Unit 6 Genetics

Chapters 14-15

Let me tell a story

Once upon a time* there was a monk who enjoyed gardening. He wondered why some plants were tall while others were short. He also wondered why some peas were green and others were yellow. So he started experimenting in his garden….

*1857

Basic terms

Character- an inherited feature w/multiple variations

Trait- each variant True breeding- create all offspring with the

same traits Hybridization- mating/crossing of 2 true-

breeding individuals

Pic 14.2

Mendel’s 4 Hypotheses

1. alternate versions of genes (alleles) account for variations in inherited characters

Mendel’s 4 Hypotheses

2. for each character, an organism inherits two alleles, one from each parent.

3. If the two alleles differ, then the dominant allele is fully expressed in the organisms appearance, the recessive allele has no noticable effect on the organism’s appearance.

Mendel’s 4 Hypotheses

4. two alleles for each character segregate during gamete production

LAW OF SEGREGATION

Old Vocab reminder

Homozygous Heterozygous Phenotype Genotype Testcross- homozygous recessive +

dominant trait (unk if it is homo or hetero)

Testcross

Dihybrid crosses

Remember law of independent assortment– Every possible combination

What ratio of results is always produced with a dihybrid cross when homo. Dominant + homo recessive are used?

__________________

Pic 14.7

Practice Punnett Squares

Using the human traits discussed in class, we will make up scenarios and use punnett squares to solve the problems.

Using Math to skip punnett squares

1. Know your genotypes

2. PpYyRr x Ppyyrr

3. P= purple, p= white flowers

4. Y=yellow, y=green seeds

5. R=round, r=wrinkled seeds

2. Know the question: the probability of having 2 recessive traits

3. List all genotypes that would match this result

ppyyRr, ppYyrr, Ppyyrr, PPyyrr, ppyyrr

Probability lesson

2 traits- purple or white flowers

Chance the sperm will have purple allele = ½

Chance the sperm will have the white allele = ½

Same for ovum (egg)

For genotype pp ½ x ½ = ¼ probability

that this genotype will form

When things aren’t so simple

Mendel used all traits that were simple dominant or recessive, this is not that common in complex organisms

INCOMPLETE DOMINANCE:

Heterozygous organisms have a phenotype between that of the parents.

Common Misinterpretation

Dominant means “able to overpower” Dominant does NOT mean good OR more

common

Polydactyly is dominant over 5 digits/appendage. 1/400 in US born with it

Some types of dwarfism also dominant over average height.

Codominance- human blood groups

3 groups based on presence of proteins Proteins called M and N Homozygous is M or N Heterozygous is MN (both present)

Practice- blood type problems

Importance of blood types

Giving the wrong type can be deadly

Polygenic inheritance

Additive effect of several genes together Quantitative characters- traits that show

some point in a range instead of on/off Human skin color AABBCC- very dark aabbcc- very light

Genetics and Environment

The environment can sometimes determine what phenotype shows up.

2 flowers with the same genotype Different acidity results in different colors

Multifactorial

Genetics and environments influence the phenotype

Pedigrees

Informal Research RubricResults will be presented and discussed in class and collected ___________

You DO NOT need to write a formal essay for this .

Name of Inherited Disorder _______________ Direct Known Cause of Disorder _____________ Prevalence of Disorder ____________ How is the disorder passed on to other individuals? _____ What does the disorder to do the human body? _________ Is there any treatment to help those affected by the

disorder?________ Any particularly interesting facts you want to share? ____

Inherited Disorders

Cystic fibrosis 1/25 caucasians is a carrier Normal allele makes a membrane protein

that functions in movement of chloride ions Defective or absent in people with cystic

fibrosis Mucus builds up in lungs, digestive tract More bacterial infections

Untreated- fatal by age 5

Treated- live into the 20’s, some longer

Tay Sachs disease Dysfunctional enzyme fails to break down lipids

in the brain Infant seizures, blindness, loss of motor and

mental skills Unusually high number in pop. Ashkenazic Jews

whose ancestors are from central europe 1/3600 in that pop, 100x more than other

populations

Fatal within early childhood- 5 or younger.

Sickle cell anemia

1/400 african americans Substitution of 1 amino acid in hemoglobin Codominance- carriers have the sickle cell

trait- get the symptoms when blood oxygen is low for a long time

Causes resistance to malaria

DOMINANT inherited diseases

ACHONDROPLASIA- 1/10,000 people Most people are homozygous recessive

HUNTINGTON’S DISEASE

Degenerative disease of nervous system No phenotype noticable until ages 35-45 Fatal, not reversible Passed on because people have children

before they know they have the disorder

Multifactoral Disorders

Heart disease

Diabetes

Cancer

Alcoholism

Manic depressive disorder

INDEPENDENT EVENTS

The probability of passing on a trait good, bad, or indifferent is the same for each offspring!

Testing for Carriers

Is the future going to be like it is in Gattaca?

Should you have children if you have a 50% risk of passing on a genetic disorder?

Are you likely to inherit a disorder?

Amniocentesis

Amniocentesis

Done at 14-16 weeks into pregnancy Identify disorders from the amniotic fluid or

fetal cells that are floating in the fluid Results take 1-2 weeks

Chorionic Villus Sampling

Done at 8-10 weeks into pregancy Placenta cells taken and divide very quickly Results in 24 hours

Other pre-natal tests

Ultrasounds fetoscopy

PKU and Newborn screening

Phenylkenonuria- 1/10,000 -1/15,000 births in U.S.

