Download reading – study island – reproduction review

READING – STUDY ISLAND – REPRODUCTION REVIEW
Genetic Material
Reproduction is a characteristic of all living organisms. During reproduction, the instructions for inherited traits are
passed from parents to offspring through their genetic material. The passage of these genetic instructions from one
generation to the next generation is heredity.
Organization of Genetic Material
gene → DNA molecule → chromosome → genome
It is important to know how genetic material is organized. The genome of an organism refers to its complete genetic
makeup and includes the organism's entire set of chromosomes. The total number of chromosomes in a normal
human is 46: 23 chromosomes from the mother and 23 from the father.
Chromosomes, Genes, & DNA
Genetic information is encoded in DNA. DNA is located
in the chromosomes of cells. Chromosomes appear very
dark (when viewed under a microscope) and are located
in the cell nucleus, if the cell has one.
A gene is a section of DNA located on a chromosome.
Adapted from image courtesy of NIH.
A section of a chromosome that controls a particular
trait is a gene. Genes are a kind of blueprint for an
organism. They contain all the information necessary to
build, repair, and keep the organism running, including
how to make all the different proteins and other
materials the body needs.
Sexual & Asexual Reproduction
Reproduction is the process through which an organism passes its genetic information to its offspring. The ability to
reproduce is necessary for the survival of a species. The two main types of reproduction are asexual
reproduction andsexual reproduction.
Asexual Reproduction
In asexual reproduction, a single parent passes copies of its genes to each of its offspring, so all of the offspring are
genetically identical to the parent.
In general, this form of reproduction is used by simple organisms, such as bacteria, because only one parent is
required for asexual reproduction. Also, asexual reproduction is generally quicker and simpler than sexual
reproduction. However, populations of organisms produced through asexual reproduction have less genetic diversity
and are therefore less likely to have any members survive catastrophic change in their environment.
There are several different types of asexual reproduction:

Budding—Budding occurs when an offspring grows out of the body of the parent
organism. Hydra reproduce by budding.

Fragmentation —Fragmentation occurs when parts of an organism break off.The organism
fragments regrow their missing parts through regeneration and develop into new
individuals. Sea stars (starfish) reproduce through fragmentation and regeneration.

Binary Fission—Binary fission occurs when a prokaryotic organism undergoes cellular division and divides into two
new organisms. Amoeba reproduce by binary fission.

Mitosis—Mitosis is a special type of asexual reproduction that occurs in eukaryotes. During mitosis, genetic
material is replicated within the nucleus of a cell. Then, the nucleus divides into two new nuclei, and in most cases,
the entire cell divides during cytokinesis into two new daughter cells. The daughter cells are genetically identical to
each other and to the parent cell. They have the same number of chromosomes as the parent cell.
Sexual Reproduction
In sexual reproduction, two parents each contribute genetic material to their offspring. Because both parents
contribute genetic material, the offspring have traits of both parents, but they are not exactly like either parent. This
creates more diversity in a population of organisms. For sexual reproduction to occur, each parent must form a sex
cell, also known as a gamete. Gametes are formed by meiosis.
Meiosis is a two-step process of cell division which produces four cells that each contain half of the number of
chromosomes present in the parent cell. A female gamete is called an egg, and a male gamete is called
a sperm. When an egg's nucleus and a sperm's nucleus fuse, fertilization occurs, and a fertilized egg, known as
a zygote, forms. The zygote retains the
chromosomes from the egg cell and the
sperm cell, so it has the correct number of
chromosomes for the species. The zygote
divides over and over again through mitosis
and develops into a mature organism. All the
new cells produced from the fertilized egg
have the same genetic information as the
fertilized egg, although the cells
may specialize or differentiate to express
different genes and become different kinds of
cells.
Genes & Gene Expression
The instructions that are responsible for all of
the inherited traits of an organism are held
in genetic material known as DNA.
Genes
A gene can be defined as a basic unit of hereditary information. It refers to a specific segment of DNA that influences
a particular trait or group of traits. All of an organism's inherited traits are the result of the genes it received from its
parent(s). Genes are located in an organism's chromosomes.
The unique DNA code in a gene segment influences one or more traits of the organism. Depending on the segment, a
single gene can control a single trait, multiple genes can work together to control a single trait, or a single gene can
influence many traits.
Gene Expression
In order to study how genes affect an individual organism and its offspring, it is important to understand some basic
rules of heredity.
Within a population, there exists some level of genetic variation, or differences in traits among individuals. This
genetic variation is caused by the presence of different alleles, or alternate forms of
a particular gene. In diploid organisms like humans, each individual carries two copies
of each gene.These copies can be the same allele or different alleles, depending on
the organism’s parental contributions.
The genetic makeup of an organism, or the specific set of alleles that are present, is
referred to as its genotype, and the visible traits that we can observe as a result of
these alleles are its phenotype.
The visible traits of this toddler, such as curly hair and brown eyes, make up her
phenotype. The specific alleles that result in these traits make up her genotype.
Different alleles for a particular gene locus can be dominant or recessive.Dominant
alleles mask recessive alleles, meaning that when a dominant and a recessive allele are both present, the dominant
allele's phenotypic trait is observed.
For example, if an individual pea plant has a dominant allele for tallness (T) and a recessive allele for shortness (t), the
observed phenotype of the pea plant is tall. This plant is heterozygous for the height allele, and has a genotype of
(Tt). Traditionally, a capital letter represents the dominant allele and a lowercase version of the same letter
represents the recessive allele.
A homozygous plant is one that has two copies of the same allele. Both alleles may be dominant (e.g. TT - tall) or
recessive (e.g. tt - short).
Punnett Squares
In a Punnett square, one parent's alleles are written across the top and the other's are
written down the left side. The predicted alleles of offspring are listed inside the grid. In
the example shown, two parents that are both heterozygous (Bb) for a coat color trait are
crossed. Black coat color (B) is dominant over brown (b).
As can be seen, the resulting progeny will be approximately 3/4 (or 75%) black coated
(1/4 of these BB plus 1/2 Bb), and 1/4 (or 25%) will be brown coated (the homozygous
recessive offspring).
The rules of probability can also be used to determine the offspring, but the Punnett
square provides a visual method for determination.
When only one trait is under study, monohybrid crosses are performed to examine the genotypes. The above
Punnett square with the black and brown coat colors demonstrates a monohybrid cross.
Pedigrees
A pedigree chart shows how members of a certain family are related to each other, and it shows which members of the
family express a certain trait or set of traits.
In a pedigree chart,

