Intro to GeneticsNovember 2014

Heredity

•Study of the transmission of characteristics from parent to offspring.

•The field of genetics began with the work of Gregor Mendel

The Work of Gregor Mendel

• About Mendel– Began his work in 1860s– Breeded & studied garden pea

plants– Published paper in 1866 stating

that parents pass on discrete “heritable factors” (genes) on to their offspring• Remember that genes are

segments of DNA that code for traits

– Considered “Father of Genetics”

Plants used in Experiments

• Mendel probably studied pea plants because of their short generation time, plentiful offspring & visible varieties – Traits: Variations of genetic

characteristics.• These pea plants are usually self-

pollinating (sperm cells in pollen fertilize egg cells located in ovary on the carpel in the same flower)– This is an example of Sexual

Reproduction (Male & female gametes fuse to form a zygote.)

What Mendel Did• He prevented self-

pollination by cutting off male stamens (w/ pollen)

• Transferred pollen from other plants onto stigma located on carpel.

• This is called cross breeding- crossing plants w/ different characteristics

Traits Mendel Studied

Mendel’s work

• Mendel crossed “true breeding” (plants that produced offspring identical to parents after self-pollinating) tall pea plants with true-breeding short pea plants (P generation)

• All of the offspring (F1 generation) were tall

• When he allowed the first-generation tall plants to self-pollinate, three-fourths of the offspring (F2 generation) were tall and one-fourth were short.

Pea Plant Generations

• Parental generation (P) - original crossed plants

• First Filial (F1) generation- offspring of P generation

• F2 generation- second generation made when two F1 generation plants breed

Results of Mendel’s Experiments

1. Alternate forms of genes (alleles) produce variations in inherited characters

2. For each character, an organism inherits 2 alleles (one from each parent)

3. Principle of dominance4. Law of Segregation5. Law of Independent Assortment

Principle of Dominance

• Principle of Dominance- some alleles are dominant while others are recessive– Dominant- when that allele is

present that trait will be exhibited– Recessive- masked trait when

dominant allele present

Important Vocabulary

• Homozygous- organisms w/ two identical alleles for a trait (TT or tt) This is also called purebred.

• Heterozygous- have different alleles for a trait (Tt). This is called hybrid.

• Genotype- genetic makeup described with letters (TT, Tt or tt)

• Phenotype- physical characteristics described with words like tall or short

Law of Segregation• Applies to sexual

reproduction• Individuals have two

alleles of each gene. When gametes are produced, each gamete receives one of these alleles.

• During fertilization, these gametes randomly pair to produce four combinations of alleles.

Law of Independent Assortment

• Each pair of alleles assorts independently of other pairs of alleles during gamete formation

• The inheritance of one character has no effect on the inheritance of another

• For example-– Pea plant with genotype AaBb– A and a will separate from one

another and B and b will separate from each other

– These alleles can then recombine in four different ways.

Probability and Punnett Squares

• Probability- likelihood that a particular event or trait will occur.

• Probabilities are more accurate with bigger samples

• Punnett squares- diagram used to predict gene combinations

Crosses

• Hybrid: Offspring of parents that have different forms of a trait (Ex. tall & short)

• Punnett squares can represent different types of crosses– Monohybrid Cross: Two parents that

differ from each other by a single trait.– Dihybrid Cross: Cross between two

parents involving two different traits (Ex. Round/Yellow & Wrinkled/Green)

Punnett Squares

• Dominant allele is represented with a capital letter. When capital letter is present, that phenotype is usually exhibited. Always write dominant letter first.

• Recessive allele is represented with a lower case letter. Phenotype exhibited when homozygous recessive (tt)

Setting up a Punnett Square

• Monohybrid Cross (One trait)• Female genotype across top• Male genotype across side• Ex:• Mother Tt, Father Tt

T t

T

t

Problem: P Generation

Mother is homozygous tall TT

Father is heterozygous tall Tt

T

t

T T

TT TT

Tt Tt

Genotype: F1

2TT:2Tt or 1/2TT:1/2Tt or 50%TT:50%Tt

Phenotype: F1

4 tall or 100% Tall

T t

T

t

F1 Cross

Father Tt

Mother Tt

TT Tt

Tt tt

Genotype: 1TT: 2Tt: 1tt Phenotype: 3Tall:1short

1/4 TT: 1/2 Tt: 1/4 tt 3/4 Tall: 1/4 short

25%TT: 50%Tt: 25%tt 75%Tall: 25%short