Genetics A. The work of Gregor Mendel 1. Monohybrid crosses 2. Dominant and recessive alleles 3. Law of Segregation B. Mendel and meiosis

Who was this Mendel and what the heck is he doing in a monastery? born in 1822 trained himself to be a naturalist early in life worked as a substitute science teacher failed the qualifying exams to be a regular high school teacher! joined a monastery in Brunn, Austria sent to Vienna U. to study science and math

MendelWeb Mendel’s first published work: "Versuche über Pflanzen-Hybriden“ or Experiments in Plant Hybridization was a landmark in clarity and insight! Trained as a mathematician and a biologist, he figured out the laws of inheritance… mathematically!!

The work of Gregor Mendel worked with pea plants… …he called them his children! Why pea plants??? There was a long-standing tradition of breeding pea plants at the monastery where Mendel lived and worked Also allowed for study in an ‘either-or’ rather than a ‘more or less’ manner

So…they were readily available and they come in lots of varieties! …there were plants with different flower colors, seed color, flower position etc..

‘Brother Greg... We grow tired of peas again!!!’

And best of all… Pea plants flowers can reproduce by themselves This allowed Mendel to see if strains were true breeding and to produce hybrids

How Mendel made hybrids… He’d then tie little bags around the flowers to prevent contact with stray pollen.

Mendel’s hybridization experiments… Monohybrid crosses: Parental Generation True-breeding purple flower True-breeding white flower x All purple flowers (the hybrids) F1 generation Allowed F1 offspring to self-fertilize F2 generation 705 purple 224 white

The results of Mendel’s monohybrid crosses led him to propose… All organisms contain two “units of heredity” for each trait (alleles). Dominant and recessive alleles… …and organisms can have any combination of the two alleles (2 dominants, 2 recessives or a mixture 1 dominant and 1 recessive). The Law of Segregation – during gamete formation, alleles separate randomly into separate gametes.

A bit of genetic jargon… phenotype vs. genotype What the organism looks like What alleles the organism has - its genetic makeup

More jargon… homozygous vs. heterozygous P 2 of the same alleles: PP or pp 2 different alleles: Pp p

Even more jargon… True breeding: When the plants self-pollinate, all their offspring are of the same variety. Hybridization: Mating, or crossing, of two varieties. Monohybrid cross: A cross that tracks the inheritance of a single character. P generation: True breeding parents. F1 generation: (first filial) Hybrid offspring of the P generation. F2 generation: (second filial) Offspring from the self-fertilization of the F1 hybrids.

A Punnett square

A Punnett square… Gametes from one parent p P PP Pp P Gametes from other parent purple purple pp Pp p purple white Ratio: 3:1 or ¾ purple, ¼ white

Let’s relate Mendel’s findings to what we now know about gamete formation True-breeding white flower True-breeding purple flower x all purple P P p p P p P P P P p p p p

Mendel’s Laws of Genetics

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 express only the dominant trait. Example: RR (round seeds) x rr (wrinkled seeds) yields all Rr (round seeds)

Law of Segregation During the formation of gametes (meaning eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then “recombined” at fertilization, producing the genotype for the traits of the offspring. Cool – but what does this mean????

Law of Segregation (continued)

Law of Independent Assortment Alleles for different traits are distributed to sex cells (and offspring) independently of one another. Check out this example – See how not every yellow seed is round?

P P P P P P P P purple hybrid x purple hybrid F1 generation p p p p p pp - white

Join to your partner and together, work on the following… Determine the phenotypic and genotypic ratios for each of the following monohybrid crosses. Phenotypic ratio Genotypic ratio Aa x Aa 3:1 (dom:rec) 1:2:1 (hd:h:hr) AA x Aa 1:0 (dom:rec) or all dominant 1:1:0 (hd:h:hr) aa x Aa 1:1 (dom:rec) 0:1:1 (hd:h:hr) AA x aa 1:0 (dom:rec) or all dominant 0:1:0 (hd:h:hr) hd = homozygous dominant; h = heterozygous; hr = homozygous recessive

SO, NOW IT’S YOUR TURN TO WORK ALONE Click on the site below http://www.mendel-museum.com/eng/1online/experiment.htm