Fundamentals of Genetics Chapter 10 Fundamentals of Genetics Gregor Mendel, the father of modern genetics

9-1 Mendel’s Legacy Some stuff on Gregor Mendel: his parents were farmers he became ordained as a priest studied science and mathematics at the University of Vienna His research provided the basis for heredity- the transmission of characteristics from parents to offspring.

A. Mendel’s Pea Experiment He set up true-breeding plants (bred for many generations) by allowing them to self-fertilize. He controlled pollination, looking at 1 or 2 characteristics at a time.

These were the 7 characteristics he looked at:

2. He crossed a true breeding plant with a plant of the opposite trait (tall x short). He called this the Parental (P1) generation.

3. In the 1st filial (F1) generation all of the offspring showed the same trait – all tall.

Next, he let the F1 generation self-pollinate Next, he let the F1 generation self-pollinate. Results: ¾ of the F2 generation showed the dominant trait & ¼ showed the recessive trait.

Mendel’s Results

Analysis The F1 generation always displayed one trait (he later called this the dominant trait) The F1 generation must have within it the trait from the original parents - the white trait The F2 generation displayed the hidden trait, 1/4 of the F2 generation had it (he later called this hidden trait the recessive trait)- 3:1 ratio. Each individual has two "factors" that determine what external appearance the offspring will have. (We now call these factors genes or alleles)

Mendel established three principles (or Laws) from his research: 1. Law of Dominance – some alleles show and some alleles are “hidden”. The traits that mask other traits are dominant traits. EX: Tall plant X Short plant all offspring are Tall Tall is the dominant trait!

Use capital letter to represent dominant allele Use lower case letter to represent recessive allele EX: T = tall t = short

REMEMBER… Each organism has a pair of genes for each trait (diploid) TT = homozygous dominant (tall) Tt = heterozygous (tall, but carries an allele for short) tt = homozygous recessive (short)

2) Law of Segregation – during meiosis, one member of a gene pair separates into different gametes. Therefore, each gamete only carries one member of the gene pair. For example, if a pea plant is Tt for tall, during meiosis, half the gametes formed will carry the T, half will carry the t.

3) Law of Independent Assortment – A random distribution of alleles occurs during gamete formation. (Genes on separate chromosomes sort independently during meiosis)

Using symbols we can depict the cross of tall and short pea plants in the following manner:

Alleles Allele - one alternative form of a given gene pair. Tall and dwarf are the alleles for the height of a pea plant. More than two alleles can exist for any specific gene, but only two of them will be found within any individual. We use letters to denote alleles, since every gene has two alleles, all genes can be represented by a pair of letters. TT = tall, Tt = tall, tt = dwarf

Other Terms 1. Genotype- The genetic makeup of a trait. We use letters to denote alleles (BB, Dd, ee, for example). 2. Phenotype- Physical appearance (or what an organism looks like). Examples would be tall, short, blue, purple, etc.

Other Terms 3. Homozygous: when the alleles are the same, the individual is said to be homozygous, or true breeding. Letters designating a homozygous individual could be capital or lower case, as long as they are the same. Ex. AA, bb, EE, dd 4. Heterozygous: when the alleles are different, in this case the DOMINANT allele is expressed. Ex. Pp, Aa

When we cross-breed 2 things, looking at one factor, we have a: 5. Monohybrid cross = a cross involving one pair of contrasting traits. Ex. Pp x Pp. To solve it, we would use a: 6. Punnett Square: used to determine the PROBABILITY of having a certain type of offspring given the alleles of the parents

How to Solve a Punnett Square 1. Determine the genotypes (letters) of the parents. (Bb x Bb) 2. Set up the punnett square with one parent on each side. 3. Fill out the punnett square middle 4. Analyze the number of offspring of each type to calculate the genotypic and phenotypic ratios.

An example In pea plants, round seeds are dominant to wrinkled. The genotypes and phenotypes are: RR = round Rr = round rr = wrinkled If a heteroyzous round seed is crossed with itself (Rr x Rr) a punnett square can help you figure out the ratios of the offspring.

Set up your square Remember, it’s Rr x Rr                                               3/4 round, 1/4 wrinkled Set up your square Remember, it’s Rr x Rr Note that the letters get separated on the top and the side. It DOES NOT MATTER which parent goes on top or on the side.

Results 3/4 round, 1/4 wrinkled So,The Phenotypic Ratio is 3:1, Round to Wrinkled The Genotypic Ratio is 1:2:1, and refers to the letters. It is 1 RR, 2 Rr, 1 rr.

Another look at how to do it

Predicting Results of Monohybrid Crosses See your textbook for other examples, and try some on the worksheets that I will pass out to you for classwork and for homework.

Other types of crosses 1. Incomplete Dominance- There is no dominant or recessive, the heterozygous condition results in a "blending" of the two traits. Example: Snapdragons can be red, white, or pink (heterozygous)

2. Codominance Occurs when both alleles for a gene are expressed in a heterozygous offspring. Neither is dominant or recessive. Ex- White + Red horse= Roan (white and red hairs mixed together). +

Codominance- Also Observed in Blood Types Both A and B are dominant. Type O is recessive Four phenotypes Six genotypes

4 Types of Blood Type A with A antigens on the red cells and anti B antibodies in the plasma. Type B with B antigens on the red cells and anti A antibodies in the plasma. Type AB with both A and B antigens on the red cells and no blood type antibodies in the plasma. Type O with no antigens on the red cells and both anti A and anti B antibodies in the plasma ** Group O blood cannot be clumped by any human blood, and therefore people with Group O are called universal donors.

SEX CHROMOSOMES AND SEX-LINKED GENES A human male has one X chromosome and one Y chromosome A human female has two X chromosomes Whether a sperm cell has an X or Y chromosome determines the sex of the offspring

Sex-linked disorders affect mostly males Most sex-linked human disorders are due to recessive alleles Examples: hemophilia, red-green color blindness These are mostly seen in males A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected

Sex Linked Trait: Colorblindness

Dihybrid Crosses: Crosses that involve 2 traits Dihybrid Crosses: Crosses that involve 2 traits. For these crosses your punnett square needs to be 4x4

In any case where the parents are heterozygous for both traits (AaBb x AaBb) you will get a 9:3:3:1 ratio as in the previous example. If you cross other combinations, you will need to do a square. Prepare a Punnett Square in your notes, using the crossing of RrYy x rryy. You use the FOIL method to determine the letters that go on top and on the side. For example, in RrYy, the letters would be: RY, Ry, rY, ry.

Results of cross RrYy x rryy:

Now try some others from the worksheets you will receive.