Introduction to Mendelian Genetics Biology Introduction to Mendelian Genetics

Genetics - The study of heredity, how traits are passed from parent to offspring or x = or

The study of heredity started with the work of Gregor Mendel and his pea plant garden Mendel was an Austrian Monk that lived in the mid 1800’s

Mendel’s Laws of Heredity Why we look the way we look...

What is heredity? The passing on of characteristics (traits) from parents to offspring Genetics is the study of heredity

Gregor Mendel Mendel used pea plants to discover the mechanism of heredity – how traits get passed from parents to offspring.

Mendel noted that the size of pea plants varied Mendel noted that the size of pea plants varied. He cross-bred these pea plants to find some surprising results.

Why Mendel used peas... They reproduce sexually They have two distinct, male and female, sex cells called gametes Their traits are easy to isolate

Mendel crossed the Plants Fertilization - the uniting of male and female gametes Cross - combining gametes from parents with different traits

Questions What did Mendel cross? What are traits? What are gametes? What is fertilization? What is heredity? What is genetics?

What Did Mendel Find? He discovered different laws and rules that explain factors affecting heredity.

Phenotype & Genotype Phenotype - the way an organism looks Example - red hair or brown hair genotype - the gene combination of an organism AA or Aa or aa

Heterozygous & Homozygous Heterozygous - if the two alleles for a trait are different (Aa) Homozygous - if the two alleles for a trait are the same (AA or aa)

Dihybrid vs Monohybrid Dihybrid Cross - crossing parents who differ in two traits (AAEE with aaee) Monohybrid Cross - crossing parents who differ in only one trait (AA with aa)

Questions... What is the phenotype? What is the genotype? What is homozygous? What is heterozygous? What is monohybrid crossing?

Mendel’s cross between tall pea plants yielded all tall pea plants Mendel’s cross between tall pea plants yielded all tall pea plants. His cross between small pea plants yielded all small pea plants. Tall plants = X X = Short plants

Mendels’ cross between tall pea plants and small pea plants yielded all tall pea plants. = x

Mendel then crossed these second generation tall pea plants and ended up with 1 out 4 being small. x =

What Did Mendel Find? He discovered different laws and rules that explain factors affecting heredity.

Mendel’s work led him to the understanding that traits such as plant height are carried in pairs of information not by single sets of information.

Carrying the information are chromosomes Chromosomes are made up of sections called genes Genes are made up of DNA

Rule of Unit Factors Each organism has two alleles for each trait Alleles - different forms of the same gene Genes - located on chromosomes, they control how an organism develops

Rule of Dominance The trait that is observed in the offspring is the dominant trait (uppercase) The trait that disappears in the offspring is the recessive trait (lowercase)

Questions... What do we call the trait that is observed? What case (upper or lower) is it written in? What about the one that disappears? What case is it written in?

Law of Segregation The two alleles for a trait must separate when gametes are formed A parent randomly passes only one allele for each trait to each offspring

Law of Independent Assortment The genes for different traits are inherited independently of each other.

Questions... How many alleles are there for each trait? What is an allele? How many alleles does a parent pass on to each offspring for each trait

Segregation - Alleles separate Heterozygous parents Tt Tt Gametes T t T t

Fertilizaiton tt Tt Tt TT Gametes t t T T F2 Generation Parent 1

Probability The likelihood that a particular event will occur is called_______. Probability

Probability The probability that a single coin flip will come up heads is… a. 100 percent b. 75 percent c. 50 percent d. 25 percent

Probability The probability that a single coin flip will come up heads is…. a. 100 percent b. 75 percent c. 50 percent d. 25 percent

Probability Is the following sentence true or false? The past outcomes of coin flips greatly affect the outcomes of future coin flips. False

Probability Why can the principles of probability be used to predict the outcomes of genetic crosses? The way in which the alleles segregate is completely random, like a coin flip.

How do geneticists use Punnett squares? Punnett squares can be used to predict and compare the genetic variations that will result from a cross.

