeds.In each case, one of the two characteristics was dominant, and the F1 hybrids all showing the dominant characteristic.

GAMETES t t

T

T

As seen above, when the F1 hybrids are crossed, the recessive characteristic will appear in 25% of cases in the F2 generation.

GAMETES T t

T

t

C INCOMPLETE DOMINANCE

Genetics

11In Gauteng, we often see cosmos flowers on the side of roads at the end of summer. We see red white and purple flowers. Why?Mendel observed a similar phenomenon when crossing pure-breeding red snapdragons with pure-breeding white. All the offspring produced pink flowers.This blending effect is known as INCOMPLETE DOMINANCE, where one gene is not completely dominant over the other. Let R = gene for red snapdragonsLet W = gene for white snapdragons

GENOTYPE

PHENOTYPE

CRCR Red Flowers

CRCW Pink flowers

CWCW White flowers

P1 CRCR x CWCW P1 (crossed 2 homozygous plants)

CR CR CW CW (Gametes)FetilisationF1 (Punnet square)

Phenotype = All pink (incomplete dominance)Genotype = All heterozygous

If CRCW is crossed with CRCW the result will beF2

Phenotype: 1 red: 2 pink: 1 white

The genes are unaltered by this

phenomenon.

The green eye colour in humans is the result of incomplete dominance between a brown gene and a blue gene. Both produce colour pigment and the mixture of brown and blue gives green eye colour.

Genetics

GAMETES CR CR

CW

CW

GAMETES CR CW

CR

CW

12D CO-DOMINANCEOccurs when both alleles are equally dominant and are expressed in the phenotype of a heterozygote. The heterozygote has characteristics of both parents. For example, I certain cattle, parents with red and white coats produce calves that are roan (even mixture of hairs that are red and white).PARENTS Phenotype: Red x White Key: R – Red coat W – White coatGenotype: IRIR x IWIW OR RR x WW MEIOSIS MEIOSISGAMETES IR IR IW IW GAMETES R R W WFERTIISATION

GAMETES IR IR GAMETES

R R

IW W

IW WF1: Genotype:

Phenotype: Roan

P2 (F1) Phenotype:Roan Roan OR Roan RoanIR IW x IR IW RW x RW

MEIOSIS MEIOSISGAMETES IR IW IR IW GAMETES R W R WFERTIISATION FERTIISATIONGAMETES IR IW GAMETE

SR W

IR IR IR IR IW R

IW IR IW IW IW WF2: Genotype: 1 IR IR: 2 IR IW: 1 IW IW

Phenotype: 1 RED: 2 ROAN: 1 WHITE

Another example of co-dominance is that of blood groups (see later, page 15)

Genetics

X X X Y

13E SEX DETERMININATIONComplete chromosomes, called the sex chromosomes determine the sex of an individual. These chromosomes are called the X and the Y chromosome These are homologous chromosomes.Each cell contains one pair of sex chromosomes, either XX in female cells or XY in male cells. The sex chromosomes form the 23rd pair of chromosomes. The first 22 pairs are called autosomal chromosomes

The sex chromosomes are important in sex determination. The zygote which results from the fusion of the male and female gametes will contain 22 autosomal chromosomes and one sex chromosome from the father (sperm) and the same number form the mother (egg cell)WHAT ARE THE CHANCES OF HAVING A BOY OR GIRL ON THIS BASIS ALONE? _______%

MALE FEMALEPARENT XY BODY CELL XX

MEIOSISGAMETES X Y X XFERTILISATIONZYGOTE [OFFSPRING/PROGENY] XX XX XY XYF SEX-LINKED INHERITANCESome Characteristics, like the gene for colour vision are found attached to the X chromosome. This means that the gene for that characteristic is linked to the sex of the individual.

Genetics

Do you notice that the male’s X chromosome does not have corresponding loci on the Y chromosome because it is shorter. Thus even a recessive gene on the X chromosome (and not on the Y), will be expressed.

