3.7.1 Inheritance

• Gene: A sequence of DNA bases that codes for a specific polypeptide or functional RNA.
• Allele: Different version of the same gene.
• Locus: The specific position of a gene on a chromosome.
• Genotype: The genetic makeup of an organism.
• Phenotype: The observable traits of an organism, determined by its genotype and environmental factors.
• Homozygous: Having two identical alleles for a gene.
• Heterozygous: Having two different alleles for a gene.
• Dominant allele: An allele that expresses its effect even when only one copy is present (e.g., B in Bb).
• Recessive allele: An allele that is only expressed in the phenotype if two copies are present (e.g., b in bb).
• Codominance: Both alleles are expressed in the phenotype when present together (e.g., AB blood type).
• Multiple alleles: More than two alleles exist for a gene (e.g., blood groups: A, B, O).
• Alleles may be dominant, recessive or codominant.
• Pure breeding is a homozygous pair of alleles.
• Monohybrid inheritance is when a phenotype or trait is controlled by a single gene with two alleles.

  • punnett squares and genetic crosses are used to predict the genotypes and phenotypes of offspring.
• A monohybrid cross between two heterozygotes (Bb x Bb) shows a 3:1 phenotypic ratio for dominant and recessive traits in the offspring.
• Dihybrid inheritance is when two characteristics are studied and it is determined by two different genes on two different chromosomes at the same time, each with two alleles are involved in the phenotype of an organism.
• A dihybrid cross between two heterozygotes (e.g., RrYy x RrYy) typically gives a 9:3:3:1 phenotypic ratio, assuming the genes assort independently (Mendel’s Second Law).
• Sex linkage is the expression of an allele dependent on the gender of the individual as the gene is located on the sex chromosome.

  • males have the chromosomes XY
  • females have the chromosomes XX
• Most sex-linked traits are located on the X chromosome with there being no equivalent locus on the Y chromosome as the X chromosome is larger.

  • therefore, females will carry two alleles of the sex-linked gene, but males will only carry a single gene.
• Autosomal linkage is when two or more genes are located close together on the same autosomal chromosome, they tend to be inherited together because they do not assort independently.
• An autosomal chromosome is a non-sex chromosome.
• Epistasis is the interaction of different loci on the gene, where the expression of one gene is influenced by another gene. One gene can either mask or suppress the expression of another gene.
• Recessive epistasis occurs when the presence of a recessive allele prevents the expression of another allele at a second locus. It gives the ratio 9:3:4.
• Dominant epistasis is when a dominant allele at one locus completely masks the alleles at a second locus. It gives a ratio of 12:3:1.
• The chi-squared test is used to test whether observed genetic ratios significantly differ from expected ratios.

  • Degrees of freedom: df = n - 1 , where  n  is the number of phenotypic categories.
• If the chi-squared value is below the critical value (p = 0.05), the null hypothesis (no significant difference) is accepted.
• F1 offspring are the first generation of offspring.
• F2 generation are the second generation of offspring.
• When two homozygotes breed in a dihybrid cross, all the F1 generation will have the same genotype.