Mendelian Genetics

Gregor Johann Mendel (1822–1884)

Hugo de Vries inHolland

Carl Correns in Germany

Eric von Tschermak-Seyseneggin Austria

William Bateson

MONOHYBRID CROSSES: THE PRINCIPLESOF DOMINANCE AND SEGREGATION

• Mendel inferred that the hybrids carried a latent genetic factor that was masked by the expression of another factor

recessive = latent factordominant = expressed factor

• The factors are separate from each other during reproduction thus the reappearance of the latent factor on the next generation


Monohybrid cross = when single trait is being studiedGenes = the heritable factor that existed in 2 forms (dominant or recessive) that controls each trait (Wilhelm Johanssen, 1909)Allele = alternate forms of a gene; the dominant or recessive forms


• Genes come in pairsDiploid & Homozygous = when each of the parental strains carries two identical copies of a gene

• However during production of gametes, two copies are reduced to one (haploid)

• He recognized that diploid gene would be restored during fertilization, and hybrid would inherit 2 different alleles (1 from mother, 1 from father)

Heterozygous = inheriting 2 different alleles


• Mendel realized that the different alleles that are present in a heterozygote must coexist even though one is dominant and the other recessive, and that each of these alleles would have an equal chance of entering a gamete when the heterozygote reproduces.


Phenotype = allelic constitution of each strainGenotype = physical appearance of each strainAllele segregation = neither allele is changed when coexisting as heterozygous, rather they are separate


Summary of Mendel’s Analysis

• The Principle of Dominance: In a heterozygote, one allele may conceal the presence of another.

• The Principle of Segregation: In a heterozygote, two different alleles segregate from each other during the formation of gametes.

DIHYBRID CROSSES: THE PRINCIPLEOF INDEPENDENT ASSORTMENT

• Crossing plants that differed in two traits

DIHYBRID CROSSES: THE PRINCIPLEOF INDEPENDENT ASSORTMENT

This analysis is predicated on two assumptions:

(1)that each gene segregates its alleles

(2) That these segregations are independent of each other.

Summary of Mendel’s Analysis

• The Principle of Independent Assortment: The alleles of different genes segregate, or as we sometimes say, assort, independently of each other.

Fork Line MethodPunnett Square or the Probability Method

1. Two highly inbred strains of fish, one with black caudal fin and the other with gray caudal fin, were crossed, and all of the offspring had black caudal fin. Predict the outcome of intercrossing the offspring.

2. A crab heterozygous for three independently assorting genes, Aa Bb Cc, is self-fertilized. Among the offspring, predict the frequency of (a) AA BB CC individuals, (b) aa bb cc individuals, (c) individuals that are either AA BB CC or aa bb cc, (d) Aa Bb Cc individuals, (e) individuals that are not heterozygous for all three genes.

3. Two true-breeding strains of peas, one with tall vines and violet flowers and the other with dwarf vines and white flowers, were crossed. All the F1 plants were tall and produced violet flowers. When these plants were backcrossed to the dwarf, white parent strain, the following offspring were obtained: 53 tall, violet; 48 tall, white; 47 dwarf, violet; 52 dwarf, white. Do the genes that control vine length and flower color assort independently?

Mendelian Genetics

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