GENETICS

Genetics

• Purpose: to understand how traits in our DNA are passed on (parent to child)

• Used to predict possible outcomes of a genetic cross.– This means that what we predict and what we see could

be different!

History of Genetics (as a science)

• Gregor Mendel

• “Father of genetics”

• Conducted experiments– Used phenotype and probability to determine

the principles of genetics

• Studied many plants including– Pisum sativum (peas)

Why peas (why choose this model)?

• The Garden pea - Model system to study heritability

– small– easily cultured – short life span– exhibits great variability– true-breeding strains – dominant/recessive alleles

Mendel’s Experiments

• Looked at seven characteristics– Characteristics are an inheritable factor, such

as color, size, seed texture, etc.

• Each characteristic occurred in only two contrasting traits– A trait is a genetically determined variant of a

characteristic

Question Asked:

• What will happen if I breed two different plants with different versions of a characteristic?

• Started with parents that were True breeding

• - means that when they self fertilize, all offspring look like parents (for that trait)

How do plants make offspring?

• What is “natural pollination”? – Pollination

• Pollen grains produced on anther are transferred to the stigma (top of the female reproductive system)

• Self pollination:– Pollen from a plant pollinates a stigma on the same plant

(same flower or different flower)

• Cross pollination– Pollen from a plant pollinates a stigma on a totally

different plant.

Results of pollination

• Flowers bloom- produce a pistil a stamen

• Female pistil:– Stigma (sticky top)– Style– Ovule (seeds form)

• Male stamen– Anther– Filament

Fertilization

Embryo formation

What are plant embryos?

• Seeds! These are 2N!

Mendel’s method

• Manual pollination (Selective breeding)– Occurs when anthers are removed from

the flowers of a plant (contain the pollen grains at the top). Then you choose which flowers to pollinate.

His Scientific Method• Utilized monohybrid crosses

– ONE characteristic, two alleles, selective breeding

• Carefully recorded his data (PHENOTYPES).– Parental characteristics and offspring characteristics

– 3 or more generations (P, F1, F2)

• Formed testable hypotheses.• Tested hypotheses “statistically”• Utilized seven traits in the garden pea.

Characteristics studied

• Height tall or short

• Flower position axial or terminal

• Pod color green or yellow

• Pod appearance inflated or constricted

• Seed texture smooth or wrinkled

• Seed color yellow or green

• Flower color purple or white

Parents: (both true breeding)

white x purple

Expect???

What he got:

So… he crossed two of them….

Expect???

What he got:

A carpal is another name for_________?

These crosses showed that there were “factors” being passed from parent to offspring even if it wasn’t being “used”

Now we call these factors GENES

Genes – control a heritable feature; characteristic

Example of characteristic: Hair color, seed shape, height;

Allele – controls the variation of a feature (characteristic) – AKA trait.Example of trait: brown, blonde, black hair

Characteristic/Gene?

Trait/Allele?

CHARACTERS (characteristics) AND VARIANTS (traits)

TRAITS

?

TRAITS

?

RARE DOMINANT PHENOTYPE - Polydactyly

A chromosome = folded up string of many genes

What are alleles?

Variations of a gene that occupy the same locus on homologous

chromosomes

Locus = position on a chromosome.

P

pT

t

LONG

SHORT

GENE = FLOWER COLORGENE = STEM LENGTH

Terms – apply these to genetics and Punnett squares

• Diploid (2n)• Haploid(n)• Egg• Sperm• Parent• Meiosis• Testes

• Gamete

• Zygote

• Progeny

• Offspring

• Fertilization

• Ovary

Linking vocabulary

• Review Mendel’s process and substitute in all words on the previous slide, (used in mitosis and meiosis) to describe how Mendel arrived at the F1 generation.

• Punnett squares should have been covered in grades 6-8.

• Draw a Punnett square and link terms on the previous page to the Punnett square

Mendel’s laws of genetics

1. Law of segregation: only one allele for each gene is passed from a parent to the offspring.

Why? Has to do with separation of homologous chromosomes during meiosis.

Segregation of Alleles

Tongue Rolling

2. Law of independent assortment:

Alleles for one gene are passed to offspring independently of alleles from other genes.

The result is that new combinations of genes present in neither parent is possible.

