Fundamentals of Genetics

Chapter 9 (pgs 164-178)Mrs. McNamara - Biology

Heredity and Genetics

Parents pass certain traits to their offspring.

Landon (left) at 6 months and Gavin (right) at 3.5 months.

On the left, cousins; on the right, brothers! Notice the similarities and differences!

PART 1

Key Concepts

• Genes are particulate and are inherited according to Mendel’s laws.

• Mendel learned about heredity by conducting experiments.

• Genes are carried on chromosomes.• Alleles and genes interact to produce

phenotypes.

Heredity•Your combination of traits received for each characteristic are unique to you.

•Your mother and father each contribute to your genetic makeup.

•For example: the color of your hair, the size of your feet and the shape of your nose are some of your traits.

Heredity•Heredity- The passing of traits from parents to offspring.

•Genetics-The branch of biology that studies heredity.

•Geneticists-Biologists who study heredity.

• Look at the photographs to the right.

• What traits have these babies inherited from their parent?

DNA & HEREDITY

•Remember that traits are passed from parents to offspring (inherited) by the passing of DNA from parents to their offspring.

Gregor Mendel

•Called the “Father of Genetics”

•In the late 1800’s Mendel began studying the passing of traits from parents to offspring.

•He, too wanted to know why certain patterns of traits showed up in living things.

•Analyzed pea plants to look for patterns in how traits were passed from parents to offspring.

Gregor Mendel

While trying to develop new color variations of flowers Mendel began experimenting…..

Gregor Mendel’s Experiment• He studied 7 characteristics (genes), each with 2

contrasting traits (alleles) available.• CHARACTERISTIC-a distinguishing quality that an

organism exhibits.• Ex: height, hair color, eye color, skin color.

• TRAIT- specific hereditary options available to be inherited for each characteristic.• Ex: tall height/short height, brown

hair/blonde hair, brown eyes/blue eyes, Dark skin/ light skin.

CHARACTERISTICS TRAITS

1. Plant Height Tall vs. Short

2. Seed Color Yellow vs. Green

3. Seed Shape Round vs. Wrinkled

4. Pod Color Green vs. Yellow

5. Pod/Flower

Location

Axial vs. Terminal

6. Pod Shape Inflated vs.

Constricted

7. Flower Color Purple vs. White

Gregor Mendel’s Experiment• He started by growing plants that were PURE-having

a trait and always passing that trait to their offspring.

Gregor Mendel’s Experiment

•Then, Mendel bred plants that had different traits to see what the offspring would look like.

•The first plants he crossed he called the “Parental Generation” (P)

•In his first crosses, Mendel found that only one of the two traits appeared in the offspring plants (F1 generation – children of P generation).

•For example, when he crossbred tall pea plants with short pea plants, the offspring (F1) were always tall.

Gregor Mendel’s Experiment

•After his first crosses, Mendel took those offspring plants (F1) and crossed them.

•In these second crosses, both traits showed up again in the F2 generation.(F2 GENERATION-offspring of the F1 generation).

Gregor Mendel’s Experiment

•The same results happened in every experiment. One trait, like being tall, was always present in the first generation (F1).

•The other trait, like being short, seemed to go away; only to reappear again in the second generation.

•This happened with every set of traits that Mendel studied in the same proportions..

Gregor Mendel’s Experiment

EXAMPLES:Plant Height

CrossSeed Color

CrossSeed Shape

Cross

Parent P

Tall x Short

All Tall Plants

¾ Tall¼ Short

Yellow x Green

All Yellow Plants

¾ Yellow¼ Green

Round vs. Wrinkled

All Round Plants

¾ Round¼ Wrinkled

F1

F2

MENDEL’S 3 CONCLUSIONS:

• 1. Law (Principle) of Dominance and Recessiveness• 2. Law (Principle) of Segregation• 3. Law (Principle) of Independent Assortment

1. Law of Dominance and Recessiveness

From these experiments, Mendel discovered that traits from one parent may hide traits from the other parent.

He also found that certain traits would always show themselves over other traits.

• These traits he called DOMINANT-can mask or dominate the other trait and is displayed most often.

• The hidden or masked traits he called RECESSIVE-the trait that can be covered up; is displayed less often.• Ex: the trait (allele) for tall is dominant over

the trait (allele) for short, so the short allele would be the recessive allele.

