Genetics Patterns of Inheritance

Genetics is the study of heredity Heredity: the transmission of traits from parents to offspring. Originally, people believed in a “blending hypothesis”, where the genetic information from the two parents mixes together in the offspring. Because organisms have many traits, the overall phenotype (appearance) did indeed seem like a blending of the traits of the parents.

“Father of Genetics” Gregor Mendel (copy all): Brought an experimental and quantitative approach to genetics. Was a monk. 1857: Began breeding pea plants Carefully controlled pollination and performed experiments to follow the inheritance of a trait through 3 generations.

Mendel’s Experiments P Generation = Parental Generation Mendel used “true” or “pure” breeding plants. For example, a purple-flowered plant, that when it self- pollinates, always produces purple-flowered offspring. Mendel bred (hybridized) two contrasting true- breeding strains. Examples: purple flowers x white flowers or tall x short height F 1 Generation = 1 st generation of offspring

Mendel’s Results: All of the F 1 plants showed the characteristic of only one of the parents. Conclusion: one form of a gene must be dominant, the other is recessive. The genes do not “blend”.

Mendel’s Experiments, cont’d F 2 Generation = 2 nd Generation of offspring Mendel allowed the F1 offspring to self- pollinate (since they all had the same genotype and phenotype, cross-pollination wasn’t necessary). Mendel crossed hundreds of pea plants and kept careful records of his experiments and results.

Mendel’s Results: ¾ of the offspring had the dominant phenotype, ¼ had the recessive phenotype. (3:1 ratio) Conclusion: genes are “particles” of inheritance. A recessive gene is unchanged by its coexistence with a dominant gene.

Mendel observed the 3:1 ratio in the F 2 generation for many traits

Mendel developed a model to explain this pattern of inheritance Key aspects of inheritance: There are different forms of each gene (alleles). The alleles differ in N- base sequence, but are located at the same place on the same chromosome type.

Key aspects of inheritance, cont’d An individual inherits two alleles for a particular trait, one from each parent. The alleles exist on homologous chromosomes. Homozygous = 2 alleles that are the same (AA or aa) Heterozygous = 2 different alleles (Aa)

Key aspects of inheritance, cont’d The alleles of a pair separate during meiosis (formation of gametes). “Law of Segregation” Only 1 chromosome of each homologous pair will be in a gamete, therefore, each parent only donates one allele for a trait.

Mendel’s model explained his results The purple-flowered plants had only the “P” form of the flower color gene. They had the genotype PP. The white-flowered plants had only the “p” form of the flower color gene. They had the genotype pp. All of the F1 plants had the genotype Pp. The recessive white allele was hidden by the dominant purple allele. So all the plants had purple flowers.

Mendel’s model explained his results, cont’d ½ of the F1 gametes will have the “P” allele and ½ of the gametes will have the “p” allele. When two F1 gametes meet during fertilization, the following combinations are possible: PP, Pp, pp So, some of the F2 plants have white flowers.

Modeling Reproduction You folded the chromosomes in ½ to simulate that only one of the two alleles would be in a particular gamete. You threw the chromosomes in the air to simulate that it is random as to which chromosome ends up in the gamete that ultimately results in a new child.

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Independent Assortment Mendel discovered that the inheritance of the 7 different characteristics he studied was independent. If an offspring inherited the “P” allele for flower color, it had no influence over what alleles it inherited for stem length, pod color, etc.

Modeling Reproduction By placing each allele pair on a different pair of homologous chromosomes, you simulated independent assortment. When you threw the chromosomes in the air, there were many possible combinations of alleles that would’ve been in the gamete that ultimately produced the offspring.

Key aspects of inheritance, cont’d If genes for 2 different traits are located on different chromosomes, their inheritance is “independent”. “Law of Independent Assortment”. So inheriting purple flowers didn’t mean that the pea plant would also be tall in height. Mendel studied more than 7 traits and had some “ambiguous” results. Later it was discovered that the traits were controlled for by genes located on the same chromosome.