- Slides: 47
Variation and Genetics.
Variation is the different characteristics that organisms have. Some characteristics are environmental. They are not passed on from one generation to the next Other characteristics are genetic. These are passed from one generation to the next.
Genetic Variation 1 Inside every cell is a nucleus that controls the cell. Inside the nucleus are threads called chromosomes
The chromosomes are made up from an enormously long molecule called DNA acts as the code for all the characteristics.
The DNA in a chromosome is divided up into sections called Genes. Each gene contains the DNA code for a different characteristic.
Normal human cells have 46 chromosomes - in 23 pairs. They are in pairs because we get half of them from each of our parents. i. e. 23 from the egg and 23 from the sperm.
Pairs 1 – 22 contain the genes that control the body’s normal characteristics (about 100, 000 of them!) The last pair are called the sex chromosomes. They determine what sex you are. . This person is female because they have two X chromosomes A man would have one X and one Y chromosome.
Gametes (Sex Cells) … but gametes (eggs and sperm) only have 23 single chromosomes. … this is because gametes fuse together at fertilisation to produce a zygote (fertilised egg). This then grows into a new person 46 Normal body cells have 46 chromosomes (23 pairs) 46 23 23 46
Cell Division. We need new body cells for growing and to replace old or damaged cells. It is important that the new cells have exactly the same set of chromosomes as the original cells. This is done by a type of cell division called Mitosis which occurs throughout the body. Gametes only have half the normal number of chromosomes in them so they need a different type of cell division to produce them. This type of cell division is called Meiosis only occurs in the ovaries or testes in animals and the anthers or ovaries in plants.
Mitosis During mitosis a cell divides once to produce two new identical cells. Original body cell (46 chromosomes) Two new body cells (46 chromosomes each)
Meiosis 2 nd division During meiosis there are two cell divisions which half the number of chromosomes. Gametes (eggs or sperm) 23 chromosomes each Cell in the testes or ovaries. (46 chromosomes) 1 st division
Boys or Girls? Sperm, however, will have either X or Y Because gametes only have half the normal number of chromosomes, each egg will only contain a single X chromosome. XX X X XX XX X Y XY XY XY As any sperm can fertilise any egg, this gives an equal chance of having a boy or a girl
Key Points What someone looks like is called their Phenotype This person’s phenotype is female. XX X X Y XY X
Key Points The genes that control the phenotype are called the genotype This person’s genotype is XX. XX X X Y XY X
Key Points Remember that gametes only contain half the normal number of genes / chromosomes X Y XY Two sex chromosomes XX Single sex chromosomes X X
Key Points XX X X Y XY X This is called a Punnett Square. It shows all the possible new combinations the gametes can make.
Genes and Alleles A gene is a section of DNA on a chromosome that contains the code for a particular characteristic. Genes For instance, we have a gene for eye colour, another for being able to roll our tongue etc.
Genes and Alleles Genes can come in different versions called Alleles. These people have both got genes for eye colour. … but they have inherited different versions (alleles) of the gene. … so they have ended up with different phenotypes.
Because our chromosomes are in pairs we actually have two copies of each gene. Copy 1 Copy 2 These two copies of the gene could be the same allele (called homozygous) Or they could be different alleles for the gene (called heterozygous)
One allele is usually stronger than the other. The stronger (dominant) one will completely overshadow the weaker (recessive) one if they are mixed together. The allele for brown eyes is dominant. The dominant allele is always given a capital letter: B The blue eye allele is recessive. Recessive alleles always get a lower case letter: b.
Phenotypes and Genotypes Phenotype = brown eyes Genotype = BB ( Homozygous dominant ) or Bb (heterozygous) Phenotype = blue eyes Genotype = bb ( Homozygous recessive ) Hint: If someone has the recessive phenotype they must be homozygous for that allele!
Pure breeding? This woman’s genotype has to be bb (i. e. two blue-eye alleles). All her eggs will contain a single b (one blue-eye allele) – so she is pure breeding for blue eyes. This woman’s genotype, however, could be BB or Bb. • If she is BB then she is pure breeding for brown eyes. • If she is Bb, however, half of her eggs will have a brown-eye allele and half of them a blue-eye allele. She is a carrier for blue eyes.
Pure Breeding 2 In rabbits the allele for brown fur is dominant to the allele for white fur. Gene = Fur colour Brown fur allele = B White fur allele = b The white rabbit has to be bb But the brown rabbit could be pure breeding BB or it could be Bb. How could you find out?
A Back Cross (also called a Test Cross) A back cross is used to find out if an organism with the dominant phenotype is pure breeding (homozygous) or not. To do this you breed it with another one with the recessive phenotype. The mixture of phenotypes you get in the offspring will tell you what the parent’s genotype was. For instance:
We know that this rabbit can only produce gametes with the white coat allele (b). If this rabbit is pure breeding brown (i. e. BB). All its gametes would be B b b B Bb Bb B All the offspring will be brown.
On the other hand…. . This rabbit can still only produce gametes with the white coat allele (b). If this rabbit is heterozygous (i. e. Bb) half its gametes would be B and half would be b b b B Bb Bb bb bb b Now half the offspring are brown and half are white.
