3.4 Inheritance Flashcards
What were Mendel’s experiments?
They were done using a variety of pea plants. Tallness of pea plants was dominant. Smallness of pea plants was recessive.
What are gametes?
Gametes are haploid so contain one allele of each gene. They have one of the paired chromosomes.
What is a zygote?
A zygote is the result of 2 gametes, a male and a female, fusing together and doubling the chromosome number. So they contain a gene from each parent for each characteristic.
What does co-dominant mean?
Co-dominant alleles are ones where both alleles will have an effect when they are present together. For example a red and white flower making a pink flower.
What is an example of a co-dominant allele?
The ABO blood groups. The genotype IAIA gives blood group A and the genotype IBIB gives blood group B, but neither is dominant over the other so someone with the genotype IAIB has a different blood group AB. O is recessive.
The reason they are co-dominant is because:
- All of the three alleles cause the production of a glycoprotein in the membrane of the red blood cells.
- IA alters the glycoprotein by addition of acetyl-glalactosamine. This protein is absent from people who do not have the IA allele so if exposed to it the make anti A antibodies.
- IB alters the glycoprotein by addition of galactose. This altered protein is absent from people who do not have the IB allele so if exposed to it make anti B antibodies.
- So genotype IAIB causes both acetyl galactosamine and galactose to be added so neither A or B antibodies are produced.
Why are A and B blood group alleles co-dominant?
The reason they are co-dominant is because:
- All of the three alleles cause the production of a glycoprotein in the membrane of the red blood cells.
- IA alters the glycoprotein by addition of acetyl-glalactosamine. This protein is absent from people who do not have the IA allele so if exposed to it the make anti A antibodies.
- IB alters the glycoprotein by addition of galactose. This altered protein is absent from people who do not have the IB allele so if exposed to it make anti B antibodies.
- So genotype IAIB causes both acetyl galactosamine and galactose to be added so neither A or B antibodies are produced.
What causes genetic diseases?
Many genetic diseases in humans are caused by a recessive allele of a gene. It passes down through carriers who have one dominant and one recessive so they carry it but do not show signs.
Give an example of a co-dominant genetic disease?
Sickle cell anaemia. Individuals who have one hbA and one hbS do not have either sickle cell anaemia or not sickle cell anaemia. As they do not have the same characteristics as those who have two copies of either allele the alleles are co-dominant.
AA
- susceptible to malaria
- not anaemic
AB
- increased resistance to malaria
- mild anaemia
BB
- susceptible to malaria
- severe anaemia
What is cystic fibrosis?
It is due to a recessive allele of the CFTR gene. This gene is located on chromosome 7 and the gene product is a chloride ion channel that is involved in secretion of sweat, mucus and digestive juices.
The recessive allele result in these chloride channels not functioning properly. Sweat containing excessive amounts of sodium chloride is produced, but digestive juices and mucus are secreted with insufficient sodium chloride. As a result not enough water moves into the secretions making them very sticky. Sticky mucus builds up in the lungs causing infections and the pancreatic duct is usually blocked so digestive enzymes secreted by the pancreas do not reach the small intestine.
What is Huntington’s disease?
Huntington’s disease is due to a dominant allele of the HTT gene. This gene is located on chromosome 4 and the gene product is a protein called Huntingtin. The function is still being researched.
The dominant allele of HTT causes degenerative changes in the brain. Symptoms usually start between 30 and 50 years old.
Changes to behaviour, thinking and emotions become increasingly severe. After they start showing symptoms the life expectancy is only 20 years. A person with the disease eventually needs full nursing care and usually succumbs to heart failure, pneumonia or some other infectious disease.
What is sex linkage?
An inheritance pattern where the ratios are different in males and females and the genes are directly tied to the sex chromosomes.
Almost all examples of sex linkage are due to genes on the X chromosome as there are very few genes on the Y chromosome.
Give an example of sex-linked inheritance?
Almost all examples of sex linkage are due to genes on the X chromosome as there are very few genes on the Y chromosome.
- Red Green colour blindness is caused by a recessive allele of a gene for one of the photoreceptor proteins. These proteins are made by cone cells in the retina of the eye and detect specific wavelength ranges of visible light. Males only have one X chromosome and if that X chromosome carries the red-green colour-blindness allele then the son will be red-green colour blind.
- Hemophilia is cause by an inability to make Factor 8, one of the proteins involved in the clotting of the blood. Life expectancy is only about 10 years if hemophilia is untreated. Treatment is by fusing Factor 8 from donors blood into theirs. The gene for hemophilia is located on the X chromosome and it is recessive so men will have it if they have inherited the gene from their mother, whereas women have another chance to have another dominant allele for something else.
What causes red-green colourblindness?
- Red Green colour blindness is caused by a recessive allele of a gene for one of the photoreceptor proteins. These proteins are made by cone cells in the retina of the eye and detect specific wavelength ranges of visible light. Males only have one X chromosome and if that X chromosome carries the red-green colour-blindness allele then the son will be red-green colour blind.
What increases the chances of mutation?
Radiation and mutagenic chemicals increase the mutation rate and can cause genetic disease and cancer. A mutation is a random change to the base sequence of a gene. Two types of factor can increase the mutation rate.:
- Radiation increases the mutation rate if it has enough energy to cause chemical changes in DNA. Gamma rays and alpha particles from radioactive isotopes, short-wave ultraviolet radiation and X-rays are all mutagenic.
- Some chemical substances cause chemical changes in DNA and so are mutagenic. Examples are benzo(a)pyrene and nitrosamines found in tobacco smoke and mustard gas used as a chemical weapon in WW1.
What is the origin of genetic diseases?
Mutations in body cells, including those that cause cancer, are eliminated when the individual dies, but mutations in cells that develop into gametes can be passed on to offspring. This is the origin of genetic diseases. It is therefore particularly important to minimise the number of mutations in gamete producing cells in the ovaries and testes.
