1. Mendel and Heredity
Chapter 8

2. 8.1 The Origins of Genetics
Heredity is the passing of characters from parents to offspring
Used throughout history to alter crops and domestic animals
Gregor Johann Mendel – Austrian Monk
Used pea plants and bred different varieties
Developed rules to accurately predict patterns of heredity

3. Why Peas? 2 characters have clearly different forms
Character = inherited characteristic (color)
Trait = single form of character (purple)
Male and female reproductive parts are in same flower
Can control fertilization
Flower can fertilize itself (self-fertilization) or can cross pollen from 1 plant to another (cross-pollination)
Peas are small, grow easily, mature quickly, and produces many seeds so results obtained quickly

4. Traits Expressed as Simple Ratios
Mendel started by looking at 1 characteristic (monohybrid), such as color, with 1 pair of contrasting traits, purple or white flowers
Only allowed plants to self-pollinate for many generations
True-breeding – all offspring show only 1 trait
Parental (P) generation
Cross pollinated 2 P generation plants with contrasting traits
Offspring called filial (F1) generation
Counted numbers of each trait

5. Allowed F1 generation to self pollinate
Offspring called F2 generation
Each characterized and counted

6. Mendel’s Results
F1 showed only 1 form of character other had disappeared
When F1 self pollinates other trait reappears in some of F2
Found ratio of traits to be 3 to 1
3 white flowers to 1 purple flower
Same ratio found for any trait he studied

7. 8.2 Mendel’s Theory We used to think offspring were blend of traits
Tall x short = medium
Mendel’s experiments showed us this is not entirely true

8. Mendel’s Hypothesis
There are 2 copies of a gene, one from each parent, for each inherited characteristic
There are different versions of genes called “alleles”
Tall or short
When both versions are present one may be dominant (completely expressed) and the other may be recessive (not expressed when dominant is present)
When you form gametes, alleles separate independently so only one allele in each gamete

9. Mendel’s Finding in Modern Terms
Use letters to show alleles
Capitol = dominant (T, P, Y, etc…)
Lower case = recessive (t, p, y, etc…)
Homozygous = letters are same
Homozygous dominant = TT, PP
Homozygous recessive = tt, pp
Heterozygous = letters are different
Tt, Pp
Only dominant allele is expressed

10. Genotype = set of alleles
What you actually have
TT, Tt, or tt
Phenotype = what is expressed
How it looks
Tall, Tall, or Short

11. Mendel’s Laws of Heredity
Law of Segregation
2 alleles for a character segregate when gametes are formed
Behavior of chromosomes during meiosis
Law of Independent Assortment
1 character does not affect another
Alleles of different genes separate independently of on another
Now know this only applies to genes located on different chromosomes or that are far apart on same chromosome

12. 8.3 Studying Heredity: Punnett Squares
Breeders want certain characteristics when they breed (cross) animals
Horticulturists produce plants with specific characteristics

13. Punnett Square Used to predict outcomes
Shows all possible combinations of gametes
Put 1st parents genotype on top
Put 2nd parents genotype on side
Do the cross TT Tt tt Tt

14. So in Mendel’s F1 generation, a pure Tall plant bred with a pure short plant can only give 1 kind of offspring due to dominance of tall allele

15. Determining Unknown Genotypes
How do you know if a tall plant is homozygous or heterozygous? They both look tall
Can do a Test Cross
If dominant phenotype is shown with unknown genotype, cross it with homozygous recessive

16. Test Cross Results
If unknown is homozygous dominant, all offspring of test cross will have dominant trait

17. Test Cross Results
If unknown is heterozygous, offspring of test cross will have 2 dominant and 2 recessive phenotypes

18. Can use probability calculations to predict results of genetic crosses
Probability is the likelihood a specific event will occur
Probability = # of 1 kind of possible outcome divided by total number of possible outcomes
We will express these as fractions
Chance a coin will come up heads
1 head / 2 sides = ½

19. DD = ¼
Dd = 2/4 or ½
dd = ¼

20. Dihybrid Cross
Uses a Punnett Square to determine outcomes of 2 traits at one time
Example: Surface and Color
Surface: RR, Rr, or rr round or wrinkled
Color: YY, Yy, yy yellow or green
What are the possible combinations?
RY, Ry, rY, ry

21. So if you have 2 purebred homozygous parents RRYY and rryy and you mate them, what do you get?
All offspring will be RrYy
What if you have F1 breed?
Make a Punnett Square of possible gametes for each parent
What possible combos can parents offer?
Do you remember FOIL?
RY, Ry, rY, ry

