Chapter 3.4 Inheritance Flashcards
Describe Gregor Mendel’s experiment
Experiment subject: different varieties of pea plants, each of which reliabily had the same characteristics when grow independently
Method: hybridization (transferring the male pollen from one variety to the female parts of flowers of another variety), the resulted pea seeds were grown to find out their characteristics – one cross
Each cross was repeated with thousands of plants –> ensure accuracy and reliability –> statistically significant results at high confidence level, anomalous results are also less likely to distort the whole set of data
7 crosses were tested in total
Name of the scientist who did inheritance experiments with pea plants
Gregor Mendel
Genotype
The combination of alleles of a gene carried by one organism in a diploid cell.
Phenotype
The physical expression of the alleles of a gene processed by an organism. It is the resulted feature of one’s genotype.
Dominant
An allele that is expressed whether its paired allele is identical or different (i.e. regardless if the organism is heterozygous/homozygous of the gene in question).
Masks the effects of recessive alleles.
Recessive
Alleles that are only expressed if two copies of the same recessive allele are found in the genotype (i.e. homozygous recessive).
Codominant
Pairs of alleles that are both expressed when present (e.g. blood type A and B alleles) –> joint effects.
Loci
Specific locations on chromosomes where a gene is located.
Centromere
Joins two chromatids together during cell division (mitosis and meiosis).
Alleles
Different forms of the same gene.
Carrier
A heterozygous individual carrying a recessive disease-causing allele, who does not personally suffer from the disease.
Use of punnett grids
Predicting the outcomes of monohybrid (crossing a single trait) genetic crosses.
Steps of constructing a punnett grid
- Start with homozygous parents with different alleles (e.g. TT, tt).
- Write down both the genotype and phenotype of the parents.
- Write down the allele that would be contained in their gametes (e.g. T, t).
- Write down the genotype and phenotype of the F1 generation (e.g. Tt, tall stem).
- Construct a punnett grid (Tt * Tt) and write down the genotype and phenotypes for each grid.
- Calculate the ratios between genotypes and phenotypes.
Why blood testing is necessary to determine the blood group of a donor before a transfusion?
RBCs have agglutinogens (antigens) on their surfaces, which can be used to identify self/non-self cells by the immune system. The body of a patient who receives a wrong blood transfusion would react patally, as his lymphocytes would secrete antibodies which causes blood to clot.
Blood AB is known as…
universal recipient (the recipient’s RBCs have both type A and B agglutinogens)
Blood O is known as…
universal donor (no agglutinogens on type O RBCs)
Why is ABO blood groups often used as a textbook example?
Only one gene determines one phenotype, whereas more than one gene is often necessary to determine a phenotype in many other scenarios.
Why ABO blood groups is an example of codominance?
Both type A and B agglutinogens are present on RBCs for a IAIB person (type AB).
An example of genetic disease caused by a recessive allele
Cystic fibrosis (CF)
An example of genetic disease caused by a dominant allele
Huntington’s disease
An example of genetic disease caused by a codominant allele
Sickle-cell anemia
Examples of a sex-linked genetic disease
- RG colour blindless
2. Hemophilia
Cystic fibrosis
The P of a person being a CF carrier is relatively high, but since CF is caused by a recessive gene, both parents need to be carriers to have a 0.25 P of having a child with CF –> therefore overall P is low.
Caused by a recessive allele of the CFTR gene on chromosome 7.
The CFTR gene involves in the production of chloride ion channels that involve in secretions of sweat, digestive juices, and mucus.
The disease-causing allele results in production of faulty chloride ion channels –> instead of being a lubricant –> viscious sweat, digestive juices and mucus –> block tubes/ducts/air passages –> infections in lungs as sticky mucus accumulates + prevents digestive enzymes from reaching the duodenum from the pancreas.
Life expectancy: 35~50 years
C –> normal; c –> CF allele
Huntington’s Disease
Caused by a dominant allele of the HTT gene on chromosome 4. The gene product is huntingtin (protein).
Causes degenerative changes in the brain –> changes in behaviour, thinking and emotions become increasingly severe –> patients will eventually need full nursing care and often suffer/die from heart failure and/or infectious diseases such as pneumonia (lung infection)
Symptoms usually show up when the person is between 30~50 years old –> late onset –> the diagonised patient may have already had children by that age
H –> HD allele; h –> normal
Sickle-cell anemia
Hb(S) –> SC allele; Hb(A) –> normal
Three possible scenarios:
- Hb(S)Hb(S): susceptible to malaria, severe anemia (WORST SITUATION!)
- Hb(S)Hb(A): increased resistance to malaria, mild anemia
- Hb(A)Hb(A): susceptible to malaria, not anemic
Why sex-linked genetic diseases are more common in males?
Caused by recessive alleles on the sex-linkage: sex chromosomes are not homologous, there are some genes present on X but not on Y.
A male processing a disease-causing allele is affected, regardless whether the allele is dominant/recessive, as he does not have an alternative allele on the Y chromosome to combat the disease-causing allele on the X chromosome.
RG colour blindness
Caused by a x-linked, recessive allele of the RG gene which codes for a photoreceptor protein in cone cells.
Locus of the gene is Xq28, the non-homologous section of the X chromosome.
XA –> normal
Xa –> colour blindness
Hemophilia
Caused by a recessive x-linked mutation in hamophilia which causes one of the clotting factors not being produced.
The sufferer could bleed to death. X^X
Treatment: injecting commercial blood clotting factors produced by genetic modification. Restriction enzymes cutting out the genes which codes for blood-clotting factors –> inserted into a sheep embryo’s milk gene –> clotting factors produced along with sheep milk –> extraction and purify
XH –> normal
Xh –> hamophilia
General rules when interpreting a pedigree diagram
- Male – squares, female – circles
- Generations are indicated by roman numerals (I, II etc.)
- Individuals are indicated by letters (A, B etc.)
- Sufferers are shaded, carriers are half-shaded –> KEY
Uses of pedigree charts
Trace family history of genetic-diseases
Shows carriers, sufferers, non-sufferers
Predict the probability of a new-born child having a genetic disease.
Causes of genetic mutations
- Exposure of short WL radiations: UV, X-ray, gamma ray, alpha particles from radioactive isotopes
- Chemicals: carcinogens in tobacco smoke, mustard gas (WW2)
Once a person dies, his mutations are eliminated from the human gene pool. Yet if that person has offspring, his genetic mutations would be passed on by gametes.
Define autosomal genetic disease and give examples.
Genetica diseases caused by fatally genes from non-sex chromosomes (autosomes).
Examples: Hungtington’s disease, cystic fibrosis, sickle-cell anemia.
