Mendelian Genetics Chocolate yellow and black Labrador retriever

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 pollenproducing 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 selffertilized. • True-breeding parents in experiments like this are called the P generation.

• The hybrid offspring of this monohybrid cross, the F 1 generation, only consisted of purple flowers. – A monohybrid cross involves parents that differ in one trait only.

• When individuals from the F 1 generation were crossed, the resulting F 2 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 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 (Yy. Rr). • Second, he mated two of the heterozygous F 1 plants.

• The resulting F 2 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 F 1 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 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 graybodied 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.

• 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 Xc. Xc is red-green colorbind. • In males, the Y-chromosome is shorter and does not contain the allele. – XCY his normal, Xc. Y 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.