1 Chapter 4-1 Mendel ’ s Work

2 Some Important Terms Heredity – the passing of physical characteristics from parents to offspring Trait – a characteristic that an organism can pass on to its offspring through its genes Genetics – the scientific study of heredity

3 Gregor Mendel Priest and gardener at a monastery in Europe Mid-1800s Worked with pea plants

4 Mendel ’ s Experiments Crossing Pea Plants Began by crossing plants with contrasting traits Example: tall and short plants Used purebred plants

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6 F1 Offspring F1 = first filial (son/daughter) generation The offspring of a cross of purebred tall and purebred short plants Parent plants are called P generation All offspring in F1 were tall, despite 1 parent being short

7 F2 Offspring F2 = second filial Plants from F1 were bred Offspring were a mixture of tall and short plants ¾ were tall; ¼ were short

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10 Experiments with other traits Mendel also did experiments with other contrasting traits In all crosses, only 1 form of the trait appeared in the F1 However, in F2, the “ lost ” trait always reappeared in ¼ of the plants

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12 Dominant and Recessive Mendel reasoned that individual factors, or sets of genetic “ information ” must control the inheritance of traits in the peas These factors exist in pairs Female gives 1 part of pair Male gives 1 part of pair A factor can “ mask ” or “ cover ” another factor

13 Genes and Alleles Gene – factors that control a trait Alleles – different forms of the gene Example: in the gene that controls height, there is an allele for tall and short Each plant inherits 2 alleles; one from the mother and one from the father

14 An organism ’ s traits are controlled by the alleles it inherits Some are dominant; others recessive Dominant – trait always shows up Recessive – hidden whenever dominant is present This trait will only show up if the organism doesn ’ t have the dominant

15 Alleles in Mendel ’ s work P generation – tall had 2 dominant, short had 2 recessive F1 generation – all plants had 1 of each F1 all hybrids Hybrid – organism that has 2 different alleles for a trait F2 generation – ¾ dominant and ¼ recessive

16 Symbols for Alleles Geneticists use letters to represent alleles Dominant alleles are capital letters (ex: T) Recessive alleles are lowercase (ex: t)