Inheritance and Selection

Inheritance and Selection

1. Studying Inheritance

   • Definitions
     • Genotype: is the genetic constitution of an organism
     • Phenotype: the characteristics of an organism, often visible, resulting from both its genotype and the effects of the environment
     • Dominant alleles: a term applied to an allele that is expressed in the phenotype of an organism
     • Recessive allele: the conditions in which the effect of an allele is apparent in the phenotype of a diploid organism only in the presence of another identical allele
     • Codominant allele: condition in which both the alleles for one gene in a heterozygous organism contribute to the phenotype
     • Multiple alleles: term used to describe a gene that has more than two possible alleles

2. Monohybrid Inheritance

   • Monohybrid inheritance is the inheritance of a single gene
   • Organism that are homozygous for that particular gene are said to be pure breeding
   • In diploid organisms, characteristics are determined by alleles that occur in pairs
     • Only one of each pair of alleles can be present in a single gamete
   • Punnet Square

3. Sex inheritance and Sex linkage

   • Haemophilia
     • Caused by a recessive allele
       • The defective allele is linked to the X chromosome, males always inherit it from their mother.
       • Some females are carriers because they can carry the allele without showing any sign of the character in their phenotype
     • People who suffer from haemophilia are unable of producing a protein that is required for blood clotting
     • Males cannot pass on the defective allele to their sons but they can pass it on to their daughters
   • Pedigree charts
   • Males produce both X and Y chromosomes
     • It is which chromosome that combines with the female gamete that determines the sex of the offspring
   • Any gene carried on the X or Y chromosome is said to be sex linked
     • The X chromosome is much longer than the Y, this means that most of the X chromosome doesn’t have an equivalent portion on the Y chromosome
       • This means that recessive alleles found on this portion of the X chromosome will be more likely to be expressed; because of this recessive phenotypes are more likely to be present in men.

4. Co-dominance and multiple alleles

   • Co-dominance
     • Both alleles of the same gene are expressed
   • Multiple alleles
     • Sometimes a gene has more than two alleles
   • Multiple alleles and a dominance hierarchy
     • When there are multiple alleles, some are more likely to be more dominant than others.
       • They are they then arranged in a hierarchy according to which alleles they are dominant over.

5. Allelic frequencies and population genetics (The Hardy-Weinburg Principle)

   • The principle predicts that the proportion of dominant and recessive alleles of any gene in a population remain the same from one generation to the next provided that the conditions are met.
     • Conditions
       • No mutations
       • There is no emigration or immigration
       • There is no selection
       • The population is large
       • Mating within the population is random

6. Selection

   • Selection is the process by which organism that are better adapted to their environment survive and breed, while those less adapted fail to do so.
   • Directional Selection
     • Directional selection results in phenotypes at one extreme of the population being selected for and those at the other extreme being selected against
   • Stabilising Selection
     • Stabilising selection results in phenotypes around the mean of the population being selected for and those at both extremes being selected against
   • There is genetic variation in a characteristic within a population
     • Individuals with characteristics that make them less well adapted to their environment will die young from predation, disease or competition.
       • Individuals with characteristics that make them well adapted to their environment will survive and reproduce.
         • The allele frequency will change in each generation.

7. Speciation

   • Speciation is the evolution of new species from existing species
     • A species is a group of individuals that share similar genes and are capable of breeding with one another to produce fertile offspring
   • A population becomes separated into two groups that are reproductively isolated, so that there is no gene flow between the groups.
     • The two groups’ environments are different, so natural selection favours different characteristics.
       • The allele frequencies in the two groups will change in different ways.
         • Eventually the two populations will be unable to interbreed, so will be different species.

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