Fundamentals of Genetics CHAPTER 9 Fundamentals of Genetics

Mendel’s Legacy Genetics is the field of biology devoted to understanding how characteristics are transmitted from parents to offspring. Genetics was founded with the work of Gregor Johann Mendel.

Gregor Mendel Known as the “Father of Genetics” Born in 1822 to a poor family in the Czech Republic Mendel excelled in school and eventually became a monk. In 1843, at the age of 21, Mendel entered a monastery in Brunn, Austria (Brunn is now in the Czech Republic).

Gregor Mendel In addition to his monastery duties, he taught high school science. Mendel also was in charge of the monastery garden.

Site of Gregor Mendel’s experimental garden in the Czech Republic Mendelian Genetics 5/21/2018 Site of Gregor Mendel’s experimental garden in the Czech Republic Fig. 5.co

Gregor Mendel Mendel worked with garden peas and studied how their characteristics are passed in a very predictable manner to their offspring. Between 1856 and 1863, Mendel cultivated and tested about 28,000 pea plants! He found that the plants' offspring retained traits of the parents.

Why Peas? Mendel used peas to study inheritance because: Peas are easy to grow Peas produce many offspring Peas have many easy to observe traits including: Seed color - Green or yellow Seed shape - Round or wrinkled

Why Peas? Pod color - Green or yellow Pod shape - Smooth or constricted Flower color - White or purple

Why Peas? Flower position - Axial or terminal Plant size - Tall or short

Why Peas? Mendel was able to control how the pea plants were pollinated. Mendel knew that The male part of each flower produces pollen, (containing sperm). The female part of the flower produces egg cells. Pollination occurs when pollen grains produced in the male reproductive parts called the anthers are transferred to the female reproductive part called the stigma.

Why Peas? Self-pollination occurs when pollen is transferred from the anthers of a flower to the stigma of either the same flower or a flower from the same plant.

Why Peas? Cross-pollination involves fertilization from the flowers of separate plants. Cross-pollination

Why Peas? Pea flowers are constructed in such a way that they typically self-fertilize. If scientists snip off the anthers, they can prevent the pea plant from self-fertilizing, and place pollen from another plant on the stigma to cross-pollinate the pea plant. This gives scientists the ability to control which pea plants will pollinate with each other. Anthers Pea flower Pea flower Stigma

Why Peas? By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas.

Why Peas? By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas. Snip

Why Peas? By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas. .

Why Peas? By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas. .

Why Peas? By removing the anthers of one flower and artificially pollinating using a brush, crosses can be easily controlled in peas. .

Mendel’s Experiments Mendel began by growing plants that were true-breeding for each trait. Plants that are true-breeding, or purebred, for a trait always produce offspring with that trait when they self-pollinate. For example: Pea plants that are true-breeding for the purple flower color self-pollinate to produce offspring with purple flowers. Purebred white flowered plants self-pollinate to produce offspring with white flowers.

Mendel’s Experiments Mendel produced true-breeding plants for each of the 14 contrasting traits that he observed by self-pollinating the pea plants for several generations. He called these true-breeding plants the P generation. 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 The next part of Mendel’s experiment involved cross-pollinating true-breeding P generation plants with contrasting forms of a trait. For example: He cross-pollinated a pure purple flowered plant with a pure white flowered plant. What do you think the F1 generation looked like?

Mendel’s Experiments He observed that all of the F1 offspring had purple flowers despite the fact that one parent was true-breeding for white flowers!

Mendel’s Experiments Mendel repeated this procedure for the other six traits. This table shows Mendel’s crosses and results.

Mendel’s Experiments Next, Mendel allowed the F1 plants to self-pollinate. What do you think the F2 generation looked like? He observed that 75% of the F2 generation had purple flowers and 25% had 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. Mendel hypothesized that each trait was inherited by means of a separate factor. Because the characteristics studied had two alternative forms, he reasoned that a pair of factors must control each trait. Mendel did not know that the “factors” were actually genes.

