Introduction to Genetics

Chapter 11 pages 308 to 321

What is inheritance?Scientifically known as heredity and genetics.

Early views

Blending Hypothesis (1800s) Idea that genetic material contributed by two

parents will mix to produce a “blended” offspring

If a blue flowered plant and a yellow flowered plant reproduced they would produce a green flowered plant

Predicted that over several generations a freely mating population will become uniform in their traits

Gregor Mendel

Father of Genetics Austrian monk – studied at

University of Vienna Spent 14 years teaching high

school and working in the monastery garden What did Mendel complete his

studies on? Why these plants? Small plants that were easy to grow

and maintain. Produced hundreds of offspring at once.

Peas are a “Model System”

The Experiment

Garden was full of “true-breed” plants Plants reproduce by sexual

reproduction Self-pollinated to produce identical

offspring with identical traits. Mendel performed cross-pollination to

allow fertilization to occur. Sperm in pollen, Eggs in ovule

Produces a new cell known as an …. Embryo

The embryo grew to be a full size plant that Mendel could study.

Keeping Track

Mendel studies 7 different traits with contrasting characteristics Ex. Green seeds or yellow seeds, wrinkled seeds or smooth seeds,

etc.

The offspring of each cross between parents is called a hybrid.

To help keep track of crosses, each original pair of plants is said to be P or parental

Each generation after that is kept track of with F1, F2, etc. First filial, second filial, etc.

Results

F1 generation – had the characteristics of only one of its parents. Mendel concluded that: An individual's

characteristics are determined by factors that are passed from one parental generation to the next.

We call these characteristics genes, but Mendel called them “heritable units”

Different forms of genes are known as alleles.

Results

Principle of Dominance Some alleles are dominant and others are recessive.

Explains why only one parents traits showed in the F1 generation.

Recessive allele will only show when the dominant allele is not present

Parental Equivalence

Mendel also noticed that when crossing a dominant trait with a recessive trait the sex of the dominant and recessive alleles did not cause a change in expression

Next Question

Had the recessive alleles simply disappeared, or were they still present in the new plants? Mendel tested this by allowing the F1 generation to self-

pollinate Produced the F2 generation

Recessive alleles reappeared in the F2 generation ¼ of the offspring showed the recessive alleles.

How did this happen??

The Explanation

At some point during the self-pollination, the allele for one trait must have separated from the allele of the opposing trait Called segregation

Mendel suggested that this happened during the formation of sex cells called gametes.

Conclusion – During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene. F1 gametes either had a tall allele or a short allele

Mendel’s Mathematics

Used a lot of stats to keep track of his results and analyze his data

Probability can be used to explain results.

What is the probability of flipping a coin and having it land heads up?

1/2

What is the probability of flipping a coin three times and it landing heads up each time?

½ X ½ X ½ = 1/8

Why did we multiply to get this answer?

Past events do not affect future ones

Probability Multiplication Rule:

If you have two mutually exclusive events the probability of an occurrence is unique for each event

Example: If a couple are both carriers for cystic fibrosis (F = normal; f

= CF) then what is the probability that they will have a child with CF?

Egg has a ½ chance of getting the f allele

Sperm has a ½ chance of getting the f allele

The couple would then have a 25% (1/4) chance of having a child with CF

Probability

Addition rule: Two parents who are carriers for CF want to have a

child what is the probability that their child will also be a carrier for CF? F = normal ; f = CF

Ff x Ff ¼ FF; ¼ Ff; ¼ fF; ¼ ff

Since Ff and fF both create carriers you would add their probabilities

You would have a ¼ + ¼ = ½ chance of having a child that is a carrier for CF

Probability with Pea Plants• Remember that each

allele segregates

For each of Mendel’s experiments, ¾ of the plants showed traits controlled by the dominant allele and ¼ of the plants showed traits controlled by a recessive allele.

How do the combo of alleles differ between dominant and recessive?• Tt – Heterozygous – Tall plant• TT – Homozygous Dominant – Tall

plant• tt – Homozygous Recessive – Short

Plant

Genotype v. Phenotype

Genotype – The genetic makeup of an organism TT

Phenotype – The physical characteristics expressed as a result of the genotype Tall

Two organisms may have the same phenotype, but two different genotypes

Punnett Squares

Can help us easily predict the outcome of genetic crosses

One-Factor (monohybrid) and two factor (dihybrid) crosses

Bb X Bb

TtGg X TtGg

I bought a prized purebred black guinea pig… but I am not too sure about its validity

Black fur is dominant over white fur

So what can I do to predict its genotype?

Our “mystery” black pig is either BB or Bb. 

Our black guinea pig parent is on the left, and the white parent is up above. Notice that the

offspring will inherit only "b's" from the white parent.

Let's fill-in the boxes with all the alleles that we KNOW ...

The hybrid ("Bb") baby guinea pigs across the top will be black. 

For the offspring in that bottom row, their phenotype depends on what that second ("?") allele is in our black guinea pig parent.

There are two possibilities ...

As you can see, if our mystery genotype pig has a "B" in the "?" spot, all of the offspring from the test cross will be heterozygous (Bb) & have the dominant phenotype --- black fur.

There is NO WAY white guinea pigs can be produced

On the other hand, if the mystery allele "?" = "b", then we can predict that half (2 of 4 boxes) of the offspring from the test cross are going to have the recessive phenotype; i.e. have white fur.

Test CrossCan determine the genotype of an individual

showing the dominant phenotype

AA or Aa ???? A ?

If there are many offspring and they all show

dominant trait then probably AAIf any offspring would show recessive trait then

AaIf ANY recessives show up in the offspring then the

unknown allele would have to be recessive. The individual in question is HETEROZYGOUS

Back Cross Another way to determine the genotype of an

unknown hybrid is to cross the offspring back to their parent This is a back cross

This will allow you to determine the exact genotype of the offspring

You will only cross back to the homozygous parent

Does the segregation of one allele affect another? Two-factor cross: F1

Crossed true-breed round yellow peas with wrinkly green peas. All F1 produced round yellow

Which is dominant and which is recessive?

Does not provide us with confirmation of independent assortment.

Two-factor: F2 Crossed F1 generation

Produced 556 seeds – 315 seeds were round yellow, 32 seeds were wrinkled green, 209 seeds were a combo of parental genotypes

Confirms Independent Assortment – genes from different traits can segregate independently during formation of gametes.

THIS IS TRUE AS LONG AS THEY ARE NOT ON THE SAME CHROMOSOME!

Summary of Mendel’s Principles The inheritance of biological characteristics is

determined by individual units called genes, which are passed from parents to offspring.

Where two or more forms (alleles) of the gene for a single trait exist, some alleles may be dominant and others may be recessive.

In most sexually reproducing organisms, each adult has two copies of each gene – one from each parent. These genes segregate from each other when gametes are formed.

Allele for different genes usually segregate independently of each other.

Mendel Paves the Path

Thomas Hunt Morgan tested these principles on Drosophila melanogaster Quickly learned that all of Mendel’s principles

applied to fruit flies and other organisms – including humans.