Simple Medelian Genetics Competency 12.00. Genetic Terminology.

Simple Medelian GeneticsCompetency 12.00

Genetic Terminology

DNA

• Deoxyribonucleic Acid

• The major nucleic acid in organisms

• Carries genetic information and is responsible for the transmission of traits.

Gene

• A segment of DNA that codes for a specific trait in an organism.

Allele

• An alternative form of a gene/trait.

• Example: Eye color alleles are blue, green, hazel, brown

• Can be homozygous or heterozygous

Homozygous Allele

• Organism with identical alleles for a given trait

• Can be dominant or recessive

• Example: TT or tt

Phenotype

• The physical appearance of a trait in an organism

• Determined but not always indicative of the genetic makeup of the organism

• Example: tall or short

Genotype

• The genetic composition of an organism for a given trait

• Often cannot be determined by looking at an organism

• Example: Tt or TT, both are tall

Recessive Gene/Allele

• Variation of a trait that can only be expressed in the absence of a dominant allele

• Heterozygous individuals are carriers for recessive alleles.

Dominant Gene/Allele

• Variation of a trait that is expressed over other variations of the same trait

• Most common forms in natural populations

• Some traits can be co-dominant or exhibit incomplete dominance

Chromosome

• Long condensed strand of DNA forming in the nucleus of a cell prior to cell division

• Form cells that when split, create an exact copy of DNA in the daughter cell

Chromatid Pairs

• X-shaped structures that serve as the mechanism for the transmission of genetic material during cell division.

• They are pulled apart in the process of mitosis and meiosis.

Heterozygous Allele

• Organism with different alleles for a given trait

• Example: Tt

Heredity

Gregor Mendel

• 1863• Austrian monk who

conducted the first genetics experiments using pea plants in the mid 1800s.

• Often considered the founder of genetics and heredity.

What is Heredity?

• Heredity is best described as the manner in which inheritable characteristics (traits) are passed from parents to offspring.

Heredity

• A direct outcome of the RANDOM genetic recombination resulting during reproduction– Ensures genetic diversity

Heredity

• Determines the genetic potential of an animal, but . . .

• Heredity and environmental influences determine the overall quality of the animal.

• Nature versus Nurture

Types of Heredity

• Simple Heredity• Complex Heredity

– Polygenic Inheritance– Incomplete Dominance– Codominance

Simple Heredity

• One gene controls one trait—alleles are either dominant or recessive.– Example: height and color in

pea plants

Complex Heredity

• Polygenic Inheritance– One trait is controlled by

several genes and possibly environmental factors

– Genes may even be located on different chromosomes

Complex Heredity

• Polygenic Inheritance– This is a slow process

requiring many generations to achieve desired results.•Example: Height in humans

Complex Heredity

• Incomplete Dominance– Multiple alleles for a given

trait are not expressed over one another, but in combination.•Example: RR (Red Flower) x

WW (White Flower) = RW (Red and White Striped Flower)

X =

Codominance

• Similar to incomplete dominance, except characteristics of alleles blend instead of remaining distinct– Example: RR (Red Flower) x

WW (White Flower) = RW (Pink Flower)

X =

Heredity in Agriscience

• Heredity is a huge factor in successful agricultural selective breeding programs.

• Heredity is manipulated to create high quality HYBRID offspring.

Heredity in Agriscience

• Plants and animals are inbred through several generations to isolate a specific trait or traits.– No more than 7 generations

are inbred to prevent genetic disorders.

Heredity in Agriscience

• The final generation of two different lines inbred for different traits are crossed producing offspring with the beneficial traits of both lines.– Resulting offspring

possesses hybrid vigor

Hybrid Vigor

Heredity in Agriscience

• Hybrid vigor usually lasts only one generation, as hybrid organisms rarely express traits true to type in offspring.– Alternative forms of the gene

resurface in the new cross.

Heredity in Agriscience

• Punnett Squares, Pedigree Charts, Genetic Mapping and DNA analysis can be used to predict heredity.

Punnett Squares

What is a Punnett Square?

• A method utilizing the known genotypes of parent offsprings to predict the expression of a given trait or traits in offspring.– Must know the genotype of

parents and the inheritance pattern of the trait.

Using Punnett Squares

• When using Punnett Squares the Dominant Trait is always represented by an uppercase letter

• Recessive is lowercase– Example: T=Tall, t=short

Using Punnett Squares

• A box should be drawn with one space for each allele expressed by both parents.

• In simple heredity boxes are 2x2.

Using Punnett Squares

• The alleles for one parent should be placed above each column at the top, with the alleles for the other placed beside each row on one side.

T T

T

t

Using Punnett Squares

• The alleles of each parent should be distributed across and down the box.

T T

T

t

TT TT

Tt Tt

Using Punnett Squares

TT-Homozyous Dominant

Tt-Heterozygous

Genotype Ratio:3:1:0Phenotype Ratio:4:0

T T

T

t

TT TT

Tt Tt

Using Punnett Squares

• When crossing using complex heredity boxes are 4x4 and two different traits are being crossed.

Using Punnett SquaresTG Tg tG tg

Tg

Tg

Tg

Tg

TTGg TTgg TtGg Ttgg

TTGg TTgg TtGg Ttgg

TTGg

TTGg TTgg

TTgg TtGg

TtGg

Ttgg

Ttgg

Genotype Ratio-0:4:4:4:4:0

Phenotype Ratio- 0:8:8:0

Assignment

Go to the career center and look up the following website. Complete the assignments listed on the corresponding worksheet.

http://www2.edc.org/weblabs/WebLabDirectory1.html