Topic #9: Introduction to Genetics

Mendel and His Work How does your hair know how to be the color it is or your eyes the color they are? Why are your hair and eyes the color they are? Are your hair and eye colors the same as one or both of your parents?

Mendel and His Work Gregor Mendel was an Austrian Monk who is credited with founding the study of genetics. – Genetics is the study of heredity (the passing on of traits or characteristics).

Mendel and His Work He selectively cross pollinated pea plants to identify trends in the way they passed on traits.

Mendel and His Work He was very lucky to choose pea plants. They are considered a model organism because they carry traits which can be crossed and predicted as basic traits.* * This will make more sense when we discuss the mechanics of genetic crosses.

Mendel and His Work What did Mendel need to know to perform his crosses? Fertilization is the process of joining male and female reproductive cells. He used tiny brushes to take the pollen from the male portion of the flower and put it on the female portion of the flower of a different pea plant.

Mendel and His Work Through this pollination plants characteristics or traits are handed down. A trait is a specific characteristic, such as seed color or plant height.

Mendel and His Work Mendel determined how these traits were passed on by measuring the number of offspring that resulted from each of his crosses. The different offspring that resulted from these crosses are called hybrids.

Mendel and His Work Factors which are passed on from parent to offspring are called genes. Each of the genes Mendel looked at occurred in two different forms called alleles.

Mendel and His Work The Principle of Dominance states that some alleles are dominant and others are recessive. – Dominant, the trait which will overshadow the recessive trait and be sown in the offspring – Recessive, the trait which is overshadowed and not shown but still carried.

Mendel and His Work Mendel first identified his traits and their alleles and he bred the plants to guarantee these traits were pure. (He called this the P or parent generation) He cross bred the plants with the same trait but different alleles and identified the dominant allele. (He called this the F1 or first filial generation)

Mendel and His Work He then bred each F1 plant with itself to prove that the recessive alleles did not disappear but in fact were still hidden in the plants DNA.

Applying Mendel’s Work What are the chances of flipping a coin and it landing on heads? What does this have to do with genetics?

Applying Mendel’s Work Probability of an event occurring is the number of times an event may occur in a total number of possible outcomes. So in the coin toss there is one side of the coin which has heads and two possible sides for the coin to land on. Probability = 1:2

Applying Mendel’s Work To find the probability of an occurrence use the following general equation: Probability = Number of times and event is expected to happen Number of opportunities for an event to happen

Applying Mendel’s Work The creation of an offspring is like flipping a coin in that there are a given number of possible outcomes and a random selection of what the offspring will have.

Applying Mendel’s Work The genetic makeup of an organism is its genotype (letters), which consists of the alleles that the organism inherits from its parents. – Genotypes can be: Homozygous Dominant (AA), Heterozygous Dominant (Aa), Homozygous Recessive (aa).

Applying Mendel’s Work The appearance of an organism is called its phenotype (looks). – This is the expression the organisms genotype

Applying Mendel’s Work A cross between individuals that involves one pair of contrasting traits is called a monohybrid cross. Scientists use Punnett squares to aid in predicting the out come of a monohybrid cross.

Applying Mendel’s Work A dihybrid cross is a cross between individuals that involves two pairs of contrasting traits. Looking back at Mendel’s pea plants you could do a dihybrid cross for traits such as color of the pea and wrinkled or smooth peas.

Applying Mendel’s Work These two traits can be predicted using one Punnett square. The cross for these traits would be: RRYY  rryy

Other Types of Inheritance Patterns The simplest outcome of the cross of parental genes results in one trait being dominant over another. This is called complete dominance.

Other Types of Inheritance Patterns But alleles can mix and blend their characteristics, this is called incomplete dominance. + =

Other Types of Inheritance Patterns Another expression of genes is co- dominance which occurs when both alleles for a gene are expressed in a heterozygous offspring.

Other Types of Inheritance Patterns An example of this would be roan coats in horses, the coat is the result of a white horse mating with a red horse. The roan coat has both red and white hairs in it.

Other Types of Inheritance Patterns Many genes have more than two possible outcomes, for example hair color in humans. These are said to be multiple allele traits.

Other Types of Inheritance Patterns Many traits are also very complex, meaning they are controlled by many genes. These are said to be polygenic traits. An example of this would be human skin tone.

Gene Linkage We commonly see traits handed down together such as hair and eye color. This implies a linkage of some sort. If genes are located on the same chromosome then the traits tend to be handed down together.

Gene Linkage The frequency at which traits are handed down can be used to estimate their location on a chromosome. The higher the percentage of the time they stay together the closer together they are located on the chromosome.