X X X Y

BB bb bb

YXXBBXXBB XXbbYY

Normal Normal Female Female

Normal female Normal female possible egg cellspossible egg cells

X

BB

XX

bb

Colour Colour Blind Blind MaleMale

Normal Female x Colour Blind MaleNormal Female x Colour Blind Male

Colour blind male Colour blind male possible sperm cellspossible sperm cells

X

BB

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YX XX

Normal Female but Normal Female but carries the (b) colour carries the (b) colour blind geneblind gene

Normal MaleNormal Male

bb BBBB

XXBBXXBB

Normal Normal Female Female

XXbbYY

Colour Colour Blind Blind Male Male

Normal Female x Colour Blind MaleNormal Female x Colour Blind Male

11

XX

bb

Y

X

BB

14In humans, the gene for colour vision is sex-linked. The gene is linked to the X chromosome. The gene for normal colour vision (B) is dominant over the gene for colour blindness (b).

If the female parent (XX) has normal vision (XBXb) and the male (XbY) is colour blind (b – only on the X)… How do we link the colour blind genes to the sex chromosomes?

Genetics

XBXb XbY

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G INHERITANCE OF BLOOD GROUPS (Multiple Alleles and Co-dominance

The main blood groups are: A, B, AB and OFor every one of these there are two genes.If you are group A, your genes are AA (IAIA) or AO (IAIO)If you are group B, your genes are BB (IBIB) or BO (IBIO)If you are group AB your genes are AB (IAIB) [universal recipient]If you are group O, your genes are OO (IOIO) [universal donor]Neither A nor B is dominant over the other, but both are dominant over Oe.g., Cross a homozygous group A man with a heterozygous group B women to find the F1PHENOTYPE A (IA IA) x B (IBIO)GENOTYPEGAMETES

GAMETES

F1 GENOTYPE: ____________________________________________________PHENOTYPE: ________________________________________________

The table below indicates the possible blood groups of children from all possible combinations of parents:

BLOOD TYPE OF PARENTS

BLOOD TYPE OF CHILDREN

AB x AB A, AB, B AB x A A, AB, B

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16 AB x B A, AB, B AB x O A, B A x A A, O A x B AB, A, B, O A x O A, O B x B B, O B x O B, O O x O O

A knowledge of blood types can be used to determine if someone is the parent of a child. However, they can only be used to determine:(a) whether someone is NOT the father or(b) whether someone MIGHT BE the father.

Mrs. Hey claims that Mr. Bee is the father of her child. Their blood is tested and it turns out that Mrs. Hey belongs to Group B, the child to Group AB and Mr. Bee to Group O. Could Mr. Bee be the father of the child?

The pedigree diagram below shows the blood groups of individuals of a family. The blood groups are indicated inside the circle or square. The blood groups of individuals W and X are not indicated.

1 Write down all the possible genotypes of individuals:

(a) W(b) X

(8)

2 Haemophilia is a blood clotting disorder. Explain why mainly males suffer from this disorder.

(4)______________________________________________________________________________________________________________________________________________________________________________________________________________________________

BLOOD TRANSFUSIONBefore a person (recipient) receives blood, the blood of the donor has to be first tested to ensure that it is not infected and is of the right type.The table below shows the safe donor for recipients of the various blood types:

BLOOD TYPE OF

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RECIPIENT ANTIBODIES

DONOR

A B A, O

B A B, O

AB NONE A, B, AB, O

O A & B O

A

person with the blood type O can donate blood to a person with any blood type. Such persons are said to be UNIVERSAL DONORS. A person with the blood type AB can safely receive blood from a person with any blood group. Such a person is said to be a UNIVERSAL RECIPIENT.If blood of a wrong group is transfused into a recipient, the blood cells can clump together, reduce blood flow and even cause death.

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H DIHYBRID CROSSESThese crosses involve TWO PAIRS of contrasting genese.g., TALL WITH BLUE EYES X SHORT WITH BROWN EYES

AN EXAMPLE I pea plants a plant produces seeds that are Round (dominant) or wrinkled (recessive) and Yellow (dominant) or green (recessive)

Let R = gene for round seeds r = gene for wrinkled seedsLet Y = gene for yellow seeds y = gene for green seeds

If a Round, yellow seed is crossed with a wrinkled green seed:

Genotype of parents (P1) RRYY x rryy

meiosis

Gametes RY RY ry ry

Genotype of offspring (F1) RrYyPhenotype of offspring: All Round, Yellow seeds

Genotype of parents (P2) RrYy x RrYy

meiosis

Gametes RY Ry rY ry RY Ry rY ry

Genotypes of the offspring (F2)

GAMETES RY Ry rY ry

RYRyrYry

Phenotype of offspring (F2): Round Yellow: ………Round Green: ............Wrinkled Yellow: …….....Wrinkled Green: ……….

