Traits, Genes, and Reproduction Basics

Traits and Alleles

Traits are an organism's characteristics or features. They are encoded into an

organism's D

N

A.

Traits include physical aspects such as hair color, eye color, and height.

Traits also include less visible characteristics such as disease resistance, blood

type, and other internal processes.

Even behavioral patterns and instincts can be considered traits, as they are

influenced by genes.

Traits are specified by genes. A gene is a segment of D

N

A that codes for a particular

trait.

Genes come in different versions, called alleles.

An allele is a specific form of a gene that determines a particular aspect of a

trait.

When an organism has two of the same alleles (two dominant or two recessive), it is

called homozygous for that trait. When the organism has two different alleles (one

dominant and one recessive), it is called heterozygous.

Homozygous: An organism that has two of the same alleles

Heterozygous: An organism that has two different alleles

Dominant vs. Recessive Alleles

Alleles can be either dominant or recessive.

If there are two different alleles, the stronger allele is the dominant allele.

The dominant allele is always expressed.

The allele that gets masked is called the recessive allele.

The recessive allele is only expressed when both alleles are the recessive form.

Scientists use letters to represent alleles:

A capital letter represents the dominant allele.

A lowercase letter represents the recessive allele.

For example, "R" can represent the dominant allele for roundness in pea plants, while

"r" represents the recessive allele for wrinkledness.

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Genotype vs. Phenotype

When one allele is dominant and the other is recessive, the dominant allele gets

expressed in the trait, while the recessive allele doesn't. Therefore, we can't predict

an organism's actual alleles just by looking at its traits. The genes an organism has

are called its genotype, and the traits it expresses are called its phenotype.

Genotype: The genes an organism has.

Phenotype: The traits an organism expresses.

You can't observe an organism's genotype, but you can observe its phenotype.

DN

A and Inheritance

Traits such as hair or eye color get passed down from parents to offspring through

D

N

A, which stores hereditary information.

D

N

A is tightly coiled around protein molecules to form chromosomes. D

N

A can be

thought of as a zipper. During cell replication, the D

N

A is unzipped. Each side of the

zipper is later paired up with a new complementary zipper half that is identical to the

other missing side of the zipper. The result is two new identical zippers, and each

complete zipper contains half of an old zipper and half of a new zipper.

Sexual vs. Asexual Reproduction

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Genetic

Variation

Sexual Reproduction Asexual Reproduction

Results in more variation. Each

offspring has a unique

There is no genetic variation.

combination of genetic material.

Survival in

Different

Environments

More variation means that

offspring will have different traits

that help them survive in different

environments.

If there were a parasite that

targeted the organism, the

population might die off quickly

because every organism is

identical.

Requires more effort and energy

because the organism must find a

Energy Required

mate. If an organism is

unsuccessful at finding a mate, it

will not reproduce.

Population

Growth

Population will not grow as

quickly

Cell Structures and Organelles

Requires less energy and can

be done with just one

organism.

A population can grow quickly

with asexual reproduction.

Animal cells and plant cells are similar in many ways, but they have their differences.

Cell Wall

Every plant cell has a cell wall, the rigid barrier made out of cellulose that surrounds

the cell membrane.

If animals had these, we might be crunchy!

Chloroplasts

Plant cells have chloroplasts to produce food from sunlight.

If animals had these, we could just photosynthesize in the sun for lunch!

Vacuoles are used for storage (to store food and water).

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A lysosome is like the cell's stomach; it helps to break down food, cell waste, and

worn-out parts. Lysosomes also help to protect the cell from foreign invaders like

bacteria and viruses, as lysosomes can digest and destroy dead cells or old tissues.

Mitochondria are organelles that convert the energy in food into a form that the cell

can use by carrying out a reaction with oxygen. Cells that need more energy, like

muscle cells, have a larger number of mitochondria.

The endoplasmic reticulum (

ER) is the transport facility for the cell. It is made of

folded membranes, and it processes and moves materials.

Golgi bodies are the packaging, sorting, and distribution centers of the cell. They

process and package proteins and other molecules produced from the ER.

Microtubules are protein tubes that maintain structure and help things move around

the cell.

Ribosomes are the protein-making factories of the cell. Proteins are essential parts of

every cell; they are structural components and also a part of enzymes or reactions

that take place in the cell. Ribosomes receive directions from hereditary material to

make certain proteins.

The nucleus is the control center of the cell. It houses the cell's chromosomes, which

are strands made of D

N

(deoxyribonucleic acid). The nucleus also includes the

nucleolus, which is a small structure found inside the nucleus that makes ribosomes

and transports them.

A

Every cell has a cell membrane on the outside of the cell that holds the cell together

and controls the flow of materials in and out of the cell.

The cytoplasm is the jelly-like material that fills the cell. Organelles are suspended

in the cytoplasm.

Cell Types

There are two main types of cells: prokaryotes and eukaryotes. The main difference

is that prokaryotes do not have a membrane-bound nucleus, mitochondria, or

organelles, while eukaryotes do.

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Prokaryotic cells are very simple and are found in single-celled bacteria.

Eukaryotic cells are much more complex and are found in protists, fungi,

animals, and plants.

Reproduction in Single-Celled Organisms

Asexual reproduction is when one parent organism produces offspring that are

genetically identical to the parent. Single-celled organisms, such as bacteria and

protists, use asexual reproduction.

Fission is a type of asexual reproduction used by prokaryotes (single-celled

organisms). During fission, the cell duplicates its genetic material, and the cell splits

down the middle, producing two new cells that are identical to their parent cell.