Genes, Chromosomes and DNA
Genes, Chromomes and DNA:
- Most cells in your body have a nucleus, nucleus contains the genetic material.
- The genetic material is arranged into chromosomes.
- Human cell nucleus contains 23 pairs of chromosomes
- Each chromosome is one very long module of DNA (Coiled)
- Gene is a short length of a chromosome
- Genes control different characteristics (hair colour, eye colour)
- Genes give instructions to cells
- An organisms genotype describes what genes its got
1 of 6
Genes and Variation
Genes and Variation:
- Sperm and egg cells have half the normal amount of DNA
- each cell has 23 single chromosomes so when they fuse together they have 46 or 23 pairs
- The two chromome pairs always carry the same genes and each gene is always found in the same place on the two chromosomes
- Because the two chromosomes in a pair come from different parents, thye might have different alleles of these genes.
- Alleles are different versions of the same gene.
- Children can look simular to both there parents but they're not identical
- Genes are shuffled together randomly to make egg and sperm cells
- The two chromosomes in a pair are never identical because they have different alleles
- When they go into different sex cells, each of the two cells get different allels
- each of the 23 pairs of chromosomes seperates independentl
- There is millions of different chromosomes combinations that can be produced from the seperation of 23 pairs
- This means that all the sex cells produced by one individual will probably be geneticaly different
- This is why brothers and sisters look simular but there isn't a big chance of them being identical
2 of 6
Inheritance and Genetic Diagrams
Inheritance and Genetic Diagrams:
- The combination of alleles determines the Phenotype
- when you've got two copies of a gene, usually only one of them can be expressed in the phenotype
- alleles are different versions of the same gene
- most of the time you have two alleles, one from each parent
- If you're homozygous for a trait, you have two alleles that are the same for that particular gene
- if you're heterozygous for a trait, you have two different alleles for that particular gene
- alleles can be dominant or recessive. If you have two dominant alleles of one dominant and one recessive only the dominant one will be shone (stronger!)
- To show the characteristics of a recessive there must be two recessives because they're weaker!
- In genetic diagrams, letters are used to represent alleles. alleles that produce dominant characteristics are always shown with a capital letter, and alleles that are recessive are lower case letters
3 of 6
Genetic Diagrams and Sex Chromosomes
Genetic Diagrams and Sex Chromosomes:
- Your chromosomes control if you're male or female
- The 23rd pair of chromosomes are labelled XY, these are the chromosomes that decide on which sex you are
- all men have x and y chromosome: XY, this is because the Y chromosome cause male characteristics
- and all women have two X chromosomes :**, this is because there isn't a Y chromosome so they can't be male
- Y chromosomes cause embryos to develop male characteristics . And females have to X chromosomes so they develop differently
- There is a 50% chance of having a male baby or female and you can see this in the diagram
- The gene that makes you a male produces a specific protein, and when the embryo's reproductive system starts to develop this protein causes the development of testes.
- The testes then create male sex hormones, which make the rest of the male reproductive system develop
- in females the protein isn't released so this causes them to create ovaries
4 of 6
- genetic disorders are caused by faulty alleles
- some can be inherited- one or both parents carry a faulty allele and they pass it on to their children
- Cystic fibrosis and Hunting-ton's disease are both caused by faulty alleles of a single gene
- some genetic disorders are caused by recessive alleles
- most of the defective alleles that are responsible for genetic disorders are recessive- so they won't show, they'll just be carriers, but if the carry meets with another carrier the offspring can have the disorder
- Cystic fibrosis is a genetic disorder of the cell membrane caused by a faulty recessive allele.
- Cystic fibrosis can have lots of nasty symptoms such as thick sticky mucus in the air passages making breathing very difficult. They can get chest infections and they have a difficulty digesting food.
- Cystic fibrosis can be carried in some people without any symptoms so you could be unaware you had this genetic disorder.
- one person in twenty five carries cystic fibrosis.
- For a child to have a chance of getting cystic fibrosis both their parents must be carriers of sufferers.
- There's a one in four chance of getting cystic fibrosis if both parents are carriers
- other genetic disorders are cause by dominant alleles, such as huntington's disease.
- Huntington's disease isn't very nice either with symptoms such as tremors, clumsiness, memory loss, mood change and poor concentration.
- There is NO cure to Huntington's disease.
- The dominant allele means there's a 50% chance of each child inheriting the disease if just one of the parents is a carrier.
- With this disorder if your a carrier you will have the disease too, unlike cystic fibrosis.
- The symptoms normally occur after the age of 40 and by this time most people have passed it on to their children so this is one of the main problems!
5 of 6
- Genetic testing can be used on embryos, children and adults
- When embryo's are produced using IVF (in vitro fertilisation), doctors can test the embryo's to check if they've got certain genetic disorders. This is known as pre-implantation genetic diagnosis and is especially important if there's concern that one of the parents might carry alleles for a genetic disorder.
- When the embryos are tested, only healthy ones are chosen to be implanted into the mother's womb - embryos with a genetic disorder are discarded.
- children and adults can be checked too see if they carry alleles for genetic disorders. For example, couples might do this to find out if there children are likely to inherit a particular genetic disorder.
- Children and adults can also be tested before certain drugs are prescribed to them, this can show how a drug is likely to affect an individual, e.g. if the drug will be an effective treatment, or if the person will have an adverse reaction to it.
- There are issues surrounding genetic testing such as, they can't be 100% accurate so there are often errors due to things like samples getting contaminated of misinterpretation of results. This means that healthy people could be told they have a genetic disorder when in fact it's incorrect-this is called a false positive where they are told positive but it's false.
- or they can be told they don't have the disorder when they do, this is called a false negative. if people were told a false negative this could lead to all sorts of problems such as making decisions based on the result.
- it isn't completely safe to have the genetic test during pregnancy, such as the fluid around the fetus can cause a miscarriage.
- Genetic testing can cause lots of argument between different types of people such as, some people don't agree with abortion in any circumstances. Some people believe its okay to have an abortion if it's what suits the parents lifestyle (like if the parents weren't in a position to bring up a disabled child) and some people don't know what to think. These opinions can be lead on by different religions too!
6 of 6
Similar Science resources: