Mendelian Genetics & Beyond Chapter 10.2 & 10.3 Chapter 11.2

Mendelian Genetics & BeyondChapter 10.2 & 10.3

Chapter 11.2

Today, we understand much about the inheritance of traits.

But this was not always so…

150 years ago we had no knowledge of how traits were

passed from parents to offspring…..

UNTIL….

GREGOR MENDEL – 1822-1884 Born in Austria - monk - loved nature Interested in plants, meteorology and theories of

evolution

discovered 3 basic laws which govern the passage of traits

today he is known as the… “FATHER OF GENETICS”

(postmortem)

GREGOR MENDEL

studied - pea plants observed - 7 traits (each with 2 forms) by tracing these 7 traits, Mendel discovered 3 basic laws which govern the passage of traits

PEA PLANT

before Mendel could start his experiments…..

he had to get pure (or true-breeding) plants

ALWAYS produce

offspring with the same trait

Why was this important?

Mendel got pure plants

by self-pollinating

(or inbreeding)

plants for several

generations.

eventually, he had 14 pure strains

(7 traits X 2 contrasting forms)

each pure strain he called a parental generation (P)

Now, he was ready to begin his experiments

MENDEL’S EXPERIMENTS

Mendel crossed pure (P) X contrasting pure (P)

P X P = F1 (first filial generation)

RESULTSonly 1 form of the trait appeared in the

F1 generation

Mendel repeated his experiment s many times – all with the same results

then, he crossed… F1 x F1 = F2 (second filial generation)

RESULTS both forms of the trait appeared

in a ratio of 3:1

Mendel repeated his experiment s many times – all with the same results

MENDEL concluded that the patterns of inheritance are

governed by 3 principles

PRINCIPLE OF . . . . 1. Dominance and Recessiveness

2. Segregation 3. Independent Assortment

Principle of Dominance and Recessiveness

Mendel concluded…each trait is controlled by a pair (2) of

factors… a dominant factor will prevent a

recessive factor from being

expressed

PRINCIPLE OF SEGREGATION

Mendel concluded…each pair of “factors” must segregate

(separate) during the formation of gametes

So that….. only one “factor” is inherited from each

parent

Principle of Segregation

PRINCIPLE OF INDEPENDENT ASSORTMENT

Mendel concluded… the inheritance of 1 trait is

independent of the inheritance of another trait

* the factors for different traits are distributed independently from one another

* this principle requires the observation of 2 or more traits at the same time

Principle of Independent Assortment

Important Terms to know Genetics: study of how

traits are passed from parent to offspring (heredity)

Gene: unit of inheritance that usually is directly responsible for one trait or characteristic

Allele: an alternate form of a gene; formerly called factors by Mendel Represented by letters

yellow = Y purple = P

Examples of Alleles

WrinkledSmooth

Alleles can be…Homozygous or Heterozygous When alleles from each parent are the

same, they are called homozygous (pure) Written as double letters that are the same

size Ex: PP, pp, YY, yy, BB, bb

When alleles from each parent are different, they are called heterozygous (hybrid) Written as double letters that are different

sizes Ex: Pp, Yy, Bb

Alleles can be…Dominant or Recessive A dominant allele is expressed no matter

what the second allele is Represented by a capital letter

Ex: PP, Pp, YY, Yy, BB, Bb

A recessive allele is only expressed when the second allele is the same Represented by a lower case letter

Ex: pp, yy, bb

Phenotype vs. Genotype Phenotype is the

physical expression or appearance of the trait Ex. Purple flower yellow seeds blond hair

Genotype refers to the alleles (genes) pairs Ex. PP, Pp YY, Yy bb

This may be changed.

(Coloring your hair)

(Plastic surgery)

This can NEVER be changed.

Mendel’s studies Studied garden peas

because: they grew fast made lots of

offspring with short generation times

few traits that were easily seen

traits showed complete dominance

Usually self-pollinate/fertilize

7 well-defined garden pea traits

Counted offspring of each phenotype and analyzed the results mathematically – saw patterns

Trait Dominant Recessive

Flower color

Purple White

Flower position

Axial Terminal

Seed color Yellow Green

Seed texture

Smooth Wrinkled

Pod color Green Yellow

Pod texture

Inflated Constricted

Height Tall Short

You MUST KNOW THESE!!

