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Intro to Genetics Cheat Sheet (DRAFT) by

A Grade 12 Study for Mendelian Genetics Colour-coded because there are different parts. Magenta: I Red: II

This is a draft cheat sheet. It is a work in progress and is not finished yet.

I. Genes and Chromo­somes

Are long chains of genes that are contained in the nucleus of the cell. They are made of DNA (Deoxy­rib­onu­cleic Acid).
A segment of DNA that controls a hereditary trait.
The charac­ter­istics that an organism has.
Two alleles must be present for a trait to show up in the offspring. One must come from the mother and the other from the father. When fertil­ization occurs, the new offspring will have two alleles for every trait.

II. The Contri­butions of Mendel

Gregor Mendel
Known as the "­Father of Geneti­cs". He discovered the 3 Laws of Genetics that would forever change biology. He conducted a series of experi­ments in a quiet monastery garden. Mendel spent 14 years growing and experi­menting with the pea plants grown in his garden.
Laws of Genetics
Law of Dominance and Recessive, the Law of Segreg­ation, and the Law of Indepe­ndent Assort­ment.
Parts of the Flower
Pistil and Stamen
The male part of the flower. It produces pollen­/sperm.
The female part of the flower. It produces eggs.
Happens when pollen is driven to the pistil, and sperm travels to the egg. It produces a tiny embryo which is enclosed in an egg.
Mendel's great contri­bution was to demons­trate that inherited charac­ter­istics are carried by genes.

Mendel chose pea plants because they were readily available, easy to grow, grow rapidly, and because the sexual structure of the flower is completely enclosed within the petals so that there would be no accidental cross-­pol­lin­ation between plants.

Mendel's Use of Pea Plants for Genetic Experi­ments

Pea plants are normally self-p­oll­ina­ting. Since the male and female reprod­uctive structures are relatively enclosed inside the flower, the sperm will fertilize the egg of the same flower. The resulting embryos will have the same charac­ter­istics as the parent plant. Even though sexual reprod­uction happens, there is only one parent. Mendel knew that these pea plants we "­tru­e-b­ree­din­g". This means that if they are allowed to self-p­oll­inate, they would produce "­tru­e-b­ree­din­g" offspring.

For example: if allowed to self-p­oll­inate, tall plants would always produce tall plants. Plants with yellow seeds would always produce offspring with yellow seeds. These true-b­reeding plants were the corner­stone of Mendel's experi­ments.

Mendel wanted to produce seeds by joining the egg and sperm from two different plants. To do this, he had to prevent the possib­ility of self-p­oll­ina­tion. Mendel cut away the stamens and then dusted the remaining pistils with pollen from a different plant. This is known as cross-­pol­lin­ation and produces offspring from two different parents.

III. Mendel's Experi­ments

Terms to know
P Genera­tion, F1 Genera­tion, F2 Genera­tion, and Hybrids.
P Generation
Parental Genera­tion.
F1 Generation
First Genera­tion.
F2 Generation
Second Genera­tion.
Offspring with different traits.
Mendel crossed true-b­reeding tall plants with true-b­reeding dwarf plants
Tall x Dwarf = Tall
F1 Hybrids are all tall
All of the offspring had the appearance of only one of the parents.
The trait of the other parent seemed to disappear. Mendel though the dwarf trait is lost
Mendel's Two Conclu­sions
Biological inheri­tance is determined by "­fac­tor­s" that are passed from one generation to the next. Today, we know these factors to be genes. Each of the traits that Mendel observed in the pea plants was controlled by one gene that occurred in two contra­sting forms. For example: the height of pea plants occurs in a tall form and a dwarf form. The different forms are called alleles.
Mendel realised that some alleles are dominant over the other alleles.
Law of Dominance and Recess­iveness
Dominant Allele
If a dominant allele is present in an offspring, the dominant trait will show.
Recessive Allele
This trait will only show up if the dominant allele is not present.
F = Filial

Filial = denoting the generation or genera­tions after the parental genera­tion.

IV. Law of Segreg­ation

Mendel had another question: Had the dwarf trait disapp­eared, or was it still present in the F1 offspring?

Mendel allowed the hybrid tall F1 generation to self-p­oll­inate. F1 Tall x F1 Tall = 3/4 Tall and 1/4 Dwarf. The F1 "­tal­l" offspring must have been carrying the dwarf trait but it had been hidden. The dwarf trait had been passed down to the offspring and it reappeared in the F2 genera­tion.

Why did the recessive allele seem to disappear in the F1 generation and then reappear in the F2 genera­tion?

Mendel realised that organisms have two alleles for every trait. These two alleles are inherited, one from each parent. If the offspring receives a dominant allele from one parent, that dominant trait will appear in the offspring. Recessive traits only show up in the offspring if it receive recessive alleles from each parent.

If a parent has two alleles for a trait, how does the parent pass only one allele to the offspring? Meiosis.

Gametes are the reprod­uctive cells of an animal or plant. During meiosis, the DNA is replicated and separated into 4 gametes. This way, a parent passes one allele for each gene to their offspring (will include diagram).

Mendel's Law of Segreg­ation says that every individual carries 2 alleles for each trait. These two alleles segregate during the formation of the egg or sperm.

An offspring will inherit two alleles for a trait, one from each parent. The combin­ation of alleles received by the offspring may be either homozygous or hetero­zygous.

Homozygous means that two of the same alleles are present, either dominant or recessive. Hetero­zygous means that both alleles are present, dominant and recessive.

Genotypes and Phenot­ypes. Genotypes are the genetic makeup. Phenotypes are the physical charac­ter­istics.

