Fundamentals of Inheritance

  • Inheritance is the process by which genetic information is passed from parents to offspring.
  • Each individual has a pair of alleles for each characteristic, one from each parent.
  • These alleles can be dominant or recessive, determining how the trait is expressed.

Genotype and Phenotype

  • The genotype is the underlying genetic make-up of an individual, represented by letters (e.g., BB, Bb, bb).
  • The phenotype is the expressed trait or traits as a result of the genotype and the individual’s environment.

Mendel’s Laws

  • Gregor Mendel’s work laid the foundation for understanding inheritance.
  • Mendel’s First Law, the Law of Segregation, states that during gamete formation, the two alleles for each trait separate, so each gamete carries just one allele for each characteristic.
  • Mendel’s Second Law, the Law of Independent Assortment, says that alleles of different genes assort independently of one another in the formation of gametes.

Punnett Squares

  • Punnett squares are used to predict the probability of specific genotypes and phenotypes in offspring.
  • Each box represents an equally likely product of fertilisation.
  • Punnett squares consider one characteristic (monohybrid) or two (dihybrid).

Co-dominance and Multiple Alleles

  • In co-dominance, both alleles contribute to the phenotype in separate, distinguishable ways.
  • Some genes have multiple possible alleles, known as multiple alleles.
  • The ABO blood group system is an example of multiple alleles and co-dominance at work.

Sex-linked Traits

  • A sex-linked trait is one where the gene responsible for the trait is located on the sex chromosomes.
  • More common in males as they have only one copy of the X chromosome (e.g. haemophilia and colour blindness).

Autosomal and Recessive Disorders

  • Some genetic disorders are caused by abnormalities in the autosomal chromosomes and can be dominant or recessive.
  • Dominant disorders only need one copy of the defective gene for the disorder to develop (e.g. Huntington’s disease).
  • Recessive disorders require two copies of the defective gene (e.g. cystic fibrosis).

Pedigree Analysis

  • A pedigree is a diagram that depicts the biological relationships between an organism and its ancestors.
  • It’s used often in genetics to study the inheritance pattern of traits or diseases.

DNA and Chromosomes

  • DNA carries the genetic code that determines the characteristics of living organisms.
  • DNA is packaged into structures called chromosomes, each carrying many genes.

Genetic Variation

  • Mutations in the DNA sequence contribute to genetic variation.
  • Factors like crossing-over during meiosis and random fertilisation of gametes also add to diversity.
  • Variation allows populations to adapt to changing environments via natural selection.