Fig. 10.4, Mendel’s 7 garden pea characters.

Practice assigning alleles1. PP2. TT3. GG4. Gg5. ss6. AA7. Aa8. aa9. yy

1. Homozygous purple flower

2. Homozygous tall

3. Homozygous green pod

4. Heterozygous green pod

5. Homozygous wrinkled seed

6. Homozygous axial flower

7. Heterozygous axial flower

8. Homozygous terminal flower

9. Homozygous green seeds

Probability

The chance that an event will occur

Probability = # of 1 kind of event total # of events

What is the probability of… A coin landing on heads? Drawing a king from a deck of cards? Having a baby boy? A die landing on the number “3”?

Punnett Squares Dr. Reginald Punnett, early 1900s Graphical way to show probability 5 steps:

Assign P genotypes Remember: use the letter of the dominant trait

(homozygous = same size; heterozygous = different sizes)

Split alleles Perform cross Report F1 genotypes Report F1 phenotypes

Types of Crosses Monohybrid cross = cross of two different

alleles for a single trait. Ex: crossing eye color x eye color Ex: crossing hair color x hair color

Dihybrid cross = cross of two different alleles for two traits. Ex: crossing eye & hair color x eye & hair color

4 squares

16 squares

Monohybrid Crosses can be used with… Complete dominance of traits Incomplete dominance of traits Codominance of traits Sex-linked traits Multiple allele traits

Let’s Practice

Step 1. P = SS x ss

Step 2. Split alleles

S S

Ss Ss

Ss Sss

s

Step 3: perform Punnett

Step 4: F1 genotype

100% Ss

Step 5: F1 phenotype

100% smooth seeds

Some practice – Show all 5 steps1. In gerbils, brown fur is completely

dominant over white fur. Cross a heterozygous brown-furred gerbil with a white-furred gerbil.

2. Cross two heterozygous tall plants. 3. In pigs, curly tails are completely

dominant over straight tails. Cross a homozygous curly-tailed pig with a heterozygous curly-tailed pig. What are the possible phenotypes of the offspring?

INCOMPLETE DOMINANCE when neither allele is completely dominant, both alleles influence the trait there is a BLENDING of the alleles

EXAMPLE – Four O’clock Flowers red (r) and white (w) exhibit incomplete

dominance a heterozygous individual (rw) will be pink

CODOMINANCE when both alleles are dominant both alleles are fully expressed – NO BLENDING

EXAMPLE - Some cattle and horses exhibit codominance in their coat color.

red (R) and white (W) are codominant alleles in the heterozygous individual (RW), both be fully

expressed

SEX-LINKED GENES….are genes that are linked to (found on) the sex chromosomes

X chromosome is LARGE + carries many genes

genes on the X chromosome are called X linked genes

discovered by Thomas Hunt Morgan when working with the fruit fly, Drosophila melanogaster

Sex-linked Traits Males affected more often; cannot be

heterozygous Females less affected; can be

heterozygous (carriers)

Examples: Red-green colorblindnessHemophilia (blood clotting disorder)

Duchenne Muscular Dystrophy

COLORBLINDNESS a recessive trait found on the X chromosomeresults in an inability to distinguish certain colors

Duchenne MUSCULAR DYSTROPHY a recessive trait found on the X chromosomes results in the weakening and wasting away of muscle tissue

MULTIPLE ALLELES occur when there are 3 or more forms of a

trait although there are 3+ alleles, only 2 are

inherited

EXAMPLE – 3 alleles (A, B and o) influence blood type Alleles A and B are codominant. Allele i is recessive.

4 possible blood types –

What is a person’s blood type if their allele pair is?

AA, Ao, BB, Bo, AB, oo

Sometimes with multiple alleles Epistasis occurs

One allele hiding the effect of another allele

Example: coat pigmentation on animals – see p. 305

POLYGENIC TRAITSTraits that result from the interaction of several

genes (several allele pairs).

Skin color, hair color, eye color are polygenic traits.

