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: Finge