Fundamentals of Genetics

Genetics The field of Biology devoted to understanding how characteristics are transmitted from parents to offspring.

Gene A segment of DNA that contains coding for a polypeptide or protein A unit of hereditary information.

Heredity The transmission of characteristics from parents to offspring.

Pollination Self -Pollination Cross-Pollination Web Link

Discussion How are traits inherited? Where does a person get their eye color, hair color, and so on? What if one parent has brown hair and the other one is blond? What hair color will their children have?

Gregor Mendel Austrian monk who experimented with garden peas Used knowledge of statistics and studied heredity

Mendel Video

Mendel’s First (P1 to F1) In the first set of experiments, Mendel conducted cross-pollination between a pure breeding tall plant and a pure breeding dwarf plant. He collected the seeds from this cross pollination and allowed them to germinate. All the resulting plants were found to be tall. P1 to F1 generation

Next he… In a similar pollination between a pure breeding plant with axial flowers and a pure breeding plant with terminal flowers, all the resulting plants of the next generation produced only axial flowers. F1 generation

Principle of Dominance Based on these results, Mendel came to the conclusion that in a cross-involving two contrasting characters, only one character expresses itself in the next generation. Mendel called the character, which expressed as dominant character and the character, which failed to express, as recessive character. This idea came to be known as the principle of dominance (first law).

F2 Generation At this stage, Mendel wanted to know whether the tall plants resulting from a cross between tall and dwarf plants, were similar to the tall plants of the P1 generation. Hence, he allowed the tall plants of the F1 generation to undergo self- pollination. In the next generation, Mendel found both tall plants and dwarf plants, approximately in the ratio 3:1. The results were most surprising since the recessive character dwarfness had reappeared in the next generation. (F2 generation)

Mendel’s Garden Peas Observed 7 characteristics of pea plants. Each characteristic occurred in 2 contrasting traits. Characteristic Trait Plant height long or short stems Flower position axial or terminal Pod color green or yellow

P= Parent F1=first Filial cross F2 =second Filial Cross Mendel’s monohybrid

Mendel’s Garden Peas Observed that seeds grown from purple flowered plants produce plants with purple flowers and plants with white flowers. Tall seeds produced tall plants and short plants. Wanted explanations.

Mendel’s Methods Able to document the traits of each generation’s parents by carefully controlling how plants were pollinated. Pollination: occurs when pollen grains produced in the male reproductive parts of a flower (anthers) are transferred to the female reproductive parts (stigma).

Self-pollination: occurs when pollen is transferred from the anthers of a flower to the stigma of either the same flower or a flower of the same plant. Cross pollination: involves flowers of two separate plants. Mendel was able to control pollination by transferring pollen manually and removing the anthers from the flower.

Mendel’s Experiments Studied each characteristic and its contrasting traits individually. Began by growing plants that were pure for each trait. Strain: plants that are pure for a specific trait. Eventually produced strains for each of 14 characteristics. Called each strain a parental generation or P1 generation.

Mendel’s Experiments Cross-pollinated pure strains (P1) for contrasting traits, planted seeds and recorded the number of each type of offspring produced. Called the F1 generation (first filial generation) Allowed F1 generation to self-pollinate and collected seeds. Grew the plants F2 generation (second filial) and recorded the number of each type produced. Performed and documented the results of hundreds of crosses.

Mendel’s Results and Conclusions Crossed green pods with yellow pods (P1) All green pods produced (F1) ¾ green pods and ¼ yellow pods (F2)

Mendel’s Results and Conclusions Hypothesized that something within the pea plants controlled the characteristics he observed – called these “factors” Said that each trait was inherited by the means of a separate factor. Reasoned that there must be a pair of factors controlling each trait because the characteristics he studied had 2 alternative forms.

Recessive and Dominant Traits When Mendel crossed strains, one of the P1 traits failed to appear in the F1 plants but reappeared in a 3:1 ratio in the F2 generation. Mendel concluded that one factor in a pair may prevent the other from having an effect.

Dominant factor: masks the other factor for a specific characteristic Recessive factor: has no observable effect on an organism’s appearance when paired with a dominant factor. The hidden factor.

The Law of Segregation States that a pair of factors segregate or separate during formation of gametes (reproductive cells). Each gamete receives only one factor of each pair When two gametes combine during fertilization, the offspring have two factors controlling a specific trait

The Law of Independent Assortment States that factors for different characteristics are distributed to gametes independently.

The Law of Independent Assortment Mendel crossed plants that differed in two characteristics such as flower color and seed color. Data showed that traits from dominant factors do not necessarily appear together. Factors for different characteristics are not connected.

Most of Mendel’s findings agree with what biologists now know about molecular genetics.