• Mendel hypothesized that something in the pea plants was controlling the characteristics that came through

• He called these controls “factors”

(We now know that thesecharacteristics are controlled byGenes- pieces of DNA that code for a characteristic (protein that is made).)

Genes

•Mendel also figured out that for each trait, the plant had two factors, or a pair of factors, controlling the expression of each trait.

•Each pair consists of alternate forms (alleles) of the same gene; one from mother and one from father.

Genes

•Dominant traits were controlled by dominant alleles and recessive traits were controlled by recessive alleles.

Genes

•Letters are used to represent the alleles that carry the trait found on genes. A letter represents a trait.

•CAPITAL if the gene that controls the trait is dominant.•Lowercase if the gene is recessive.

• T- represents a dominant allele for tallness• t – represents a recessive allele for lack of tallness, or shortness

Genes

•Genotype- The genetic combination, or pair of alleles an organism inherits for a certain trait.

•Phenotype- The appearance of a trait in an organism.

•Ex: A plant's genotype is TT. Its phenotype is tall.

Genotype and Phenotype

b b

Blue alleles

Genotype (bb)

Phenotype (blue eyes)

Like and Unlike Alleles

• HOMOZYGOUS- organism has 2 of the same alleles for a trait.• Homozygous Dominant-has 2 dominant alleles;

dominant trait is displayed• Ex: TT = tall pea plant

• Homozygous Recessive-has 2 recessive alleles; recessive trait is displayed• Ex: tt = short pea plant

• HETEROZYGOUS-organism has 1 dominant and 1 recessive allele; the dominant trait is displayed.• Ex: Tt = tall pea plant

b b b B

Blue Allele Brown AlleleBlue alleles

Homozygous – alleles are the same

Heterozygous – alleles are different

X =

X =

Two Like Alleles

•If you crossbred homozygous tall plants (TT) (purebred) with homozygous short plants (tt) (purebred)…All of the offspring are tall! WHY?

X =

Two Unlike Alleles

All of the offspring had received a tall gene (T) from one parent and a short gene (t) from the other parent.

•Organisms with different genotypes may have the same phenotype.

•For example, a homozygous tall plant (TT) and a heterozygous tall plant (Tt) have different genotypes.

•However, they have the same phenotype, which is tall.

TTtt

Tt Tt

TtTT

ttTt

TallTall

Tall

Tall Tall

Short

Traits of HumansRemember, the combination of alleles makes you look the way you do.

You get traits from your parents, just like pea plants do.

For example, the trait of earlobe shape in humans is inherited.

There is an allele for free earlobes and an allele for attached earlobes.

The allele for free earlobes is dominant, and the allele for attached earlobes is recessive.

This is one of 9000 traits in humans controlled by a single gene.

MENDEL’S LAWS OF INHERITANCE Mendel developed the following three laws to describe how an organism's various traits are inherited and expressed.

1. The Law of Dominance and Recessiveness.

2. The Law of Segregation.

3. The Law of Independent Assortment.

2. The Law (Principle) of Segregation:

•Organisms have 2 copies of each gene (one from mom and one from dad), but put only 1 copy in each sperm or egg.

• During fertilization, the alleles come together and new combinations of alleles are randomly formed.

•The trait observed in an individual depends on the two copies of the gene it inherits from its parents.

3. The Law (Principle) of Independent Assortment:

• The alleles for different genes, on different chromosomes, are not connected.

• The chromosomes are distributed into gametes independently (randomly) during meiosis, not based on the distribution of any other chromosome.

• EX: in pea plants, a gamete that receives a dominant (yellow) allele for pea color can receive either a dominant (round) or recessive (wrinkled) allele for pea shape

• However, if two genes are located on the same chromosome, they are inherited together .

Principle of Independent Assortment

3. The Law (Principle) of Independent Assortment:

http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

PART 2

Key Concepts

• Alleles and genes interact to produce phenotypes.

• Observing and individual’s phenotype is not sufficient for determining its genotype.

• The outcome of a genetic cross can be predicted.

AFTER MENDEL• Today, Geneticists rely on Mendel’s work to

predict the likely outcome of genetic crosses.

•Probability is the likelihood that a chance event will occur.

•The value of studying genetics is in understanding how we can predict how likely it is to inherit a particular trait.