Some ways to check yourself…. In monohybrid crosses

heterozygous x heterozygous crosses: Genotypic ratio ALWAYS = 1:2:1 Phenotypic ratio ALWAYS = 3:1

In dihybrid crosses heterozygous x heterozygous crosses:

Phenotypic ratio ALWAYS = 9:3:3:1

TRY SOME INTERACTIVE PUNNETT SQUARES @http://glencoe.mcgraw-hill.com/sites/dl/free/

0078695104/383934/BL_05.html

Dihybrid Punnett Squares Follow the same steps as in monohybrids Extra step (FOIL the alleles)

Assign P genotypes Remember: use the letter of the dominant trait

(homozygous = same size; heterozygous = different sizes)

FOIL alleles Split alleles Perform cross Report F1 genotypes Report F1 phenotypes

Practice with FOILing1. Homozygous purple & axial x white & terminal

P= PPAA x ppaaFOIL = PA PA PA PA x pa pa pa pa

2. Homozygous smooth & yellow seeds x heterozygous smooth & yellow seedsP= SS YY x Ss YyFOIL = SY SY SY SY x SY Sy sY sy

3. Heterozygous green & inflated pods x heterozygous green & inflated podsP= Gg Ii x Gg IiFOIL = GI Gi gI gi x GI Gi gI gi

Cross #1

PpAa PpAa PpAa PpAa

PpAa PpAa PpAa PpAa

PpAa PpAa PpAa PpAa

PpAa PpAa PpAa PpAa

P= PPAA x ppaaFOIL = PA PA PA PA x pa pa pa pa

PA PA PA PA

pa

pa

pa

pa

F1 genotypes:

100% PpAa

F1 phenotypes:

100% purple & axial flowers

Cross #2P= SS YY x Ss YyFOIL = SY SY SY SY x SY Sy sY sy

SSYY SSYY SSYY SSYY

SSYy SSYy SSYy SSYy

SsYY SsYY SsYY SsYY

SsYy SsYy SsYy SsYy

SY SY SY SY

SY

Sy

sY

sy

F1 genotypes:

25% SSYY, 25% SSYy

25% SsYY, 25% SsYy

F1 phenotypes:

100% smooth, yellow seeds

Cross #3P= Gg Ii x Gg IiFOIL = GI Gi gI gi x GI Gi gI gi

GGII GGIi GgII GgIi

GGIi GGii GgIi Ggii

GgII GgIi ggII ggIi

GgIi Ggii ggIi ggii

GI Gi gI gi

GI

Gi

gI

gi

F1 genotypes:

1 GGII, 2 GGIi, 2 GgII, 4 GgIi, 1 GGii, 2 Ggii, 1 ggII, 2 ggIi, 1 ggii

F1 phenotypes:

9 green & inflated pods

3 green & contricted pods

3 yellow & inflated pods

1 yellow & constricted pods

(9:3:3:1)

There should always be a 9:3:3:1 phenotypic ratio in a heterozygous x heterozygous dihybrid cross.

Polyploidy One or more extra sets of all

chromosomes in an organism Triploid organism (3n) Rarely occurs in animals Always fatal in humans Plants OFTEN exhibit polyploidy

Exhibit more vigor and size oats & wheat = 6n Sugar cane = 8n

Genetic Recombination understoodAfter studying meiosis and

After Mendel’s studies…

Why don’t you look identical to any other human being?

*the possible number of allele combinations is 223 x 223 70 trillion (not including variation from crossing over)

Scientists now use this knowledge to artificially recombine genes to breed plants and animals with desirable traits.

Gene Linkage Is an exception to the Law of Independent

Assortment Genes that are closer together on a

chromosome are more likely to travel together during gamete formation.

Scientists studied the Drosophila melanogaster (fruit fly) to demonstrate gene linkage

Chromosome maps are used to show this frequency 1 map unit = 1% cross over

CHROMOSOME MAP (GENE MAP)A “map” showing the relative location of genes on a

chromosome.The “HUMAN GENOME PROJECT” was an effort to

map the human genome (the human chromosomes).

How to map genes…Frequency between gene X and Y = 10%Frequency between gene Y and Z = 15%Frequency between gene X and Z = ?

X Y Z10 map units 15 map units