Gregor Mendel Father of Genetics
Genetics Branch of biology which deals with principles of variations in traits (distinguishing characteristics) and inheritance Allows us to predict patterns of inheritance within family lines and determine the probability of inheriting certain traits
So why do family members look alike anyway?
So why do family members look alike anyway?
Gregor Mendel Augustinian Monk at Brno monastery in Austria (now Czech Republic) Well-trained in math, statistics, probability, and physics Interested in plants and heredity
Mendel worked with Peas (Pisum sativum) Commercially available throughout Europe Easy to grow and mature quickly Sexual organs of plant enclosed in flower (so the plants selfpollinate), allowing Mendel to control which plants reproduced Different varieties have different observable traits
Mendel s Peas
Mendel s Experiments Obtained purebred plants for the trait he wanted to study (ex. height) Produced through selective breeding Designated parent, or P generation Crossed purebred tall pea plant with purebred short pea plant Offspring designated first filial, or F1 generation Also called hybrid, as they were the result of a cross between two different purebred plants This is a monohybrid cross, because it involved only one trait (height)
Results of the Monohybrid Cross (F1 generation) Expected to obtain plants of medium height Instead, found 100% of the plants were tall Concluded that trait for tall plants is dominant Dominant traits are always expressed (or always appear) in an individual Recessive traits (ex. shortness in pea plants) are present, but usually inactive
Principle of Dominance When individuals with contrasting traits are crossed, the offspring will express only the dominant trait.
The Punnet Square Organizes the factors or alleles of a particular gene to determine possible combinations of alleles in offspring Helps us calculate the probability of inheriting a particular trait
Mendel s Experiments (Part 2) Allowed hybrid F1 plants to cross This produced the Second Filial, or F2 generation
Law of Segregation Each parent in F1 generation starts with two factors (alleles) - one is dominant and the other is recessive The alleles separate, and only one allele from each parent is contributed to the offspring
Law of Segregation Each offspring (F2 generation) inherits one allele from each parent. If the dominant factor is present, it will be expressed even if the recessive factor is also present The recessive factor will be expressed only if two recessive factors are present
Solve the Punnet Square for the F2 Generation TT Tt Tt tt Gametes formed by each parent are listed along the top and side Possible genotypes are determined by combining gametes from each row and column in the square
So the genotypes are TT Tt Tt tt 25% TT (homozygous tall) 50% Tt (heterozygous tall) 25% tt (homozygous short)
And the phenotypes are: TT Tt Tt tt 75% (3/4) tall Since tall is dominant, any time there is a T in the genotype, the offspring will be tall 25% (1/4) short Since short is recessive, it is only expressed when the genotype is homozygous tt 3 tall : 1 short
Another Example of Mendel s Work P Green seeds gg Homozygous recessive x Yellow seeds GG Homozygous dominant Phenotype Genotype F1 F2 All Yellow Gg Heterozygous G g G GG Gg g Gg gg Phenotype Genotype Phenotype Genotype 3/4 Yellow 1/4 Green
Mendel as a Scientist Mendel repeated his crosses over and over, experimenting with different traits - the results held true The ratio of 3:1 is known as the Mendelian Ratio
Remember: Phenotype vs. Genotype Phenotype Genotype P TT x tt F1 All Tt F2 TT, Tt, Tt, tt
Genotype vs. Phenotype Trait Dominant/ Recessive Phenotype Genotype Homozygous black BB Fur colour Black/ White Homozygous white bb Heterozygous black Bb Homozygous horned HH Presence of horns Horned/ No horns Homozygous no horns hh Heterozygous horns Hh
Solving Genetics Problems 1 S s S SS Ss s Ss ss In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds? 3/4 of the offspring should have spherical seeds.
Solving Genetics Problems 2 The ability to taste the bitter chemical PTC is determined by the dominant allele T and inability to taste by the recessive allele t. Suppose two heterozygous tasters (Tt) have a large family.
Solving Genetics Problems 2 a) Predict the proportion of their children who will be tasters and nontasters. Use a Punnett square to illustrate how you make these predictions. T t T TT Tt t Tt tt 3/4 of their children will be tasters
Solving Genetics Problems 2 b) What is the likelihood that their first child will be a taster? 3/4 or 75% c) What is the likelihood that their fourth child will be a taster? 3/4 or 75% In these questions, each child is independent of the other d) What is the likelihood that all of the first three children of this couple will be nontasters? 1/4 for each child 1/4 x 1/4 x 1/4 = 1/64 chance that all three will be non-tasters In this question, the probability of the event is dependent on each of the children being nontasters
Pedigrees We can t do experimental crosses on humans, so how can we study patterns of inheritance? We use medical, historical, and family records to study crosses that have already occurred
The Test Cross Used to determine the genotype of an individual Cross an individual of unknown genotype with a homozygous recessive individual Offspring s phenotypes will allow you to determine if the parent of unknown genotype was homozygous dominant or heterozygous
The Test Cross In dogs, there is a hereditary deafness caused by a recessive gene, d. A kennel owner has a male dog that she wants to use for breeding purposes. The dog can hear, so the owner knows his genotype is either DD or Dd. If the dog s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd).
The Test Cross Draw the Punnet squares to represent the two possible crosses If the male dog is homozygous dominant (DD), all the offspring will be normal (Dd) If the male dog is heterozygous, half the offspring will be heterozygous normal (Dd), and half will be deaf (dd) d d d d D Dd Dd D Dd Dd D Dd Dd d dd dd
Practice Page 128 Practice Problems #3 & 4 (you may want to read the section entitled Probability and Genetics ) Page 130 Practice Problems #1-3 Practice Problems from Handout and Text to be announced!