Genetics

Chapter Introduction How are traits passed from parents to offspring?

Heredity– the passing of traits from parents to offspring. Gregor Mendel Gregor Mendel is known as the father of genetics—the study of heredity.

Mendel studied genetics by doing controlled breeding experiments with pea plants… Why peas? Grow quickly Come in many varieties Mendel could control pollination

Plant Pollination There are two types of pollination: Self pollination Cross pollination

Lesson 1 Self-Pollination

Lesson 1 Cross-Pollination

Mendel’s Experiments Used plants that he knew were true- breeding True breeding—when a plant self- pollinates it will always produce more plants with the same traits; ( is homozygous) (a.k.a. purebred) X =

Mendel’s Experiments continued… Mendel crossed (mated) a true-breeding purple flowered pea plant with a true- breeding white flowered pea plant All of the offspring had purple flowers! X = (Parents) (Offspring)

What happened to the white flowers? (Transparency demo) Dominant trait—the trait that appears if two traits are mixed (capital letter) Recessive trait—trait that is hidden if 2 traits are mixed (lowercase) Purple flowers are dominant over white flowers

Mendel’s Experiments continued… Mendel also cross-pollinated hybrid plants. X = Parents Offspring 75% 25%

Mendel’s Experiments continued… The trait for white flowers showed up again! He observed that offspring of hybrid crosses always showed traits in a 3:1 ratio. 75% 25%

Mendel’s Experiments continued… Mendel concluded that two factors, one from each sperm and one from each egg, control each inherited trait.

The factors that Mendel talked about are called genes A gene is a section on a chromosome that has genetic information for one trait. Gene

Genes continued… One trait/gene may have a few different varieties Example: hair color The different forms of a gene are called alleles brown red blonde

Lesson 2

Genotype The GENetic code (or combination of alleles) of an organism is called its GENotype Shown as 2 letters per trait Example: someone with brown hair might be BB One allele from mom One allele from dad

Lesson 2 When the two alleles of a gene are the same, the genotype is true breeding, or homozygous. If the two alleles of a gene are different, the genotype is hybrid, or heterozygous. What Controls Traits (cont.)

Lesson 2 The PHenotype is the PHysical appearance of the organism Example: brown hair, purple flowers, blue eyes Phenotype

Check this out… B= brown hair b=blonde hair BB Bb bb Brown Hair Blonde Hair Brown Hair Homozygous (true-breeding) Heterozygous (hybrid) Homozygous (true-breeding)

Lets see how quick you are…. P = purple flowers in pea plants, p= white flowers in pea plants PP Pp pp Purple White

What letters do you use? Choice of letter code is up to you… Usually the first letter of the trait Capital letter means the trait is dominant Lower case means it is recessive

A Punnett Square is a model used to predict possible genotypes and phenotypes of offspring. Bb x Bb B B b b B BbBb BbBb b

Probability- the mathematical chance an event will occur Bb x Bb Probability of Brown: 3 out of 4; 75% Probability of blue: 1 out of 4; 25% B B b b B BbBb BbBb b brown blue

Phenotypic Ratio Bb x Bb Probability of Brown: 3 out of 4; 75% Probability of blue: 1 out of 4; 25% Phenotypic Ratio is 3:1 (Brown:blue) B B b b B BbBb BbBb b brown blue

Genotypic Ratio Bb x Bb Probability of BB: 1 out of 4; 25% Probability of Bb: 2out of 4; 50% Probability of bb: 1 out of 4; 25% Genotypic Ratio is 1:2:1 (BB:Bb:bb) B BbBb b b B B BbBb b brown blue

Lesson 2 A pedigree shows phenotypes of genetically related family members. Modeling Inheritance (cont.)

Lesson 2 Sometimes traits appear to be blends of alleles. Alleles show incomplete dominance when the offspring’s phenotype is a blend of the parents’ phenotypes. Complex Patterns of Inheritance

Snapdragons Red, white and pink flowers R=red r=white

Snapdragons continued… How do you get pink flowers? RR Rr rr Red White Pink

Incomplete dominance Mixing of two traits (blending) One dominant allele is only “half-strong” Red + white = pink R=red r=white

Lesson 2 Codominance occurs when both alleles can be observed in a phenotype. Complex Patterns of Inheritance

Cattle Fur can be red or white Mixing the two gives roan fur

Codominance Both alleles are expressed at the same time in the phenotype Red fur + white fur = roan fur (red and white mixed) R=red r=white

Websites Incomplete & Codominance