1 Fundamentals of Genetics Chapter 9 Topics:Mendel’s Legacy (9-1) Genetic Crosses (9-2)

2 Gregor Mendel  Austrian monk/scientist  Studied statistics (science of the collection, organization, analysis, & interpretation of data)  Studied garden pea (Pisum sativum)  Grew many generations of the plant & applied statistical analyses to results to research the transmission of characteristics from parent to offspring (heredity)

3 Understanding Pisum sativum  Pea characteristics observed: (pg 167) Plant height: long stem or short stem Flower position on stem: axial or terminal Pod appearance: inflated or constricted Pod color: green or yellow Seed (cotyledon)color: yellow or green Seed texture: smooth or wrinkled Flower Color: purple or white  Mendel noted that for each of these characteristics, the plants exhibited one of 2 possible traits  Mendel studied pea generations taking meticulous data to set the stage for genetics

4 Pisum sativum Characteristics

5 Important vocabulary terms  Genotype – genetic make-up of an organism  Phenotype – the external appearance of an organism based on the genetic make- up  Trait – characteristics of an organism, what is “seen”  P 1 generation – parental cross (mating)  F 1 generation – offspring from parental cross  F 2 generation – offspring from F 1 cross

6 Mendel’s Experimental Set-up  Anther – male reproductive part of a flower  Pistil – female reproductive part of a flower  Pollen – the male gamete, must move to fuse ovum (female gamete)  Pollination – union of pollen & ovum Self pollination – flower that self fertilizes Cross pollination – pollen from one flower fertilizes ova of another flower  Peas normally self pollinate, but cross pollination is possible Insects, weather, animals, etc all help in cross pollination of plants  Mendel “controlled” all these possibilities & controlled which pollen pollinated which ova

7 Pea Experiments  Mendel began with peas pure for the 7 traits he chose to study; purebred means that the offspring from that parent will always yield that characteristic (trait)  Each strain of pure pea he obtained for each trait he named it Parental 1 (P 1 ) generation  He then proceeded to manually cross pollinate his P 1 plants to obtain offspring, named Filial 1 (F 1 ) generation (each trait studied separately)  The F 1 plants were allowed to self pollinate creating the F 2 generation, and the seeds collected  F 2 seeds grown to see traits

8 Cross Results  One cross example: pod color Crossed ( P 1 ) pure green w/ pure yellow All offspring (F 1 ) had green pods (100% green), no yellow F 1 plants allowed to self pollinate for F 2 generation F 2 plant pods: ¾ green, ¼ yellow  All other crosses for other traits yielded similar results….. interesting

9 Cross conclusions  When Mendel saw P 1 traits fail to be seen in F 1 plants, he found that in F 2 the traits were always in a 3:1 ratio  He concluded: “something” was responsible for the trait which he called “factors” “factors” came in a pair of alternative forms controlling the trait these alternative forms were either dominate or recessive For a recessive trait to be expressed, it must have both recessive “factors” For a dominate trait to be expressed, it may have either 2 dominate “factors” or 1 dominate & 1 recessive “factor”

10 Law of Segregation  Mendel concluded that each gamete receives only 1 “factor” from each parent, so the resulting cell has 2  So, he proposed the Law of Segregation which states: A pair of factors is segregated or separated during gamete formation

11 Law of Independent Assortment  Mendel further crossed plants that differed in 2 traits P1 - flower color (purple) x seed color (yellow) Might hypothesize that F1 will all have purple flowers with yellow seeds  He found that this was not always the case The traits of the offspring do not necessarily appear together; green seeds could be from white flowered plants, etc.  He suggested the Law of Independent Assortment which states: factors for different traits are distributed to gametes independently Wait a minute! What does this sound like? Didn’t we just learn about independent assortment of something????

12 Chromosomes & genes  DNA strand composed of genes which code for a trait  Alleles – alternative forms of a gene Mendel’s “factors” are alleles  Homologous pairs of chromosomes contain the genes from maternal parent & paternal parent; each gene is an allele

13 Genotype v. Phenotype  Genotype – the alleles the organism inherited from both parents  Phenotype – the appearance of the organism; how the 2 alleles manifest themselves in the outward appearance of the organism  Alleles may be dominate or recessive (careful, all alleles are not just dom or rec)  Use letter to denote a trait; capitol letter for dominant trait & lower case letter for recessive trait

14 Alleles  Homozygous alleles – alleles are alike Homozygous dominate – 2 dom alleles Homozygous recessive – 2 rec alleles  Heterozygous alleles – alleles are different

