The science of genetics explains why you have inherited certain traits from your parents. Geneticists study heredity. By the end of this unit you will.

The science of genetics explains why you have inherited certain traits from your parents. Geneticists study heredity.By the end of this unit you will understand how traits were passed on to you.

Gregor Mendel was an Austrian monk who first predicted how traits were transferred from one generation to the next.

Up until the time of Mendel (1822-1884) the actual mechanisms governing inheritance were unknown.  Darwin had no knowledge about how traits were passed on.

Knowledge of the complex genetic mechanisms finally came as a result of careful laboratory breeding experiments carried out over the last century and a half.

Mendel’s simple laws and ideas were published in 1866 but largely went unrecognized until 1900, which was long after his death. 

One possible explanation of heredity is a “blending” One possible explanation of heredity is a “blending” hypothesis that Darwin implicated in his work.hypothesis that Darwin implicated in his work.

This is the idea that genetic material contributed This is the idea that genetic material contributed by two parents mixes in a manner analogous to by two parents mixes in a manner analogous to the way blue and yellow paints blend to make the way blue and yellow paints blend to make green.green.

X =

This may be true for some traits which are governed by incomplete dominance, a complex form of inheritance pattern, but for most it is not.

An alternative to the blending model is the An alternative to the blending model is the “particulate” hypothesis of inheritance: the “particulate” hypothesis of inheritance: the gene ideagene ideaParents pass on discrete heritable units known as Parents pass on discrete heritable units known as

genes. Mendel knew nothing about genes, or genes. Mendel knew nothing about genes, or alleles. He called these “particles” governing alleles. He called these “particles” governing inheritance, “unit factors”. How did Mendel come inheritance, “unit factors”. How did Mendel come up with it?up with it?

Gregor Mendel documented a Gregor Mendel documented a particulate mechanism of particulate mechanism of inheritance through his inheritance through his experiments with garden experiments with garden peaspeas

Describe the “blending” hypothesis of inheritance vs. the particulate hypothesis.

Blending has traits from mother and father mixing equally to produce a medium range within the offspring, whereby particulate has each parent contributing a discrete unit called a gene, which may or may not be observable in the offspring.

Mendel chose the garden pea (Pisum sativum) for his experiments for various reasons.

• because they can be grown easily and quickly

• reproduce in large numbers

• their reproduction can be manipulated. 

The transfer of pollen (male gamete) to the female reproductive organ (ovary) is called pollination.

?Pollination

Plants AnimalsWhen Mendel wanted to breed two plants, he dusted the female organ of one plant with pollen from the plant he wished to cross it with.

In a typical breeding experiment:In a typical breeding experiment: Mendel mated two contrasting, true-breeding varieties Mendel mated two contrasting, true-breeding varieties

in a process called in a process called hybridizationhybridization

The true-breeding parents are called the “P” generationThe true-breeding parents are called the “P” generation

The hybrid offspring of the P The hybrid offspring of the P generation are called the Fgeneration are called the F11

generation (F stands for filial)generation (F stands for filial)

True breeding individuals are those who when bred together, always produce offspring of the same type.

When FWhen F11 individuals self- individuals self-

pollinate the Fpollinate the F22 generation is generation is

producedproduced

Through careful control, Mendel was able to use his experiments to show how traits were transferred from one generation to the next.

• He only analyzed one trait at a time. For this reason, they were called monohybrid crosses.

The Traits Mendel Studied:

• seed shape (round or wrinkled)

• seed color (green or yellow)

• flower color (purple or white)

• flower position (Axial or terminal)

• pod color (green or yellow)

• pod shape (inflated, or constricted)

• plant height (tall or short)

Why do geneticists studying heredity through experimentation use organisms such as peas and fruit flies?

These organisms reproduce and produce new generations quickly; they reproduce abundantly with many offspring; they are easily manipulated to reproduce.

What is a true-breeding individual?An organism that when bred with another true-

breeding individual, will only produce the same type of offspring as themselves.

What is hybridization?When true-breeding individuals with contrasting

traits are crossed, the result is a hybrid.

