Mendel and the Laws of Inheritance

BiologyDobson High School

Hope Finzer

Who was Gregor Mendel?

Austrian monk and scientistHis work marks the start of modern geneticsUsed pea plants to study the passing of traits from generation to generationFirst person to succeed in predicting how traits are transferred from one generation to the next

Why did Mendel choose pea plants?

Pea plants have both male and female gametes in each plantThey reproduce sexually by the transfer of pollen grains from the male gamete to the female ovuleThe plants reproductive structures are easily identifiedShort 90 day life cycle Distinct traits: seed shape, seed color, pod shape, pod cover, and height; are some examples

Pea Traits

How did Mendel control his experiment?

By manually transferring pollen from one plant to another plant with traits that he wanted to test

What is a cross?

Manually transferring pollen from a designated plant with the desired trait to another plant with another desired trait

What is pollination?

The transfer of the male pollen grains to the female organ of flower, resulting in a fertilized mature seed

Mendel’s Investigations

His goal was to explain the patterns of inheritance at the time, no explanation could accurately explain heredity

What is a hybrid?

An organism that receives different genetic information for a trait from each parent

Mendel’s Experiment

Pure-bred – organisms that when allowed to self-fertilize, only produce offspring the parental trait(s)

Mendel started by crossing pure-bred tall pea plants with pure-bred short pea plant

What sort of plants did Mendel get in his first generation?

He called the first pair the (P) or parental group and the first generation the first filial group or the F1 generationThe F1 generation were all tall but each offspring had both the tall and short genesHe then allowed the F1 group self-fertilize and they produced a F2 generation of 3/4th tall plants and 1/4th short plants

Parent, F1 and F2 generations

How did this happen?

What did Mendel determine?

That one trait must be stronger than the other

What are the types of traits?

Dominant – the trait that shows in a hybridRecessive – trait that does not show in a hybrid

Dominant &

Recessive traits

Mendel’s Hypothesis

Each trait is controlled by something he called a “factor” within the organismWe now call this factor a “gene”Gene – genetic factor that controls a trait Since 2 parents contribute to the offspring, there must be 2 genes that separate when gametes form Since traits can disappear for a generation and return unchanged to the next, they must be separate and distinct

What is an allele?

Gene form for each variation of atrait of an organism

Mendel’s pea plants The pure-bred plants have 2 of the same alleles; example Mendel’s tall pea plants had 2 alleles for tallness

The hybrid crosses have alleles of 2 different traits Example Mendel’s F1 generation had 1 allele for tallness and 1 allele for shortness

Laws of Segregation

The 2 alleles for each trait must separate when gametes formA parent passes on at random only 1 allele for each trait to each offspring

What is a phenotype? The way an organism looks and behaves – the outward appearance regardless of what is in the genetic code Example Mendel’s F1 generation looked tall but had the gene for shortness

What is a genotype? The gene combination an organism contains; example even though Mendel’s F1 generation was all tall plants, they all had for both tallness and shortnessIf the organism has the same alleles or traits; example having both tall alleles, usually written as TT; then the organism is considered homozygous

What is a genotype?

If the organism has different alleles or traits; example having one tall allele and one short allele, usually written as Tt; then the organism is considered heterozygous

What is a Dihybrid cross?

A cross involving two different traits

What happens in the F1 generation?

Mendel took a pea plant with round, yellow seeds (these are the dominant traits) and crossed this plant with a pea plant with wrinkled, green seeds (these are the recessive traits)

What happens in the F1 generation?

The genotypes of these are:

RRYY and rryyThe entire F1 generation were genotype – RrYyThe entire F1 generation had the phenotype - round, yellow seeds

What happens in the F2 generation?

Mendel allowed the plants in the F1 generation to self-pollinate andMendel found that there were some plants with round, yellow seeds and some wrinkled, green seeds

What happens in the F2 generation?

He also found that some plants had round, green seeds and some wrinkled, yellow seedHe found that the traits appeared in a fixed ratio of:9 round, yellow3 round, green3 wrinkled, yellow1 wrinkled, green

What is Mendel’s Law of Independent Assortment?

Genes for different traits are inherited independently of each other

What is a Punnett Square?

Short hand way of finding the expected proportions of possible genotype in offspring

Monohybrid crosses in thePunnett Square

The Punnett Square is 2 boxes tall and 2 boxes wide. One parents’ genotype is separated and placed on the top of the Punnett Square and the other parents genotype is separated and placed along the left side of the boxes

Monohybrid crosses in thePunnett Square

The alleles are then written into the squares that are underneath or beside them and then all possible offspring can be determined

Examples:

Exceptions to Mendel’s Laws

Incomplete dominanceCodominanceSex-linked traits

Incomplete Dominance

Shows the appearance of a third phenotypeExample: crossing a red flowering snapdragon with a white flowering snapdragon produces pink flowering snapdragons in the F1 generation

Incomplete Dominance

Codominance

Both alleles are expressed in the F1 generationExample: Cross a black feathered chicken with a white feathered chickenThe F1 generation will show both black and white feathers in a checkerboard pattern

Codominance

Sex Linked Traits

The expression of traits from genes found on the sex chromosomes – X or YExample eye color in fruit flies

Examples: Incomplete Dominance

Examples: Codominance

Examples: Sex-linked traits