# Unit 6 Genetics Chapters 14-15.

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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
Know your genotypes PpYyRr x Ppyyrr P= purple, p= white flowers Y=yellow, y=green seeds 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 Rubric Results 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 yyrr Y- yellow R-round y-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.

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

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

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

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
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.