Chromosomes are thread like gene carrying structures found in the nucleus. Each chromosome consists of one DNA molecule and associated proteins (ghr.nlm.nih.gov). Genes are segments of DNA. These are the units which make up chromosomes, responsible for the inheritance of specific characteristics. Alleles are alternate versions of genes. A dominant allele is the version of that gene that is always shown as the characteristic. A recessive allele is only shown if there is no dominant allele present inherited information is passed on in the form of each genes specific sequence of nucleotides that make up DNA. Most genes programme cells to synthesise proteins that produces an organisms inherited traits. Traits are organisms’ characteristics.
The transmission of hereditary traits is based on the replication of DNA which produces copies of genes that can be passed on from parents to offspring.
Humans have 46 chromosomes in almost all cells (apart from sex cells for example sperm and egg cells which are haploid or have half the number of chromosomes)
Sexual reproduction results in greater degree of variation. Two parents give rise to offspring that have unique combinations of genes inherited from two parents.
Sexual reproduction generates genetic variation by homologous parings (half of the chromosomes form the mother and half from the father).
The process crossing over is the exchange of genetic material which enhances variation in a species; it produces recombinant chromosomes, which combine genes inherited from two parents. Crossing over occurs early in meiosis, as homologous chromosomes pair loosely along their lengths (N.A Campbell& J.B Reece).
(252 words)
References
ghr.nlm.nih.gov, 05/2013, what is a chromosome? Available at: http://ghr.nlm.nih.gov/handbook/basics/chromosome
Biology sixth edition By N.A Campbell and J.B Reece (2002)

TAQ 2 – Describe Mendelian Laws of hereditary: mono and dihybird inheritances (criteria 1:2)
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments. Mendel used peas because they are available in many varieties and it gave Mendel strict control over which plants mated with which.

Mendel discovered two rules of inheritance
• The law of segregation – two alleles for a characteristic are packed into separate gametes
• The law if independent assortment – each pair of alleles segregated into gametes inde-pendently.

In typical breeding experiments Mendel used true-breeding varieties to ensure that when they self-pollinate they will produce offspring of the same variety. The crossing (or mating of two different true bred varieties is called hybridisation) the parents are called P generation and the hybrid offspring are F1 generation. F1 generation offspring are called F2 generation. For example Mendel crossed a true breeding purple flower with a true breeding white flower on pea plants and observed the results. Mendel used large samples and found that data fitted a 3:1 ratio. Mendel found that the heritable factor for white flowers did not disappear in F1 plants, but only the purple flower factor was affecting flower colour in the hybrids. This means that when a purple and white true breeding flower bred they made all purple flowered offspring (F1) but when F1 generation bred that produced 3:1 ratio of purple:white flowers.

Mendel did the same experiment using other characteristics of peas (smooth, round or wrinkled peas)

Mendel found that the gene for colour exists in two versions (purple and white) this is an allele. The offspring inherits an allele from each parent. If two alleles differ then the dominant allele is fully expressed in the plants appearance. A recessive allele has no effect on an organism if the dominant allele is present.
This shows that if the F2 generation has a 3:1 purple:white ratio then the purple allele is dominant the white allele is recessive. This is a mono hybrid inheritance example as only one characteristic is followed.

P = Purple allele (dominant) p = White (Recessive)

P p

P PP Pp

p Pp pp

The Punnett square above shows the 3:1 purple : white flower ratio.
Law of separation – Mendel’s first law, stating that alleles pairs separate during gamete (sex cell) formation, then randomly reform as pairs during fusion at fertilisation.
Law of independent assortment – each allele pair segregates independently during gamete for-mation, applies when genes for two characteristics are located on different pairs of homologous chromosomes. Mendel’s results show that each characteristic is independently inherited. A 9:3:3:1 ratio was always observed.

This shows that Yellow is a dominant trait and green is recessive. Smooth peas are dominant alleles and wrinkled is recessive.

