Fundamentals of Genetics

Chapter 9

Mendel’s Legacy

• Genetics is the study of how characteristics are transmitted from parents to offspring– Founded with the work of Gregor Mendel, an

Austrian monk

Gregor Mendel

• Mendel conducted experiments at the monastery with pea plants

• He researched heredity – the transmission of characteristics from parents to offspring

Mendel’s Garden Peas

• Mendel observed seven characteristics of pea plants

• Each characteristic occurred in two contrasting traits – genetically determined variants of a characteristic

Mendel’s Garden Peas

Characteristic• Position of flowers along

stem

• Height of plant

Trait

Mendel’s Garden Peas

Characteristic• Pod Appearance

• Pod color

Trait

Mendel’s Garden Peas

Characteristic• Seed Texture

• Seed color

• Flower color

Trait

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

GENETIC CROSSESSection 2

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

• FWFW X FBFB (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