Intro to Genetics Cheat Sheet

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.

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

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.

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.

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.

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.

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.

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

Three alleles determine blood type: I^A, I^B, i. Alleles I^A and I^B are codominant and i is recessive. Codomi­nance is when both alleles are apparent in the phenotype of the hetero­zygous alleles.

I^A I^A = A
I^A i = A
I^A I^B = AB
I^B i = B
I^B I^B = B
i i = O

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), c^ch (light grey, c^h (albino with black), c (albino).

Full colour: CC, Cc^ch, Cc^h, Cc
Chinch­illa: c^chc, c^ch, c^chc^ch, c^chc
Himalayan: c^hc^h, c^hc
Albino: cc

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.

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.