Mendel and Principles of Genetics

for Monday quiz

Heredity

Passing of characteristics from parents to offspring

Genetics

the study of heredity

Characteristics are called:

traits

Who carried out the first important studies of heredity during the mid 1800’s?

• Gregor Mendel – a monk from Austria

• Had vast gardens in the monastery where he lived

• Studied math and botany

• Experimented using the scientific method

• Took meticulous (careful) notes

Mendel studied pea plants because they reproduce sexually. Why is this important?

• Pea plants have two sex cells (male and female).

• He wanted to see which traits each offspring inherited from each parent.

• He could control the crosses

• Pea plants have many simple traits

How do pea plants reproduce sexually?

1. Sex cells are called gametes.

• Male gamete = pollen

• Female gamete = ovule

2. Fertilization is the uniting of male and female gametes

3. Sperm is carried to the ovary in pollen grains

4. The fertilized ovule becomes a seed

Name the process used to hand-transfer gametes from one individual to another.

Types of crosses:

• monohybrid cross: 1 trait

• dihybrid cross: 2 traits

Mendel’s basic experiment: monohybrid cross

• Ex: height

• Mendel controlled his experiments!

• He crossed true bred (homozygous) tall plants with true bred (homozygous) short plants.

Mendel crossed one tall pea plant (6 ft.) and one short pea plant (2 ft). Describe the offspring.

They were all tall! …the short trait disappeared

Mendel crossed two offspring plants with each other. What did their offspring look like?

Mendel crossed two offspring plants with each other. What did their offspring look like?

P1

“parent” or first generation (1 tall, 1 short)

F1

first “filial” generation (all tall) (P1 offspring)

F2

second “filial” generation (3 tall, 1 short) (offspring of F1)

What happened when Mendel repeated his experiment?

In every case one trait seemed to disappear in the F1 generation only to reappear in ¼ of the F2 plants.

Mendel concluded that each organism has two factors for each trait. What did he call this?

• These factors are genes.

• Genes are on chromosomes.

• Genes have different forms called alleles.

What is an example of an allele?

A plant could have:

• 2 alleles for tallness OR

• 2 alleles for shortness OR

• 1 allele for tallness and 1 for shortness

How does a plant get two different alleles?

• 1 from female parent

• 1 from male parent

If a plant has two alleles, why do we only see one of them in the F1 generation?

• The rule of dominance

• One trait (tallness) is dominant

• One trait (shortness) is recessive

P1: tall plant (TT) x short plant (tt) =

F1: Tt (tall) all offspring

F1: tall (Tt) x tall (Tt) =

F2: 3 TT (tall), 2 Tt (tall), 1 tt (short)

How did short plants reappear in the F2 generation?

• The law of segregation

• The two alleles for each trait must separate and a parent only passes on one allele for each trait to each offspring.

Phenotype

the way an organism looks- physical appearance

Genotype

the gene combination of an organism- list the letters for the trait

Homozygous

two alleles for the trait are the same (ex: TT or tt)

Heterozygous

two alleles for the trait are different (ex: Tt)

How did Mendel explain the four different kinds of seeds?

The law of independent assortment: genes for different traits are inherited independently of each other.

What is a Punnett Square?

• If you know the genotypes of the parents you can predict the possible genotypes of their offspring.

• After you know the genotype, you can determine the phenotype.

Next Mendel did a dihybrid cross with two traits.

Mendel crossed round yellow seeds (RRYY) and wrinkled green seeds (rryy).

What happened?

F1: Round yellow seeds were dominant so all offspring had round yellow seeds.

F2: 4 different kinds of seeds appeared. They had a ratio of 9 round yellow seeds: 3 round green seeds: 3 wrinkled yellow: 1 wrinkled green (9:3:3:1)

How do we explain organisms having 2 alleles for each trait?

Most chromosomes come in pairs: one half is from the mother, the other half is from the father. A cell with 2 of each kind of chromosome is called a diploid cell (2n).

How does an organism pass on only one chromosome to offspring?

Gametes contain only one of each kind of chromosome.

• This is called a haploid cell (n).

Male gametes:

sperm

Female gametes

eggs

What are homologous chromosomes?

Paired chromosomes are called homologous chromosomes. They have genes arranged in the same order, but the alleles can be different.

Why can’t cells always reproduce using mitosis?

• Remember that when cells divide by mitosis the daughter cells have exactly the same number and kind of chromosomes as the parent cell.

• Each species has a special number of chromosomes. If both parents gave an offspring a complete set of chromosomes, the offspring would have twice the number of chromosomes and wouldn’t be the same species!

What cell division produces gametes with half the number of chromosomes as a parent’s body cell?

Meiosis.

• Meiosis I: starts with 1 (2n) cell

• Meiosis II: ends with 4 (n) cells

Meiosis is called:

reduction division

What is sexual reproduction?

• The production and subsequent fusion of haploid sex cells.

• Eggs and sperm are haploid.

• After fertilization, the zygote is diploid.