Define codominance (inheritance of blood groups).
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| Term | Definition |
|---|---|
Define codominance (inheritance of blood groups). | Some alleles are both expressed in the same phenotype, this is called codominance. |
How are codominant alleles shown? | Both codominant alleles are shown with upper case letters in genetic diagrams, but the letters used are different. For example, feather colour in hens may be white, black or speckled (it has both white feathers and black feathers). The alleles can be shown as W for white and B for black. |
Codominance: What are the 3 possible genotypes? | There are three possible genotypes: WW BB and BW. |
Codominance: What are the 3 possible phenotypes? | There are also three possible phenotypes: WW = white, BB = black, and BW = speckled. |
Blood groups: The gene controlling human ABO blood groups has three alleles, not just two. | IA and IB are not dominant over one another - both are dominant over IO. |
Diploid human body cells have how many pairs of chromosomes, where? | Diploid human body cells have 23 pairs of chromosomes in the nucleus. |
22 pairs of human body chromosomes are known as what? What is the function and name of the 23rd pair? | Twenty two pairs are known as autosomes, and control characteristics, but the 23rd pair carries genes that determine sex and are known as the sex chromosomes. |
A genetic diagram, like a Punnett square, shows what? | A genetic diagram, like a Punnett square, shows how alleles may combine in zygote. The diagram below shows how sex is inherited. |
Sex-linked disorders | All genetic conditions are called disorders and those that inherit them are called sufferers. Most genetic disorders, like cystic fibrosis, require two recessive alleles to be inherited (one from the mother and one from the father). So if a person inherited both recessive alleles they would be a sufferer. If a person receives only one recessive allele, their one dominant allele means they do not have symptoms of the disorder. However, they are able to pass it to their children. They are called a carrier. If alleles for a disorder appear on the sex chromosomes they are called sex-linked. Men have one X chromosome and one Y chromosome. The Y chromosome is smaller than the X chromosome and does not possess as many genes. If a gene is contained on the part of the X chromosome that is missing from the Y chromosome, men only have one allele for that gene, rather than the usual two. If the allele on the X chromosome codes for a disorder, the man will always have the disorder because there isn't another allele on the Y chromosome to mask the effect of the allele. This means that even conditions caused by a recessive allele will be inherited by the man because there cannot possibly be a dominant allele on the X chromosome to mask the effect of that recessive allele. |
What is the genome of an organism? | The genome of an organism is the entire genetic material of that organism. Each of your diploid body cells (not sperm, eggs or red blood cells) has one copy of your entire genome. |
The mapping of the whole human genome has great importance for medicine. In order to exploit its secrets, it is vital that the human genome is fully understood. It enables us to: | Search for genes linked to different types of disease, understand inherited disorders and their treatment, trace human migration patterns from the past. |
What is selective breeding? | Selective breeding or artificial selection is when humans breed plants and animals for particular genetic characteristics. Humans have bred food crops from wild plants and domesticated animals for thousands of years. |
Main steps in selective breeding: Selective breeding takes place over many generations. These are the main steps for both plants and animals: | 1) Decide which characteristics are important enough to select. 2) Choose parents that show these characteristics from a mixed population. They are bred together. 3) Choose the best offspring with the desired characteristics to produce the next generation. 4) Repeat the process continuously over many generations, until all offspring show the desired characteristics. |
Characteristics can be chosen for usefulness or appearance. Desired characteristics in plants: | Disease resistance in food crops, wheat plants that produce lots of grain, large or unusual flowers. |
Characteristics can be chosen for usefulness or appearance. Desired characteristics in animals: | Animals that produce lots of milk or meat, chickens that lay large eggs, domestic dogs that have a gentle nature. |
Reasons the new varieties produced by selective breeding are good. | The new varieties may be economically important. For example, they may provide more or better quality food, or allow farmers to feed more people. |
Problems with selective breeding | Because of selective breeding, future generations of selectively bred plants and animals will all share very similar genes which will reduce variation. Genes and their different alleles within a population are known as its gene pool. Inbreeding can lead to a reduced gene pool, making it more difficult to produce new varieties in the future. This also makes organisms prone to certain diseases or inherited defects. |
Benefits of selective breeding include: | New varieties may be economically important, by producing more or better quality food. Animals can be selected that cannot cause harm, for example cattle without horns. |
Risks of selective breeding include: | Reduced genetic variation can lead to attack by specific insects or disease, which could be extremely destructive. Rare disease genes can be unknowingly selected as part of a positive trait, leading to problems with specific organisms, eg a high percentage of Dalmatian dogs are deaf. Creation of physical problems in specific organisms, eg large dogs can have faulty hips due to not being formed correctly. |
Cloning in plants: overview | Clones are genetically identical individuals. The cloning of plants has many important commercial implications. It allows a variety of a plant with desirable characteristics to be produced cheaply, quickly and on a large scale. Cloning often follows genetic engineering. It allows many copies of the new organism to be produced. Cloning expensive food crops has been carried out for many years, and causes the public fewer ethical and moral concerns than animal cloning. |