Mendelian Genetics Chocolate, yellow, and black Labrador retriever puppies.

Gregor Mendel Most of our understanding of the process of heredity, the process of transferring traits from parent to offspring, started with Gregor Mendel. Mendel was a scientist and monk who was trying to find a pattern of probability in heredity.

The advantage of studying with pea plants was that the matings could be strictly controlled. Mendel focused on studying the inheritance of seven characters, or properties that vary among individuals. Each character had two traits or variations.

Mendel would cut off the pollen- producing anthers and stamens of his experimental plants, then use a brush to introduce pollen from another plant. This allowed him to be completely sure of the parents of each of his new pea plants.

In one experiment, Mendel selected purple and white-flowered pea plants to cross. These plants were true-breeding, meaning they all produced the same color when they were self- fertilized. True-breeding parents in experiments like this are called the P generation.

The hybrid offspring of this monohybrid cross, the F1 generation, only consisted of purple flowers. A monohybrid cross involves parents that differ in one trait only.

When individuals from the F1 generation were crossed, the resulting F2 generation had a predictable ratio of colors: 705 out of 929 had purple flowers (about 75%). 224 out of 929 had white flowers (about 25%). These same results occurred with the other 6 pea plant characteristics he studied.

Mendel’s Four Hypotheses There are alternative versions of genes that account for the different traits within a character. These are called alleles. For each character, an organism inherits one allele from each parent. If the alleles differ, only the dominant allele will be expressed. The recessive allele has no noticeable effect. A sperm or egg will only carry one allele for inherited character due to the segregation of chromosome pairs during meiosis. This is the Law of Segregation.

The possible combinations and probabilities of any monohybrid cross can be visually shown with a Punnett Square. The two possible alleles from the father are shown along the top. The two possible alleles from the mother are shown along the bottom. Capital letters represent dominant alleles. Lower-case letters represent recessive alleles.

Each box of the Punnett square shows a different possible genotype, or genetic makeup of the offspring. 25% are homozygous dominant, or BB. 50% are heterozygous, or Bb. 25% are homozygous recessive, or bb.

The phenotype is the actual physical result of those traits. 75% purple (BB or Bb). 25% white (bb).

When the phenotypes of the homozygous dominant and heterozygous offspring look identical, the only way to identify their genotype is by conducting a test cross. The unknown genotype is crossed with a homozygous recessive individual. If the unknown individual is homozygous dominant, all of the offspring will show the dominant characteristic. If the unknown individual is heterozygous, half the offspring will show the dominant trait, the other the recessive trait.

Dihybrid Crosses Mendel also wanted to study whether the inheritance of one gene could affect another. He set up a dihybrid cross, where he first mated true-breeding parental varieties that had two differences in traits. He crossed a plant with yellow, round seeds (YYRR) with one that had green, wrinkled seeds (yyrr), creating offspring with all yellow, round seeds (YyRr). Second, he mated two of the heterozygous F1 plants.

The resulting F2 generation showed a 9:3:3:1 ratio of the different possible phenotypic combinations. These traits were inherited from genes on different chromosomes and did not affect each other’s inheritance. This is called the Law of Independent Assortment.

Incomplete Dominance Not all traits follow the simple dominant and recessive pattern. When true-breeding red and white snapdragons are crossed, the resulting F1 generation has only pink hybrids. This is called incomplete dominance because the hybrids have a phenotype somewhere in-between that of the true-breeding parents.

Codominance Some genes have more than two alleles within a population. The primary blood types in humans are the result of three alleles that cause certain carbohydrates to appear on the surfaces of their blood cells. A (dominant) and B (dominant) O (recessive) An individual with A and B alleles will express both carbohydrates on their membranes. This is codominance, as both alleles are dominant and both are expressed. An individual with only O alleles will not express any carbohydrates.

A type O individual must have two recessive O alleles. A heterozygous individual with both A and B alleles will express a both types of carbohydrate on the membranes of their red blood cells. Type AB. Individuals with two of the same dominant alleles or heterozygous with one allele O will only express one of the carbohydrates. Type A (AA or AO) Type B (BB or BO) A type O individual must have two recessive O alleles.

Polygenic Inheritance Some characteristics are polygenic and vary across an entire spectrum within a population because they are influenced by multiple genes. For example, assume that human skin color is controlled by three genes. Each gene has two alleles, one for darker skin, and one for lighter skin.

An individual with all six dark alleles would have the darkest skin color. An individual with all six light alleles would have the lightest skin color. All other individuals would fall somewhere in the middle of the spectrum.

Chromosome Theory of Inheritance Each gene occupies a specific loci, or position on a chromosome. Because chromosomes undergo independent assortment during meiosis, it is during this time that the different possible alleles within gametes are formed. This is the Chromosome Theory of Inheritance.

In pea plants, seed color and texture are on different chromosomes, so there is an equal chance of each allele making it to a gamete.

Genes on the same chromosome tend to be inherited together and are called linked genes. Linked genes do not follow the law of independent assortment.

Fruit Fly Experiments Thomas Hunt Morgan studied the chromosomal basis of inheritance by mating and counting fruit flies. Fruit flies breed at a high rate (every two weeks) and only have four pairs of chromosomes, so they were ideal test subjects. Hunt studied body color and wing size, which were linked traits and should have been inherited together.

One cross performed by Morgan was between a heterozygous gray- bodied fly with normal-shaped wings and a homozygous recessive fly with a black body and small wings. Most of the offspring (83%) had the same phenotypes as the parents, as expected with a linked gene.

A smaller percentage of the offspring (17%) had phenotypes that did not match either of the parents. These are called recombinant phenotypes.

Recombinant phenotypes should not have been possible unless alleles were somehow moving from one chromosome to another. Scientists later discovered that this was due to crossing over taking place in meiosis I.

In females, sex-linked traits follow a pattern of simple dominance. Alleles present on the sex chromosomes (X and Y in humans) take on unique pattern of inheritance and are called sex-linked traits. In females, sex-linked traits follow a pattern of simple dominance. XCXC is normal, XCXc is normal, and XcXc is red-green colorbind. In males, the Y-chromosome is shorter and does not contain the allele. XCY his normal, XcY is red-green colorblind.

Pedigree Charts When studying sex-linked traits, it is helpful to set up a pedigree chart. Males are indicated by boxes, females by circles. Affected indiviuals are colored in, non-affected are left blank. Heterozygous females, also called carriers, may be shown as half-colored in.

One of the most famous pedigree charts helped to determine the carriers of hemophilia, a sex-linked blood clotting disorder, in an English royal family.