C13: Inheritance (5088 COMBINED BIOLOGY)

INHERITANCE Gene - A unit of inheritance, made up of a segment of DNA containing a sequence of nucleotides that codes for one specific polypeptide / protein that controls a trait in an organism. Allele - alternative forms of the same gene. - alleles occupy the same relative position on a pair of homologous chromosome. Dominant allele - traits are expressed in a homozygous or heterozygous condition - in heterozygous condition, dominant allele will mask / suppress the expression of the recessive allele. Recessive allele - Traits are expressed only in the presence of an identical allele in a homozygous condition. Homologous chromosome - A pair of chromosome, where one chromosome in the pair comes from the female parent and the other from the male parent. - They have the same shape, length and position of centromere. - They have the same sequence of gene loci. Genotype - combinations of alleles for a particular gene a) Homozygous: having two identical alleles controlling the trait (e.g. DD or dd). b) Heterozygous: having two different alleles controlling the trait (e.g. Dd). Phenotype - Expressed physical trait of an organism - Can be seen externally: eye/hair/skin colour, presence of dimples, chin shape, types of earlobes etc - Cannot be seen externally: blood types, resistance to certain diseases etc Phenotype is the expressed physical traits of an organism whereas genotype is the specific combination of alleles of a gene.

Pure-bred : Individuals who are homozygous at a particular gene locus. Selfing: fertilisation of gametes from same individual.

RATIOS - The observed ratio often differs from the expected ratio of 3:1 because of small sample size and the random nature of fertilisation. and

Sex determination in man - Chromosomes that determine gender of an organism is known as sex chromosomes. The rest of the chromosomes are known as autosomes. - Humans have a pair of sex chromosomes. The Y chromosome determines that the person is a male. Females have XX chromosome at position 23 Males have XY chromosome at position 23 VARIATIONS - Variations refers to the differences in traits between individuals of the same species. - The traits of an individual is dependent on interactions between the genes and the environment. - Genetic variation is heritable, but variations due to the environment are not. Discontinuous Variation - Clear cut phenotypes - There are few or no intermediate forms of the phenotype. Challenge: Give some other examples of discontinuous variations in humans. Answer: Fingerprints, gender, ability to roll tongue Continuous Variation - There are many intermediate forms. - It is controlled by the additive effect of numerous genes. (note: discontinuous variation is controlled by one or a few genes)

Genetic Diagrams vs Pedigree chart - Genetic diagrams show the probability of genotypes/phenotypes of the offspring due the crossing of two individuals. - Pedigree chart shows the actual phenotypes of the offspring due to the crossing of two individuals. In a pedigree chart, the actual genotype of the individual can only be determined when there are enough information of the individual’s parents or offspring. To deduce the genotype of the individual, explanation should include the allele passed on by the individual to his offspring or the allele that he inherited from his parents.

MUTATIONS Mutation is the sudden, random change in gene structure or chromosome number. Mutation results in varieties of individuals in a species. - Mutations may be inherited by the next generation if they occur during gamete production. - Mutations that occur in somatic cells will not be passed down from parents to offspring. - Dominant mutations are easily detected unlike recessive mutations, which may not be detectable for generations. - Mutations may be caused by change in number of chromosome or structure of gene. TYPES OF MUTATION Chromosome mutation - change in number of chromosomes (e.g. Down syndrome) Gene mutation - change in sequence of nucleotide bases resulted in a change in gene structure - may affect amino acids produced → may affect protein formed - variation introduced because of new alleles of genes - e.g. albinism and sickle-cell anaemia Sickle cell anaemia - Gene mutation caused by recessive allele. - Individuals who suffer from sickle cell anaemia are homozygous recessive. > when oxygen concentration is low, RBC of affected individuals are sickle-shaped - reduced surface area to volume ratio results in slower rate of transport of oxygen around the body - can be fatal (change in sequence of bases > change one amino acid > haemoglobin S (HbS) protein is produced instead of haemoglobin A (HbA) > changes three dimensional shape of haemoglobin molecule > HbS molecules tend to clump together > red blood cell appear sickle-shaped)

Down Syndrome - a chromosome mutation - individual has three copies of chromosome 21 - linked to reduced life span, varying degrees of mental retardation, heart defects SYNDROMES - low muscle tone - small stature - upward slant to eyes - flat facial profile - single deep crease across centre of palm - prone to gum disease and dental problems, problem with upper part of spine, poor development of small bowel and blockage of the large intestine and with an inability to pass stool, reduced fertility in males, prone to ADHD, autism, slow learning etc