Genetics & Punnett Squares First let’s look at two basic kinds of genes: Dominant - A gene that is always expressed and hides others Recessive - A gene that is only expressed when a dominant gene isn’t present

Dominant and Recessive Genes A dominant gene will always mask a recessive gene. A “widows peak” is dominant, not having a widows peak is recessive. If one parent contributes a gene for a widows peak, and the other parent doesn’t, the off-spring will have a widow’s peak Widows Peak

Genetics Punnet Square - A tool we use for predicting the traits of an offspring Letters are used as symbols to designate genes Capital letters are used for dominant genes Lower case letters are used for recessive genes Genes always exist in pairs

Genetics A Widows Peak, dominant, would be symbolized with a capital “W”, while no widows peak, recessive, would be symbolized with a lower case “w”. Father-No Widows Peak ‘w’ Mother-Widows Peak ‘W’

Genetics All organisms have two copies of each gene, one contributed by the father, the other contributed by the mother. Homozygous - Two copies of the same gene Heterozygous - Two different genes

Genetics For the widows peak: WW - has a widows peak Homozygous dominant Ww - has a widows peak Heterozygous ww - no widows peak Homozygous recessive

Since Herman has no widows peak, he must be “ww”, since Lilly has a widows peak she could be either “WW” or “Ww” Definitely Homozygous recessive ww Either Heterozygous Ww or Homozygous dominant WW

Genetics We can use a “Punnet Square” to determine what pairs of genes Lilly has A Punnet Square begins with a box 2 x 2 One gene is called an “allele” One parents pair is split into alleles on top, the other along the side Each allele is crossed with the other allele to predict the traits of the offspring Assume Lilly is heterozygous Ww W w Assume Herman is homoozygous recessive ww w Ww ww w Ww ww

Genetics Notice that when Lilly is crossed with Herman, we would predict that half the offspring would be “Ww”, the other half would be “ww” Half “Ww”, Heterozygous, and will have a widows peak Half “ww”, Homozygous, and will not have a widows peak W w w Ww ww w Ww ww

Genetics Another possibility is that Lilly might be “WW”, homozygous dominant. Notice that all the offspring are heterozygous and will have a widows peak Assume Lilly is homozygous dominant WW W W Assume Herman is homoozygous ww w Ww Ww w Ww Ww

Genetics So which is true? Is Lilly homozygous dominant (WW) or is she heterozygous (Ww)? W w W W w Ww ww w Ww Ww w Ww ww w Ww Ww

If Lilly were heterozygous, then 1/2 of their offspring should have a widows peak, 1/2 shouldn’t If Lilly were homozygous, all of their children will have a widows peak W w W W w Ww ww w Ww Ww w Ww ww w Ww Ww

Recall that Herman and Lilly had another offspring, Marylin Recall that Herman and Lilly had another offspring, Marylin. She had no widows peak, therefore, Lilly must be heterozygous.

Genetics & Punnett Squares Now let’s look at two other basic kinds of genes, Incomplete dominance and Codominance: Incomplete dominance - Genes that work together to produce a third trait where the alleles are blended Like a red flower crossed with a white flower produces a pink flower

Genetics & Punnett Squares Codominant - Genes that work together to produce a third trait where both alleles contribute to the trait Like a red flower crossed with a white flower produces a red and white flower

Genetics Hair color can be an example Prince Charming is blond Snow White has black hair

Genetics What color hair will the offspring of Prince Charming and Snow White have?

Genetics Hair color is different from widows peak, no color is truly dominant. In Fairy tales… Brown and blond are two, true traits Homozygous conditions produce either brown or blond hair Heterozygous conditions produce red hair

Genetics For Snow White to have brown hair she must be homozygous dominant, “BB”, a blond Prince Charmin must be homozygous recessive, “bb”. B B b Bb Bb b Bb Bb

Genetics All the offspring from Prince Charming and Snow White will therefore be heterozygous, “Bb”, and since hair color is codominant….. all their children will have red hair. +

That’s all for now!