17.1 Studying Inheritance

• Genotype is the genetic constitution of an organism. It describes all of the alleles that an organism has.
• The genotype determines the limits within which the characteristics of an individual may vary.
• Phenotype is the observable or biochemical characteristics of an organism. It is the result of the interaction between the expression of the genotype and the environment.
• A gene is a length of DNA (a sequence of nucleotide bases) which codes for a particular polypeptide.
• Genes can exist in two or more different forms called alleles.
• The position of a gene on a particular DNA molecule is known as the locus.
• An allele is one of the different forms of a gene.
• Only one allele of a gene can occur at the locus of any one chromosome. However, in diploid organisms the chromosomes exist as homologous pairs. There are therefore two loci that carry one allele of a gene.
• In a homologous pair, if the allele on both chromosomes is the same, then the organism is said to be homozygous.
• In a homologous pair, if the allele on both chromosomes is different, then the organism is said to be heterozygous.
• In most cases where two different alleles are present in the genotype (heterozygous), only one is expressed in the phenotype.
• The allele of the heterozygote that expresses itself in the phenotype is dominant, whilst the one that isn't expressed is recessive.
• A homozygous organism with two dominant alleles is called homozygous dominant, whereas one with two recessive alleles is known as homozygous recessive.
• If two alleles both contribute to the phenotype, they are codominant. In this situation, the phenotype is either a blend of both features or both features will be represented.
• If a gene has more than two allelic forms, it is said to possess multiple alleles.
• Pure-breeding means that the organisms being bred are both homozygous (two of the same alleles).
• The law of segregation states that in diploid organisms, characteristics are determined by alleles that occur in pairs. Only one pair of alleles can be present in a single gamete.
• Actual results of genetic crosses may differ from predicted results because the sample size was too small (not representative).
• Monohybrid inheritance is the inheritance of a single gene.
• Dihybrid inheritance is the inheritance of two different genes located on different chromosomes.
• The law of independent assortment states that each member of a pair of alleles may combine randomly with either of another pair.
• An example of multiple alleles is the inheritance of the human ABO blood groups. There are three alleles associated with the gene I (IA, IB and IO) which lead to the presence of different antigens on the cell-surface membrane of red blood cells.
• IA and IB are codominant, whereas IO is recessive.
• Any gene that is carried on either the X or Y chromosome is said to be sex-linked.
• Haemophilia is an X-linked genetic disorder, caused by a defective gene on the X chromosome. With this condition, blood clots very slowly and consequently there can be slow and persistent internal bleeding.
• One of the causes of haemophilia is a recessive allele with an altered sequence of DNA nucleotide bases that code for a faulty, non-functioning protein. Therefore, a haemophilic individual may not be able to produce a functional protein needed for the clotting process.
• Autosomes refer to all 22 chromosomes that are not sex chromosomes.
• Autosomal linkage occurs when two or more genes are carried on the same autosome.