This only applies to SOME genes, not all.

3. Law of complete dominance – some alleles overpower others. So even if both alleles are present, we only “see” the dominant one.

- the “hidden” allele is called recessive

This only applies to SOME genes, not all.

Remember Mendel’s pea plants?

- Purple was crossed with white and we got ALL purple. So which allele is dominant?

Genotype: the alleles that an organism has.

- alleles are abbreviated using the first letter of the dominant trait. (with some exceptions that we will get to)

- a capital letter represents the dominant

ex: P for purple flower allele

- a lower case represents the recessive.

ex: p for white flower allele

All diploid organisms have two alleles for each trait:

- you can have two of the same alleles

Ex: PP or pp

- such an individual is described as Pure or Homozygous.

OR

All diploid organisms have two alleles for each trait:

- you can have two different alleles

Ex: Pp

- such an individual is described as hybrid or heterozygous

Phenotype: physical appearanceExamples: brown hair, widows peak

- the trait that “shows” in the case of complete dominance;

- depends on the combination of alleles

P generation: “parents;” First generation in the cross

F generations: results of the cross;

- F1 – 1st generation; offspring of P generation

- F2 – 2nd generation; offspring of F1 generation

Terminology for Genetic Crosses

Monohybrid cross: cross that focuses on the alleles of a single characteristic;

How do we show the possibilities?

- punnett square

PUNNETT SQUARE

Allele in Egg 1

Zygote formed if sperm 1

fertilizes egg 1

Allele in Egg 2

Allele in sperm 1

Allele in sperm 2

Zygote formed if sperm 2

fertilizes egg 1

Zygote formed if sperm 1

fertilizes egg 2

Zygote formed if sperm 2

fertilizes egg 2

In pea plants, tallness is dominant to short or dwarf. Cross a pure tall male to a pure dwarf female pea plant. Show both ratios phenotype & genotype for the offspring. Now cross two of the F1.

• Take it step by step until you “get it”• Step 1: what are the parent’s genotypes?

–Mom?–Dad?

tt

TT

• Step 2: Set up Punnett Square

t t

T

T

Tt Tt

TtTt

• Step 3: ANSWER THE QUESTION

t t

T

T

Tt Tt

TtTt

Offspring genotypes:

Offspring phenotypes:

• Step 4: Part II

T t

T

t

TT Tt

t tTt

Offspring genotypes:

Offspring phenotypes:

Inheritance Patterns:

Every gene demonstrates a distinct phenotype when both alleles are combined (the heterozygote)

Complete dominance is one - when both alleles are present, only the dominant trait is seen.

This is the dominance pattern seen in the characteristics Mendel used.

Other dominance patterns

• Incomplete Dominance– Still use Capital and Small letters– Heterozygous offspring ARE blended

Other Inheritance Patterns:

Incomplete dominance - when both alleles are present, the two traits blend together and create an intermediate trait (Red + White = Pink)

Codominance- When both alleles are present you see both traits of the characteristic are visible. Red + White = Red and White

INCOMPLETE DOMINANCE

Inheritance Patterns:

Co-dominance - when both alleles are present, both traits are visible

Different notation: Use first letter of the feature with a superscript for the trait. Example: CW or CB for white coat or black coat;

Inheritance Patterns:

Co-dominance - when both alleles are present, both traits are visible

Inheritance Patterns:

Each gene has a specific inheritance pattern. - you will either be told or be given a hint; look at the heterozygote!

Still more inheritance patterns

• Sometimes depend on the gender (male/female)

• Reason: males have “non-homologous” sex chromosomes

Women have two X’s but men only have one.

How do we deal with the genes on the X chromosome?

Sex-linked traitAlleles for the trait are located on the X chromosome in humans.

- works the same in women as all the other traits.

BUT –

- men only inherit one such allele.

Sex-linked traitFor females: have to figure out phenotype based on inheritance pattern.

For Males: phenotype is that of whatever allele they inherit.

Example: color blindness

Seeing color (XC) is dominant to being color blind (Xc)

Identify the sex and trait of the following:

XCY XCXc XCXC

XcXc XcY

Example: Color Blindness

Set up a punnett square crossing a heterozygous normal female with a normal male:

- what is mom’s genotype?

