Genetics And Heredity Introduction

Gregor Mendel Made many observations that led to the model of inheritance that we use today

Mendel Used pea plants to make observations for several reasons o Pure varieties of peas were available o Earlier investigators had done work with them o Small and easy to grow o Easy to control reproduction

Monohybrid Crosses (Punnett Squares) The cross of two variations of a single trait o Example would be White vs Purple flowers

Blending Inheritance disproved When crossing a white and purple flower, a light purple was not created as previously thought Instead Mendel got purple and white flowers at specific ratios.

Types of Traits Dominant – the trait that if present it will be displayed (R) Recessive – the trait that only appears if a dominant trait is not present (r)

Mendel’s Experiment Took two pea plants with specific traits (Purple vs White flowers) Fertilized them and observed the traits of the offspring (F 1 ) Then fertilized the offspring with itself (F 2 )

His Results Found that the traits appeared in specific ratios or percentages each time Out of 929 flowers in the offspring (F 2 ) 705 (75.9%) had purple flowers 224 (24.1%) had white flowers 3:1 Ratio

What does this mean? Blending theory was not correct There was a dominant and recessive trait The traits appear in predictable patterns

Mendel’s five element model 1. Parents do not transmit physiological traits directly to their offspring. Rather they discrete information for traits (genes) 2. Each individual receives one copy of each gene from each parent 3. Not all copies of a gene are identical. The alternative forms of a gene are called alleles.

4. The two alleles remain discrete – they neither blend with nor alter each other. 5. The presence of a particular allele does not ensure that the trait that it encodes will be expressed. (If recessive trait is expressed in a parent it may not be expressed in the offspring even though that offspring contains the allele)

Principle of Segregation Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization. Example o If genotype for a trait is Pp o The probability of getting a P is 1:2 and the probability of getting a p is 1:2 o The two letters or variations of the trait are segregated during the fertilization process

Humans A number of human traits have been shown to display both dominant and recessive inheritance Researchers cannot perform controlled crosses in humans the way Mendel did with pea plants Instead we look at a pedigree or family history

Pedigree

Dihybrid Cross Mendel asked the question “Do two traits behave independently of each other?” So he looked at two traits together and observed the results. To do this he created the “dihybrid cross” and found 9:3:3:1 ratio

Law of Independent Assortment Two different traits do not affect each other. Example:

Testcross Mendel came up with the concept of a testcross to help determine the genotype of a parent In a testcross the unknown parent is crossed with a homozygous recessive The results will determine the genotype of the unknown parent

Incomplete Dominance The heterozygote is intermediate in appearance between the two homozygotes This is still not blending because you can still go back to the original color in future generations

Codominance The heterozygote shows traits of both the recessive and dominant allele