Presentation is loading. Please wait.

Presentation is loading. Please wait.

Chapter 8 Introduction to Genetics

Similar presentations

Presentation on theme: "Chapter 8 Introduction to Genetics"— Presentation transcript:

1 Chapter 8 Introduction to Genetics

2 The Work of Gregor Mendel
What is Genetics? the study of heredity Gregor Mendel’s Peas Pollen: plant’s sperm Egg Cells: plants reproductive cells Fertilization: joining of pollen + egg cells  develops intos embryo in a seed Born in 1822. His work with pea plants laid the foundation for Genetics.

3 Working with pea plants…
Self-pollinating: pollen fertilizes egg cells in the SAME flower (single parent reproduction) True-breeding: offspring genetically identical to parents due to self-pollination Cross-pollination: combining reproductive cells from 2 DIFFERENT parent plants Mendel could cross- breed a purple flower with a white flower… What do you think is the color of the offspring?

4 Genes and Dominance TRAIT: specific characteristic (seed color, plant height, etc) What did Mendel do in his pea plant experiments? Studied 7 different traits each with contrasting characters (ex) Height, short or tall He crossed the plants (with contrasting characters) and looked at their offspring

5 P = parental generation = original pair of plants
F1 = first filial generation= first generation Hybrids: offspring from parents with different traits

6 Tracking Generations Parental generation P mates to produce
First-generation offspring F1 mate to produce Second-generation offspring F2

7 P GENERATION: purple x white flowers F1 GENERATION: all purple flowers
CROSS-POLLINATION: Mendel cut the male parts of one flower (ouch!) and dusted the female parts with pollen from another flower. P GENERATION: purple x white flowers F1 GENERATION: all purple flowers HYBRID PLANTS

8 Inheritance is determined by factors that are passed down
What happened in Mendel’s crosses? All the offspring only had one of the parent’s characters…the other parent’s character disappeared!! Mendel’s Conclusions: Inheritance is determined by factors that are passed down GENES: the factors that determine traits Contrasting characters are different forms of a gene called ALLELES

9 Mendel’s Principle of Dominance
some alleles are dominant, some are recessive DOMINANT ALLELE: form of trait that will always be exhibited; usually expressed in capitals RECESSIVE ALLELE: form of trait is only exhibited when the dominant allele is NOT present (ex) Allele for tall is dominant for and the allele for short is recessive

10 What happened to the recessive allele?
Mendel wanted to know if the recessive allele disappeared from the F1 plants. F1 CROSS: He self-crossed the F1 generations to make F2 offspring

11 THE F2 GENERATION… The recessive traits reappeared!!
~¼ plants had white flowers, the recessive trait Summary of Crosses: tall plants X short plants  tall plants P P F1 tall plants self-pollinating  ¼ short, ¾ tall F1 Cross F2

12 Explaining the F1 Cross Why did the recessive allele reappear? At some point, the recessive allele had to separate from the dominante allele. This is called… SEGREGATION: separation of alleles  occurs during formation of gametes (eggs & sperm) in anaphase II of meiosis F1 plants inherited 1 tall allele & 1 short allele from parents When gametes are formed, the two alleles segregate from each other  each gamete has 1 copy of each gene So, 2 different types of gametes are formed (one w/ tall allele, one w/ short allele)


14 Probability & Punnett Squares
Mendel realized that the principles of probability can explain the results of genetic crosses. PROBABILITY: likelihood an event will occur (ex) Flip coin 3x in a row, 1/8 chance it will be heads all 3 times ( ½ x ½ x ½ ) The pattern in which alleles segregate is random…just like a coin flip! So which ever allele gametes receive is also random.

15 Punnett Squares What is a Punnett Square?
A diagram showing the possible genetic combinations from a particular cross Can be used to predict and compare the genetic variations that will result from a cross What do the letters represent in a punnett square? Letters represent alleles: capital = dominant lowercase = recessive Homozygous: has two identical alleles for a trait (ex) TT or tt Heterozygous: has two different alleles for the same trait (ex) Tt

16 Punnett Square for TT x Tt
Punnett Square for YY x yy

17 Genotype vs Phenotype GENOTYPE: the genetic makeup of an organism (ex) TT PHENOTYPE: the physical characteristics exhibitied (ex) tall plant In the Punnett Square shown What is the genotype of the offspring? What is the phenotype?