Inability to break down amino acid phenylalanine

Too much of it can build up and result in mental retardation

Solution- diet low in phenylalanine

CHAPTER 15

Chromosomal Basis of Inheritance

In 1900 scientists doing research on plants discovered that Mendel had recorded the same discoveries about inheritance 35 years earlier.

LESSON OF THE DAY- DO YOUR RESEARCH FIRST!!!!!

Chromosomal Theory of Inheritance

The genes Mendel studied have specific loci on chromosomes

Chromosomes independently assort and segregate during meiosis

The first experimental Mutant in Thomas Morgan’s experiments

Drosophila melanogaster

Fruit fly Harmless Breed quickly and in large numbers 3 pairs autosomes, 1 pair sex chromosomes

Wild type has red eyes, the “normal” phenotype

SAY CHEESE!!

WILD TYPE

Sex Linked Traits

White eyes in flies had to be on the X chromosome

Morgan’s Evidence

P- Male white eye mutant, Female wild type F1- 100% red eye flies, male or female

– White must be recessive F2- 100% females red eyes,

– 50% males white eyes– 50% males red eyes

– Eye color trait must be on X chromosome

Punnett Square Evidence

Linked genes

Each chromosomes has 100’s or 1000’s of genes

Genes often overlap on chromosomes Genes near each other on a chromosome are

usually passed on together as a group

Genetic Recombination

Production of offspring with new combinations of traits

Parental types- offspring that have the same phenotype as parents

Recombinants- offspring with different phenotypes as parents

YyRr x yyrrY- yellow R-roundy-green r- wrinkled

Complete the punnett square to see what the seeds of the next generation will look like.

How many are recombinants? How many are the parental types?

Correct Results

50% recombinants Therefore 50% frequency of recombination

Any 2 genes on different chromosomes have this recomb. frequency

Linked genes can become recombinant in crossing over

Genetic maps

Alfred Sturdevant calculated where genes were based on recombinant frequencies

Closer together = smaller frequency of recomb.

Called Linkage Maps

1 map unit= 1% recombination frequency

Aka centimorgan

Genes on 1 chromosome with 50% r.f. or higher are equivilent to being on different chromosomes.

Cytological maps

Show actual location of genes on chromosome based on visible traits

Mapping Human Genes: HGP

I have a more updated human gene map in the classroom…but still several years old.

Sex Chromosomes

X and Y rarely cross over and do not match but count as homologous chromosomes

Gene SRY discovered 1990 w/o SRY gonads form into ovaries SRY is a trigger causing many other genes

to operate

Not all organisms use the XY system, but they have variations in sex chromosomes to identify genders.

Sex Linked Traits-aka X linked traits

On X chromosome resulting is distinct patterns of expression in Males vs. females

Trends in Sex Linked traits

If the trait is sex linked and recessive you will see it more in males because they only have 1 x chromosome

Less common in females because they can inherit the dominant allele and hide the recessive trait

Human Sex Linked Traits

Color blindness Hemophilia Duchenne muscular dystrophy

Hemophilia records

X Inactivation

Females have 2 X chromosomes, but one condenses and becomes a Barr Body

Most genes on barr body are never activated At the time of X inactivation each cell

present randomly “assigns” one X to become the barr body.

Therefore not all cells show the same phenotype

How is X inactivated?

Methyl group (CH3) attached to cytosine in DNA

Only one X has XIST gene (X inactive specific transcript)

Causes the DNA to be covered in RNA Starts the inactivation Still don’t know it all though

Chromosome errors

Nondisjunction- tetrads/chromatids fail to separate in Meiosis I or II

Aneuploidy- abnormal # chromo. Trisomic- 3 copies of a chromo. Monosomic- only 1 copy of a chromo. Polyploidy- more than 2 complete chromo.

Sets (common in plants)

Mutations to Chromosome structure

Specific Cases

Down Syndrome- trisomy 21

Klinefelter Syndrome- XXY

Turner Syndrome- XO

Cri du Chat- deletion in chromosome 5, but normal 46 chromo.

Chronic Myelogenous Leukemia- part of #22 switched with part of #9

More complications

Sometimes the phenotype depends on which parent provided the allele

Prader Willi and Angelman syndromes

Both caused by deletion of a specific section of #15

Prader-Willi- deletion from father– Mental retardation, obesity, short stature, small hands

and feet Angelman- deletion from mother

– Spontaneous uncontrollable laughter, jerky movements, motor skills problems, mental disabilitiesa

Genomic imprinting

Allele on one chromosome is silenced Caused by methylation, same as X

inactivation

Fragile X Syndrome

Piece of the X chromosome is dangling on 1/1500 males, 1/2500 females The most common genetic cause of mental

retardation More commonly inherited from mom

Passing On Mitochondrial DNA

Does not follow mendelian rules Passed on from mother Rare human disorders caused by this Mitochondrial myopathy- weakness,

intolerance of exercise, muscle deterioration May contribute to alzheimers disease

Clearly, we could spend more time on genetics! But, I have MUCH more to cover in the world of biology this year!

This is a summary of what the college boards states is on the AP exam.