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Organism contains two determinants (genes) for a trait, one paternal and one maternal. Different forms of a gene exist within a population and these are termed alleles. Some alleles are dominant (T) and some recessive (t). Genotypes come in two general classes: homozygotes where both alleles are the same (TT or tt), and heterozygotes where the two alleles are different (Tt). If two heterzygotes breed (in a monohybrid with one gene responsible for a trait), the four possible outcomes would be as follow (Rules of segregation): Tt X Tt

T t

T TT Tt

t Tt tt

TT : Tt : tt1 : 2 : 1

With 3 different genotypes (1:2:1 ratio) and with 2 different phenotypes (3:1 ratio) possible.

However, dominance is not universal and sometimes the heterozygotes will be distinctly different from the two homozygotes and it is called partial dominance (three phenotypes 1:2:1).

More than 1 allele may also be present.

The rule of independent assortment states that alleles from one gene can segregate independent from the alleles from another gene.

But often several genes contribute to the same phenotype (1 gene – monohybrid; 2 genes – dihybrid; 3 genes – trihybrid; etc). This gives a small insight into the complexity of genomitry. A single phenotyphical trait can be the result of various genes with various possible alleles, as well as their dominance, recessiveness or partial dominance, etc.

The pattern for inheritance on the heteromorphic sex chromosomes (XY) is different from the pattern of autosomal chromosomes. Sex linked traits refers to loci on the X chromosome. The term Y linked refers to loci found on the Y chromosome, which control holandric traits (traits found only in males). Traits found on both X and Y are called pseudoautosomal.

Inbreeding is when there is common ancestry in the family tree. The extent of inbreeding is thus a function of the degree of common ancestry shared by the parents of an inbred individual. The first observable effect of inbreeding is the expression of hidden recessives. Dominance acts to mask the expression of deleterious alleles. Inbred animals can often be smaller and less fertile.

Mutation is a change in one of the base pairs (see structure of DNA) which could result from either an error in complimentarity during replication or damage to the DNA that was not repaired by the time of the next cycle of DNA replication.

Diploid Chromosome numbers (2n) differ between species.

Organism (2n)

Pig 38

Human 46

Sheep 54

Cattle 60

Horse 64

Dog 78

Chromosome sizes also differ between pairs and between species.

Structure of DNA

DNA consists of two very long polymeric chains that twist around each other to form a double helix (imagine a ladder that is twisted in a long spiral). Nitrogen basis are part of the structure. Important to us is the fact that they form the pairing of the double helix structure that is the horizontal steps of the ladder). These four nitrogen basis are:

Thymine (T) Cytosine (C) Adenine (A) Guanine (G)

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There are precise base-pairing and only A-T (or then T-A) pair and C-G (or then G-C) pair. The complimentarity of the two chains, guide its own replication of tissue cells into identical daughter cells in the process of mitosis. It is also the manner in which these base pairs are arranged. Different alleles of a gene will have a different order of base pairs giving rise to the differentiation in phenotype.

A A T C C G C T A T T A G G C G A T

Replication is when the double helix unwound and each half act as a template for a new helix. Each new double helix will then be the same as the original. A chromosome is therefore replicated into two sister chromatids.

Transcription is when the DNA double helix partially unwound and allowing one strand to be the template in the syntheses of RNA.

Division of cells

New cells do not arise de novo but come only by cell division of an existing cell into two daughter cells. Cell division implies that both the cytoplasm (cytokinesis) as well as the nuclei that contains the DNA (karyokinesis) divide. Karyokinese has got two forms, mitosis and meiosis whereby the chromosomes are apportioned to daughter cells. There is the nonreductional mitosis in which the mother and two daughter cells have exactly the same genetic complement. Cells are then diploid (also known as the 2n state) which means that cells have a full complement of chromosomes in pairs which means that there is two of each chromosome. Then there is the reductional meiosis in which the products (gametes – sperm or eggs) have half the genetic material of the parent cell. This ensures that the zygote (the product of two gametes – fertilised egg) will have the same genetic complement from generation to generation. Gametes are then haploid (n) and have only one copy of the chromosomes and a zygote is once again diploid (receiving 1 set of chromosomes from the father and one set from the mother).

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MITOSIS is a continuous process but explained in steps.

Interphase

Meiosis is a two division process re-sulting in 4 cells from one parent cell.