Molecular Genetics The study of the structure and function of chromosomes and genes

Chromosomes and Genes Chromosome: a threadlike structure made up of DNA Gene: the segment of DNA on a chromosome that controls a particular hereditary trait. Chromosomes occur in pairs. Genes occur in pairs. Allele: each of several alternative forms of a gene (Mendel’s factors) Letters are used to represent alleles. Dominant is capital ex. G = green pod color Lowercase is recessive. Ex, g= yellow pod color

In meiosis, gametes receive 1 chromosome from each homologous pair In meiosis, gametes receive 1 chromosome from each homologous pair. When gametes combine in fertilization, the offspring receives 1 allele for a given trait from each parent. PTC Tasting Characteristic

Mendel’s Law of Independent Assortment is supported by the fact that chromosomes segregate independently to gametes during meiosis. But it is observed only for genes located on separate chromosomes or located far apart on the same chromosome where crossing over can occur.

Modern Versions Law of Segregation: states that pairs of alleles are separated during meiosis. Law of Independent Assortment: states that alleles located on separate chromosomes or far apart on the same chromosome separate independently of one another.

Ted-Ed How Mendel’s pea-plants helped us understand genetics Video

Genetic Crosses Genotype and Phenotype Probability Predicting Results of Monohybrid Crosses Predicting Results of Dihybrid Crosses

Genotype and Phenotype Genotype: the genetic makeup of an organism. Consists of the alleles that the organism inherits from its parents Purple flowered plants PP or Pp White flowered plants pp

Phenotype: the appearance of an organism as a result of its genotype. PP or Pp = purple flowers pp = white flowers

Human phenotypes can appear to be altered: hair dye, colored contact lenses, surgery, etc but they do not alter the individual’s true phenotypes or genotype. Colored Contact Lenses

Homozygous: when both alleles of a pair are alike. PP= homozygous dominant pp= homozygous recessive Heterozygous: when the two alleles in the pair are different. Pp= heterozygous for flower color. Video (1.5 minutes)

Probability Probability: the likelihood that a specific event will occur. May be expressed as a decimal, %, fraction, or ratio Probability= # of actual times # of opportunities

Example: Mendel’s experiments – the dominant trait of yellow seed color in F2 generation occurred 6022 times, the recessive trait of green seed color appeared 2001 times. Total # of individuals = 8023 Probability of Dominant trait 6022 / 8023 = 0.75 or 75%, ¾, 3:4 Probability of Recessive trait 2001 / 8023 = 0.25 or 25%, ¼, 1:4

Predicting Results of Monohybrid Crosses Monohybrid: a cross between individuals that involves one pair of contrasting traits. Punnett Square: a diagram used to predict the probability that certain traits will be inherited by offspring

Examples

Homozygous x Homozygous Predict the possible outcome of a monohybrid cross between a pea plant homozygous for purple flower color (PP) and a pea plant homozygous for white flower color (pp). PP purple flower x pp white flower

Showing Results of Cross Genotypic ratio: the ratio of the genotypes that appear in offspring. Ex. 1BB:2Bb:1bb Phenotypic ratio: the ratio of the offspring’s phenotype. Ex. 4purple

Homozygous x Heterozygous Cross a guinea pig that is homozygous dominant for the trait of black coat color (BB) and a guinea pig that is heterozygous for this trait (Bb).

Showing Results of Crosses Genotypic ratio: the ratio of the genotypes that appear in offspring. Ex. 2BB:2Bb Phenotypic ratio: the ratio of the offspring’s phenotype. Ex. 4black

Heterozygous x Heterozygous Cross two rabbits that are both heterozygous (Bb) for coat color (B=black, b=brown)

Showing Results of Crosses Genotypic ratio: the ratio of the genotypes that appear in offspring. Ex. 1BB:2Bb:1bb Phenotypic ratio: the ratio of the offspring’s phenotype. Ex. 3black:1brown

Test Cross An individual of unknown genotype is crossed with a homozygous recessive individual (bb). Can be used to determine the genotype of any individual whose phenotype is dominant. If one of the offspring turns up with the recessive trait, then you know the unknown parent is heterozygous (Bb).

Complete, Incomplete and Co- Dominance Video 8 minutes Amoeba Sisters

Complete Dominance One allele is completely dominant over another (Mendel’s pea plant crosses) Ex. Heterozygous plants and homozygous dominant plants are indistinguishable in phenotype. PP and Pp are both purple.

Incomplete Dominance Occurs when 2 or more alleles influence the phenotype resulting in a phenotype intermediate between the dominant trait and the recessive trait. Ex. 4 o’clock flowers RR=red, rr=white, Rr=pink

Codominance Occurs when both alleles for a gene are expressed in a heterozygous offspring. Neither allele is dominant or recessive The alleles do not blend in the phenotype. Ex. RR= red coat, R’R’ = white coat, RR’= roan coat or both red and white

Predicting Results of Dihybrid Crosses Dihybrid Cross: a cross between individuals that involves 2 pairs of contrasting traits. Considers how the four alleles from each parent can combine.

Examples

Homozygous x Homozygous Predict the results of a cross between a pea plant that is homozygous for round, yellow seeds and one that is homozygous for wrinkled, green seeds.