What is a monohybrid cross?A cross between two individuals that only looks at

one trait.

Mendel’s studies found that when crossed, one trait of a pair seemed to disappear in the F1 generation, and then reappear in the F2 plants.

As you can see, Mendel was extremely lucky in that each of the traits he studied had two clearly distinct forms.

Round X Wrinkled= RoundRound X Wrinkled= Round

When Mendel crossed these two individuals, the results were:

P1 Generation

F1 GenerationRound X Round= Round, and wrinkled both

Because of this curiosity, Mendel came up with his rule of unit factors.

x =

x =

The rule of unit factors:• Mendel concluded that each organism has two factors that control each of its traits. We now call these factors alleles.

For example, each of Mendel’s pea plants had two alleles of the gene that determined its seed shape. A plant could have inherited two alleles for round, two alleles for wrinkled, or one for each trait.

An organism’s two alleles for each trait are inherited from each of its parents. One from female, and one from male.

Because of Mendel’s work, he was also able to come up with a rule of dominance.

Allele for purple flowers

Locus for flower-color geneHomologouspair ofchromosomes

Allele for white flowers

The gene for flower color

Mendel called the observed trait the dominant trait, and the trait that disappeared, the recessive trait.

Mendel concluded that the allele for round seeds was dominant to the allele for wrinkled seeds, and when the two different alleles were presented together, the round one would always win!

Round x Wrinkled = Round

When recording the results of crosses, it is customary to use the same letter for the different alleles of the same gene. Usually, one would use the dominant allele’s letter for both traits, too. You would use an upper case to represent the dominant allele, and a lower case for the recessive. This is not always the case, however, as with fruit flies. se/se; how/how…etcThe dominant allele is always written first, so the alleles for seed

shape would look like this: R=round, r=wrinkled

X =

In pea plants with purple flowers, the genotype In pea plants with purple flowers, the genotype may not be immediately obvious.may not be immediately obvious.

A A testcrosstestcross a allows us to determine the genotype of an llows us to determine the genotype of an organism with the dominant phenotype, but unknown organism with the dominant phenotype, but unknown genotypegenotype

A testcross crosses an individual with the dominant A testcross crosses an individual with the dominant phenotype (and unknown genotype) with an individual phenotype (and unknown genotype) with an individual that is homozygous recessive for a trait that is homozygous recessive for a trait

ppParents: x PP or PpIf the resulting offspring are all purple, then the genotype in question must be homozygous dominant. If the resulting offspring are both purple and white, then the genotype in question must have been heterozygous.

Put the “rule of unit factors” in your own words.

Mendel said that each organisms contains two factors that govern each trait…one from mother, and one from father. It is the combination of these two “units” that determine the phenotype of the offspring.

Put Mendel’s rule of dominance in your own words.

Mendel said that when these two “units” were presented together, one may “mask” the other, and dominate over it. This would be the dominant trait. The masked trait would be called the recessive trait.

Describe what a testcross is used to determine.

A testcross would be used to determine an unknown genotype, when the phenotype is dominant. The cross ALWAYS involves a homozygous recessive individual x unknown genotype.

He was then curious about what would happen if he crossed the offspring from that first monohybrid cross with each other.

The results were surprising, but they helped him to determine one of his two laws. The Law of Segregation.

Round X Round = Both round, and wrinkledOffspring appeared in a three to one ratio looking like this: 3 round, and 1 wrinkled

x =

Mendel crossed his parental generation (P) of true-breeding peas, and studied his results.

• Every individual has two alleles of each gene. • When gametes are produced through meiosis, they each receive one of these alleles.• During fertilization when gametes unite, they randomly pair to produce four possible combinations of alleles.This law is best expressed by observing a simple hereditary tool called a Punnett Square, which is a tool used by geneticists to predict possible genetic outcomes.

R r

R

r

RR Rr

Rr rr

The results of Mendel’s crosses were to prove that not all pea plants with round seeds were the same. Some with round seeds when crossed with one another, would produce only offspring with round seeds.