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I Polygenic inheritancePolygenic inheritance, also known as quantitative or multifactorial inheritance refers to inheritance of a phenotypic characteristic (trait) that is attributable to two or more genes and their interaction with the environment. Unlike monogenic traits, polygenic traits do not follow patterns of Mendelian inheritance (separated traits).Instead, their phenotypes typically vary along a continuous gradient depicted by a normal bell curve (for example, height, as shown below).An example of a polygenic trait is human skin color. Many genes factor into determining a person's natural skin color, so modifying only one of those genes changes the color only slightly. Many disorders with genetic components are polygenic, including autism, cancer, diabetes and numerous others. Most phenotypic characteristics are the result of the interaction of multiple genes.

J ABINISMAn albino is person who has no pigment in the skin and so is very pale. This condition is caused by a recessive gene. An albino man marries a normal woman one of whose parents was an albino. How likely is it that their first child will be an albino? Give your reasons in full. Certain individuals in this family are ‘carriers'. Which ones are carriers and what does this word mean?

The woman must be Aa (because parents were aa x AA or aa x Aa)Albino man normal woman

Xaa Aa

all a sperms x ½ A eggs ½ a

½ Aa ½ aa

Normal children Albino children

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22Their first child has a 50:50 chance of being albino. The normal woman and the normal children are carriers of the albino gene, even they are not albino.

On the left are two brothers, one normal (Aa) and the other albino (aa)

K DOWN’S SYNDROME

SEX CHROMOSOMES: Is this male or a female?L VARIATION FROM GENERATION TO GENERATIONIt is easy to understand why an individual resembles one or both of his parents. But very often an individual resembles neither of his parents. There are a number of different reasons for this:1 Non Inherited variations1.1 Environmental variations

An offspring may find itself in a different environment to that of its parents. For example, mealie plants, which are given fertiliser and much water, will probably become bigger, much healthier plants than their parents, if the parents had been grown in a dry soil, lacking in nutrients and adequate light. In the drawing below, an experiment was done whereby two seeds from the same plant were grown in different environmental conditions, one favourable for growth (left hand plant), and the other unfavourable (right-hand)

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Similarly, an animal’s growth, development and immunity to disease may depend on environmental factors such as having sufficient food, water and the correct environmental temperature.In humans, although it is necessary to inherit a certain amount of intelligence to succeed in life, environment plays an important role. Many people in South Africa are born with the intelligence, but, because of unfavourable environmental factors, such as poverty, inadequate schooling, lack of a proper diet, etc., they do not reach their true potential.

1.2 Birth Defects also called congenital disorders, any abnormalities of structure or function that is present at birth. The process of foetal development can be disrupted by a variety of external factors such as exposure to radiation, heat, chemicals, infectious agents, or maternal disease. An external agent that alters foetal development is called a teratogen (Greek teratos, “monster”; genes, “born”). In other words, instead of inheriting the defective gene from parents, the genes are mutated in the foetus. This is not inheritedDevelopmental malformations can also be the result of abnormalities in the genetic makeup of the foetus, or they can be caused by a combination of genetic and environmental influences.Twenty percent or more of malformed foetuses are spontaneously aborted; the rest result in a new-born with a birth defect. Although each single type of birth defect is rare, taken together they make up almost 5 percent of all live births and cause about 20 percent of infant deaths in the period immediately after birth. About one in ten developmental disorders is hereditary and arises from an abnormality in a single gene. Another 5 percent of birth defects arise from physical abnormalities in the chromosome.

2 Inherited Variations2.1 Mutations (SEE LATER SECTION)

Some genetic disorders have symptoms that are manifest at birth. These disorders may result from mutations in a single gene or from more general chromosomal abnormalities. A mutation occurs when the order of nucleotides in the D.N.A. is changed. This could be only one nucleotide, or more. The changed nucleotide causes the gene to become defective because the gene will now produce a “bad” or “wrong” protein. This may lead to diseases or physical or metabolic deformities.

