The Work of Gregor Mendel:

Monohybrid, Dihybrid, Incomplete, Codominance, Multiple Alleles, Polygenic

Traits

Genetics: study of hereditary

Every living being- plant, animal, microbe or human being, has a set of characteristics inherited from its parent or parents.

Genetics: study of hereditary

Gregor Mendel’s Peas:

Austrian Monk in charge of the garden, Studied peas

What he knew:

fertilization- during sexual reproduction a male sperm (pollen) and female egg (ova) joined which produced a new cell, which began to develop into a tiny embryo encased within a seed.

When Mendel took control of the garden, he had a garden full of self-breeding garden peas which were true-breeding

True-breeding-

pure genes, if allowed to self pollinate, these plants would produce identical copies of themselves.

What he had to work with

One stock of seeds: produced Tall plants,

One stock of seeds: produced Short Plants,

One line produced: green Seeds,

Another line produced: yellowyellow Seeds

cross pollination

Basis of Mendel’s Experiments: Tall, Short, green Seeds, yellow seeds

He wanted to cross breed these plants called cross pollination

Genes and Dominance:

Mendel studied 7 different pea plant traits

Trait: a specific characteristic, such as seed color or height

1. Form of ripe seed Smooth Wrinkled

2. Color of seed albumen

Yellow Green

3. Color of seed coat Grey White

4. Form of ripe pods Inflated Constricted

5. Color of unripe pods Green Yellow

6. Position of flowers Axial Terminal

7. Length of stem Tall Dwarf

Mendel crossed these plants and studied their offspring He called each of the original plants the P

(Parental) generation

F1 generation

He called the offspring, F1 or “first filial” generation

Filius and filia are the Latin words for “son” and “daughter” Therefore the child in the picture below is the F1 generation of

those parents

Hybrids

The offspring of crosses between parents with different traits are called hybrids

Ex: toyota prius

Hybrids in other areas

Mythology: Centaurs Biology: Zeedonk, Liger

So, what were the results? Did they have a mixture of all the traits? NO, all the hybrids had the characteristics of

only ONE of the parents. In each cross, the character of the other

parent seemed to disappear!

First Conclusion:

biological inheritance is determined by factors that are passed from one generation to the next

Today, we called these GENES Different forms of a gene are called Allele

Second conclusion: Principle of Dominance P of D: states that some

alleles are dominant and others are recessive

An organism with a dominant form will ALWAYS show the trait

Mendel’s Findings

Tall plant was Dominant and Short plant was recessive/

Yellow seeds Dominant and green seeds are recessive

Some common human dominant traits

Segregation:

Mendel wanted to know what happened to the recessive traits. Did they disappear?

He took the F1 generation and crossed them with one another and made the F2 generation

F2 generation:

the recessive traits reappeared ¼ or 25% of all the plants had the recessive

traits

He concluded the

F1 plants produced gametes (sex cells), the 2 alleles segregated from one another so that each gamete carried a single copy of the gene

In the F1 generation, each gamete had 1 copy of the Tall gene and one copy of the short gene.

Genetics and Probability:

Probability: the likelihood that an event will occur

Coin toss: 2 possibilities: head or tails The probability or chances are equal, 1 in 2

chance That is ½ or 50% chance If you flip a coin 3 times in a row what are the

chances that you will get heads every time: ½ x ½ x ½ = 1/8 1 in 8 chance of flipping heads 3 times in a row!

So what?

The principles of probability are used to predict the outcomes of genetic crosses

Punnett Squares

the gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett Square

Very important terms to know!

Homozygous: 2 identical alleles (TT or tt) considered true-breeding

Heterozygous: 2 different alleles (Tt) considered Hybrids

Phenotypes: physical characteristics, like tall or short

Genotypes: genetic characteristics like TT, Tt, or tt

Tutorial

The figure represents a monohybrid cross of F1-hybrid plants.

Both parent plants are heterozygous (Ss) for an allele that determines seed shape.

Presence of the dominant allele (S) in homozygous (SS) or heterozygous (Ss) plants results in spherical seeds.

Homozygous recessive (ss) plants have dented seeds.

Setting up a Punnett square

1. Set up a 2 by 2 Punnett square.

2. Write the alleles for parent 1 on the left side of the Punnett square.

Each gamete will have one of the two alleles of the parent.

In this particular cross, half of the gametes will have the dominant (S) allele, and half will have the recessive (s) allele.

3. Write the alleles from parent 2 above the Punnett square.

For this heterozygous parent (Ss), half of the gametes will have the dominant (S) allele, and half will have the recessive (s) allele.

4. Fill the squares for parent 1.

Fill each square with the allele from Parent 1 that lines up with the row.

5. Fill the squares for parent 2.

Fill each square with the allele from Parent 2 that lines up with the column.

Interpreting the results of a Punnett square We now have the information for predicting

the outcome of the cross. The genotypes in the four boxes of the

Punnett square are each equally likely to occur among the offspring of this cross.

We may now tabulate the results.