18 Probability and Segregation
F2 generation from Tall F1 plants  ¾ tall, ¼ short 3:1 ratio of tall to short plants Punnet squares work to predict outcomes, so Mendel’s ideas about segregation are accurate!

19 Exploring Mendelian Genetics
Does the gene that determines flower color have anything to do with the gene for height? (ex) Do all tall plants have purple flowers? Mendel performed TWO-FACTOR CROSSES: crossing 2 different genes and following traits as they pass from one generation to the next

20 Two-Factor Cross: F1 Two Genes: shape of pea & color of pea
The Cross: Round yellow peas x wrinkled green peas RRYY x rryy What are the possible alleles parent 1 can pass? RY What are the possible alleles parent 2 can pass? ry Draw a Punnett Square for this cross. All F1 were RrYy (round and yellow) or HYBRIDS This cross does not answer question, but provides hybrids for next cross

21 Two-Factor Cross: F2 F1 Generation = RrYy How would these alleles segregate when F1 self-pollinated? RrYy x RrYy Do the two dominant alleles stay together? What are the possible alleles each parent can pass on? There are 4 possible combinations: Ry, RY, rY, ry Draw a Punnett Square for this cross.

The F1 Hybrid cross produces a 9:3:3:1 phenotype ratio Mendel found that the 2 alleles (seed shape & seed color) don’t influence each other’s inheritance This is called the principle of Independent Assortment: genes for different traits can segregate independently during the formation of gametes

23 Independent Assortment
Metaphase I: A A a a A A a a B B b b b b B B Metaphase II: A A a a A A a a B B b b b b B B Gametes: B B b b b b B B A A a a A A a a 1/4 AB 1/4 ab 1/4 Ab 1/4 aB

24 Summary of Mendel’s Principles
Inheritance of characteristics is determined by genes which are passed to offspring If 2+ alleles of a trait exist, some alleles may be dominant, others may be recessive Sexually reproducing organisms have 2 copies of each gene which segregate during gamete formation Alleles for different genes segregate independently

25 Beyond Dominant and Recessive Alleles
Genetics is more complicated Some alleles are neither dominant nor recessive Many traits are controlled by multiple alleles or multiple genes

26 Other Inheritance Patterns…
Incomplete Dominance Codominance Multiple Alleles Polygenic Traits

27 Incomplete Dominance Homozygous parent (RR) X Homozygous Parent (rr) When one allele is not completely dominant; recessive allele is not totally masked Heterozygous phenotype is in between the two homozygous phenotypes (ex) Red snapdragon flowers (RR) X snapdragon white (rr) flowers  pink hybrid flowers (Rr) All F1 are heterozygous X F2 shows three phenotypes in 1:2:1 ratio

28 Incomplete Dominance X All F1 offspring heterozygous for flower color:
homozygous parent X homozygous parent All F1 offspring heterozygous for flower color: Cross two of the F1 plants and the F2 offspring will show three phenotypes in a 1:2:1 ratio:

29 Codominance Both alleles contribute to the phenotype
Heterozygous genotype expresses both phenotypes (ex) Feather colors in chickens: white feathers X black feathers  speckled chicken (ex) Horse coats: red X white roan coat

30 Codominance: ABO Blood Types
Alleles that controls blood type are codominant Two alleles A & B are both exhibited when paired, a third allele (i) is recessive to others AA or Ai = Type A Blood BB or Bi = Type B Blood AB = Type AB Blood ii = Type O Blood

31 Multiple Alleles > 2 possible alleles for a gene
Individuals can still only have 2 alleles each but more than 2 alleles exist in a population (ex) coat color in rabbits  lots of options due to 4 different alleles (ex) blood type is determined by multiple alleles

32 Polygenic Traits Traits controlled by the interaction of 2+ genes
(ex) Fruit fly eye color (3+ different genes) (ex) Skin color in humans (4+ different genes), eye color, height, weight

33 Applying Mendel’s Principles
Early 1900’s Morgan used Mendel’s principles to study fruit flies  advanced study of genetics Mendel’s Principles also apply to study the inheritance of human traits and to calculate the probability of traits appearing in the next generation.

Download ppt "Chapter 8 Introduction to Genetics"

Similar presentations

Ads by Google