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24X-rays, excessive exposure to the sun’s heat, exposure to harmful chemicals, radiation form nuclear bomb explosions are some of the causes of mutated genes.When you go to hospital for an X-ray, it may cause a gene mutation in your sex cells. This deformity may be passed on to your offspring, even though you will not show any symptoms of the problem.Many diseases and conditions are inherited in a recessive manner: Neither parent may have the defect even though they both carry the causative gene. When both parents have a dominant gene A and a recessive gene a, their offspring may inherit one of four different combinations: AA, Aa, aA, or aa. If the recessive gene a is defective, the statistical probability is that one in four of the offspring will bear the defective trait (aa). Sickle-cell anaemia is an example of a mutant recessive gene

NORMAL RED BLOOD CELL SICKLE CELL

Persons who have sickle-cell anaemia have some red blood cells shaped like a half moon (or sickle). The cells cannot carry oxygen effectively and so the person suffers from anaemia (shortage of blood to the cells)

2.2 HybridisationHere characteristics of two unlike but closely related species of parents are combined to make a new individual by means of cross-breedingHybrid, strictly defined, an offspring that is a cross between different species, genera, or, in rare cases, families.

More loosely defined, a hybrid can also be a cross between parents of different subspecies or varieties of a species. For example, a horse and a donkey are crossed to produce a mule, with good characteristics of both parents (running speed of the horse, strength of the donkey).

In getting a new hybrid strain, we might choose one parent because of its strong growth while the other parent may be selected because of its fine quality of fruit or flower. Hybridisation is artificial and is often used by humans to increase food production. It was Africans who experimented with hybrids of grasses to make the crops we know today (mealies, maize, etc).

2.3 Genetically modified (GM) foods are foods derived from genetically modified organisms. Genetically modified organisms have had specific changes introduced into their DNA by genetic engineering, unlike similar food organisms which have been modified from their wild ancestors through selective breeding (plant breeding and animal breeding) or

Genetics

25mutation breeding. GM foods were first put on the market in the early 1990s. GM foods have been modifies to: increase the crop yield; make crops resistant to herbicides (so that weeds can be eliminated); resistance to insects which may eat the crop; production of specific nutrients (like vitamins); produce drought-resistance crops; improve the taste of certain foods; Critics have objected to GM foods on several grounds, including perceived safety issues (may cause diseases), ecological concerns (genes from GM foods may mix with non-GM foods and cause unfvourable changes in crops) and economic concerns raised by the fact that these organisms are subject to intellectual property law (premium on price for these seeds).

M THE HUMAN GENOMEThe human genome is the genome of Homo sapiens, which is stored on 23 chromosome pairs. Twenty-two of these are autosomal chromosome pairs, while the remaining pair is sex-determining. The haploid human genome occupies a total of just over 3 billion DNA base pairs. The Human Genome Project (HGP) produced a reference sequence of the euchromatic human genome, which is used worldwide in biomedical sciences.The haploid human genome contains ca. 23,000 protein-coding genes, far fewer than had been expected before its sequencing, intact, only about 1.5% of the genome codes for proteins, while the rest consists of non-coding RNA genes, regulatory sequences, introns, and (controversially named) "junk" DNA.There are estimated to be between 20,000 and 25,000 human protein-coding genes. The estimate of the number of human genes has been repeatedly revised down as genome sequence quality and gene finding methods have improved.Earlier predictions estimated that human cells have as much as 200,000 genes.

Genetics

26N STEM CELLS

Stem cells are cells found in all multi cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiate into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Ernest A. McCulloch and James E. Till at the University of Toronto in the 1960s.The two broad types of mammalian stem cells are:1. embryonic stem cells that are isolated from the inner cell mass of blastocysts and the umbilical cord blood;2. adult stem cells that are found in adult tissues (bone marrow).In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing specialized cells, but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.Stem cells can now be grown and transformed into specialized cells with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture. Highly plastic adult stem cells from a variety of sources, including umbilical cord blood and bone marrow, are routinely used in medical therapies.Stem cells can replace tissues or organs which may be damaged. They will not be rejected by the body’s immune system as they originate from your own body.There are criticisms of the use of stem cells like the idea that scientists are destroying human life. The research is still new and not much is known about the long term effects of their use.

Genetics