Introduction to Mendelian

Genetics

Biology

Genetics - The study of heredity, how traits are passed from

parent to offspring

x =or

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. 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. His cross between small pea plants yielded all small pea plants.

X =

X =

Tall plants

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

Tt TtHeterozygous parents

T Tt tGametes

Fertilizaiton

T Tt t

Gametes

TtTT

Parent 1

Parent 2

ttTt

F2 Generation

ProbabilityThe 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.

Punnett Squares• 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

GeneticsPunnet 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

GeneticsA 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’

GeneticsAll 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

GeneticsFor the widows peak:WW - has a widows peak Homozygous dominantWw - has a widows peak Heterozygousww - 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

GeneticsWe can use a “Punnet Square” to

determine what pairs of genes Lilly has

Ww ww

Ww www

w

W w

Assume Lilly is heterozygous Ww

Assume Herman is homoozygous recessive ww

• 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

GeneticsNotice 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

Ww ww

Ww www

w

W w

GeneticsAnother possibility is that

Lilly might be “WW”, homozygous dominant.

Ww

Www

w

W W

Assume Lilly is homozygous dominant WW

Assume Herman is homoozygous ww

Ww

Ww

Notice that all the offspring are heterozygous and will have a widows peak

GeneticsSo which is true? Is Lilly

homozygous dominant (WW) or is she heterozygous (Ww)?

Ww

Www

w

W W

Ww

Ww

Ww ww

Ww www

w

W w

Ww

Www

w

W W

Ww

Ww

Ww ww

Ww www

w

W w

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

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?

GeneticsHair 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”.

Bb

Bbb

b

B B

Bb

Bb

GeneticsAll 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!