Each character is independently inherited, the two alleles for seed colour segregate independently of the two alleles for shape.
(474 words)

References
Biology sixth edition By N.A Campbell and J.B Reece (2002)
Biologyabout.com, Available at: http://www.biologycorner.com/APbiology/inheritance/11-1_mendel.html TAQ 3 – Explain linkage, sex determination and crossing over in chromosomes and their role in transmission (criteria 1:3)
1. Genes located on sex chromosomes are called sex-linked genes. Genes located on same chromosomes tend to be inherited together in genetic crosses because the chromosome is passed along as a unit. Linked genes don’t sort independently (as stated by Mendel) because they are located on the same chromosomes and tend to move together through meiosis and fertilisation. If genes are located on the same chromosome, they are said to be linked. Gene linkage can be demonstrated by using a test cross. In humans the term sex linked usually refers to the genes on the X-chromosome. Fathers pass all sex linked alleles to their daughters but none to their sons. In contrast, mothers can pass the sex linked alleles to both sons and daughters. In sex-linked traits, such as colour-blindness, the gene for the trait is found on the X chromosome (a sex chromosome). Sex-linked traits affect primarily males, since they have only one copy of the X chromosome (male genotype: XY). Females, who have two copies of the X chromosome, are affected only if they are homozygous for the trait. Females can, however, be carriers for sex-linked traits, passing their X chromosomes on to their sons. Sex-linked inheritance works as follows: if a female carrier and a normal male give birth to a daughter, she has a 1 in 2 chance of being a carrier of the trait (like her mother). If the child is a son, he has a 1 in 2 chance of being affected by the trait (for example, colour-blindness). If a female carrier and an affected male give birth to a daughter, she will either be affected or be a carrier. If the child is a son, he will either be affected or be entirely free of the gene. See the following Punnett squares (The letters X and Y represent their respective normal chromosomes; X underlined represents the colour-blindness allele). (library.thinkquest.org)

All of Mendel's traits were caused by a single gene, or locus. They were also expressed dominantly. Mendel’s second law, or the law of the independent assortment, is valid for genes located in different chromosomes. These genes during meiosis segregate independently.

Mendel’s second law however is not valid for phenotypical features conditioned by genes located in the same chromosome (genes under linkage), since these genes, known as linked genes, do not separate in meiosis (except for the phenomenon of crossing over).

Certain alleles of one gene were somehow coupled with certain alleles of another gene; however, they were not sure how this could occur. This phenomenon is now known as genetic linkage, and it generally describes an inheritance pattern in which two genes located in close proximity to each other on the same chromosome have a biased association between their alleles. This, in turn, causes these alleles to be inherited together instead of assorting independently. Genetic linkage is a violation of the Mendelian principle of independent assortment. (nature.com)

Whenever two genes are linked because of their location on a chromosome, their alleles will not segregate independently during gamete formation. As a result, test crosses involving alleles of linked genes will yield phenotypic ratios that stray from the classic Mendelian ratios. Also in the case of linked genes, the phenotypic ratio will show higher numbers of offspring with the parental genotypes than offspring with the recombinant genotypes (nature.com)

2) Sex determination is the determination of the gonads and is strictly chromosomal and is not usually influenced by the environment (ncbi.nlm.nih.gov).
Humans have an additional pair of sex chromosomes for a total of 46 chromosomes. The sex chromosomes are referred to as X and Y, and their combination determine a person’s sex. Human females have two XX chromosomes while males possess an XY pairing. This XY sex-determination system is found in most mammals.
Whether a person has XX or XY chromosomes is determined when a sperm fertilizes an egg. Unlike the body’s other cells, the cells in the egg and sperm — called gametes or sex cells — possess only one chromosome. Gametes are produced by meiosis cell division, which results in the divided cells having half the number of chromosomes as the parent, or progenitor, cells. In the case of humans, this means that parent cells have two chromosomes and gametes have one.
During fertilization, gametes from the sperm combine with gametes from the egg to form a zygote. The zygote contains two sets of 23 chromosomes, for the required 46.
All of the gametes in the mother’s eggs possess X chromosomes. The father’s sperm contains about half X and half Y chromosomes. The sperm are the variable factor in determining the sex of the baby.
A gene has been identified for the development of testes. The presence of the SRY gene is essential for the development of testes. In the absence of the SRY gene the gonads develop into ovaries. Therefore if the sperm carries an X chromosome, it will combine with the egg’s X chromosome to form a female zygote. If the sperm carries a Y chromosome, it will result in a male (Livescience.com).

