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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
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.