Chapter 4 Section 1&2: Mendel’s work• Key concepts:– What were the results

of Mendel’s experiments, or crosses?

– What controls the inheritance of traits in organisms?

• Key terms:– Heredity– Trait

– Genetics– Fertilization– Purebred– Gene– Alleles– Dominant allele– Recessive allele– Hybrid– Phenotype– Genotype– Codominance

The basics

• Heredity is the passing of physical characteristics from parents to offspring

• A characteristic of a living thing is a trait. Examples: eye color, hair color, plant stem height, seed color

• Genetics is the study of heredity

There were no good memes for heredity.

Gregor Mendel (Germany, 1800s)

• “Don’t let my comb overfool you. I’m actually quitesmart.”

“I like pea plants and I cannot lie.”

Mendel’s experiments

• Fertilization – when sperm and egg join to form a new organism

• Purebred – offspring of many generations of organism that have the same trait– Example: purebred short pea plants always come

from short pea plants

Mendel’s experiments

• In one experiment, Mendel crossed purebred tall plants with purebred short plants (he did not allow self-pollination (meaning, plants HAD to “mate” with another plant) in this round of experiments). The mating plants were called the parental, or P, generation.

• Their offspring were the F1 generation. All offspring were tall.

• When the F1 plants were full grown, Mendel allowed them to self-pollinate (fertilize themselves). The F2 generation that resulted from this had a mix of tall and short plants. The shortness trait had reappeared, even though none of the F1 plants were short.

• About 3/4s of F2 were tall, and ¼ were short.

Mendel did other stuff, too…

• He tried other traits, like seed shape, seed color, pod shape, pod color.

• In ALL of Mendel’s crosses, one form of the trait appeared in the F1 (first) generation. However, in the F2 (2nd) generation, the other form of the trait reappeared in ¼ of the plants.

• What does this meaaaaan??

Dominant and recessive alleles

• Gene – a factor that controls a trait. (found on chromosomes within DNA sequences)

• Allele – different forms of a gene– Example, brown and blue eyes are alleles for the

gene that controls eye color– Or, in Mendel’s case, an allele for short or tall

plants in the gene that decides plant height

Alleles

• An organism’s traits are controlled by the alleles (forms of a gene) in inherits from its parents. Some alleles are dominant (brown eyes, tall pea plants), some are recessive.

• A dominant allele is one whose trait always shows up when the allele is present.

• A recessive allele is hidden whenever the dominant allele is present.

In Mendel’s case

• The purebred tall plants in the P generation had two (dominant) alleles for tall stems. The purebred short plants had two (recessive) alleles for short stems. When crossed, the resulting generation, F1, had one dominant and one recessive allele, which showed the dominant allele, which is tall.

However,

• The F1 generation had both sets of alleles, dominant and recessive, so when the F1 generation had babies, F2, we saw a few plants retain the recessive allele ONLY. And that is why some short plants showed back up.

• In a genetic cross, the allele that each parent will pass on to its offspring is based on probability.

Punnett square examples:

Punnett square example: The first cross of tall vs short plants

First and second generations.

Phenotypes and genotypes

• Phenotype – physical appearance• Genotype – allele combinations

• Examples: phenotype – tall plants; genotype – TT or Tt

• Example: phenotype – blue eyes; genotype - bb

Finally,

• Codominance – alleles are neither recessive nor dominant. As a result, both alleles are expressed in the offspring.