Gregor Mendel“Father of Genetics”

3 Laws from the 1860’s

Law of Dominance

• Every gene has two alleles that can code for a trait.

• One allele is dominant, meaning it will always show.

• One allele is recessive, meaning it will be masked by the presence of the dominant allele.

Mendel crossed two pure strains of pea plants with purple and white flowers and discovered that the first filial generation were all purple.

Law of Dominance States…

• Hybrids will always show the dominant phenotype.

• Ex: PP = purple pp = white

Pp = purple

Law of Segregation

• Mendel arrived at this conclusion by performing monohybrid crosses.

• These were cross-pollination experiments with pea plants that differed in one trait, for example height of pea plant.

Law of Segregation States…

• The alleles for a trait separate when gametes are formed. These alleles can then randomly united at fertilization.

Anaphase I of Meiosis Anaphase II of Meiosis

Law of Independent Assortment

• Mendel began to wonder what would happen if he studied plants that differed in two traits (dihybrid).

• Data showed that traits produced by dominant factors do not have to appear together in offspring.

• Ex: A green seed pod (dominant) could appear in a white-flowering pea plant (recessive).

• In other words, dominant traits don’t have to travel together when traits are passed from parents to offspring.

• Mendel’s law of independent assortment applies only to traits carried on different chromosomes, i.e.unlinked genes

• independent assortment occurs as a result of the alignment of homologues during metaphase I, determining which maternal and paternal chromosomes assort to each daughter cell

• each pair of alleles separates independently of every other pair of unlinked alleles during gamete formation

The data…

• Mendel performed dihybrid crosses in plants that were true-breeding for two traits. For example, a plant that had green pod color and yellow seed color was cross-pollinated with a plant that had yellow pod color and green seeds. In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive. The resulting offspring or F1 generation were all heterozygous for green pod color and yellow seeds (GgYy).

• After observing the results of the dihybrid cross, Mendel allowed all of the F1 plants to self-pollinate.

• He referred to these offspring as the F2 generation.

• Mendel noticed a 9:3:3:1 ratio.

Law of Independent Assortment States…

• allele pairs separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another.

References

• http://biology.about.com/od/mendeliangenetics/ss/independent-assortment.htm

• http://en.wikipedia.org/wiki/File:Independent_assortment_%26_segregation.svg

• http://intranet.canacad.ac.jp:3445/BiologyIBHL1/3094