•One of the easiest ways to calculate themathematical probability of inheriting aspecific trait was invented by an early20th century English geneticist namedReginald Punnett .

Punnett Squares

Punnett Squares• MONOHYBRID CROSS-cross between 2 individuals

involving 1 pair of contrasting traits (2 alleles).

•His technique employs what we now call a Punnett square.

•Punnett square-chart that shows the possible allele combinations for potential offspring of two parents whose genotypes are known.

Punnett Squares

•The results of Mendel’s experiments with two homozygous plants can be shown using a Punnett Square…Let’s try it!

STEP 1 - Draw a chart with four boxes.

STEP 2 - Above the two boxes going across, write thesymbol for a the first parent’s alleles (in this case…homozygous tall plant (TT)) with oneletter over each box.

How to Use a Punnett Square

T T

STEP 3 Write the symbol for the other parent’s alleles (in this case…a homozygous short plant (tt)) going down the left side with one letter next to each box.

STEP 4 Fill in the left side of each box with the letter at the top of the column (T).

STEP 5 Fill in the right side of each box with the letter at the left of the row (t).

How to Use a Punnett Square

t

t

T T

T

T

T

T

t t

t t

You should see that in all of the boxes are the alleles for a heterozygous tall (Tt) offspring.

How to Use a Punnett Square

t

t

T T

T

T

T

T

t t

t t

The combination of genes from organisms with heterozygous genes can also be shown in a Punnett square.

Lets cross two heterozygous tall pea plants (Tt).

So, two heterozygous tall plants could potentially produce tall and short offspring!

Predicting Gene Combinations

T

T

T TT

t

T

tt

t

t

t

A Punnett square can help you predict the chances that a certain combination may occur.This Punnett Square shows there is a 1/4 (25%) chance that an offspring from this cross will be homozygous tall (TT)

Predicting Gene Combinations

T

T

T TT

t

T

tt

t

t

t

It also shows a l out of 4 (1/4) chance that the offspring will be homozygous short (tt).

There is also a 2 out of 4 chance (2/4 or 1/2) that each offspring will be heterozygous tall (Tt). The probability ratio for the genotypes is 1:2:1

Predicting Gene Combinations

T

T

T TT

t

T

tt

t

t

t

You can show probability as a percent by using this formula:

Number of times an event occurs x 100 = Probability %

Number of total possible events

For example the probability for heterozygous tall (Tt) is:

2 x 100 = 2 x 100 = 200 = 50 %4 4 1 4

The Punnett square also shows that there is a 25% chance (1/4 X 100) that the offspring could be homozygous tall (TT) and a 25% chance the offspring could be homozygous short (tt).

Predicting Gene Combinations

EXAMPLE :HETEROZYGOUS X HOMOZYGOUS

T= tall plant TALL X SHORT (Tt x tt)t = short plant

Genotype = 2 Tt; 2 ttPhenotype = 2 tall plants; 2 short plants

Probability: 2/4 tall plants; 50% tall plants 2/4 short plants; 50% short plants

T

tt

tTt tt

Tt tt

TESTCROSS

What if we only know what the parents LOOK like, but we aren’t sure of their genotypes?

• Testcross- Cross to determine the “unknown” genotype of a parent with a dominant phenotype (trait displayed).

• Use to determine if the unknown parent’s genotype is heterozygous or homozygous dominant.• Ex: A plant with green seed pods could have a

genotype of GG or Gg. We don’t know yet!!• To find out, we cross the unknown parent with a

homozygous recessive parent.

G? X gg

The results of the crosswill tell us the unknown genotype!

If only dominant offspring result- then the ? must be dominant.

If both dominant and recessive offspringresult- then the ? is recessive.

G

gg

?Gg ?g

Gg ?g

PART 3

Key Concepts

• The effects of both alleles in a genotype can show up in the phenotype.

Much of Mendel's success was due to the traits he studied. The traits Mendel picked to study were each determined by single genes (pieces of DNA). The height of the pea plants is due to a single gene that occurs in two different versions, or alleles.

But are people either tall or short?Are there only two skin colors?Do people have only blonde or brown hair?

Of course not. In humans, and in most organisms, almost all traits do not follow the patterns of heredity that Mendel found.

