Mendelian inheritance and the role of meiosis in determining the pattern of inheritance

Gregor Mendel: The Father of Genetics

• Discovered dominant and recessive traits.

• Introduced the concept of the gene ("heritable factor").

• Formulated the basic laws of inheritance through pea plant experiments

Character

Heritable features e.g. flower colour

Trait

Variant of a character (e.g., purple or white)

Alleles

Alternative forms of a gene

Mendel’s law of segregation

The two alleles for a gene segregate (separate) during gamete formation

Features of Mendel’s law of segregation

• Each organism has two alleles per gene.

• During meiosis, alleles segregate so each gamete gets one allele.

• Formulated via monohybrid crosses.

• F1 generation: shows only dominant trait.

• F2 generation: 3:1 ratio (dominant:recessive)

Mendel’s law of independent assortment

Alleles of different genes assort independently during gamete formation

Features of Mendel’s law of independent assortment

• Formulated through dihybrid crosses.

• F2 generation: 9:3:3:1 ratio.

• Independent assortment creates recombinant phenotypes.

• If not independent, only parental combinations would appear

Punnett square

• Used to predict offspring genotypes and phenotypes.

• Reveals how different genotypes can produce the same phenotype

Testcross

• Used to determine the genotype of a dominant phenotype.

• Cross with a homozygous recessive individual

Chromosome theory of inheritance

• Sutton (1902)

• Chromosomes occur in pairs and segregate/assort during meiosis

Sutton (1902)

Chromosomes carry Mendel’s "heritable factors"

Mitosis

• Somatic cells

• 1 division

• 2 diploid identical cells

• Role is growth and repair

Meiosis

• Germ line

• 2 divisions

• 4 haploid unique gametes

• Genetic variation

• Synapsis

• Role is sexual reproduction

Law of Segregation explained

• Alleles are on homologous chromosomes.

• During Anaphase I, homologs (and thus alleles) separate

Law of Independent Assortment explained

• Chromosomes line up randomly at the Metaphase I plate.

• Leads to independent inheritance of gene pairs.