Genetics Origin and Theory

Gregor Mendel

Gregor Mendel

Pea Plant Characters and Traits

Wrinkled

Short

Character

Trait

Mendel’s Experiments

Began with monohybrid crosses Crosses that involve organisms

that are different in only one trait Ex- tall pea plant with a short

plant (P1)

3 steps…

1st step…

• Allow self-pollination for several generations

• Result: true-breeding P generation (parent generation)• Purple flowers plants

will only produce purple flowered offspring

2nd step….

• Cross-pollination of P generation

• Result: F1 generation (first filial generation)

• All same trait

…3rd step

• Self-pollination of F1 generation

• Result: F2 generation (second filial generation)

• Some purple, • Some white

His data

Mendel’s Hypotheses1. For each inherited character, an individual has two

copies of the gene-one from each parent.

2. There are alternative versions of genes: alleles

3. When two different alleles occur together, one of them may be completely expressed (dominant )

while the other may have no observable effect on the organism’s appearance (recessive)

4. When gametes are formed, the alleles for each gene separate independently of the other (law of independent assortment

Monohybrid cross- plant height

• Two traits (alleles) tall and short

• Dominant trait: tall• Recessive trait: short

In genetics problems we generally assign a letter to each trait with the upper case being the dominant and lower being the recessive allele.

Ex. Height T = Tall & t = short

Monohybrid cross

Example of a Monohybrid Cross with seed shape

Create a visual of Mendel’s Experiment with plant height.

F2 Generation

From this cross his 2nd generation were. 2nd generation 75% tall and 25% short

(F2) Why was the short gene apparent in the

2nd and not in the 1st Generation of offspring?

T t

TT tt

Fertilization

Back to his hypotheses

From this experiment Mendel concluded that for each gene we carry there are two factors called Alleles.

One allele from each homologous chromosome One from Mom and one from Dad

Mendel Cont.

This brought up the terms recessive and dominant For a recessive trait to show both alleles must

be recessive. For a dominant trait to show only one

dominant allele needs to be present Can be a carrier if you have one

dominant and one recessive (carrier for recessive) – The recessive trait does not show.

Passing on the Trait

When gametes (sex cells) are formed only one of the alleles is passed on from each parent

The allele combination that the child receives is called the genotype. (Tt)

Genotype= genomic composition The genotype may be homozygous or

heterozygous homozygous have the same alleles (TT, tt) Heterozygous have different alleles (Tt)

The trait that physically appears is the phenotype (tall or short)

Write out the genotype for pea height ex for….

Homozygous dominant, homozygous recessive, and heterozygous

Punnett squares

The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment.

Named after Regi Punnett Is a summary of every possible combination

Draw the punnett squares for the height example

Mendel’s Laws

Law of Segregation: Gene pairs separate when gametes form.

Alleles separate during meiosis Alleles are found on the homologous

chromosomes so when they separate the Gene Pairs separate.

Mendel’s Law

Law of Independent Assortment: Gene pairs segregate into gametes randomly and independently of each other regardless of how they are connected in the organism. Note: they have to be on different

chromosomes. These allele pairs are then randomly

united at fertilization.

YyPp

Independent Assortment

YP Yp yP yp

Y = Yellowy = green

P = Purplep = White

Review

What is an allele? What is a phenotype? What is a genotype? Define dominant and recessive based upon

allele presence. Explain Law of Segregation Explain Law of Independent Assortment.