LIGHT – electromagnetic radiation



CH 14 & 22 Mendel

Gregor Mendel discovered the basic principles of heredity by breeding pea plants in carefully controlled experiments. He probably chose pea plants because there were many varieties of characteristics, they have a short generational time, they produce many offspring, and the crossing of one plant with another was easy to control.

Genetics Vocabulary:

1. Character(istic): a heritable feature. Ex: flower color

2. Trait: a variation of the character Ex: purple flowers

3. Gene: the section of DNA that codes for the production of a specific protein that controls a characteristic.

4. Allele: a form of a gene. Genes may have 2 or more possible forms that vary in the base sequence of the gene.

- Gregor Mendel identified a type of inheritance called simple dominance / recessiveness, in which:

A dominant allele will determine the trait in an individual. It is generally represented by a capital letter.

A recessive allele has no effect on the trait unless it is the only allele present in the organism. It is generally represented by a lower case letter.

The same letter is used for one gene. For example Purple flowers (dominant) = P

White flowers (recessive) = p

5. Genotype: refers to the alleles within a cell of an organism.

- Homozygous describes a genotype made up of the same two alleles.

Also called “pure”.

Ex: Homozygous dominant: PP

Homozygous recessive: pp

- Heterozygous refers to a genotype made up of two different alleles.

Also called “hybrid”.

Ex: Pp

6. Phenotype: the appearance or expression of the genotype.

Ex: Purple flowers or Type A blood

Mendel’s Experiments

1. Mendel always began his breeding experiment with pure-breeding (homozygous) parents, one with the dominant alleles and one with the recessive alleles. These were called the parental generation, P1

2. Crossing the two homozygous parents would always produce a heterozygous or hybrid offspring. This generation is called the F1 or first filial generation.

3. He would then cross two F1 plants together to produce the F2 or second filial generation.

His F2 generation always showed 3 dominant : 1 recessive phenotype ratio.

He applied mathematics to his experimental data to try and determine how the traits were being inherited.

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4. Mendel came up with the following conclusions:

a. The “heritable factor” for the one of the traits, which we now know is an allele or gene, is not lost in the F1 generation, but was somehow hidden.

b. Variation within a characteristic is caused by different forms of a gene (alleles).

c. An organism inherits two alleles, one from each parent.

d. The dominant allele will determine the trait if present. The recessive allele will determine the trait only if it is the only allele present.

e. Law of Segregation: the two alleles for a characteristic segregate during gamete formation and end up in different gametes.

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5. Monohybrid cross: is a cross between two heterozygous (hybrid) individuals. The results are always a 3 dominant: 1 recessive ratio.

(applies only to simple dominance/recessiveness)

An example is the F1 x F1 cross in the diagram above.

6. Punnett square: a method used to predict the expected offspring from a specific mating.

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The possible gametes from each parent are written either across the top or down the left side of the box.

The gamete allele from the column is combined with the gamete from that row to produce the genotype of a possible offspring. Note that the dominant allele is traditionally written first, if present.

The expected offspring can be expressed as a ratio or a percentage.

7. Test cross: is a type of cross (mating) used to determine the genotype of an individual that shows the dominant trait.

This is done by mating the mystery parent (who may be homozygous dominant, AA, or heterozygous, Aa) to an individual showing the recessive trait (aa).

An analysis of the offspring will identify whether the parent in question is homozygous or heterozygous.

Test cross example: Black fur is dominant to brown fur in mice. Draw two Punnett squares to show the expected outcome of a test cross.

(one with the mouse homozygous, and one with the mouse heterozygous)

CH 22 Descent with Modification & Darwin

Charles Darwin never used the term “Evolution”.

He labeled his theory as “Descent with Modification”, his explanation of how current species are descendants of ancestral species.

Darwin developed his theory to try and explain his observations that:

• Organisms had adaptations suited to their particular habitat

• The shared characteristics of many organisms.

• The diversity of life on Earth

With our knowledge of genetics, we can define evolution as the change in genetic composition of a population over time.

Evolution is both a pattern and a process

• The pattern of changes seen in organisms over time is revealed in observation of the natural world and supported by data from biology, chemistry, physics, and geology.

• The process (or how) involves the natural mechanisms that produce the observed patterns of change.

A brief history of people and ideas leading up to our current understanding of evolution…

1. Paleontology – the study of fossils, was largely developed by the French scientist Georges Cuvier in the early 1800’s.

- Fossils – the remains or traces of organisms from the past.

- Strata – layers of sedimentary rock.

In undisturbed sedimentary rock, a lower layer is older than an upper layer (The Law of Superposition).

- Looking at fossils from one layer of rock to the next, Cuvier noticed that some new species appeared while others disappeared.

- He opposed the ideas that species changed, but acknowledged that extinction was common. He explained appearance and disappearance of fossils in the strata as being due to catastrophic events that killed organisms, followed by a time when new organisms moved in from nearby.

