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 F 1 progeny exactly resemble only one of the parents (not a blend of both)

Mendel planted and allowed the F 1 plants to self-fertilize: Both smooth and wrinkled seeds appeared in the F 2 generation (3:1 ratio). But how can a trait present in the P generation disappear in the F 1 generation and then reappear in the F 2 ?

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 F 1 progeny resembled one of the parental strains (indicating dominance).  In the F 2 generation, the parental trait that had disappeared in the F 1 generation reappeared. 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