Intro to Genetics

Heredity: the transmission of biological traits from parent to offspring and from generation to generation

Gene: basic unit of heredity

Genetics: the study of heredity and the variation of inherited characteristics

Character: any discrete, observable, physical feature of an organism. Ex: seed shape or flower color

Trait: specific form of a character. Ex: purple flowers, white flowers, round seeds, wrinkled seeds

Phenotype: organism’s observable physical properties

Genotype: individual’s genetic composition

Allele: different forms of the same gene (have different DNA sequences)

Theory of Blending of Inheritance: two parents’ offspring had some intermediate type of phenotype

Gregor Mendel: father of modern genetics, performed pea plant experiments

True breeding: one that has biological traits that are passed on to all of its offspring when it is self-crossed, only has one type of allele at the locus that produces the phenotype)

Why peas were lucky:

• easy to do controlled crosses with pea plants
• many varieties of pea plants which bred true for particular traits (round vs wrinkled seeds, purple vs white flowers)
• many discrete, variable characters to look at (either or characteristics: flower color, seed color)

no gene-environment interactions: phenotypic effects of interactions between genes and their environment

Parental (P) generation: first generation of a cross

First filial (F1) generation: offspring of the parental generation

Second filial (F2) generation: grand-offspring of parental generation

Pea Experiment:

P: smooth and wrinkled (true-breeding)

F1: all 4 smooth (Rr)

F2: 3 smooth, 1 wrinkled

Mendel performed similar experiments for seed color, pod shape, pod color, flower color, flower position, stem height = all generated 3:1 ratio

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Particulate inheritance: Mendel’s theory

Diploid organism: homologous pairs of chromosomes

Homozygote: same alleles

Heterozygote: different alleles

Mendel’s law of segregation:

• the gametes of an individual with identical alleles at a locus will all have that allele

half of the gametes of an individual with two different alleles at a locus will have one allele, and the other half will have the other allele

Punnett square: tool invented by Reginald Punnett to determine the probability that an offspring will have a particular genotype

Mendel’s Legacy:

Why Mendel’s work did not gain attention at first:

• he did not propose an apparent mechanism for this pattern of inheritance (were not able to identify and visualize chromosomes)
• his statistical arguments did not appeal to naturalists (data-driven mathematical arguments did not appeal to contemporaries)

he lacked academic standing (monk, not scientist)

Test cross: determine unknown genotype by crossing organism with known homozygous recessive

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