MEIOSIS I Phrophase ILeptotene

MEIOSIS I continues...Metaphase I

In the diplotene phase there is a cross-ing over where homologous chromo-somes exchange parts. This results in maternal & paternal alleles being ran-domly combined in each gamete.

Therefore, it will be 25% of your mother’s genes and 25% of your fa-ther’s genes making up the total 50% contribution you make towards the genetic makeup of your child. The combination of traits is random.

Recombination thus increase allele combinations and create diversity.

Zygotene

Anaphase I

Anaphase II

Telophase

Interphase

MEIOSIS IIMeiosis II does not reduce the chromo-some number as in Meiosis I but does

reduce the genetic material in half.Metaphase II

In males each meiosis produce 4 equally sized sperm cells in the process called spermatogenesis. In the female oogenesis takes place and an ovum is produced once a month under hormo-nal control. Meiosis I yields a second-ary oocyte and a polar body (which re-ceived very little cytoplasm). Meiosis II (of the secondary oocyte) yields another polar body and an ovum. The polar bodies disintegrate.

Pachytene

Diplotene

Diakinesis

Early Prophase

Middle Prophase

Metaphase

Anaphase

Telophase

Interphase

ReferencesTamarin, R.H. 1993. Principles of genetics. Fourth edition. Wm. C. Brown Publishers, United States of America.

Diagrams also taken from “Principles of genetics”.

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Die effek van bos op weiding en die gevolglike ekonomiese implikasies

INLEIDING

Bosverdigting is ongetwyfeld een van die mees opsigtelike probleme as gevolg van wanbalanse wat in savanna ekosisteme voorkom. Die probleem het dan ook reeds ‘n geweldige impak op ekonomiese, volhoubare veeboerdery in geaffekteerde gebiede gehad, veral vanweë die negatiewe invloed op die produktiwiteit van die graslaag. Hierdie negatiewe impak kan hoofsaaklik toegeskryf word aan die kompetisie-effek om beskikbare grondwater tussen bos en gras, aangesien grondwater in ariede en semi-ariede gebiede die bepalende faktor met betrekking tot grasproduksie bly. Dit is ook ‘n alombekende feit dat veral gewenste en hooggewenste grasse beskikbare water baie meer doeltreffend benut as bv bos.

Die toepassing van bosbeheerpraktyke is gevolglik ‘n belangrike hulpmiddel in die strewe om die produksiepotensiaal van ons savanna gebiede te herstel en moontlik ten volle te ontsluit. Dit is egter krities dat die gekose metode ‘n positiewe reaksie, ekologies sowel as ekonomies, op die bepaalde sisteem sal hê.

Huidige situasie en tendense ten opsigte van bosverdigting

Indien verslae van die vroeë sestigs tot hede bestudeer word, is dit baie duidelik dat daar ‘n geweldige toename in die digtheid van sekere spesies was. So bv. het beraamde Swarthaak infestasies in die doringstruikveld toegeneem vanaf ± 860 duisend ha tot ± 3.5 miljoen ha. Hierdie neiging sal ook geld in die geval van ander probleemspesies soos Driedoring, Suidwesdoring en selfs Vaalbos, alhoewel al hierdie spesies nie noodwendig proporsioneel tot dieselfde mate sou verdig het nie.

Dit word aanvaar dat Swarthaak, wat ‘n aggresiewe verdigter is, gevolglik tot ‘n groter mate kon toeneem as wat met ander spesies die geval sou wees. In savanna gebiede is hierdie verdigting van houtagtige plante ‘n natuurlike neiging aangesien dit ‘n hoër stadium van suksessie verteenwoordig.

Dit beteken egter nie dat dit die mees gewenste situasie verteenwoordig nie soos wat duidelik blyk uit navorsing in Suid Afrika waar grasproduksie met tot soveel as 88% onderdruk is by bosdigthede van 2000 bosekwivalente en meer.

Onder toestande van ernstige verdigting, soos waarna hier verwys word, kan die situasie hom in die meeste gevalle nie vanself omkeer nie. In sulke gevalle is daar dus geen ander uitweg as om met ‘n aanvanklike drastiese stap die houtagtige komponent te verminder nie. Dit is die bekende benadering waar die plantegroei by die dierefaktor aangepas word. Chemiese bosdoders soos Molopo en/of Limpopo is in die situasies, ‘n logiese of in baie gevalle die enigste alternatief, en kan òf per vastevlerk vliegtuig òf met die hand toegedien word.