He concluded that two organisms can look alike, but still have different underlying allele combinations.

Others when crossed, had the potential to produce both round, and wrinkled seeded offspring.

• The way an organism looks and behaves is known as PHENOTYPE

• The allele combination an organism contains is known as GENOTYPE

Round

Rr

An organism is homozygous for a trait if both of its alleles are exactly the same.

RR, or rr

An organism is heterozygous for a trait if both of its alleles are different.

Rr

An organism with the genotype of RR, is said to be homozygous dominant, because of the presence of the dominant allele. An organism with the genotype of rr is said to be homozygous recessive, because of the presence of the recessive allele

Organisms with this heterozygous allele combination will express the dominant trait in their phenotype.

Mendel’s Law of Segregation says what?• Every individual has two alleles (unit factors) for each gene• When the individual produces gametes, these unit factors separate randomly into the different sex cells.• During fertilization, these traits reunite in four possible combinations.

What tool is best used to illustrate this law?A simple punnett square

The last of Mendel’s laws is best expressed when observing something called a dihybrid cross.

While most of Mendel’s crosses were monohybrid in nature, it took the dihybrid cross to show his Law of Independent Assortment.

Whereas Mendel’s monohybrid crosses used only one trait when studying genetic outcome, his dihybrid crosses used two.

Because of this particular cross, Mendel was able to determine that the two traits were still inherited independently of each other.

The dihybrid cross that Mendel first looked at studied the two traits of pea seed shape, and color.

Mendel crossed a round yellow pea, with a wrinkled green pea

How do you think he showed a dihybrid cross such as this in a Punnett Square?

X

In this cross, remember all the alleles sort themselves independently from one another, so you must show all possible alleles.

The parents are both heterozygous for seed shape and color.

The gametes from RrYy Parent

R = Roundr = wrinkled

Y = Yellowy = green

The

gam

etes

fro

m R

rYy

Par

ent RY Ry rY ry

r

y

r

Y

R

y

R

Y

As you can see, the gametes will separate from each other, and then recombine or assort independently

from one another in four different ways.

RRYY

RRYy

RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

Using the Law of Independent Assortment as a guide, determine the possible gametes formed from these parents. VVPp ZzBb MMPP ffMm NNtt

VP, and VpZB, Zb, zB, and zbMP

fM, and fmNt

So, how many squares would you have to make to do a tri-hybrid cross? How does one determine probability in a far more complex cross…say one that uses four traits? There is a simpler mathematical way for doing a tri-or even a quadri-hybrid cross.In the following cross, the traits observed are: Red/brown eyes (alleles R,r); Normal/vestigial wings (alleles V, v); Long/short sex combs (alleles L, l); and Normal/ebony body color (alleles E, e).

These characteristics are found within fruit flies (Drosophila melanogaster), a long-time favorite of the heredity scientists. Here is the cross we will be performing:

RrVvLlEe X RrVvLlEe

Question: What is the probability of producing offspring with the following genotype: RRvvLLEe?

64

RrVvLlEe X RrVvLlEe = RRvvLLEe

First, take each trait for each parent individually. Look at eye color first. Offspring genotype is RR. Both parents, however, are Rr (they have red eyes, however they are both heterozygous.) What is the probability that a cross of this trait alone, will produce an RR individual? If you have to do a small Punnett Square, you can…but eventually, you should be able to see it in your mind.

The chances are 25%, or 1/4

Now do the same thing for each trait, individually, determining the probability of each cross.

Vv x Vv = vv (again=25% or ¼)

Ll x Ll = LL (again = 25% or ¼)

Ee x Ee = Ee (= 50% or ½)

At this point, your task is simple. Multiply all the fractions together, and your result is the probability of producing that genotype!

So... ¼ x ¼ x ¼ x ½ = ?

The probability of producing the above genotype is 1/128!

1/128

Challenge:

Given the following cross:

RRVvLLEe X RrvvLlee

What is the probability of the following combination?

RrVvLLee?

½ x ½ x ½ x ½ = 1/16

RRVvLLEE?

½ x ½ x ½ x 0 = 0