males are represented with squares, and females are represented with
circles.

mates are connected by a horizontal line between their symbols.

offspring and siblings are represented by vertical lines that connect to
the horizontal line between the parents.

symbols that are shaded represent individuals that express a certain trait or set of traits.
The above pedigree chart shows two generations. In the first generation, a female that expresses a certain trait mates
with a male that does not express that trait. They have two children—50% express the trait—the female does not
express the trait, but the male does express the trait. Both of the children mate with individuals that do not express
the trait.
Heredity
Features of an offspring that result from genes passed on by parents are called inherited traits.
The length and quality of a human life is determined by many factors, including genes, gender, and interaction with the
environment. Faulty genes can result in disease or dysfunction in body parts or systems. Poor environmental conditions,
such as inadequate nutrition, medical care, housing, and sanitation can also cause disease or premature death. A third
factor that influences the length and quality of life is personal health behavior. All of these factors can be categorized as
either inherited or acquired traits.
Inherited Traits
An organism's physical features are called traits or characteristics. Inheritedtraits are determined before birth and
cannot be permanently changed. Eye color, hair color, skin color, freckles, and dimples are inherited characteristics of
humans. Some diseases are inherited, or at least influenced by the inheritance of certain genes.
The way that parents pass on genes to their offspring means that the offspring often look similar to their
parents. However, this does not mean that children will look exactly like their parents. One reason is that some of the
genes that are passed on to children stay hidden.
Acquired Traits
Features that a person gets through choices or interactions with the environment are called acquired traits. Scars,
tattoos, pierced ears, and dislikes of certain foods are acquired traits because they are not inherited.
The clothing and hairstyle that people choose to wear are also acquired characteristics. In addition, anything that a
person has learned is acquired, rather than inherited. For example, parents might pass on a love of music to their
children. However, this is something that the children have learned and not something they inherited at birth.
This girl inherited her hair color from her mother, but the girl will have to
learn to play the guitar.
People and some animals acquire behaviors. For example, people can learn
to brush their teeth after meals because it helps prevent tooth decay. Dogs
can learn to play fetch. Some animals in changing environments can learn to
find new food sources or move to a location where the food is more
plentiful. The ability to acquire or learn behaviors can help determine an
organism's likelihood of survival.
Inherited Traits in Plants
Plants inherit traits from their parent plants. Some common
inherited characteristics are flower color, flower position, seed
color, seed shape, pod shape, pod color, leaf pattern, and stem
length.
This pepper plant inherited its pod shape and color from its
parent plants.
The characteristics that are seen in plant offspring (sometimes
called daughter plants), depend on the genes of the parent plants
and how they combine in the daughter plant. This means that some characteristics will usually be shared with parent
plants, while others will differ.
Inherited Traits in Animals
Animals also inherit traits from their parents. Some common inherited
characteristics are fur color, fur length, eye color, height, length of tail, ear shape,
and patterns such as spots, stripes, or patches.
A horse's color, the color of the mane, and the horse's height are all inherited characteristics. These are features that
differ among horses. However, all horses inherit an ability to eat and digest certain foods, such as grass.
This horse inherited its dark color and white leg markings from its parents.
Animal characteristics such as being overweight or having docked tails (tails that have been cut short) are acquired
rather than inherited.
Natural Selection
Some organisms have favorable traits that are well-suited to their immediate environment. Organisms with this
advantage are more likely to thrive, reproduce, and pass their traits to future generations than organisms without those
favorable traits. This process is known as natural selection.
Mutations Increase Genetic Diversity
An accidental change in the sequence of an organism's genetic material is known as a mutation. Mutations can occur
in any cell in an organism's body.Mutations that occur in the sex cells of an organism may be passed on to their
offspring. Mutations lead to greater genetic diversity of the species.
Mutations may be helpful, harmful or have no effect on an organism at all.They may change an organism's
appearance, physiology, or behavior.
A mutation can cause a new color pattern. This increases the diversity of the population.
Having a wide variety of traits in a population is known as genetic diversity. It is valuable for a population to have a
large genetic diversity. When the environment changes, many individuals may die. But if there is a large genetic
variety, then at least some individuals may have traits that help them survive.
The following are sources of genetic variation among living organisms.