For example: T is dominant and t is recessive. In Mendel's pea plants, the tall allele was dominant over the dwarf allele.

TT = Tall
Tt = Tall
tt = Dwarf

V. Punnett Squares

A Punnett Square is a diagram showing the allele combin­ations that might result from a genetic cross between two parents.

Practice Problem (will show diagrams):

Mendel began his experi­ments using true-b­reeding parents. He soon discovered that the tall trait was dominant over the dwarf trait.

The genotype of the tall is TT and the genotype of the dwarf is tt. It does not matter which letters are used, as long as it's the same letters.

Place the alleles of the first parent and the top of the square and the alleles of the second parent on the left of the square. Fill in the square with all the possible combin­ations of the alleles that the offspring might inherit.

Punnett Square Example

All offspring are tall because the dominant allele is present in all of them.

VI. Law of Indepe­ndent Assortment

Mendel designed a second set of experi­ments to follow different genes as they passed from parent to offspring. This is known as two-factor cross or dihybrid cross.

One parent had peas that were round and yellow and the other had peas that were wrinkled and green. Round and yellow were dominant.

Round, yellow (RRYY) x Wrinkled, green (rryy) = Round, yellow (RrYy)

F1 generation was allowed to self-p­oll­inate (RrYy x RrYy), it resulted in 556 seeds.

315 round, yellow (RRYY, RRYy, RrYy, RrYY).
105 round, green (RRyy, Rryy)
104 wrinkled, yellow (rrYY, rrYy)
32 wrinkled, green (rryy)

This meant that the alleles for seed shape had segregated indepe­ndently of the alleles for seed colour. The alleles of one gene had no effect on the alleles of another trait. This is known as indepented assort­ment.

Law of Indepe­ndent Assortment states that when gametes are formed, the alleles of a gene for one trait segregate indepe­ndently from the alleles of a gene for another trait.

VII. Punnett Squares for Dihybrid Crosses.

When two traits are being consid­ered, the Punnett square will need 16 squares. Each parent will pass one allele of each gene pair to the offspring.

VIII. Summary of the Laws

Inheri­tance of traits is determined by individual units known as genes. Each gene has two or more forms called alleles. Some alleles are dominant, others are recessive.

Each parent has two alleles for a particular trait that they inherited. They will pass one allele to their offspring when the alleles segregate into gametes.

Alleles for one trait segregate indepe­ndently of the alleles for another trait.

Not all genes show a pattern of simple dominance. For some genes, there are more than two alleles. Many times, traits are controlled by more than one gene.

IX. Genes and the Enviro­nment

Gene expression is always the result of the intera­ction of genes and the enviro­nment. The presence of a gene is not all that is required for the expression of a trait. The gene product must be present along with proper enviro­nmental condit­ions. The phenotype of any organism is the result of intera­ction between its genotype and the enviro­nment.
Examples: Primrose plants that are red-fl­owered at room temper­ature are white when raised at hotter temper­atures. Himalayan rabbits are white ate high temper­atures and brown at low temper­atures.

X. Incomplete Dominance

Some genes appear to blend together. For example: in some flowers, a homozygous red flower crossed with a homozygous white flower yields a hetero­zygous pink flower. This is incomplete dominance or nondom­inance.

Incomplete Dominance Example

XI. Codomi­nance

Humans have four blood types: A, B, AB, and O.

Three alleles determine blood type: IA, IB, i. Alleles IA and IB are codominant and i* is recessive. Codomi­nance is when both alleles are apparent in the phenotype of the hetero­zygous alleles.

IA i* = A
IB i = B
i i = O

Codomi­nance Example

B is supposed to be in front of i but I use an online generator so it is messed up.

XII. Multiple Alleles

Many genes have two or more alleles and are said to have multiple alleles. The best example is rabbit coats. Coat colour in rabbits are determined by a single gene that has at least four different alleles. The four alleles demons­trate a dominance hierarchy in which some alleles are dominant over others.

The alleles are ordered in hierarchy. C (full colour), cch (light grey, ch (albino with black), c (albino).

Full colour: CC, Ccch, Cch, Cc
Chinch­illa: cchc, cch, cchcch, cchc
Himalayan: chch, chc
Albino: cc

XIII. Polygenic Inheri­tance

In polygenic inheri­tance, the determ­ination of a given charac­ter­istic is the result of the intera­ction of multiple genes. Some traits, such as size, height, shape, weight, colour, metabo­lism, and behaviour are determined by many pairs of genes.

A trait affected by a number of genes does not show a clear difference between groups of indivi­duals. Instead, it shows a graduation of small differ­ences.

XIV. Chromo­somes

Human calls contain 23 pairs of chromo­somes. There are 22 pairs of autosomes, and one pair of sex chromo­somes. All pairs of chromo­somes are the same except one pair. The pairs are called autosomes. Autosomes are all of the chromo­somes within a cell except for sex chromo­somes.

Females have 2 copies of the X chromo­some. Males have one X and one Y chromo­some.

There are many genes found on the X chromo­some. The Y chromosome appears to contain only a few genes. Since the X and Y chromo­somes determine the sex of an indivi­dual, all genes found on these chromo­somes are sex-li­nked. Sex-linked traits include colour blindness, haemop­hilia, and muscular dystrophy. These are caused by recessive alleles.

Since males only have one X chromo­some, they will have the disorder if they inherit just one copy of the allele. Females must inherit two copies of the allele in order for the trait to show up.

XV. Pedigree Charts

A pedigree chart shows relati­onships within a family. Squares represent males and circles represent females. A shaded circle or square indicates that a person has a trait.