Mendelian Genetics

Unit 1

Genetics Science that deals with the structure &

function of genes and their transmission from one generation to the next (heredity) Genes

factors that control traits

Genotype Genetic makeup of an organism

Phenotype Observable characteristic of an organism

produced by the interaction between its genotype and the environment

Genotype & Phenotype Genes provide only the POTENTIAL

for developing a particular phenotype.

The extent to which it is realized depends upon Interactions with other genes & their

products

Environmental influences

Random developmental events

Genes are only a starting point for determining structure and function of an organism.

Gregor Mendel Father of modern genetics

Experimented using garden pea plants (Pisum sativum)

Good choice:

Easy to grow

Bears flowers and fruit in the same year a seed is planted

Produces a large number of seeds

Each trait had two easily distinguishable, alternative appearances

Mendel’s Work Pea plants are normally self-fertilizing

He allowed each strain to self-fertilize for many generations to ensure that the traits he wanted to study were inherited (true-breeding strains).

Then he prevented self-fertilization and cross fertilized true-breeding strains of peas that differed in a single trait

This is a MONOHYBRID CROSS

Used reciprocal crosses to show that the trait does not depend on the sex of the organism

Example:

Cross smooth female with wrinkled male

Cross wrinkled female with smooth male

Cross of the P (parental) generation:

All the F1 progeny exactly resemble only one of the parents (not a blend of both)

Mendel planted and allowed the F1 plants to self-fertilize:

Both smooth and wrinkled seeds appeared in the F2 generation (3:1 ratio).

But how can a trait present in the P generation disappear in the F1 generation and then reappear in the F2?

Mendel concluded… The alternative traits in the cross were

determined by what we now call genes.

Factors transmitted from parents to progeny that carry hereditary information

Each existed in alternative forms (which we now call alleles)

A true-breeding strain of peas must contain a pair of identical factors (and each F1 must have contained both factors).

Because only one of the traits was seen in F1, the expression of the missing trait must somehow have been masked by the visible trait: DOMINANCE The allele for purple (P) is dominant to the allele for

white flowers (p).

The allele for white flowers is recessive because it is masked.

Individuals that contain two copies of the same specific allele of a particular gene are said to be HOMOZYGOUS for that gene.

Individuals that have two different alleles of a particular gene are said to be HETEROZYGOUS.

Punnett Square Matrix that describes all the possible genotypes of

progeny resulting from a genetic cross

Mendel’s Conclusions: Results of all reciprocal crosses were the same.

All F1 progeny resembled one of the parental strains (indicating dominance).

In the F2 generation, the parental trait that had disappeared in the F1 generation reappeared.

From this Mendel proposed his 1st law:The principle of segregation

The two members of a gene pair (alleles) segregate (separate) from each other during

the formation of gametes.

• We now know genes are on chromosomes and the specific location of a gene on a chromosome is called its locus.

• Gene segregation parallels the separation of homologous pairs of chromosomes at anaphase I in meiosis.

Confirming the Principle of Segregation Test Cross

Cross of an individual of unknown genotype (usually expressing the dominant phenotype) with a homozygous recessive individual to determine the unknown phenotype