Mutations are a significant source of genetic variation. Mutations are responsible for introducing new traits and
new versions of existing traits.

Sexual reproduction results in an offspring that has a combination of genetic material from both parents, which
contributes to genetic variation.
Favorable Traits
Favorable traits are traits that increase an organism's chance of survival in a particular environment. Organisms with
favorable traits are more likely to reproduce and pass on their traits than organisms without favorable traits. In this
way, forces in nature determine, or "select" which traits (or even which types of organisms) will continue to exist in
that ecosystem.
Favorable traits may include physical traits, such as the body of a sea lion storing extra fat, or they may be instinctive
behavioral traits, such as a bird building a nest to protect its young.
If an organism dies before reproducing, then its unique traits will be eliminated from the population. On the other
hand, if an organism has favorable traits that allow it to survive and produce many offspring, then its traits will be
more common within the population.
Natural Selection Occurs Over Time
Genetic diversity makes natural selection possible. If all organisms within a population were identical, then no
organisms would have a better chance of survival than another. A population with little or no diversity is less likely to
survive changes in the environment.
Over time, favorable traits are likely to increase within a population and unfavorable traits are likely to decrease. In
this way, natural selection plays an important role in the way species evolve over time. When natural selection results
in changes in a species, these changes generally develop gradually, over many generations, rather than suddenly.
However, in some cases, rapid evolution can occur. This has been observed with few species of insects and bacteria.
Natural Selection Can Lead to Speciation
If enough significant changes or adaptations occur in the inherited traits of a population, natural selection may result
in a new species. The emergence of a new species is called speciation. Speciation occurs when members of a
population can no longer successfully reproduce with other members of the species. This is due to changes in their
genetic material.
For example, there are currently thirteen different species of ground finches in the Galapagos Islands. These thirteen
species can be traced to a single species from the mainland on South America. After a group of finches moved onto
the Islands thousands of years ago, they had to find new food sources.
Luckily, the species had natural variations in the sizes and shapes of their beaks. The birds specialized in eating
different food sources. Finches with short, stout, strong beaks ate
primarily seeds, which had to be cracked open.Finches with long,
slender beaks used them to get nectar from the throats of flowers
or to reach insects in small, narrow holes. Finches with medium
beaks used them to grasp and hold insects.
Over time, each variety of finch began to interact less with the
other varieties.Therefore, they began to mate only with other
similar finches. Eventually, each variety had gained enough genetic
changes that they could no longer breed with the other
varieties. This is how speciation occurs.
Selective Breeding
Selective breeding occurs when humans breed organisms with specific qualities that will result in a specific type of
offspring.
Genetic mutations occur naturally in all populations of organisms. These mutations produce new characteristics in
individual organisms. A breeder can select those individuals to breed with others that have the same desirable
characteristics. This helps ensure that the mutated characteristics will be passed on to offspring. The process of
breeding individuals with the selected characteristics continues until a new variety or breed exists.
Selective breeding is done with many types of domesticated animals, such as dogs and cows. For example, humans
have used selective breeding to create an astonishing number of dog breeds, ranging from the giant Great Dane to
the tiny Pomeranian.
Selective breeding has resulted in a huge variety of dog breeds. These breeds can be big, small, fluffy, hairless, or
anything in between.
Selective breeding has also been used to create specific varieties of crops. For example, farmers have used selective
breeding to create seedless varieties of many popular fruits, such as oranges, grapes, and watermelons. This was
done by selecting fruits with little to no seeds, breeding them together, collecting the offspring with the fewest,
smallest seeds, and repeating the process over again. Over many generations, a new, consistently seedless variety of
these fruits resulted.
Seedless watermelons are the result of selective breeding.
A negative result of selective breeding is that it reduces the genetic diversity of a population. This makes the
population more susceptible to diseases and changing environmental conditions.