MENDEL’S LAWS OF INHERITENCE

LSE

-03

: L

EC

TU

RE

-2

02

-02

-20

15

BASIC GENETIC TERMINOLOGY

• Gene: A genetic factor (region of DNA) that helps determine a characteristic

• Allele: One of two or more alternate forms of a gene• Locus: Specific place on a chromosome occupied by

an allele• Genotype: Set of alleles that an individual organism

possesses• Heterozygote: An individual organism possessing

two different alleles at a locus• Homozygote: An individual organism possessing two

of the same alleles at a locus• Phenotype (trait): The appearance or manifestation

of a character• Character: An attribute or feature

MENDEL’S EXPERIMENTAL ORGANISMTHE GARDEN PEA

Mendel chose the garden pea, Pisum sativum, to investigate the natural laws that govern plant

hybrids.What advantages were provided by Mendel’s choice of the garden

pea in his experiments?

MENDEL STUDIED SEVEN CHARACTERS THAT BRED TRUE

CROSSING PEA PLANTS

MONOHYBRID CROSS

THE PRINCIPLE OF SEGREGATION OF MENDELIAN FACTORS

WHAT MONOHYBRID CROSSES REVEAL

Mendel’s monohybrid crosses revealed the principle

of segregation and the concept of dominance.

The principle of segregation (Mendel’s first law) states that each individual diploid organism possesses two alleles for any particular characteristic. These two alleles segregate (separate) when gametes are formed, and one allele goes into each gamete.

The concept of dominance states that, when two different alleles are present in a genotype, only the trait encoded by one of them—the “dominant” allele—is observed in the phenotype.

THE TESTCROSSA testcross is a cross of an individual of unknown genotype, usually expressing the dominant phenotype, with a known

homozygous recessive individual to determine the genotype of the unknown individual.

WHAT RESULTS ARE POSSIBLE FROM A TWO-FACTOR CROSS?

Two hypotheses to explain how two different genes assort during gamete formation

MENDEL’S ANALYSIS OF TWO-FACTOR CROSSES

In this experiment, Mendel began with two types of true-breeding pea strains that were different with regard to two

characters. One strain produced round, yellow seeds (RRYY); the other strain produced wrinkled, green seeds (rryy).

MENDEL’S ANALYSIS OF TWO-FACTOR CROSSES

Mendel’s results from many two-factor experiments rejected the hypothesis of linked assortment and, instead, supported the hypothesis that different

characters assort themselves independently.

MENDEL’S LAW OF INDEPENDENT ASSORTMENT

The Principle of Independent Assortment states that the alleles of different genes segregate (assort)

independently of each other.

THE TRIHYBRID CROSSThe trihybrid cross demonstrates that Mendel’s principles apply to inheritance of multiple traits.

INTERCROSS INVOLVING THREE INDEPENDENTLY ASSORTING GENES

TESTCROSS INVOLVING THREE INDEPENDENTLY ASSORTING GENES

SIMPLE MATHEMATICAL RULES USEFULIN WORKING GENETICS PROBLEMS

REVIEW QUESTIONS

In cucumbers, orange fruit color (R) is dominant over cream fruit color (r). A cucumber plant homozygous for orange fruits is crossed with a plant homozygous for cream fruits. The F1 are intercrossed to produce the F2. Give the genotypes and phenotypes of the parents, the F1 and the F2.

Why was Mendel’s approach to the study of heredity so successful?

In summer squash, white fruit color (W) is dominant over yellow fruit color (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d). If a squash plant true-breeding for white, disk-shaped fruit  is crossed with a plant true-breeding for yellow, sphere-shaped fruit,  what will thephenotypic and genotypic ratios be for the F1 and F2 generation?