Genetics Mendel And His Peas

Genetics – the scientific study of heredity Gregor Mendel – 1800s His monastery garden pea plant experiments laid the foundations of the science of genetics.

Summary of the pea plant experiments: 1) Mendel developed pure-bred (true-breeding) plants by allowing plants to self-pollinate. (always produce identical offspring)

(a) stigma – traps pollen e) stamen = anther + filament (b) anther – makes pollen (f) carpel (or pistil) = stigma, and style (c ) ovary – eggs and seeds g) sepal (bud leaf) form here (d) filament – lifts anther

2) Mendel crossed-pollinated pure bred plants and then crossed their offspring. (anther cut off one plant to eliminate its pollen, and then dusted pollen from another plant.) Modern Biology Textbook (Holt)

3) After numerous experiments, Mendel analyzed data and drew conclusions about inheritance.

WHAT EXACTLY DID MENDEL DO? First, Mendel crossed a pure-bred (true breeding) TALL plant with a pure-bred SHORT plant. Result: The offspring were all TALL. about 6 feet tall under two feet

Second, Mendel crossed two of the TALL offspring. Result: ¾ of the offspring were TALL ¼ of the offspring were SHORT

(This same result happed for other characteristics such as seed shape, seed color, position of flowers etc. --pg 264) Seven pea plant traits studied by Mendel Modern Biology Textbook (Holt)

WHY DID THIS HAPPEN? Mendel found that some genes for specific traits are dominant over other genes. Contrasting genes for a particular trait are called alleles, so they are dominant alleles. The genes that are not expressed because dominant alleles are present, are said to be recessive alleles.

What if Mendel crossed an F1offspring pure-bred tall Try this: What if Mendel crossed an F1offspring pure-bred tall (heterozygous) with a (homozygous) plant. X What would be the phenotypic & genotypic ratios?

Monohybrid & Dihybrid Crosses

“The principle of dominance” – some alleles are dominant and some are recessive. Dominant allele blocked the expression of recessives. (Mendel called alleles or genes “factors”)

“The principle of segregation” – alleles separate during the formation of gametes. “The principle of independent assortment”- genes for different traits can segregate independently during the formation of gametes. (where one allele goes does not affect where the other goes.)

He was a mathematician and statistician – he was better able to analyze his data. He worked with large numbers of plants etc.. He luckily chose plant traits to observe, in which the alleles for those traits were on separate chromosomes, or at least not close together. Modern Biology Textbook (Holt)