Back Cross Summary Pure breeding (homozygous) b Hybrid (Heterozygous) b b b Bb Bb bb bb B B Bb Bb b B Bb Bb All dominant phenotype Half dominant, half recessive.
Other common test questions…. . 1. Can two organisms with the dominant phenotype produce offspring with the recessive one? i. e. Could two brown rabbits have white offspring? 2. Can two organisms with the recessive phenotype produce offspring with the dominant one? i. e. Could two white rabbits have brown offspring? 3. If a couple already have two boys, what is the chance that their next child will also be a boy?
1. Could two brown rabbits have white offspring? Yes – If they are both heterozygous. Bb B B Bb b b On average, one in four of their offspring will be white. This is called the 3: 1 ratio.
2. Could two white rabbits have brown offspring? No, all white rabbits have to be homozygous bb b b
1. If a couple already have two boys, what is the chance that their next child will also be a boy? It is still a 50: 50 chance. A man produces about 100, 000 sperm a day – that’s 50, 000 with an X chromosome and 50, 000 with a Y chromosome. So the fact that he has already used 2 sperm with a Y chromosome makes no difference whatsoever.
Pedigrees Having a pedigree means that the animal’s family tree is known (also called a pedigree chart). It allows us to work out which organisms are true breeding for a characteristic. BB How good is this rabbits pedigree?
The parents are both brown so their genotypes have to be Bb … so this rabbit could be BB or Bb BB … so this rabbit could also be BB or Bb. This rabbit has to be bb so it must have got one b from each of its parents
Bb BB Bb B? bb B? B?
Using Genetics 1. For centuries, man has been selectively breeding his plants and animals to improve their quality. 2. Cloning is making exact copies of an organism. It is often easy to clone plants, but more recently man has been able to clone animals too. 3 Man is also now able to genetically modify organisms to suit his needs. This is also called genetic engineering.
Selective Breeding By only allowing the organisms with the best characteristics to breed, man can “weed out” unwanted alleles, leaving only the ones he wants. As a result he is left with pure breeding pedigrees. We do this with both plants and animals as the next couple of slides show.
Problems • The problem with selective breeding is that the number of alleles in the population gets steadily less and less. • This can lead to the problems of inbreeding as all the animals or plants that are left are genetically closely related to each other. • Also, once an allele has been lost from a population it is gone forever, so if tastes change, or a new disease arrives the old “best” may not be good enough anymore. • These problems are even more exaggerated with cloning and genetic modification.
Cloning is the production of a new organism that is genetically identical to the one that produced it. Many plants clone themselves naturally by asexual reproduction. For instance, a potato plant will produce many potatoes underground by mitosis – so they are all genetically identical. If they are allowed to grow they will all be clones of the original plant. It was cloned potatoes that caused the Irish potato famine in the 1800 s in which over 1 million people died and 2 million were forced to emigrate. All the potatoes were susceptible to the same disease.
Cloning Animals (For this example, assume sheep have the same number of chromosomes as humans. ) 1. Take a body cell from a sheep. 6 7 2. Remove its nucleus (46 chromosomes) 3. Take an egg cell from the sheep. 1 2 3 4. Remove its nucleus (23 chromosomes) 5. Put the body cell nucleus into the egg cell. (the egg now has 46 chromosomes – as if it had been fertilised). 5 6. Implant the egg into the sheep’s womb. 4 7. The lamb that is born is a genetic clone of its mother.
Genetic modification This is when a gene from an organism of one species is inserted into an organism of a different one. This generally produces genotypes that could never occur in nature. Examples of where these this technique has been used are: • Making bacteria produce human insulin to treat diabetes. • Making bananas produce human hormones to treat various deficiency diseases • Making crops resistant to a particular weedkiller (herbicide)
How is it done? Example: Producing human insulin from bacteria 1. Cut a chromosome out of a human cell and from a bacteria cell. 1 2. Cut up the chromosomes using an enzyme. 3. Join the human piece containing the gene for insulin into the bacteria chromosome. . 2 3 5. Grow lots of copies of the bacteria. 4 4. Insert the joined chromosome back into the bacteria. The bacteria will now produce human insulin.
A few little extra bits…. Genetics is full of terminology and rules. Here a few of them…. The type of inheritance where you look at just one gene is called Monohybrid inheritance. Homozygous = pure breeding. Heterozygous = hybrid = carrier (for the recessive allele) Offspring = f 1 generation Grandchildren = f 2 generation Sexual reproduction = 2 parents (using gametes from meiosis) Asexual reproduction = 1 parent (using mitosis)
The last two slides just show mitosis and meiosis in a bit more detail – too much detail for NCEA level 1, but useful if you are going on to do Biology Level 2
Mitosis 1. The chromosomes copy themselves and the nucleus disappears 4. Two new identical cells are produced 2. The chromosomes split apart to opposite ends. 3. The cell splits in half
Meiosis 3. The cell divides. 1. The chromosomes copy themselves 4. The chromosomes are pulled apart 2. The chromosome pairs are pulled apart. 5. The cells divide again to produce 4 gametes.