Chapter 11 Genetics

Gregor Mendel

• Used pure breed pea plants to understand inheritance

• Used self pollination to cross breed– Pollen from the male

part of the pea plant fertilizes the female egg of the same flower

Cross Pollination

• Cross Pollination– Mendel cut away the male parts of the

plant and then dusted pollen from another plant on the flower

– Seed that come from cross-pollination inherit all of the characteristics from two parents

Genes and Dominance

• A trait is a specific characteristic that varies from one individual to another

• The chemical factors that determine traits are called genes

• The different forms of a gene are called alleles

• Video Clip!!

Mendel’s Work

• Mendel crossed two pure breed plants with different forms of the same trait– One plant was tall and one plant was short

(trait: plant height)

• The offspring plants were called hybrids– Hybrids are the offspring of crosses between

parents with different forms of a trait

Principle of Dominance

• The principle of dominance states that some alleles are dominant and others are recessive– An organism with a dominant allele for

a specific trait will always show that form of the trait.

– An organism with a recessive allele for a specific trait will only show that form only when the dominant allele for that trait is not present.

Segregation

• Segregation is the separation of alleles during gamete formation

• Gametes are specialized cells involved in sexual reproduction

Probability and Punnet Squares

• Principle of probability can be used to predict the outcomes of genetic crosses.

• Probability is the likelihood that a particular event will occur.– Probability can predict the outcome of

genetic crosses because alleles segregate randomly.

Punnet Squares

• These can be used to predict and compare the genetic variations that will result from a cross.

TT

50%

TT

50%

Tt

50%

Tt

50%

T T

T

t

Punnet Square, cont.

• Homozygous – TT or tt, organisms that have two identical alleles for a particular trait.

• Heterozygous – Tt, organisms that have different alleles for the same trait

• Phenotype – physical characteristics, – TT = tall, Tt = tall, tt = short

• Genotype – genetic makeup, TT, Tt, tt.

TT

(tall)

Tt

(tall)

Tt

(tall)

tt

(short)

T t

T

t

Ratios:

Phenotype- 3 Tall: 1 Short

Genotype- 1 TT: 2Tt: 1tt

Probabilities Predict Averages

• Probabilities predict the average outcome of a large number of events.

• Probability cannot predict the precise outcome of an individual event

Mendelian Genetics

• Mendel found that the gene controlling seed shape did not affect the gene controlling seed color.

• Principle of Independent Assortment– Genes for different traits can segregate

independently during the formation of gametes

Independent Assortment

• Independent Assortment accounts for the many genetic variations observed in plants, animals, and other organisms

Beyond Dominant and Recessive Alleles

• Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

Incomplete Dominance

• One allele is not completely dominant over the other.

• The heterozygous phenotype is somewhere between two homozygous phenotypes

Codominance• Both alleles contribute to the

phenotype.• For Example: In chickens, the allele

for black feathers is codominant with the allele for white feathers. These chickens will have both black and white feathers.

Multiple Alleles/Polygenic Traits

• Multiple alleles are when there is more than two possible alleles that exist for each trait in a population.

Example: blood type (A, B, AB, O)• Polygenic traits are traits that are

controlled by 2 or more genes. Example: variations in human skin color; height

Meiosis

• The daughter cells (haploid) produced contain only HALF of the number of chromosomes as the original parent cell (diploid).

• How does this happen? Meiosis is a process of reduction division—that is, the number of chromosomes is cut in half. No replication occurs.

Meiosis

Crossing-over• During meiosis I,

chromosomes pair up with their corresponding homologous pair chromosomes. They form a tetrad.

• These tetrads exchange alleles and form new allele combinations in a process known as crossing-over.

• Video Clip!!