Mendelian Genetics

The Pea Plant• Peas reproduce sexually (they

produce two different gametes - male and female)

• Pollination= transfer of male pollen to pistil

• Fertilization= uniting of male and female gametes

POLLEN + OVULE SEED

• Self-pollination= flower pollinates itself; male and female gametes come from same plant

• Cross-pollination= male and female gametes come from different plants

Gregor Mendel

• 1850’s Austrian monk and gardner

• “Father of Genetics” (study of heredity)

• Heredity= passing on of traits from parent to offspring

• Traits= characteristics that are inherited

Mendel’s garden

Mendel’s Mendel’s Experimental MethodsExperimental Methods

• Mendel Mendel hand-pollinatedhand-pollinated flowers using a flowers using a paintbrushpaintbrush

– He could He could snip the snip the stamensstamens to prevent to prevent self-pollinationself-pollination

– Covered each flower Covered each flower with a cloth bagwith a cloth bag

• He traced traits through He traced traits through the the several generationsseveral generations

Mendel’s Studies

• He first studied one trait at a time (the first trait he studied was the height of pea plants)

• Monohybrid cross= mating between parents that only differ by ONE trait

(Other traits he studied: flower position, flower color, seed color, pod color, seed shape, pod shape)

Mendel’s Studies

Mendel’s Studies

P1 (Parent) Generation:

X

F1 (filial) Generation:

Hybrids= offspring of parents with different forms of the same trait

F2 Generation:

Cross-pollinate

Self-pollinate

Mendel tested 6 more traits. In every cross, one trait disappeared in F1, then reappeared in a ration of ¼ in the F2 plants.

*Remember- a good science experiment must give reproducible results!

Mendel’s LawsFrom his pea plant experiments,

Mendel came up with rules or “laws” about how traits are passed down from parent to offspring:

1. Rule of Heritable Factors:EACH ORGANISM HAS 2 COPIES OF A GENE THAT CONTROLS EACH TRAIT; ONE COPY CAME FROM THE MALE PARENTS AND ONE COPY CAME FROM THE FEMALE PARENT

• Each chromosome has genes (a gene is a piece of DNA that controls a trait- ex. height)

Mendel’s Laws

• There can be different forms of the same gene; these different forms are called alleles.ex. For the height gene, the alleles are tall and short

• The offspring can inherit any combination of alleles from the parentsex. Tall + tall, short + short, tall + short, or short + tall

Mendel’s Laws2. Rule of Dominance

ALLELES CAN BE DOMINANT OR RECESSIVE; THE DOMINANT ALLELE, IF PRESENT, COMPLETELY MASKS THE RECESSIVE ALLELE

• Dominant Trait- remained in the F1 generation (ex. tall)

• Recessive Trait- disappearned in F1 (ex. short)

• So we say that tall is dominant to short.

• When writing alleles, follow these rules:– Use the same letter for different

forms of the same gene.– Use uppercase letter for dominant

allele.– Use lowercase letter for recessive

allele.– Always write dominant allele first.

Example: height

T = tall t = short

A plant can be:

TT tall

Tt tall

tt short

Punnett SquarePunnett SquareUsed to help Used to help

solve genetics solve genetics problemsproblems

Mendel’s Laws3. Law of Segregation

THE TWO ALLELES THAT CONTROL EACH TRAIT WILL SEPARATE DURING GAMETE FORMATION

TT X tt

Result: Tt (all tall)

ThenTt X Tt

Result: TT, Tt (tall) and tt (short)

Mendel’s Laws

4. Law of Independent AssortmentTRAITS ON DIFFERENT CHROMOSOMES ARE INHERITED INDEPENDENTLY OF ONE ANOTHER

• This can be seen in dihybrid crosses. For example, – Round Yellow seeded hybrid

crossed with a Round, Yellow seeded hybrid

(RrYy x RrYy)

Other Important Information:• Phenotype = written description of

appearance/behavior– Examples: tall and short

BUT, two organisms can look the same but have different gene (or allele) combinations*:

• Genotype = 2-letter gene combination– Examples: TT, Tt, tt

*Note: TT and Tt are both genotypes for the same phenotype, tall

Other Important Information:There are 3 different types of genotypes:

homozygous dominant (TT)

homozygous recessive (tt)

heterozygous (Tt)

Homo- means same

Hetero- means different

FYI: homozygous is the same as PUREBRED

heterozygous is the same as HYBRID

JUST A LITTLE PRACTICE

Description Genotype or Phenotype?

Homo- or Heterozygous?

Red Hair

GG

mm

White eyes

Short toe

Dd

Steps to Solving Genetics Problems-When solving genetics problems, ask yourself the following:

1. What’s dominant?

2. What letter do I use? What are my possible combinations?

3. What am I crossing? (What are parents’ genotypes?)

4. How do I set up my punnett square?

5. What is the answer to the problem I’m trying to solve?

Example• Tall pea plants are dominant to short plants.

A farmer crossed a homozygous tall plant with a short plant. How many of the offspring will be tall plants?

Steps to Solving Genetics Problems1. Identify the dominant trait. (ex. Tall is dominant

to short.)2. Use the first letter of the dominant trait to

represent each possible combination of alleles in the problem. (TT= tall, Tt= tall, tt= short)

3. Identify the parents’ genotypes, and write out what the cross would be. (Homozygous tall crossed with short is TT x tt.)

4. Set up your punnett square. Put one parent’s alleles on one side, and the other parent’s on the other side.

5. Solve your punnett square. Answer the question.

t

t

T T

Tt

Tt

Tt

Tt

100% will be tall plants.