Medelian Genetics

Remember DNA? A double stranded molecule that contains the genetic information for cell growth, division and function. Made up of nucleotides Base pairing A? C?

Gregor Mendel – His Background “Father of Genetics” Austrian Monk 1822 – 1884 Studies physics, mathematics, and botany at the University of Vienna. Mendel studied the garden pea plant. This plant is self-fertilizing (produce egg and sperm). Member of the Agricultural Society and won awards for developing variety of fruits and vegetables.

8 years and thousands of individual plants. His Experiment 8 years and thousands of individual plants. Mendel chose to use peas for his experiments, because of their many distinct varieties, and because offspring could be quickly and easily produced. He cross-fertilized pea plants that had clearly opposite characteristics—tall with short, smooth with wrinkled, etc.—and from the results reached two of his most important conclusions: Law of Segregation Law of Independent Assortment Principle of Dominance

Mendel knew that the male part of each flower produces pollen, (containing sperm). the female part of the flower produces egg cells. To cross-pollinate pea plants, Mendel cut off the male parts of one flower and then dusted it with pollen from another flower. Copyright Pearson Prentice Hall

During sexual reproduction, sperm and egg cells join in a process called fertilization. Fertilization produces a new cell. Pea flowers are self-pollinating. Copyright Pearson Prentice Hall

Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves. Cross-pollination Mendel was able to produce seeds that had two different parents. Copyright Pearson Prentice Hall

Each trait has a contrasting characteristic called an allele. Dominant Recessive Ex. Trait: Pod Color Dominant: Yellow Recessive: Green

Plant Height Mendel's F2 Generation F1 Generation F2 Generation P Generation When Mendel allowed the F1 plants to reproduce by self-pollination, the traits controlled by recessive alleles reappeared in about one fourth of the F2 plants in each cross. Tall Short Tall Tall Tall Tall Tall Short Copyright Pearson Prentice Hall

Law of Segregation During the production of gametes the two copies of each hereditary factor segregate so that offspring acquire one factor from each parent. Established that there are dominant and recessive traits passed on randomly from parents to offspring.

Principle of Dominance Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait. The different forms of a gene are called alleles. Some alleles are dominant and some alleles are recessive. This is called the principle of dominance.

Punnett Squares Grids to calculate the probability of genotypes and phenotypes that will occur in offspring Probability is the measure of chance that a particular outcome will occur.

Punnett Square A capital letter represents the dominant allele for tall. A lowercase letter represents the recessive allele for short. In this example, T = tall t = short Male Female The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall

T TT t Tt Gamete Gamete

Hybrids Each original pair of plants is the P (parental) generation. The offspring are called the F1, or “first filial,” generation. The offspring of crosses between parents with different traits are called hybrids.

Alleles separate during gamete formation. During gamete formation, alleles segregate from each other so that each gamete carries only a single copy of each gene. Each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness. The alleles are paired up again when gametes fuse during fertilization. The TT and Tt allele combinations produce tall pea plants; tt is the only allele combination that produces a short pea plant. Copyright Pearson Prentice Hall

Law of Independent Assortment Established that traits were passed on independently of other traits from parent to offspring. A gene is distributed into gametes independently of how other genes are separated. Dihybrid Cross Test for the dominance relationships between alleles at the gene.

Dihybrid Crosses Ex. Seed color and shape

Dihybrid Cross AB Ab aB ab AaBb Aabb aaBb AaBb x aabb GAMETE FORMATION 1234 1234 AaBb x aabb AB Ab aB ab AaBb Aabb aaBb GAMETE FORMATION 13: AB 14: Ab 23: aB 24: ab 13: ab 14: ab 23: ab

Beyond Simple Dominance Non-Mendelian Genetics

Codomiance Two non-identical alleles of a gene are both fully expressed in heterozygote, so neither is dominant or recessive. Both alleles will show Multiple allele: three or more alleles of a gene Blood Type

Incomplete Dominance One allele of a pair is not fully dominant over the other allele The offspring’s (heterozygous) phenotype is somewhat between the two. Blending together

In a nut shell….