Chapter 11.1 - Gregor Mendel

Developed the fundamental laws of heredity
He studied science and mathematics, using statistics to support his hypotheses
Blended Concept of Inheritance (accepted at the time) vs Particulate Theory of Inheritance (Mendel's Theory)
Mendel chose garden peas (Pisum sativum)as his subjects as they are easily grown and their pollination is easily controlled. He controlled pollination by manually moving pollen between plants
Developed True-breeding plants by self-pollination, crossbreeding those to create hybrids
Mendel examined varieties of peas for heritable characters and traits for his study. (stem length, pod shape, seed shape, seed color..etc)
His publication " Experiments on Plant Hybridization" went largely ignored until long after his death

Note: Often Mendelian genetics refers to basic genetics that use Punnett Squares and statistics to predict the phenotypes of offspring. Modern Genetics usually refers to our modern-day understanding of how DNA and genes control the expression of traits. During Mendel's time, biologists did not know about DNA and genes.

Mendel's Law of Segregation (MONOHYBRID CROSS)

Mendel crossed true-breeding plants that differed for a given character
A monohybrid cross involves one (mono) character and different (hybrid) traits.
Pollen from true-breeding pea plants with purple flowers (one trait) was placed on stigmas of true-breeding plants with white flowers (another trait).
The F1 seeds were all purple; the white flower trait failed to appear at all.
Because the purple flower trait completely masks the white flower trait when true-breeding plants are crossed, the purple flower trait is called dominant, and the white flower trait is called recessive.
The F1 plants were allowed to self-pollinate.
This step was the monohybrid cross. (or the F1 cross).
The progeny, called F2, were examined: roughly 1/4 were white, and 3/4 were purple.
Mendel proposed that the units responsible for inheritance were discrete particles - particulate theory

flower cross

As Viewed by Modern Genetics

During production of gametes, only one of the pair members for a given character passes to the gamete. (LAW OF SEGREGATION)
When fertilization occurs, the zygote gets one from each parent, restoring the pair.
Mendel's units of inheritance are now called genes.
Different forms of a gene are called alleles.
Each allele is given a symbol: In the case of purple flowers, P might represent purple and p white. By convention, uppercase P represents the dominant; lowercase p represents the recessive. Never mix your letters! Choose one letter and use a capital to denote dominate and a lowercase to denote recessive.
True-breeding individuals would have two copies of the same allele: Purple would be PP. Ttwo copies of same allele = homozygous. Homo means "the same"
White true-breeding would be pp. Two copies of same allele = homozygous.
Some purple-flowered plants could be Pp, although they would not be true-breeding. Individuals that are purple, but had a white parent, are heterozygous: Pp. Hetero means "different".
When an organism is studied for three different genes and has the alleles AABbCC, it is homozygous for A and C genes but heterozygous for the B gene.
The physical appearance of an organism is its phenotype. Purple-flowered would be a phenotype.
The actual composition of the organism's alleles for a gene is its genotype: Pp is a genotype.
Organisms have many different genes some have thousands, and complex organisms have 10 times that number.

The Punnett Square

a simple box-like device that helps us to consider all genetic combinations and show the expected frequencies of genotypes.
The P and p symbols represent the single allele each gamete receives.
Fertilization provides the two alleles for the new individual, one from the male (sperm) and one from the female (egg).
The Punnett square shows that the genotypes and associated ratios for a monohybrid cross are 1 PP :2 Pp : 1 pp.
Any progeny with a P would have the dominant (purple) phenotype, so the phenotypic ratio is 3 purple to 1 white.
Now it is known that a gene is a portion of the chromosomal DNA that resides at a particular site, called a locus (plural is loci). The gene codes for a particular function or trait.
Mendel arrived at the law of segregation with no knowledge of meiosis or chromosomes. The mechanism of chromosome separation in meiosis I today explains his law of segregation.

One Trait Test Cross

A test cross can determine the genotype (heterozygous or homozygous) of an individual with a dominant trait.
It involves crossing the individual to a true-breeding recessive (homozygous recessive).
If the unknown is heterozygous, approximately half the progeny will have the dominant trait and half the recessive trait.
If the unknown is homozygous dominant, all the progeny will have the dominant trait

punnett

Mendel's Law of Independent Assortment

The second law describes the outcome of dihybrid (two character) crosses, or hybrid crosses involving additional characters.
A dihybrid is an individual that is a double heterozygote (e.g., with the genotype RrYy - round seed, yellow seed).
Mendel's second law states that the Rr alleles assort into gametes independently of the Yy alleles.
The dihybrid, RrYy, produces gametes that have one allele of each gene.
Four different gametes are possible and will be produced in equal proportions: RY, Ry, rY, and ry.
Random fertilization of gametes yields the outcome visible in the Punnett square. Note its 4x4 table construction to accommodate 16 possible phenotypes.
Filling in the table and adding the like cells reveals a 9:3:3:1 ratio of the four possible phenotypes: (Round, Yellow - Round, Green - Wrinkled, Yellow - Wrinkled, Green.)
The Law of Independent Assortment states that alleles of different genes assort independently of one another during gamete formation.
In fact, this law is not always true - The law of independent assortment is accurate for genes that are on separate chromosomes, but not necessarily for genes that are on the same chromosome.
Genes that are close to each other on the same chromosome tend to stay together, but crossing over during meiosis may separate them.

Dihybrid Cross: RrYy x RrYy

punnett square

TIP: In any cross that is dihybrid (AaBb x AaBb) you will always get a 9:3:3:1 ratio, if you memorize this, you can save the trouble of doing a giant square!

A Mathematical Alternative (LAWS OF PROBABILITY)

A punnet square is not needed to determine the ratios of genotypes and phenotypes. Simple statistics and math can save you the trouble of filling out a square.

In a monohybrid cross Pp x Pp, each parent produced P gametes and p gametes

If you wanted to determine how many of the offspring are pp: x =

Example 2:

H is dominate for long hair (h = short) and B is dominate for black eyes (b = red eyes). If the parents are...

HhBb x hhBb

How many off the offspring will be short haired and red eyed?

Task: Use mathematical analysis to determine the number of short haired, black eyed offspring from the cross above.

TWO-TRAIT TEST CROSS

Used to determine the genotype of an "unknown" by crossing it with an individual that is homozygous recessive for both traits.

In flies (Long wings is dominant to short wings, Gray body is dominant to black)

A L __ G ___ is test crossed.

The offspring are 1:1:1:1 --> What is the genotype of the unknown parent?
If the offspring are half long winged & gray, and half long winged and black --> What is the genotype of the unknown parent?