Lesson 6.1 Mendel’s Experiments

Mendel Is the Father of Modern Genetics Modern genetics began with Gregor Mendel’s experiments with pea plants

Mendel Chose Chose Peas for Good Reason Peas reproduce sexually It is possible to control which plants are allowed to reproduce Pea plants produce many offspring in a small amount of time Peas have easily identified characteristics

Cross Pollination A single pea flower can contain both male and female gametes Anthers make pollen (male gametes) Stamen contain ovules (female gametes) Fertilization results from pollen being transferred to the stamen When the pollen of one flower fertilizes the ovule of the same plant, this is self - fertilization When the pollen of one plant fertilizes the ovule of another plant, this is cross - fertilization

1 Removed stamens from purple flower White Stamens Carpel Purple PARENTS (P) OFF-SPRING (F1) 2 Transferred pollen from stamens of white flower to carpel of purple flower 3 Pollinated carpel matured into pod 4 Planted seeds from pod How Mendel Controlled Cross - fertilization - He did this to thousands of plants and kept detailed notes

Monohybrid Crosses Mendel chose parent plants from pea populations that had been tall for many generations He bred these tall plants with other plants from populations that had been short for many generations He called the parent plants the P generation All the parent plants were true breeding The true breeding tall plants cross - pollinated the true breeding short plants What do you think the offspring looked like?

All of the offspring were as tall as the tall parent plant The short trait seemed to disappear from the population Was the short trait gone for good? How could Mendel find out?

Mendel allowed all of the offspring from his first cross to self - fertilize. When the seeds sprouted he found that 3/4 were tall plants and 1/4 were short

Mendel’s Rules Mendel did similar monohybrid crosses with a total of seven different pea plant traits, always studying one trait at a time Every time, one version of the trait appeared to disappear in the F1 generation, but return in 1/4 of the F2 generation Mendel summed up his conclusions from this experiment with two rules and one law -The Rule of Unit Factors -The Rule of Dominance -The Law of Segregation

Some of the Traits Mendel Studied

The Rule of Unit Factors Each organism has two factors that control each of its traits. There are two possible forms of each of these factors. We now call these factors genes and the different forms of a gene are alleles One allele is inherited from each parent Example: All plants receive two alleles for the height gene. A plant could receive any of the following combinations Two tall alleles One tall allele and one short allele Two short alleles

The Rule of Dominance Of the two possible alleles for any gene, one allele is dominant and the other allele is recessive. The dominant allele is the one that is always expressed. Example: Short allele (t) and short allele (t) = short plant (tt) Short allele (t) and tall allele (T) = tall plant (tT) Tall allele (T) and short allele (t) = tall plant (Tt) Tall allele (T) and tall allele (T) = tall plant (TT)

The Law of Segregation T or t gametes T or t gametes During gamete formation, pairs of alleles segregate into separate gametes. During fertilization gametes pair up creating offspring with 2 alleles for each trait Example: Parent # 1 Parent # 2 Tt Tt Produces T or t gametes Produces T or t gametes Possible Offspring: TT or Tt or tT or tt

Genotype and Phenotype Genotype is the combination of alleles an individual receives for a gene, it is the type of genes they have. TT (homozygous dominant) Tt (heterozygous) tt (homozygous recessive) Phenotype describes what version of a trait the individual will express Tall Short

Mendel’s Second Experiment Next Mendel crossed pea plants that differed in two traits. This is called a Dihybrid cross. From his results Mendel Formulated the Law of Independent Assortment

Mendel’s Law of Independent Assortment Genes for different traits are inherited independently of each other. In other words the gene for pea shape and the gene for pea color are inherited independently of each other. A round, yellow parent and a wrinkled, green parent can produce any combination of round, wrinkled, yellow, or green pea offspring. Round and yellow - Wrinkled and yellow Round and green - Wrinkled and green