Genotypes that resulted from this monohybrid cross (Ss x Ss)

25% 50% 25% homozygous heterozygous homozygous

dominant dominant recessive

Phenotypes that resulted from this monohybrid cross (Ss x Ss)

Independent Assortment: Dihybrid Crosses 2 factor Cross Mendel crossed true breeding plants that

produced only round yellow peas (RRYY) with plants that produced wrinkled green peas (rryy)

All the F1: round yellow peas

Showed that the allele for yellow and round peas are dominant Provided the hybrid plants for the F1 cross to

produce the F2 generation F2: RrYy Found a grand mix of traits proving that the

genes practiced independent assortment meaning that the seed shape and color are independent of one another

F2 results//

9:3:3:1 9 yellow and round, 3 green and round, 3

yellow and wrinkled, 1 green and wrinkled

Tutorial: Dihybrid Crosses

Determine all possible combinations of alleles in the gametes for each parent.

Half of the gametes get a dominant S and a dominant Y allele; the other half of the gametes get a recessive s and a recessive y allele.

Both parents produce 25% each of SY, Sy, sY, and sy.

1. Punnett square.

Since each Parent produces 4 different combinations of alleles in the gametes, draw a 4 square by 4 square punnett square.

2. Place in Gametes from Parent 1

List the gametes for Parent 1 along one edge of the punnett square.

3. Place Gametes from Parent 2

List the gametes for Parent 2 along one edge of the punnett square.

4. Fill in Alleles from Parent 1

Fill out the squares with the alleles of Parent 1.

5. Fill in Alleles from Parent 2

Fill out the squares with the alleles from Parent 2.

The result is the prediction of all possible combinations of genotypes for the offspring of the dihybrid cross, SsYy x SsYy.

Predicting the phenotype of offspring There are 9 genotypes

for spherical, yellow seeded plants. They are:

SSYY (1/16)SSYy (2/16)SsYY (2/16)SsYy (4/16)

Predicting the phenotype of offspring Two recessive alleles

result in green seeded plants.

There are 2 genotypes for spherical, green seeded plants. They are:

SSyy (1/16) Ssyy (2/16)

Predicting the phenotype of offspring Two recessive s alleles

result in dented seeded plants.

There are 2 genotypes for dented, yellow seeded plants. They are:

ssYY (1/16) ssYy (2/16)

Predicting the phenotype of offspring A ssyy plant would be

recessive for both traits.

There is only 1 genotypes for dented, green seeded plants. It is:

ssyy (1/16)

A phenotypic ratio of 9:3:3:1

A phenotypic ratio of 9:3:3:1 is predicted for the offspring of a SsYy x SsYy dihybrid cross.

Summary of Mendel’s Principles:

Inheritance of biological characteristics is determined by genes passed from parents to offspring

In cases where 2 alleles exist: 1 is dominant, 1 is recessive

Each adult has 2 copies of each gene, one from each parent

Genes for different things usually segregate independently

Beyond Dominant and Recessive alleles: some alleles are neither dominant nor

recessive many traits are controlled by multiple alleles

or genes

If Mendel were given a mommy black mouse & a daddy white mouse & asked what their offspring would look like, he would've said that a certain percent would be black & the others would be white.  He would never have even considered that a white mouse & a black mouse could produce a GREY mouse!  For Mendel, the phenotype of the offspring from parents with different phenotypes always resembled the phenotype of at least one of the parents. 

In other words, Mendel was unaware of the phenomenon of INCOMPLETE DOMINANCE.

1. Incomplete Dominance:

F1 Generation: red flower (RR) and white flower (rr)

F2 Generation: pink (Rr) With incomplete dominance, a cross

between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. 

2. Codominance:

Both alleles contribute to the phenotype ex: chicken feathers

In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.

I remember codominance in the form of an example like so:

red x black ---> red & black spotted

In horses, gray horses (GG) are codominant to white horses (WW).  The heterozygous horses(GW) is an appaloosa horse (a white horse with gray spots on the rump and loins). Cross a white horse with an appaloosa horse.

Genotype:GW (2); WW (2)

Phenotype:

appaloosa (2)

white (2)

W W

GW

WW

GW

WW

G

W

Polygenic traits

interactions of several genes Ex: eye color, skin color

3. Multiple Alleles:

more than two alleles that code for the trait Ex: coat color of rabbits

Blood Types

A and B are codominant AA = Type A BB = Type B AB = Type AB

A and B are dominant over O AO = type A BO = type B OO = type O

#1

Show the cross between a mother who has type O blood and a father who has type AB blood.

- AO (2) BO (2)- ratio 1:1

- type A (2); type B (2)- ratio 1:1

PHENOTYPES:

GENOTYPES O O

A

B

AO

BO

AO

BO

#2

Show the cross between a mother who is heterozygous for type B blood and a father who is heterozygous for type A blood

Genotype:

Phenotype:

-AB (1); BO (1); AO (1); OO (1)- ratio 1:1:1:1

-type AB (1); type B (1) type A (1); type O (1)- ratio 1:1:1:1

GENOTYPES:

PHENOTYPES:

A O

B

O

AB

OO

BO

AO

#3

Show the cross between a mother who is homozygous for type B blood and a father who is heterozygous for type A blood

Genotype:

Phenotype:

Show the cross between a mother who is homozygous for type B blood and a father who is heterozygous for type A blood

Genotype:

AB (2), BO (2)

ratio - 1:1

Phenotype:

Type AB (2);

Type B (2)

ratio – 1:1

B B

A

O

AB

BO

AB

BO