Geneticsthe study of heredity

Gregor Mendel“Father of Genetics”

• Heredity -the transfer of characteristics from parents to offspring through their genes

• Gregor Mendel -used garden peas to study heredity

Mendel’s Experiments• cross fertilization: Mendel crossed

two parent plants with opposite traits (purple x white). This was the Parental generation (P). The First generation (F1) were identical (purple).

• self fertilization: Mendel allowed the purple flowers from the First generation (F1) to self-pollinate. Self pollination produced the Second generation (F2).

Muskopf, ShannanMuskopf, Shannan

Mendel’s Conclusions• The F1 generation all showed the purple trait (called the dominant

trait)• In the F2 generation the (white) trait reappears in ¼ of the flowers

(called the recessive trait)• Each flower has two alleles that determine the appearance • The alleles are represented by letters (uppercase letter represents the

dominant allele; lowercase letter represents the recessive allele)• P is dominant and represents purple• p is recessive and represents white• The Dominant Is Expressed No Matter What• Need 2 Copies Of The Recessive Allele In Order To Be Expressed• PP = purple flower Pp = purple flower pp = white flower

Alleles

– homozygous: organisms that have 2 identical alleles for a trait (could be two capital or two lowercase letters)• PP • pp

– heterozygous: organisms that have 2 different alleles for a trait• ex: Pp (the dominant allele P is expressed so

this flower would be purple)

• Genotype: letters used for the alleles– ex: PP, Pp, pp

• Phenotype: what an organisms looks like– ex: purple, white

Punnett square

• A Punnett square is used to show the possible allele combinations in the offspring of 2 parents.

• Monohybrid cross = cross involving only 1 trait

The four boxes represent the four possible offspring

A plant heterozygous with green peas (Gg) is crossed with a plant that has yellow peas (g).

• Step 1: Choose a letter for the alleles (green is dominant; yellow is recessive)

• G : green pea g: yellow pea

• Step 2: Write the genotypes of the parents

• heterozygous plant with green peas : Gg• plant with yellow peas: gg• parents: Gg x gg

Example of a Monohybrid Cross

G g g

g

gg ggggggGg

G g G g G g g

g

g

g

g

g

GgGg

gggg Gg

Step 3: Set up the punnett square with one parent on each side

Step 4: Fill out the punnett square middle

G g g

g

• Step 5: Look at the four boxes from Step 5 and determine the

genotypes of the four offspring – Genotypic ratio: 2 Gg: 2 gg

• Step 6: Look at the genotypes in Step 6 and determine the phenotypes;

– Green (G) is dominant over yellow (g), plants that have G in their offspring have green peas

– Phenotypic ratio: 2 green: 2 yellow

Practice-Monohybrid Crosses Cross an individual with blue eyes with an individual with

homozygous brown eyes. Brown eyes (B) is dominant to blue eyes (b).

Phenotypes:

Genotypes:

B B

b

b

Cross an individual with blue eyes with an individual with homozygous brown eyes. Brown eyes (B) is dominant to blue eyes (b).

Phenotypes: All Brown Eyes

Genotypes: All Bb

B B

b Bb Bb

b Bb Bb

Practice-Monohybrid Crosses A child is diagnosed with a recessive genetic disease. Neither

parent has the disease. What are the genotypes of the parents?

Phenotypes:

Genotypes:

N ?

? nn

N

A child is diagnosed with a recessive genetic disease. Neither parent has the disease. What are the genotypes of the parents?

Genotypes of the parents are Nn

N n

n Nn nn

N NN nn

Incomplete Dominance = BlendingIn snapdragons, there is not a dominant allele. The flower color can be red, pink, or white. A heterozygous flower (Rr) will a blending of red and white (pink).

•Muskopf, Shannan. Online Images. The Biology Corner. 20 April 2007. http://www.biologycorner.com/bio1/celldivision-chromosomes.html

Codominance: the recessive & dominant traits appear together (both are dominant, no recessive allele)

– Ex: Cross a red cow with a white cow. What will the offspring be?

R R

W

W

– phenotype: all red and white speckled– genotype: all RW

W RW RW

W RW RW

R R

Muskopf, Shannan. Online Images. The Biology Corner. 20 April 2007. http://www.biologycorner.com/bio1/celldivision-chromosomes.html

Polygenic Traits: “many genes” act together resulting in a range of phenotypes

– Ex: skin, hair, eye color

Skin color is a polygenic trait because it shows a range of colors. There is not a dominant and recessive color.

Farabee, M.J. “Skin Pigmentation.” 2001. Online Image. Online Biology Book. 5 May 2007. http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookgeninteract.html

Multiple Alleles: genes that have more than two alleles– There are four blood types (phenotypes): A, B, AB, and O– Blood type is controlled by three alleles: A, B, and O– Each individual only inherits two alleles (one from each

parent).– A and B are codominant– O is recessive, two O alleles result in type O Blood

Blood Types Possible Genotypes

Dominant A AA or AO

Dominant B BB or BO

Codominant AB AB

Recessive O OO

Practice: Cross a Type AB with a Type O.

AB x OO

A B

O AO BO

O AO BO

1. Set up punnett square with one parent on each side

2. Fill out the punnett square middle

What are the possible blood types of the four offspring?

Genotype Blood Type

2 AO 2 Type A

2 BO 2 Type B

A woman heterozygous for Type A blood marries and a man with Type AB blood. Show the cross and the possible offspring.

1. Write the genotypes of the parents:• woman heterozygous for Type A: AO

• man with Type AB: AB

2. Set up punnett square with one parent on each side and fill in the middle.

Practice

A O

A AA AO

B AB BO

Blood types of possible offspring:

AA: Type A blood

AO: Type A blood

AB: Type AB blood

BO: Type B blood

If a Type O individual marries a Type B individual can they have

offspring with Type O blood? What type of blood can the offspring have?

Practice

B ?

O

O

Blood types of possible offspring:

AA: Type A blood

AO: Type A blood

AB: Type AB blood

BO: Type B blood

If a Type O individual marries a Type B individual can they have

offspring with Type O blood? What type of blood can the offspring have?

B O

O BO OO

O BO OO

Yes, they can have a child with type O or type B blood.

If 2 individuals with Type AB blood marry, what percentage of their offspring will have Type AB blood?

A B

A

B

Practice

50% of their offspring could be Type AB

A B

A AA AB

B AB BB

If 2 individuals with Type AB blood marry, what percentage of their offspring will have Type AB blood?

Review of Terms

• Allele A form of a gene• Homozygous Both Alleles are the

Same• Heterozygous Alleles are Different• Homozygous Dominant AA• Homozygous Recessive aa• Heterozygous Aa• Genotypic Ratio 2 PP : 2 pp• Phenotypic Ratio 2 Purple : 2 White