More Complex Patterns of Heredity

All of these traits we have looked at so far have been examples of COMPLETE DOMINANCE.

• COMPLETE DOMINANCE-one allele is totally dominant over the other allele and adheres to the Principle of Dominance.

EXAMPLE: PP and Pp = purple flower plants

The way genes control traits can be complex and interact in different ways.

More Complex Patterns of Heredity

When one gene for a certain trait is not completely dominant over the other gene, a blending effect occurs.

Incomplete dominance- a type of inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a combined phenotype (expressed physical trait).

Incomplete Dominance

Incomplete Dominance can be seen in thecolor of the flowers in four o'clock plants.

Four o'clock plants can have red flowers,white flowers, or pink flowers.

Red flowers are homozygous dominant (RR). White flowers are homozygous recessive (rr).

When four o'c1ocks that have red flowers are crossed with four o'c1ocks that have white flowers, a blending effect occurs. Heterozygous pink flowers (Rr) are produced.

Incomplete Dominance

What if we cross two pink four o’clock plants?PINK ( Rr) X PINK (Rr)

Genotype = 1RR, 2Rr, 1rrPhenotype = 1 red, 2 pink, 1 white

R

rR

rRR RrRr rr

Co-Dominance

Another pattern of heredity can occur when two equally dominant genes are present for a certain trait.

Co-dominance- both variations of the gene appearing at the same time in a heterozygous individual.

EXAMPLE: R = red R’ = white

RR = red R’R’ = white RR’ = red and white

Co-DominanceExample: a homozygous chicken with black feathers (BB) and a homozygous chicken with white feathers (WW) reproduce, the offspring is heterozygous (BW).

It shows that offspring will have some black feathers and some white feathers.

Roan Cow

Blood Type A (IAIA) X Blood Type B (IBIB)

Genotype = 4 IAIB

Phenotype = 4 Blood Type AB

IA

IB

IB

IA

IAIB IAIB

IAIB IAIB

The A and B alleles are equally dominant.A child who inherits and A allele from one parent and a B allele from the other parent will have type AB blood.

What type of dominance is this?co-dominance

FYI

PREDICTING THE RESULTS OF A DIHYBRID CROSS

• DIHYBRID CROSS- a cross between two individuals that involves two pairs of contrasting traits (looking at 2 genes).• More complicated than monohybrid because more

possible combinations.• MONOHYBRID CROSS

• 2 traits/4 possible offspring• DIHYBRID CROSS

• 4 Traits/ 16 possible offspring

Dihybrid Cross Example:Heterozygous x Homozygous

Tall, Round Plant (Tt Rr) X Short, Wrinkled Plant (TT RR)

• First, we need to determine what alleles each parent could possibly give - all possible combinations of the alleles from each trait per gamete.

• TtRr

• ttrr

TR, Tr, tR, tr

tr, tr, tr, tr

Tall, Round Plant (Tt Rr) X Short, Wrinkled Plant (tt rr)

tr tr tr tr

TR

Tr

tR

tr

TtRr

Ttrr

TtRr

Ttrr

ttRrttRr

ttrrttrr ttrr

ttRr

ttrr

ttRr

Ttrr Ttrr

TtRr TtRrGENOTYPE: 4TtRr, 4 Tt rr,

4 ttRr 4 ttrr

PHENOTYPE:4 Tall, Round4 Tall, Wrinkled4 Short, Round4 Short, Wrinkled

The alleles are carried down into boxes and across into boxes like in the smaller square. Take care to keep the alleles for each trait together inside the boxes (Here, Ts come first and stay together, Rs are next)

EXAMPLE: HETEROZYGOUS X HETEROZYGOUS

Tall, Round Plant (Tt Rr) X Tall, Round Plant (Tt Rr)

TR Tr tR tr

TR

Tr

tR

tr

TTRR

TTRr

TTRr

TTrr

TtRrTtRR

TtrrTtRr ttRr

ttRR

ttrr

ttRr

TtRr Ttrr

TtRR TtRr

GENOTYPE: 1 TTRR, 2TTRr, 2TtRR, 4TtRr, 1 TTrr, 2 Ttrr, 1ttRR, 2ttRr, 1 ttrr

PHENOTYPE:9 tall, round3 tall, wrinkled3 short, round1 short, wrinkledPhenotypic Ratio= 9:3:3:1