3) A mechanism that exchanges segments between homologous chromosomes must occasionally break the linkage between the two genes. Crossing over accounts for the recombination of linked genes. The recombinant chromosomes resulting from crossing over may bring together alleles in new combinations. Crossing over is the process occurring during meiosis wherein two chromosomes pair up and exchange segments of their genetic material. This occurs at the stage when chromatids of homologous chromosomes pair up during synapses, forming X-structure (chiasma). The chromatids break into segments (of matching regions), which are then exchanged with one another. Crossing over is important because it results in new combinations of genes that are different from either parent, contributing to genetic diversity (Biology-online.org).

waynesword.palomar.edu

(715 words)
References
Biology-online.org, Available at: http://www.biology-online.org/search.php?search=genetics Library.thinkquest.org, Available at: http://library.thinkquest.org Livescience.com, 2013, Available at: http://www.livescience.com/27248-chromosomes.html Nature.com, Available at: http://www.nature.com/search/executeSearch?sp-q=genetics&sp-p=all&pag-start=1&sp-c=25&sp-m=0&sp-s=&siteCode=default&sp-advanced=true&sp-q-9%5BNG%5D=1 Ncbi.nlm.nih.gov, 2013, Available at: http://www.ncbi.nlm.nih.gov/books/NBK9967/ waynesword.palomar.edu, nd, Available at: http://waynesword.palomar.edu TAQ 4 – Distinguish with examples between continuous and discontinuation variation in biology (criteria 2:1)
Discontinuous variation - where individuals fall into a number of distinct categories, based on features that cannot be measured across a complete range. You either have the characteristic or you don't. An example would be blood: you’re either one group or another - can't be in between. Discontinuous variation is controlled by alleles of a single gene or a small number of genes. The environment has little effect on this type of variation.
Continuous variation has a complete range of measurements from one extreme to the other. Height is an example - individuals can have a complete range of heights, for example, 1.6, 1.61, 1.625m high. Other examples of continuous variation include: weight; Shoe size; Milk yield in cows.
Continuous variation is the combined effect of many genes (known as polygenic inheritance) and is often significantly affected by environmental influences. Milk yield in cows, for example, is determined not only by their genetic make-up but also significantly affected by environmental factors such as pasture quality and diet, weather, and the comfort of their surroundings.
Blood Group: Discontinuous variation diagram Height: Continuous variation diagram

(174 words)