Recessive and Dominant Traits Whenever Mendel crossed strains, one of the P traits failed to appear in the F1 plants. In every case, that trait reappeared in a ratio of about 3:1 in the F2 generation. Mendel hypothesized that the trait appearing in the F1 generation was controlled by a dominant factor because it masked, or dominated, the factor for the other trait in the pair. He thought that the trait that did not appear in the F1 generation but reappeared in the F2 generation was controlled by a recessive factor. Thus, a trait controlled by a recessive factor had no observable effect on an organism’s appearance when that trait was paired with a trait controlled by a dominant factor. This is known as Mendel’s Law of Dominance or complete dominance.

Mendel’s Results and Conclusions Trait - any characteristic that can be passed from parent to offspring Heredity - passing of traits from parent to offspring Genetics - study of heredity Today, we know that these traits are controlled by sections of DNA called genes.

Designer “Genes” Gene – a short segment of DNA that contains the instructions for a single trait Alleles – two or more alternative forms of a gene (dominant & recessive) Dominant - stronger of two genes that masks or hides the effect of the other gene; represented by a capital letter (R) Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)

More Terminology Genotype - gene combination for a trait (e.g. RR, Rr, rr) Phenotype - the physical feature resulting from a genotype (e.g. red, white)

Mendelian Genetics 5/21/2018 The Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and will display only the dominant trait in the phenotype. Example: YY x yy Yy

Mendelian Genetics 5/21/2018 The Law of Dominance In complete dominance, YY plants and Yy plants are indistinguishable in phenotype, that is, both have yellow seeds.

Genotype & Phenotype in Flowers Mendelian Genetics 5/21/2018 Genotype & Phenotype in Flowers Defining alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles control a characteristic Possible gene combinations are: Genotypes _______ _______ _______ Phenotypes _______ _______ _______ RR Rr rr RED RED YELLOW

Genotypes Homozygous genotype - gene combination involving 2 dominant or 2 recessive alleles; also called true-breeding or pure  Examples: RR = homozygous dominant rr = homozygous recessive Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid

The Law of Segregation The law of segregation states that a pair of alleles is segregated, or separated, during the formation of gametes. Each gamete receives only one allele of each pair.

The Law of Segregation When two gametes combine during fertilization, the offspring has two alleles for each characteristic (one allele from each parent).

The Law of Independent Assortment The Law of Independent Assortment states that alleles for different traits are distributed to gametes independently of one another.

The Law of Independent Assortment In other words, the genes are located on separate chromosomes, and therefore, these traits are not linked together.

Example of Incomplete Dominance in Snapdragon Flowers Red = RR White = WW Pink = RW

Example of Codominance in Cattle White = WW Red = RR Roan = RW

Example of Codominance in Cats Tan = TT Black = BB Tabby = TB

Example of Codominance in Chicken Black = BB White = WW Speckled with black and white feathers = BW

Incomplete Dominance – heterozygous offspring have phenotype between the dominant and recessive traits - The heterozygous individuals show a blend of the trait Ex – Snapdragon Flower.

Codominance – when both alleles for a gene are expressed in a heterozygous offspring Ex – AB blood type Neither allele is dominant or recessive, No blending of phenotypes

Dihybrid Crosses – when two characteristics are tracked at the same time - offspring are called dihybrids P Generation: RRYY x rryy R = Round (dominant), r = wrinkled (recessive) Y = Yellow (dominant), y = green (recessive) ry ry ry ry RY Phenotypic Ratio – All Round, Yellow Genotypic Ratio – All RrYy (Heterozygous round, heterozygous yellow) RrYy

Ex 2: P Generation RrYy x RrYy RY Ry rY ry RY Ry rY ry

Phenotypic Ratio – 9 Round, Yellow (genotypes RRYY, RRYy, ,RrYY, RrYy) 3 Round, Green (RRyy, Rryy) 3 Wrinkled, Yellow (rrYY, rrYy) 1 Wrinkled, Green (rryy)