Ch10

• 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)

• 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

• 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

• 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

• alleles on homologous chromosomes segregates at meiosis I • sister chromatids segregate at meiosis II • each gamete only carries one allele

• each of a pair of alleles on homologous chromosomes may combine with either one of another pair of alleles on different homologous chromosomes

• not easy to predict • incomplete dominance, codominance, polygenic inheritance, autosomal linkage and sex linkage

• 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

• 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

• 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

• 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

• 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)

• 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

• 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

• 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

• only be expressed at a homozygous state (homozygous recessive) • both parents must be heterozygous (carriers) • for the offspring, 75% unaffected and 25% affected

• 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)

• all heterozygous individuals will be affected • 50%

• 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

• 25% • from the mother

• 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)