Download 7 th Grade Study Island Notes for Mendel Unit

7th Grade Study Island Notes for Mendel Unit
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 → 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.
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 and sexual 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 ingenetic 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), and1/4 (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. Inherited traits 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.