Early Theories of Genetics

Thomas Hunt MorganUnited Streaming: Biologix: Introduc. To Classical Genetics – Parts 1-3

Mendelian Genetics Mendel bred, tended, and analyzed over

28 000 pea (Pisum sativum) plants in a monastery garden.

He observed many different traits or characteristics.

He let the plants pollinate with each other and called them true breeding plants

These true breeding plants, parental or P, generation.

The offspring he called, filial or F1, generation.

The F1 were then allowed to self-pollinate and their offspring were the second filial, or the F2 generation.

Expression of traits or factors

Dominant: the trait that was expressed or shown. – Shown in as a capital letter– for example, B

(black hair) Recessive: the trait that was not

expressed.– Shown as a lowercase letter—for example, b

(blonde hair)

Terminology Combination of alleles (letter

representation, different form of the gene) is called the genotype.

The phenotype is the physical form that is observed.

Homozygous – a genotype with the exact same allele, RR

Heterozygous – a genotype with different alleles, Rr

Ways to express possible traits

Punnett Square

The Law of Segregation – Mendel’s First Law

All individuals will have two copies of each factor (or gene). These copies will segregate (separate) randomly during gamete formation, and each gamete receives one copy of every factor.

Alternative versions of genes account for variations in inherited characteristics – allelesIf the two alleles, then one, that is the dominant trait is fully expressed.

The Law of Independent Assortment

Using a dihybrid cross (two genes are involved), Mendel conducted experiments that cross true breeding tall plants that had green pods (TTGG) with true breeding short plants that had yellow pods (ttgg).

• He noticed that that results showed a 9:3:3:1 ratio—producing F2 plants with the phenotypes of tall with green pods, tall with yellow pods, short with green pods, and short with yellow pods.

• Therefore Mendel concluded, that the two alleles for one gene segregate (assort) independently of the alleles for other genes during gamete formation.

Test Crosses Test crosses are often

used to determine the genotype of the parent generation, ie, to determine genotype of a dominant phenotype, or if the parent is homozygous or heterozygous.

The test cross is always performed between the unknown genotype and a homozygous recessive genotype.

Incomplete Dominance Incomplete Dominance: describes a condition in which

neither of two alleles for the same gene can be completely conceal the presence of the other.

Superscripts are used to identify the expression of the allele In this case, neither of the alleles for flower color are

completely dominant over the other; therefore, individuals with heterozygous alleles have a phenotype unlike those with either set of homozygous alleles.

Co-dominance A situation in which

both alleles are fully expressed.

Both genes are expressed at the same time.