Mendelian Genetics

Mendel & His Pea Plants

Genetics – the study of heredity

Heredity – how traits are passed from parent to offspring

A man by the name of Gregor Mendel was curious as to how traits were passed from parent to child.

– He did his own mathematical experiments with pea plants.

– For all of his work, he is known as the “Father of Genetics.”

Mendel & His Pea PlantsMendel decided to use pea plants for the following reasons:

– The plants were small

– They were easy to grow

– They were inexpensive

– They produced a lot of offspring in a short time

Mendel & His Pea PlantsHe also liked to use pea plants because they were easy to pollinate. A single plant had both male and female parts on the same flower.

– The stamen is the male part (holds sperm)

– The pistil is the female part (holds eggs)

Mendel studied 7 traits

Mendel’s Experiments

First, Mendel created a Parental (P) Generation of tall & short purebred plants.

– Purebreds will only produce offspring like themselves.

– Tall purebred plants only produce tall offspring.

– Short purebred plants only produce short offspring.

Mendel’s ExperimentsStep 1: Mendel crossed a purebred tall plant with a purebred short plant by cross pollination. This meant dusting the pollen of one flower onto the pistil of a different flower.

– All the offspring came out TALL!

– He called this generation the F1 generation.

– Mendel was thoroughly confused.

– Shortness had somehow disappeared!

Mendel’s ExperimentsStep 2: Mendel crossed two of the tall plants from the F1 generation by self pollination. This meant dusting the pollen of one flower onto the pistil of the same flower.

– ¾ of the plants were Tall and ¼ were Short!

– He called this generation the F2 generation.

– Mendel was now thoroughly frustrated.

– Shortness had reappeared!

Let’s Recap

Step 1

Step 2

All Tall!

¾ Tall & ¼ Short!

F1

F2

P

? =

? =

Mendel’s ConclusionMendel concluded that parents pass traits to their offspring via things called “factors”.

– We now know that the factors Mendel was talking about are called genes.

– Genes are pieces of your DNA.

– Genes control all your traits.

Mendel’s Conclusion

Mendel also concluded that there are different forms of the same gene.

– Different forms of a gene are called alleles

– Everyone has 2 alleles for a single gene

– One allele comes from mom, one from dad

Learning TerminologyDominant allele– The allele that always shows when present– Represented by a Capital letter– THINK! – What allele can we use to represent Tallness?

Recessive allele– The allele that is masked by the dominant one– Only shows up if the dominant allele is NOT present– Represented by a Lower-case letter of the dominant trait– THINK! – What allele can we use to represent shortness?

Homozygous for a trait– 2 of the same alleles for a gene

Heterozygous for a trait– 2 different alleles for a gene

T

t

Learning Terminology

Phenotype– What an organism looks like; physical appearance– THINK! – What do the following organisms look like?

• TT =• Tt =• tt =

Genotype– The 2 alleles an organism possesses– Think of it as the combination of letters– THINK! – What are the genotypes for the following:

• Tall = • Short =

Tall

Tall

Short

ttTT or Tt

Let’s Recap: Write the Genotypes

TT tt Tt Tt Tt Tt

TT Tt Tt tt

Tt Tt

P F1

F2