What does the Mendelian inheritance study? What did he do? What was the result?
- study only a pair of contrasting characters
- crossed pure-breeding plants and counted the offspring –> crossed F1 plants with each other –> F2 generation
- dominant phenotypes appear in F1 –> recessive phenotype re-appear in F2 –> F2 ratio 3: 1 (dominant : recessive)
What are alleles? What can they be? What is the effect of dominant and recessive alleles? What does Mendel’s first law (law of segregation) refer to?
- an alternative form of genes occurring in plants
- two alleles can be different (heterozygous) or same (homozygous)
- dominant allele determines the phenotype; recessive allele has no noticeable effect
- Mendel’s first law (law of segregation): during gamete formation, the two alleles of a gene segregate so that each gamete only has one allele
What is test cross used for? How to conduct it? What result indicates what?
- to determine the genotype of an organism as an organism with dominant phenotype can be heterozygous or homozygous
- cross the organism with a homozygous recessive individual
- all offspring are dominant –> homozygous
- half dominant and half recessive –> heterozygous
What does dihybrid cross refer to? What is Mendel’s second law (law of independent assortment) about? What is the phenotype ratio of dihybrid cross?
- a cross of two pairs of characteristics
- each pair of alleles segregates independently of other pair of alleles during gamete formation
- either of a pair of alleles can pair with either of other pair of alleles –> 9:3:3:1 phenotype ratio of F2
How does the law of segregation relate to meiosis?
- alleles on homologous chromosomes segregates at meiosis I
- sister chromatids segregate at meiosis II
- each gamete only carries one allele
How does the law of independent assortment relate to meiosis?
- each of a pair of alleles on homologous chromosomes may combine with either one of another pair of alleles on different homologous chromosomes
How to describe the relationship between genotype and phenotype? What are the examples of non-Mendelian inheritance?
- not easy to predict
- incomplete dominance, codominance, polygenic inheritance, autosomal linkage and sex linkage
Describe organisms that are incomplete dominance. Give an example of breeding and generation produced in terms of phenotype and genotype.
- heterozygous display a phenotype intermediate between the homozygotes (flower colour of snapdragons)
- homozygous red (RR) x homozygous white (rr) –> F1 offspring (Rr) are heterozygous and pink
- F2 offspring display 1:2:1 phenotype (red:pink:white) and genotypic (RR:Rr:rr) ratio
Describe organisms that are codominance. Give an example which is codominance.
- both alleles are expressed in the phenotype of heterozygotes (blood group)
- ABO blood group system: IA and IB are codominance; i is recessive (blood group AB is codominance)
- IA, IB and i determine the structure of carbohydrates on the surface of red blood cells
What is polygenic inheritance and its characteristics? Give examples of polygenic inheritance. What is pleiotropy?
- a single phenotype is controlled by multiple genes
- phenotypes show a normal distribution
- affected by environmental factors
- skin colour, height, weight and intelligence
- a single gene can influence multiple phenotypes
What are autosomes? How and what favours them to inherit together? How can they induce genetic variation?
- chromosomes other than sex chromosomes
- all alleles located close together on the same chromosome are linked –> inherit together
- alleles on the same chromosome may be separated by crossing over (prophase I of meiosis) –> creation of new recombinants –> induce genetic variation
Where do sex-linked genes locate? How are X-linked and Y-linked genes can be passed respectively?
- located on either of the sex chromosomes
- X: from mother to daughter/ father to daughter/ mother to son
- Y: from father to son (will be affected in a homozygous state)
What is the cause of genetic diseases? What are the two major types of mutations? Describe their characteristics.
- caused by mutations
- chromosome mutation
=> changes in chromosome structure
=> deletion, insertion, inversion and translocation
=> observable under microscope - gene mutation
=> changes in nucleotide sequences
=> cannot be seen under microscope
How do mutations arise?
- spontaneous mutation
=> occur naturally without any known cause - induced mutation
=> occur artificially by mutagens
=> chemical mutagens: nitrous acid, nucleotide base analogs
=> physical mutagens: ultra-violet radiation, X-rays and gamma-rays
What is cystic fibrosis? What is affected? What is its normal function? How will it be affected? What is the cause of CF?
- common single-gene disorder, affecting 1 in 2000 newborn babies
- normal allele encodes a protein, cystic fibrosis transmembrane conductance receptor (CFTR) which is a CL- channel
- CFTR is used to export CL- ions across the plasma membrane of epithelial cells lining the lung airways –> H2O follows CL- due to osmosis
- CF allele impairs the outward transport of CL- and H2) –> thick and sticky mucus build up in the lungs –> repeated bacterial infections –> long-term lung damage
- a 3-base pair deletion called ∆F508 is the most common mutation causing CF –> deletion of a single amino acid called phenylalanine (F) at position 508 –> non-functional CFTR protein
(Autosomal recessive disease) When will recessive phenotypes be expressed? What does it indicate if a child is affected but his/her parents do not? What is the probability of offspring being infected/ normal if both of his/her parents are heterozygous?
- only be expressed at a homozygous state (homozygous recessive)
- both parents must be heterozygous (carriers)
- for the offspring, 75% unaffected and 25% affected
What is Huntington’s disease? What is the cause of it? What is different when comparing affected people with normal people? What are the symptoms?
- genetic and neurological disorder, usually affects people >50 years old
- trinucleotide repeat expansion (CAG; encoding glutamine) in Hungtington (HTT) gene —> mutated Huntington (HTT) protein
- normal has around 15 repeats while mutant has >40 —> abnormally long glutamine tract in HTT —> aggregation of HTT —-> increased neuron degradation
- memory loss, depression, involuntary dance-like movements, (loss of speech and decreased ability to swallow)
Who will be affected for autosomal dominant diseases? What is the chance for heterozygous parent to produce an affected child?
- all heterozygous individuals will be affected
- 50%
What is the cause of haemophilia? What are the symptoms of haemophilia? Which sex has higher chance and what is the reason behind? What is the characteristic of X-linked diseases?
- mutation of gene encoding for blood clotting factor XIII (haemophilia A) or IX (haemophilia B) located on X chromosome —> inability to form blood clot
- excessive bleeding or spontaneous internal bleeding
- males only have one X chromosome (hemizygous) —> higher frequency of haemophilia
- most X-linked disorder are recessive —> most females are unaffected but they can still be the carrier
What will be the chance for woman who is a carrier of an X-linked recessive disorder to have an affected son (with an unaffected father)? From who the infected son received an X-linked mutated allele?
- 25%
- from the mother
What is the other name for Down syndrome? What is the cause of it? What are the symptoms?
- trisomy 21
- failure to separate homologous chromosome in meiosis I or failure to separate sister chromatids in meiosis II —> three copies of chromosome 21
- mental and growth retardation, unusual facial fractures (flat face, short neck and small head)
