Fundamentals of Genetics

Gregor Mendel (1822-1884)• Monk from Austria

• He bred pea plants in monastery gardens and used math to interpret his findings

• He is the “Father of Genetics”–Even though he was never recognized

in his lifetime!

Mendel’s Garden Peas• Mendel observed seven

characteristics when using purebred pea plants. Each characteristic trait occurred in two contrasting alleles.

• Traits – a distinguishing characteristic that is inherited such as eye color, leaf shape and tail length.

• Allele – different forms of a trait

Mendel’s Observations• Mendel observed 7 characteristics of pea

plants. ex: - seed color (green or yellow)- seed texture (smooth or wrinkled)- flower color (purple or white)

He used his knowledge of statistics to analyze his observations.

• Crossed plants (mated), collected seeds, recorded observations, then planted seeds, then recorded new plant characteristics.

Mendel’s Methods• Pollination – occurs when pollen grains

produced in the male reproductive parts of a flower (called anthers) are transferred to the female reproductive part of a flower (called stigma).

• Self-pollination – occurs when pollen is transferred from the anthers of a flower to the stigma of either the same flower or a flower on the same plant.

• Cross-pollination – involves pollen transfer with flowers of two separate plants.

• First Mendel snipped off the anthers

• Then he dusted the stigma with pollen taken from the desired father.

• Finally he tied bags over the flowers to keep out any stray pollen.

This gave him control over the pollination of his pea plants (no wind or insects could interfere!)

• He observed that purple-flowering plants grew from the seeds of purple plants, but… he noticed that some white-flowering plants also grew from the seeds of purple-flowering plants.

• Mendel wanted to find an explanation for such variations.

Mendel’s Laws• AKA – Patterns of Inheritance

Mendel Discovered Dominance• 1st Cross= pure Tall x pure Short

Tall X Short All Tall

Tallness = Dominant

Short = Recessive

• Dominance- allele expressed when present in genotype

• Recessive- allele expressed ONLY when two copies are present

Now Breed the Hybrid• 2nd Cross= hybrid Tall x hybrid Tall. Plants self

pollinate (self-fertilize)

Hybrid- containing contrasting alleles

Tall x Tall ?

Results in math:3/4 tall : 1/4 short plantRecessive trait

reappears!!!

1. Law of Segregation•Organisms inherit two copies of each gene/trait = alleles.

•Alleles segregate (separate) during the formation of sperm and eggs.

•Organism donates ONE copy of each gene in fertilization.

AllelesDef: Allele is an alternative form of a gene

(ex: tall and short for PLANT HEIGHT)• One allele T is for tallness• One allele t is for shortness• T allele is dominant over t allele.

Tt = TallTT = Tall

tt = shortCertain traits do not blend!! (no medium height)

Same Phenotype

Different Genotype

TT Tt

Homozygous – when both alleles of a pair are alike. (Ex: TT or tt)

(can be homozygous dominant or homozygous recessive)

Heterozygous – when the two alleles in the pair are different. (ex: Tt)

Punnet Square

• A grid system to predicting all possible genotypes and phenotypes that result from a cross.

• Is a shorthand way of showing how the gametes behave in a cross.

Monohybrid Cross– a cross

between one trait (ex: plant height, OR hair texture)

Tt tt

Tt tt

T t

t

t

Testcross- cross between an organism with homozygous recessive genotype

and a second organism with an unknown genotype.

• The offspring results will show whether the organism with unknown genotype is heterozygous or homozygous dominant.

Dihybrid Crossa cross between two traits (ex: plant height and seed color)

Mendel’s question… are traits inherited together?Ex: Do people with Blond hair always have blue eyes? Do purple flowers always make smooth seeds? Do seeds that are round have to be yellow???

2. Law of Independent Assortment

- Traits segregate (separate) independently during the formation of sperm and eggs.

- They then come together again in the new offspring (kid)