Mendel’s Methods• Mendel controlled pollination – Self-pollination – pollen fertilizes egg cells on the same flower – Cross-pollination – pollen fertilizes egg cells on flowers of different plants
Mendel’s Experiments• P generation – parent generation• F1 (first filial) generation – offspring of the P generation• F2 (second filial) generation – offspring of the F1 generation
Mendel’s Experiments• First he grew true- breeding plants – A plant that is true- breeding, or pure, for a trait always produces offspring with that trait when they self-pollinate
Mendel’s Experiments• Next, he cross- pollinated true- breeding parents of the P generation• What do you think the F1 generation looked like?
Mendel’s Experiments• The F1 generation was all purple flowers!
Mendel’s Experiments• Next, he let the flowers from the F1 generation self-pollinate• What do you think the F2 generation looked like?
Mendel’s Experiments• The F2 generation came out with 75% purple flowers and 25% white flowers!
Mendel’s Results and Conclusions• Mendel’s observations led him to hypothesize that something within the pea plants controlled the characteristics observed – He called these controls factors• Because the characteristics had two alternative forms, he reasoned that a pair of factors must control each trait
Recessive and Dominant Traits• Dominant – trait that masks the factor for the other trait• Recessive – trait that is masked by the factor for the other trait
The Law of Segregation• The law of segregation states that a pair of factors is segregated, or separated, during the formation of gametes
The Law of Independent Assortment• The law of independent assortment states that factors separate independently of one another during the formation of gametes
Support for Mendel’s Conclusions• A gene is a segment of a chromosome that controls a particular hereditary trait• The different forms of a gene are called alleles – Capital letters are used to represent dominant alleles – Lowercased letters are used to represent recessive alleles
Genotype and Phenotype• Genotype – an organism’s genetic makeup (PP, Pp, pp)• Phenotype – an organism’s physical appearance (purple flower, white flower)
Genotype and Phenotype• Homozygous – both alleles of a pair are alike (PP = homozygous dominant and pp = homozygous recessive)• Heterozygous – two alleles are different (Pp)
Probability• Probability – the likelihood that a specific event will occur
Predicting Results of Monohybrid Crosses• In a monohybrid cross, one characteristic is tracked• Biologists use a Punnett square to predict the probable distribution of inherited traits in the offspring
Ex 1: Homozygous X Homozygous• TT x tt (T = tall, t = short)• Possible Genotypes – 100% Tt• Possible Phenotypes – 100% tall
Ex 2: Homozygous X Heterozygous• BB X Bb (B = black hair, b = brown hair)• Possible Genotypes – 50% BB, 50% Bb• Possible Phenotypes – 100% black
Ex 3: Heterozygous X Heterozygous• Bb X Bb(B = black hair, b = brown hair)• Possible genotypes – 25% BB, 50%Bb, 25% bb• Possible phenotypes – 75% black hair, 25% brown hair
Ex 4: Testcross• How could you determine whether a black guinea pig is homozygous (BB) or heterozygous (Bb)?• By completing a testcross – an individual with an unknown genotype is crossed with a homozygous recessive individual
Ex 5: Incomplete Dominance• Incomplete dominance occurs when the phenotype of heterozygous individual is in between the two homozygous phenotypes
Ex 5: Incomplete Dominance• RW X RW (R = red, W = white)• Possible genotypes and phenotypes – 25% RR, red – 50% RW, pink – 25% WW, white
Ex 6: Codominance• In codominance, both alleles contribute to the phenotype• In some varieties of chicken, the allele for black feathers is codominant with the allele for white feathers – Heterozygous chickens are speckled with black and white feathers
Ex 6: Codominance• F W FW X F B F B (F = feathers, B = black, W = white• Possible genotypes and phenotypes – 100% FWFB, speckled
Predicting Results of Dihybrid Crosses• A dihybrid cross is a cross in which two characteristics are tracked• More complicated than monohybrid crosses because more combinations of alleles are possible
Dihybrid Homozygous X Homozygous• rryy X RRYY (R= round, r= wrinkled, Y= yellow, y= green)
Dihybrid Heterozygous X Heterozygous• RrYy X RrYy