The letter sequence of each gene contains information on building specific molecules such as proteins or hormones — both essential to the growth and maintenance of the human body. Although every cell has two copies of each gene, each cell needs only certain genes to be switched on in order to perform its particular functions.
The unnecessary genes are switched off. A change in a gene can occur spontaneously no known cause or it can be inherited. Changes in the coding that makes a gene function can lead to a wide range of conditions. Humans typically have 46 chromosomes in each cell of their body, made up of 22 paired chromosomes and two sex chromosomes.
These chromosomes contain between 20, and 25, genes. New genes are being identified all the time. The paired chromosomes are numbered from 1 to 22 according to size. Chromosome number 1 is the biggest. These non-sex chromosomes are called autosomes. People usually have two copies of each chromosome. One copy is inherited from their mother via the egg and the other from their father via the sperm. A sperm and an egg each contain one set of 23 chromosomes. When the sperm fertilises the egg, two copies of each chromosome are present and therefore two copies of each gene , and so an embryo forms.
The chromosomes that determine the sex of the baby X and Y chromosomes are called sex chromosomes. A person with an XX pairing of sex chromosomes is biologically female, while a person with an XY pairing is biologically male. As well as determining sex, the sex chromosomes carry genes that control other body functions. There are many genes located on the X chromosome, but only a few on the Y chromosome.
Genes that are on the X chromosome are said to be X-linked. Genes that are on the Y chromosome are said to be Y-linked. Parents pass on traits or characteristics, such as eye colour and blood type, to their children through their genes. Some health conditions and diseases can be passed on genetically too. Sometimes, one characteristic has many different forms. Changes or variations in the gene for that characteristic cause these different forms.
These two copies of the gene contained in your chromosomes influence the way your cells work. The two alleles in a gene pair are inherited, one from each parent. Alleles interact with each other in different ways. These are called inheritance patterns. Examples of inheritance patterns include:. An allele of a gene is said to be dominant when it effectively overrules the other recessive allele.
The allele for brown eyes B is dominant over the allele for blue eyes b. So, if you have one allele for brown eyes and one allele for blue eyes Bb , your eyes will be brown. This is also the case if you have two alleles for brown eyes, BB. However, if both alleles are for the recessive trait in this case, blue eyes, bb you will inherit blue eyes. For blood groups, the alleles are A, B and O. The A allele is dominant over the O allele.
Blood group A is said to have a dominant inheritance pattern over blood group O. If the father has two O alleles OO , he has the blood group O. However, in sexual reproduction the sperm cell combines with the egg cell to form the first cell of the new organism in a process called fertilisation.
This cell the fertilised egg has two sets of 23 chromosomes diploid and the complete set of instructions needed to make more cells, and eventually a whole person. Each of the cells in the new person contains genetic material from the two parents.
This passing down of genetic material is evident if you examine the characteristics of members of the same family, from average height to hair and eye colour to nose and ear shape, as they are usually similar. If there is a mutation in the genetic material, this can also be passed on from parent to child This is why diseases can run in families.
How is sex determined? The sex of an individual is determined by the sex chromosomes called the X chromosome and the Y chromosome. Females have two X chromosomes XX. Males have an X chromosome and a Y chromosome XY. Female gametes eggs therefore always carry an X chromosome.
Male gametes sperm can carry either an X or a Y. When an egg joins with a sperm containing an X chromosome, the result is a girl. When an egg joins with a sperm containing a Y chromosome, the result is a boy. What is a genotype? The genotype is a description of the unique genetic makeup of an individual. It can be used to describe an entire genome or just an individual gene and its alleles.
The genotype of an individual influences their phenotype. For example, if we are talking about the genotype for eye colour we may say an individual has one brown eye allele B and one blue eye allele b.
Thus, eight years later, Alfred Hershey and Martha Chase further confirmed that protein was not the hereditary material through their work with bacteriophages, which are viruses that infect bacteria Figure 1. Bacteriophages are composed of only two substances: protein and DNA. By using radioactive labels that would integrate specifically into either DNA or protein, but not both, Hershey and Chase were able to show that DNA is the only material transferred directly from bacteriophages into bacteria when the bacteria are infected by these viruses.
This observation was important, because Hershey and Chase knew that the end result of bacteriophage infection was the production of more viruses in multiple copies. But just how did the injection of viral DNA into a bacterium create new viruses?
Hershey and Chase admitted that they were unsure of the answer to this question; however, they knew it didn't have anything to do with protein, but did have something to do with DNA.
Thanks to additional research, scientists now know that the DNA in a virus can take over a bacterial cell, causing it to replicate only the viral DNA and to create new viruses Figure 2. This process is a form of hijacking , wherein the viral life-form takes over the regular machinery inside another life-form in this case, a single bacterial cell. Key Questions How do scientists explore chromosomes? How does DNA replicate?
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