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Mendel and Heredity Chapter 8.

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Presentation on theme: "Mendel and Heredity Chapter 8."— Presentation transcript:

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

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