- what is dad’s genotype?

- what gametes can each give?

- what are the offspring’s geno’s?

XC

Cross Number 1:

Xc

XC

Y

XCXC XCXc

Xc YXC Y

What % chance of having color blind daughter?

Son?

SEX-LINKED TRAITS

COLOR BLINDNESS

AFFLICTS 8% MALES AND 0.04% FEMALES.

If we are dominant, how can we figure out our genotype?

What are the possibilities?

Test cross: a cross that determines genotype of dominant parent

- Cross unknown dominant parent (possibilities BB or Bb)

with a recessive parent

then analyze the offspring.

B ?

b

b

Bb ?b

?bBb

If some of the offspring have the recessive trait, then the unknown parent has to be heterozygous

B ?

b

b

Bb ?b

?bBb

If all offspring are dominant, unknown parent HAS to be

homozygous

Multiple alleles: Some genes have more than two variations that exist, although we still only inherit 2

Example: Human blood types

Three alleles:

IA

IB

i

Genotype Phenotype

IAIA A

IAi A

IBIB B

IBi B

IAIB AB

ii 0

Polygenic –

Multiple genes each with 2 alleles

Creates additive/

quantitative effect

SKIN PIGMENTATION

Dihybrid cross:

A cross that focuses on possibilities of inheriting two traits

- two genes, 4 alleles

Black fur is dominant to brown fur

Short fur is dominant to long fur

What is the genotype of a guinea pig that is heterozygous for both black and short fur?

Dihybrid cross:

Parent genotypes: BbSs x BbSs

Figure out the possible gametes:

Then set up punnett square

Dihybrid cross:

BS Bs bS bs

BS

Bs

bS

bs

Linked Genes: genes that are on the same chromosome.

Does the law of independent assortment apply?

Can they be separated? Will they always separate?

Remember??? X-linkage

• X-linkage– Transmission of genes located on the x-

chromosome.

– Males are hemizygous (half) for all alleles on the x-chromosome.

• One recessive = recessive phenotype

Designating X-linked alleles

• Xh XH Y has no alleles–You must specify

–The chromosome (X or Y)–The gender (shown by XX or XY)–Whether the transferred allele is

dominant or recessive

X-linked Genes

• Found ONLY on X chromosomes

• Most diseases are recessive

• One “disease allele” causes disease in males

• In females, two recessive required

Gene Linkage

• Genes located on the SAME chromosome and that tend to be inherited together

• Linked genes do not follow expected inheritance patterns. No independent assortment

Gene Linkage

• Crossing over– DOES “unlink” genes

• Genes which are very close (in position) termed “highly linked”.

• Linkage seen in one phenotype/genotype being seen at a different frequency.

Linkage and Mapping in Prokaryotes and Bacterial Viruses

• Bacteria often used to study Linkage– Short generation time– Less genetic material than eukaryotes– Single “naked” chromosome– Haploid– All mutations are expressed.

Pleiotropy

• A single gene protein– Protein affects more than one phenotypic

characteristic

– Albinism andCrossed eyes

Length of rightand left leg!

Pleiotropy

• Legs are “symmetric” due to pleitropy!

Multiple Gene Interactions

• One gene affects another– On-off– Darker lighter– The combination of alleles phenotype

Epistasis

• One character affects expression of another (on-off switch)

– Recessive expression in one gene second is turned off

• Labrador retrievers• Black, chocolate and yellow labs

Labrador Retrievers

• Color of fur: – Black (BB or Bb) with “black gums” – Chocolate (bb) with “red gums”

• How do we get a Yellow lab?– Second gene (E gene)– If the E gene is recessive for both alleles (ee)

– pigment is not “expressed” in coat (but can be seen in “gums”

Red nose

Black nose

What are the

possible genotypes for each of the dogs?

Example 2

• Could two black horses produce a white horse?

• Could a white horse be homozygous dominant for the color gene? Heterozygous?

• Could you produce a black horse from the mating of a white horse and a tan horse

X-inactivation

• See Calico Cat handout

Pedigrees

• Used to study past matings and transference of specific disease alleles

Pedigree Symbols

Pedigrees and “disease”

• Pedigrees are used to look for “patterns in certain diseases.