23. You never have to count the results of a dihybrid cross between heterozygotes!
9 with both dominant traits
3 with first dominant and second recessive
3 with first recessive and second dominant
1 with both recessive traits
So 9:3:3:1

24. Inheritance of Traits Pedigree
Family history that shows how a trait is inherited over several generations
Helpful in tracking genetic disorders
Carrier – have allele for trait but show no symptoms

26. Things You Can Find From A Pedigree
Autosomal or Sex-Linked?
If autosomal it will be equal in both sexes
If sex linked generally only found in males
Y linked
Hairy ear rims
X linked
Color-blindness
Hemophilia

27. Dominant or recessive
Autosomal Dominant – every individual with condition will have parent with condition
Achondroplasia – type of dwarfism
Huntington’s Disease – brain degenerates
Autosomal Recessive – 1, 2, or no parents with condition
Cystic fibrosis
Sickle cell anemia
Albinism

28. Heterozygous or Homozygous
Autosomal homozygous dominant or heterozygous phenotype will show dominant allele
Homozygous recessive will show recessive allele
2 heterozygous of recessive allele don’t show condition but can have children that do

29. 8.4 Complex Patterns of Heredity
Complex Control
Most of the time characters display much more complex patterns than simple dominant-recessive patterns
Characters can be influenced by several genes

30. Polygenic Inheritance
Several genes affect a character
These genes may be scattered along same chromosome or on different chromosomes
Determining the effect of any one gene is difficult
Crossing over and independent assortment create many different offspring combos
Eye color, height, weight, hair, intelligence, and skin color
Usually gives a range of expression

31. Polygenic Inheritance

34. Intermediate Characters
Incomplete dominance
Phenotype that is intermediate between 2 parents, neither is completely dominant
White x red = pink
Straight hair x curly hair = wavy hair

35. Multiple Alleles Characters controlled by genes with 3+ alleles
Humans have ABO blood types
IA, IB, i
Letters A and B refer to carbohydrates on surface of red blood cells
i has neither carbohydrate
IA and IB are dominant over I, but not over each other (codominant)
Still only 2 possibilities in a person

36. Blood Types 2 forms are displayed at the same time
Codominance – both expressed, not blended
IAIB both expressed
ii = Type O

37. Characters Influenced by Environment
Plants may change color based on pH of soil
Arctic fox
Summer – enzymes produce pigments for darker fur
Winter – no enzymes, no pigments to darken fur
Siamese cats
Dark fur in cooler parts
Humans
Height related to nutrition
Skin color based on sun exposure
Twins are genetically identical, any difference is due to environment

38. Genetic Disorders
Proteins encoded by genes must function precisely for normal development and function
Genes may be damaged or copied wrong causing faulty proteins
Mutation = changes in genetic material
Rare because cells try to correct errors
Harmful effects produced by inherited mutations
Many carried by recessive alleles

39. Sickle Cell Anemia Recessive genetic disorder
Mutated allele produces defective form of hemoglobin causing red blood cells (rbc) to be misshapen
These rupture easily causing less O2 to be carried and may get stuck and cut off blood supply
Recessive allele protects heterozygous individuals from malaria
Parasites in sickle rbc die
Normal rbc still transport oxygen

40. Cystic Fibrosis
Most common fatal, hereditary, recessive disorder in Caucasions
1 in 25 has at least 1 copy of defective gene that makes a protein needed to move chloride in and out of cells
Mucus clogs organs
1 in 2,500 homozygous for cystic fibrosis
No cure

41. Hemophilia Impairs bloods ability to clot Sex-linked
Dozen+ genes code for clotting proteins
1 mutation on X chromosome causes Hemophilia A
Males only get 1 X chromosome

42. Huntington’s Disease dominant allele on autosome
1st symptoms - mild forgetfulness and irritability in 30’s and 40’s
Eventually lose muscle control, spasms, severe mental illness, and death

43. Treating Genetic Disorders
Most can’t be cured
Genetic Counseling – tells of possible genetic problems with offspring, may be treated if early enough

44. Phenylketonuria (PKU)
Lack enzyme that converts amino acid phenylalanine into tyrosine so it builds up in the body and causes severe mental retardation
Can be placed on phenylalenic diet

45. Gene Therapy Replace defective genes with normal ones
Isolate copy of gene
Put working copy into a virus
Virus infects and puts gene in
Infected cells are cured
Still trying to get this to work