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2. Jean-Baptiste Lamarck put forth a hypothesis (a possible mechanism) to explain the pattern of change in organisms seen in fossils.

- His Inheritance of acquired characteristics theory stated that parts of the body that were used extensively become larger and stronger, while parts that are not used, deteriorate. An adult organism could then pass on the altered trait (altered due to use or disuse) to its offspring.

- Although Lamarck did recognize that organisms changed gradually over time, his proposed mechanism was not supported by any evidence.

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Charles Darwin’s Work

1. The prevailing thought in Darwin’s time was that species were unchanging, remaining in the same form as they first appeared.

Charles Darwin was very much influenced by scientists whose work showed that the Earth has undergone slow, but steady changes over time, and proposed that similar slow and steady processes could produce substantial biological change – an idea he shared with several other 18th century biologists.

2. After attending Cambridge, Darwin joined the crew of the ship Beagle on a trip around the world. During the voyage he collected thousands of specimens of plants and animals.

- He described features of organisms that made them well-suited for their environment.

- He noticed that organisms from similar environments has similar features.

- Fossils he found resembled living, but different, organisms that he saw in South America.

3. At the Galapagos Islands, he was fascinated by unusual organisms. He theorized that the islands had been colonized by organisms that had strayed from South America and then diversified, giving rise to new species on the various islands.

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4. Darwin noted many examples of adaptations: inherited characteristics of organisms that enhance their survival and reproduction. He thought that a new species might arise from an ancestral form by the gradual accumulation of adaptations to a different environment.

In the Galapagos Finches, beaks and behaviors are adapted to the different foods available on their islands.

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5. Darwin published his ideas in his book “On the Origin of Species by Means of Natural Selection” in 1859.

- Alfred Russell Wallace had, concurrently, come up with the same idea.

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6. Descent with Modification: (Darwin’s pattern of evolution)

- The unity (common features) of all life are due to the descent of all organisms from a common ancestor.

- As descendants from the ancestor came to live in differing environments, they gradually accumulated adaptations.

- After long periods of time, descent with modification led to the large diversity of life on Earth today.

7. Natural Selection: (Darwin’s process of evolution)

Darwin’s theory was based on 2 observations and 2 inferences:

Observation 1: members of a population often vary in their inherited characteristics. (Variation exists in a population.)

Observation 2: All species can produce more offspring that the environment can support. Many of the offspring fail to survive and reproduce. (Production of many offspring that compete for resources)

Inference 1: Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to produce more offspring that the others. (Survival of the fittest).

Inference 2: This unequal ability of individuals to survive and reproduce leads to the accumulation of favorable traits in the population over time.

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8. Darwin compared artificial selection – the selective breeding of individuals that possess desired traits (a process that people have done since the first animals were domesticated) to a similar process occurring in nature (natural selection).

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Summary of Natural Selection

1. Natural selection is a process in which individuals that have certain heritable characteristics survive and reproduce at a higher rate than other individuals because of those traits.

2. Over time, natural selection increases adaptations an organism has to its environment.

3. If an environment changes or if the organism moves to a new environment, natural selection continues in the new conditions and may give rise to a new species.

4. Note that populations evolve, not individuals.

5. Natural selection can only occur on heritable variation in a population.

Evolution is supported by a huge amount of scientific evidence.

1. Direct observation

a. Soapberry bugs have mouthparts like a beak. As non-native species have moved into their area, changes in the length of their beaks have occurred in response to the new sources of food. In areas where the new food source has seeds located closer to the surface of the fruit, the beaks have shortened. In areas where the seeds were deeper, the soapberry bug’s beaks lengthened.

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b. Antibiotic resistance bacteria (MRSA) and viruses

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2. Homologous structures are features that share a common ancestry but not necessarily a common function as a result of natural selection on the original trait.

a. Forelimbs of humans, cats, bats, whale

b. Patterns and features of embryonic development show similarities that are not visible in adults.

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c. Vestigial structures: remnants of features that served a function in an ancestor. Example – many snakes have remnants of hip and leg bones.

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d. Molecular homologies – such as enzymes, methods of DNA transcription and translation.

[Analogous features share a common function, but not common ancestry. They are the result of convergent evolution – the result of different lineages adapting to similar environments in similar but independent ways.]

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3. Fossil Record

The fossil record documents patterns of evolution, showing that past organisms differ from present day organisms and that many species have become extinct. They also give us snapshots of evolutionary changes.

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4. Biogeography – the scientific study of the geographic distributions of species

- Distributions of organisms has been influenced by continental drift (the breakup of the single large land mass called Pangea 200 MYA)

- Islands often have species that are endemic (found nowhere else in the world) although they are closely related to species nearby.

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The pattern of evolution has been documented directly and is supported by a great deal of evidence. In addition, the process of evolution (natural selection & other molecular processes) makes sense of massive amounts of data and has been observed and tested.

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The alleles from the other parent are shown on top.

The genotypes of possible offspring are shown by filling in the allele from each row and column together.

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