Die invloed van bosverdigting op grasproduksie

Bos is ‘n inherente deel van die plantegroei van die Doringstruikveld areas in Suid-Afrika. Die probleemspesies is nie uitheems nie en daar kan dus nie gepraat word van bosindringing nie. Vanuit ‘n weidingsoogpunt beskou, is hierdie ‘n belangrike beginsel, omdat die inheemse bos verdig tot ‘n punt waar dit landbouproduksie benadeel. Die verdigting vind plaas omdat omgewingstoestande so verander dat dit die bos bevoordeel ten koste van nuttige weidingsgrasse. Dit kan aanvaar word dat ‘n bosdigtheid van 100 bosse en minder per ha nie ‘n noemenswaardige invloed op grasproduksie het nie. Teen digthede van meer as 100 bosse per ha, veral in die geval van Swarthaak, word

Chris RichterTerra Care Vegetation Consultants , e-pos: terracare@mjvn.co.za, sel: 0824584558

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grasproduksie egter progressief, nadelig beïnvloed en die effek neem toe tot by ‘n digtheid van ongeveer 2 500 bosse per ha waar grasproduksie feitlik heeltemal onderdruk word in normale reënvaljare.

Anders gestel, kan verwag word dat grasproduksie met 220 tot 700% verhoog kan word met bosbeheer op veld met so ‘n hoë bosdigtheid. Dit moet egter benadruk word dat verhoging in grasproduksie na bosbeheer in die eerste plek afhang van die bosdigtheid voor beheer.

Die invloed van bos op diereproduksie

Daar is soveel faktore wat hier ‘n rol speel dat dit nie vooraf akkuraat voorspel kan word sonder dat dit proef ondervindelik vir ‘n bepaalde veldsoort op ‘n verteenwoordigende terrein bepaal is nie. Die vraag kan op twee maniere benader word naamlik produsie per dier en produksie per hektaar.

Produksie per dier

Produksie per dier word in die eerste plek bepaal deur die hoeveelheid en gehalte voer wat beskikbaar is vir produksie. Uit die voorgaande bespreking is dit duidelik dat bosverdigting tot ‘n afname in beskikbare voer lei. Daar is dus eerstens minder voer per dier beskikbaar, en tweedens ‘n verlaagde kwaliteit van voerinname omdat die dier tot ‘n mindere mate kan selekteer vir kwaliteit. Dit het met ander woorde ̀ n baie groot impak op die tipe dier waarmee geboer word en suiwer grasvreters gaan heelwat meer geaffekteer word as struikvreters.

Teen dieselfde veelading, sal bosverdigting dus onvermydelik tot gevolg hê dat produksie /dier daal. Anders gestel, om produksie per dier te handhaaf, sal dit noodsaaklik wees om veelading te verlaag met die toename van bosdigtheid en gevolglike afname in grasproduksie of as alternatief sal drastiese beheermaatreëls toegepas moet word.

Produksie per hektaar

In Zimbabwe is gevind dat weidingskapasiteit (drakrag) oor die langtermyn (30 jaar) twee tot drie keer laer is op veld wat onderhewig is aan bosverdigting. In Namibië is beraam dat weidingskapasiteit met tot 70% afneem met bosverdigting. In die doringstruikveld van Suid-Afrika dui produksieresultate daarop dat die weidingskapasiteit viervoudig afgeneem het as gevolg van bosverdigting. In die praktyk beteken dit dat die drakrag van veld met bosverdigting kan afnaam vanaf ‘n aanvanklike 10ha/GVE tot 69ha/GVE soos in Tabel 1 aangetoon. Hierdie afnames is selfs nog hoër onder homogene stande van Swarthaak en alhoewel dit oral voorkom is dit veral meer opmerklik in die sandveld.

Ekonomiese Implikasies

Die verlaging in drakrag soos in Tabel 1 aangetoon, beteken dat ons ‘n verlaging in jaarlikse bruto-marge per ha as gevolg van bosverdigting sal kry. Volgens ekonomiese rekords behoort `n bruto–marge van ±R1 600.00 per GVE in die Doringveld gebiede gerealiseer te word met `n kalfpersentasie van 80%. Indien `n denkbeeldige plaas van 4000 ha geneem word met infestasies soos in Tabel 2 uiteengesit, sal die ekonomiese implikasies van bosverdigting `n verlies van R292 400 per jaar aan BM/jaar beloop (46 % verlies).

Bosdigtheid Geraamde drakrag BE/ha Ha/GVE

0 8

200 10

600 13

1000 22

1500 38

2000 69