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Hereditary and sex determination
Gregor Mendel (and Austrian monk) was the first to notice that different parent plants created different offspring. He noticed that
- Characteristics in plants were determined by hereditary units
- Hereditary units are passed on from both parent, one unit from each
- Hereditary units are either dominant or recessive
We now know that these hereditary units were in fact genes.
The 23rd chromosome is the sex determination chromosome. Men have XY, while women have XX.
However, there is an equal chance of the offspring being female or male. (Note Y is the dominant chromosome)
As you can see there is a 50:50 chance of the offspring being male or female
Monohybrid crosses are the breeding of a plant or animal which has one gene different (e.g. breeding a HH hamster with an Hh hamster).
The generation are; parental (the parents) F1 (the children) and F2 (the grand children).
For a recessive allele to give its phenotype it must be homozygous recessive. For a dominant allele to give its phenotype it can be either heterozygous or homozygous dominant.
Cystic fibrosis is caused by a recessive allele. It affects about 1 in 1600 people in the UK. It causes the body to over produce mucus, and to produce thick sticky mucus at that. This mucus occurs in the lungs and can block the airways as well as cause chest infections. It also occurs near the pancreas which causes digestive problems. Physiotherapy is a short term solution, but the person gradually gets more and more ill. If two parents are carriers there is a 1 in 4 chance the person will get it, as to get it you must be homozygous recessive
Sickle cell anaemia is identical genetically to cystic fibrosis. It is a condition where the red blood cells are shaped like sickles so they can get stuck in the capillaries which can deprive cells of oxygen. It is also very painful. In Africa the rate is high because the gene which causes sick cell anaemia protects against malaria and therefore it is and advantage to have it, even if some offspring die, less will die than malaria would kill.
Huntington’s chorea is caused by a dominant allele. The disorder itself causes severe shaking and mental degeneration. However, because it’s caused by a dominant allele there are no caries, but the symptoms don’t usually appear until the person is 40, by which time they may well have had children. If one parent has it there is there is a 50% chance that the children will have it.
Polydactyl is another dominant genetic disorder, and is when you have an extra digit
Sex Linked diseases
Many conditions are more likely to be present in men than in women because of the XY chromosome men possess. These include haemophilia and colour blindness. These diseases are caused by recessive alleles which are carried on the X chromosome. In men, this X chromosome is “cut” in half to form a Y, so the possibility of having a dominant allele there has gone. This means that it is completely down to the woman to not have a recessive allele there. If she is homozygous recessive then and his allele is recessive then the condition will be present in their children
(Red = normal, green = colour blind, black – has no effect)
As you can see this comes out with
- A female normal
- A female carrier
- A male normal
- A male colour blind
Co dominance
Co-dominance occurs when there are two alleles, neither which is recessive. Usually this results in the phenotype being a mixture of the two attributes
Here R = red and W = white
This means that;
- 25% are red
- 25% are white
- 50% are a mixture (i.e. pink)
There are also situations were the W chromosome may be recessive, but a pink flower appears anyway because the R chromosome isn’t dominant enough. This is called incomplete dominance.
Blood groups
Type O is recessive, while A and B are dominant
| \
| l-A
| l-o
|
| L-B
| l-AB
| l-Bo
|
| L-o
| l-Ao
| l-oo
|
Here L-O is recessive, but A and B are both dominant.
Therefore there is a
- 25% chance the person will be AB
- 25% chance the person will be B
- 25% chance the person will be A
- 25% chance the person will be O
Pedigree diagrams
They are used to show disease down a family tree. Usually
Mutation
Mutations are when organisms develop new characteristics. Usually these are bad, but occasionally they are beneficial.
A mutation is a change in a gene, DNA or number of chromosomes which lead to genetic variation. They all occur when the DNA is replicating itself and something goes wrong.
Mutations occur due to chance and radiation. However, exposure to radiation and some chemicals called mutagens increases the chance. If mutations occur in the body cells they sometimes begin reproducing uncontrollably, and this is caused cancer.
There are two main types of mutation; gene mutation and chromosome mutation. The latter is where the person has a problem with the individual chromosomes, maybe too many, or one turned around the wrong way. Gene mutation is where there is a chemical change inside an individual gene. This can often be a small change, but result is big changes, like sickle cell anaemia and cystic fibrosis.
Some mutations are good or neutral. Colour changes in birds, for example, are often neutral while bacteria mutate into bacteria which are immune to antibiotics is a beneficial mutation.
Down’s syndrome is caused by a mutation where there are 3 chromosome 21s. The problem occurs in the woman’s ovaries, where both 21s get into the egg. This leads to the child having a lower mental ability, and also to them being more susceptible to disease.
Also See
Here are the other comprehensive GCSE Biology notes by Prometheus:
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