TAQ 5 – Give examples of gene and chromosome mutation (criteria 2:2 )
1) Mutation may occur in Gene and Chromosome level. Gene Mutation is a change in chain structure of DNA and gene, meaning amino acid order is swapped/ exchanged, resulting in a different protein produced. Not all mutation may cause disease, because each amino acid has two or more codons (genetic codes). A single base substitution won’t make any difference if it results in the same amino acid. For example, Sickle Cell disease may occur if a single base substitution leads to the production of Valine instead of Glutamine. This caused a production of abnormal sickle shaped red blood which may obstruct and restrict blood flow to an organ, leading to pain and often organ damage. (112 words)
2) De Novo Mutations is a genetic difference that’s not inherited but arises due to a gene alteration appearing in one family member as a result of a mutation in an egg or sperm of parents, or in the fertilized egg itself. The mutation isn’t part of the parent's overall genetic code. For example Down syndrome is caused by an extra (third) chromosome 21. These individuals have therefore 47 chromosomes instead of 46. Whereas everyone has two chromosomes 21, they have 3. Down syndrome is also called trisomy 21, and is usually due to an abnormality of the egg of the mother: normally all reproductive cells (eggs and sperms) carry only 23 chromosomes (one of each). At fertilisation the 23 chromosomes from the female egg than join the 23 chromosomes from the male sperm to form a fertilised egg or zygote with the normal 46 chromosomes. When one of the reproductive cells has an extra chromosome 21, the fertilised egg has 47 chromosomes with 3 chromosomes 21, resulting in Down syndrome
(170 words)
3) Mosaicism describes the occurrence of cells that differ in their genetic component from other cells of the body. Mosaicism is caused when a mutation arises early in development. The resulting individual will be a mixture of cells, some with the mutation and some without the mutation. How early in development the mutation occurs will determine what tissue(s) and what percentage of cells will have the mutation. It can be germline (affecting only egg or sperm cells), somatic (affecting cells other than egg or sperm cells), or a combination of both. An example is Turner syndrome, which is a genetic condition where females don’t have the usual pair of two X chromosomes. Cells are missing all or part of an X chromosome. The condition only occurs in females. Most commonly, the female has only one X chromosome. Others may have two X chromosomes, but one of them is incomplete. Sometimes, a female has some cells with two X chromosomes, but other cells have only one.
(164 words)

4) Polymorphism - Natural variations in a gene, DNA sequence, or chromosome that have no adverse effects on the individual and occur with fairly high frequency in the general population. Polymorphic sequence variants usually do not cause overt debilitating diseases. Many are found outside of genes and are completely neutral in effect. Others may be found within genes, but may influence characteristics such as height and hair colour rather than characteristics of medical importance. However, polymorphic sequence variation does contribute to disease susceptibility and can also influence drug responses, meaning a rare disease allele in one population can become a polymorphism in another if it confers an advantage and increases in frequency. A good example is the allele of sickle-cell disease. In Caucasian populations this is a rare sequence variant of the beta-globin gene that causes a severely debilitating blood disorder. In certain parts of Africa, however, the same allele is polymorphic because it confers resistance to the blood-borne parasite that causes malaria.

(163 words)

TAQ 6 – Explain the process of protein synthesis: transcription, translation and amino acid assembly (criteria 3:1)
A gene is a sequence of bases in DNA that codes for the sequence of amino acids in a poly-peptide (protein). Protein synthesis is the formation of a 3D tertiary structure formed from amino acids coded for by genes. Proteins determine the characteristics of an organism.

The two main stages of protein synthesis are:
• Transcription
• Translation

DNA gene is copied into mRNA (transcription) mRNA is read by ribosomes to make a pro-tein (translation)

Transcription is the synthesis of RNA. Enzymes open the DNA double helix structure to ex-pose the required DNA bases (gene). mRNA is a single stranded complementary section of the desired piece of DNA. There are 2 types of RNA: mRNA and tRNA. mRNA then carries the code for building a specific protein from the nucleus to the ribosomes in the cytoplasm. Ribosomes are the site of protein synthesis in the cell (where new proteins are made). They act as an assembly line where coded information (mRNA) from the nucleus is used to as-semble proteins from amino acids.

Translation is where the protein is assembled. tRNA transfers the amino acids in the cyto-plasm to the ribosome. In the process of translation, the sequence of nucleotides in mes-senger RNA (mRNA) determines the sequence of amino acids in a protein. The sequence of nucleotides in the mRNA determines the sequence of amino acids in the protein. This step is called translation.

References
From Gene to Protein—Transcription and Translation By Dr. Ingrid Waldron and Dr. Jennifer
Doherty, Department of Biology, University of Pennsylvania, Copyright, 2010