Diseases – autosomal recessive

• Cystic Fibrosis

• Who must the alleles be inherited from?

• Can you have an allele, but not the disease?

Cystic Fibrosis

• 1 in 20 to 30 people

• Chloride transport is affected– Thick mucous forms

• Why does the gene “survive”?• Heterozygotic Advantage? (

http://bric.postech.ac.kr/science/97now/98_5now/980506b.html)

• Protection from typhoid fever??

PKU

• 1 in 50 people affected

• Symptom: mental retardation– High phenylalanine low tyrosine

• Affects neuron development (brain cells)

• Prevention: proper nutrition, infant testing• Must have low/no phenylalanine in diet

• Phenotype is affected by environment!!!!

Sickle Cell

• 1 in 10 African Americans almost 1:2 in some African countries

• REASON: protects against malaria (heterozygous)

• This is a good case of “natural selection” maintaining the allele

X-linked recessive disease

• Pedigrees often show a predominance of males with the disease, and females as carriers

• For a female to show the disease, her father must have the disease and her mother a carrier

• A male ALWAYS inherits the disease allele from her mother.

Sickle Cell Disease

Pedigrees – Autosomal Dominant diseases

• Autosomal Dominant– Requires ONE gene to show illness– Frequency the same in Male/Female

• 50% chance offspring will inherit from either parent

Example – Huntington’s Disease

• Late onset – 30s or 40s

• Progressive nerve system damage

– How many generations are shown?– Does HD show up in each generation?

• Could it skip a generation and then show up again? Explain

– What gender could be a carrier?

Practicing with Pedigrees

• Use the handout called “Hemophilia in the Descendants of Queen Victoria”.

• What type of trait is hemophilia?

• Who are carriers?

Practicing with Pedigrees

• Go through the Pedigree handout, answering all questions.

• When you are finished…answer the following:

• What patterns do you see in the pedigrees that can tell you whether the pattern is due to a recessive, dominant, or X-linked allele?

Dominant trait pedigrees

• Every Aff. Ind has at least one Aff. Parent

• Affected x unaffected 50% affected

• Affected x Affected -> 75% affected

• Generations usually not skipped– No “hidden” genes

X-linked pedigrees

• Affected males can have no affected parents

• Only a female can be a carrier of an X-linked allele

• Males are more likely to have the disease than females

Recessive

• Affected individuals may have 2 carrier parents (unaffected)

• Affected x affected 100% affected• Can be “masked” for several generation

through carriers.

History of Blood Transfusions

• 1901 discover human blood groups• 1907 cross-matching of blood suggested• 1914 preservative to allow longer preservation of blood• 1939 Rh blood groups• 1940 National blood banks established• 1947 ABO blood typing done on all donated blood• 1961 platelet concentration (chemotherapy patients• 1971 Apheresis (collect cells, not plasma)• 1983 Figured out AIDS could be transmitted• 1992 Testing of Donor blood for AIDS implemented

X-inactivation (Barr Bodies

• X-chromosome inactivation occurs early in embryonic development. In a given cell, which of a female's X chromosomes becomes inactivated and converted into a Barr body is a matter of chance (except in marsupials like the kangaroo, where it is always the father's X chromosome that is inactivated). After inactivation has occurred, all the descendants of that cell will have the same chromosome inactivated. Thus X-chromosome inactivation creates clones with differing effective gene content. An organism whose cells vary in effective gene content and hence in the expression of a trait, is called a genetic mosaic.

• http://www.nature.com/scitable/topicpage/x-chromosome-x-inactivation-323

Extranuclear Inheritance

• Maternal Inheritance-DNA from the mitochondria (chloroplast) determines offspring phenotype.

•  • Maternal Effect- Phenotypic effects on the

offspring produced by factors transmitted through the egg cytoplasm.

• Patterns (traits) established during early development.

Creating a Chromosome Map

• Determined by # of crossovers that occur– # of crossovers proportional to distance

between the genes

Genetics and Evolutionary Change

• Evolutionary change is caused by– Changes in the genetic composition of a

population• Alleles DO NOT occur with the same frequency

• Population genetics– Link between genetics and evolution

What do we know

• Individual’s genetics do not change

• BUT…– A population’s allele frequency can change

over time?

• WHAT CREATES CHANGE?