15 How to use letters to represent alleles  Flower color in peas Flowers can be purple or white Purple flowers are dominant trait White flowers are recessive trait  Pick the letter “P” to represent flower color P is purple p is white

16 Punnett Squares  Punnett square – term for the diagram representing a parental cross to predict the possible offspring  Monohybrid cross is a cross between individuals involving a pair of contrasting traits  Look at handout of Punnett squares Carry out the crosses

17 Testcross  Test crosses are used when one is trying to determine the genotype of an individual for a specific trait.  Problem: I have a black haired (phenotype) guinea pig, what is his genotype?  I know black hair is a dominate allele, brown hair is recessive allele  So a black haired guinea pig can have a genotype of BB or Bb. How do I find out? USE A TESTCROSS  In a testcross, the unknown individual is mated with homozygous recessive individual and the offspring are counted. The only 2 possibilities of genotype are BB or Bb. So, you work out what type of offspring each cross would be and match that data to the actual offspring. The data set that matches, will “tell” you what the parental genotype is.  Try the testcross example on the handout.

18 Incomplete Dominance  Occurs when 2 or more alleles influence phenotype resulting in an intermediate phenotype between dominate and recessive  Four o’clock flowers exhibit this incomplete dominance in flower color White flowers are homozygous recessive, rr Red flowers are homozygous dominate, RR Pink flowers are hererozygous, Rr

19 Four o’clock flower crosses  P 1 cross RR (red) x rr (white) F 1 is 100% Rr (pink) F 2 (Rr x Rr cross) is 25% RR, 25%rr, 50% Rr for genotype & 1:2:1 ratio of red: pink: white for phenotype  P 1 cross RR (red) x Rr (pink) F 1 is 50% RR (red) & 50% Rr (pink)  P 1 cross rr (white) x Rr (pink) F 1 is 50% rr (white) & 50% Rr (pink)

20 Codominance  Occurs when both alleles are expressed (unblended) in phenotype of heterozygous individual  Neither allele is dominate or recessive, nor do they blend  Roan coat color in horses another example: white hair (R) & red hair (R’) so RR’ is roan (shows both white and red hairs)

21 Multiple Alleles  Some traits are controlled by 3 or more genes  This is called multiple allele inheritance  Human ABO blood type is controlled by 3 alleles

22 Polygenic Traits  Polygenetic trait – trait which is controlled by many genes  Genes may be located on same or different chromosomes  Due to independent assortment and crossing over during meiosis, many phenotypes are possible from the genotypes  Human polygenic traits include: Eye color Height Weight Hair color Skin color

23 Dihybrid Crosses  Cross between individuals involving 2 pairs of contrasting traits  Use a punnett square to predict possible offspring, but it is a bit more complicated Set up punnett squares just like for monohybrid crosses, but there will be more possibilities for gametes Do sample crosses on handout

24 Dihybrid Crosses  Some things to be careful about when doing these crosses Make sure the gamete possibilities are correct Write the offspring possibilities keeping the traits in order with capitol letter first; SsRr not SRsr Write the phenotype in the box with the genotype Count carefully, making it a fraction of x/16 for both genotype and phenotype (you may reduce fractions)

25 Quiz #1  1. Alternative forms of a gene are ____.  2. The outward characteristics of an organism are known as the _____.  3. If the genotype is RR, then it is heterozygous. T/F  4. Using the first 2 letters of the English alphabet, state the genotype for a homozygous recessive for both traits.

26 Quiz #1 (continued)  5. If one parent has the genotype Ttrr, what are the possible gametes that this parent can provide?  6 – 8. Draw and fill in the Punnett square for the following cross for the trait Curly (dominant) leaves & Straight (recessive) leaves. Cc x cc

27 Quiz #1 (continued)  9. What are the possible genotypes of the offspring from that cross?  10. What are the possible phenotypes of the offspring from that cross?  Bonus: What are the ratios of the genotypes & phenotypes from that cross?

28 Quiz #1 Answers  1. Alleles  2. Phenotype  3. False  4. aabb  5. Tr, tr  6-8. C c x cc C c c CcCc cc c CcCc

29 Quiz #1 Answers  9. possible genotypes: Cc, cc  10. possible phenotypes: curly leaves (Cc) & flat leaves (cc)  Bonus: 50% Cc, 50% cc: genotypes & for phenotypes 50% curly, 50% straight leaves