Mendels Laws copyright cmassengale 1 Results of Monohybrid

Mendel’s Laws copyright cmassengale 1

Results of Monohybrid Crosses 1. Inheritable factors or genes are responsible for all heritable characteristics 2. Phenotype is based on Genotype 3. Each trait is based on two genes, one from the mother and the other from the father. 4. True-breeding individuals are homozygous ( both alleles) are the same. copyright cmassengale 2

Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds) copyright cmassengale 3

Law of Dominance copyright cmassengale 4

Law of Segregation • During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. • Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. copyright cmassengale 5

Applying the Law of Segregation copyright cmassengale 6

Law of Independent Assortment • Alleles for different traits are distributed to sex cells (& offspring) independently of one another. • This law can be illustrated using dihybrid crosses. copyright cmassengale 7

Dihybrid Cross • A breeding experiment that tracks the inheritance of two traits. • Mendel’s “Law of Independent Assortment” • a. Each pair of alleles segregates independently during gamete formation • b. Formula: 2 n (n = # of heterozygotes) copyright cmassengale 8

Question: How many gametes will be produced for the following allele arrangements? • Remember: 2 n (n = # of heterozygotes) 1. Rr. Yy 2. Aa. Bb. CCDd 3. Mm. Nn. Oo. PPQQRrss. Tt. Qq copyright cmassengale 9

Answer: 1. Rr. Yy: 2 n = 22 = 4 gametes RY Ry r. Y ry 2. Aa. Bb. CCDd: 2 n ABCD ABCd a. BCD a. BCd = 23 = Ab. CD ab. CD 8 gametes Ab. Cd ab. CD 3. Mm. Nn. Oo. PPQQRrss. Tt. Qq: 2 n = 26 = 64 gametes copyright cmassengale 10

Dihybrid Cross • Traits: Seed shape & Seed color • Alleles: R round r wrinkled Y yellow y green • Rr. Yy x Rr. Yy RY Ry r. Y ry All possible gamete combinations copyright cmassengale 11

Dihybrid Cross RY Ry r. Y ry RY Ry RRYY RRYy Rr. YY Rr. Yy Round/Yellow: 9 RRYy RRyy Rr. Yy Rryy Round/green: 3 Rr. YY Rr. Yy Rryy r. Y rr. Yy ry rr. Yy rryy copyright cmassengale wrinkled/Yellow: 3 wrinkled/green: 1 9: 3: 3: 1 phenotypic ratio 12

Dihybrid Cross Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9: 3: 3: 1 copyright cmassengale 13

Test Cross • A mating between an individual of unknown genotype and a homozygous recessive individual. • Example: bb. C__ x bbcc • • • BB = brown eyes Bb = brown eyes bb = blue eyes • • • CC = curly hair Cc = curly hair cc = straight hair b. C b___ bc copyright cmassengale 14

Test Cross • Possible results: bc b. C b___ C bb. Cc or copyright cmassengale bc b. C b___ c bb. Cc bbcc 15

Summary of Mendel’s laws LAW DOMINANCE SEGREGATION INDEPENDENT ASSORTMENT PARENT CROSS OFFSPRING TT x tt tall x short 100% Tt tall Tt x Tt tall x tall 75% 25% Rr. Gg x Rr. Gg round & green x round & green 9/16 3/16 pods 1/16 pods copyright cmassengale tall short round seeds & green pods round seeds & yellow wrinkled seeds & green wrinkled seeds & yellow 16

Incomplete Dominance • F 1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties. • Example: snapdragons (flower) • red (RR) x white (rr) r r • • RR = red flower R rr = white flower R copyright cmassengale 17

Incomplete Dominance r r R Rr Rr produces the F 1 generation All Rr = pink (heterozygous pink) copyright cmassengale 19

Incomplete Dominance copyright cmassengale 20

Codominance • When both alleles contribute to the phenotype of an organism Ex. Speckled Chickens

Codominance • Two alleles are expressed (multiple alleles) in heterozygous individuals. • Example: blood type • • 1. 2. 3. 4. type A type B type AB type O = = IAIA or IAi IBIB or IBi I AIB ii copyright cmassengale 22

Codominance Problem • Example: homozygous male Type B (IBIB) • x heterozygous female Type A (IAi) IA i IB I AI B I Bi copyright cmassengale 1/2 = IAIB 1/2 = IBi 23

Another Codominance Problem • Example: male Type O (ii) x female type AB (IAIB) IA IB i I Ai I Bi copyright cmassengale 1/2 = IAi 1/2 = IBi 24

Codominance • Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? • boy - type O (ii) X girl - type AB (IAIB) copyright cmassengale 25

Codominance • Answer: IA IB i i I AI B ii Parents: genotypes = IAi and IBi phenotypes = A and B copyright cmassengale 26

Human Blood Types Phenotype Genotype A IAIA or IAi B IBIB or IBi AB IAIB O ii

Question • A man is suing his wife on grounds of infidelity. The man claims that the child is blood type O and therefore must be fathered by someone else. Can he use this evidence in court if he and his wife both have heterozygous B genotypes? • Show the cross of the two parents

Know that : • Human hair is inherited by incomplete dominance. Human hair may be curly (CC) or straight (cc). The heterozygous genotype (Cc) produces wavy hair. Show a cross between two parents with wavy hair

Curly Hair

Early geneticists reported that curly hair was dominant and strait hair was recessive. More recent scientists believe that more than one gene may be involved.

Polygenic Traits • Traits controlled by two or more genes Ex. ) eye color, skin color

Sex-linked Traits • Traits (genes) located on the sex chromosomes • Sex chromosomes are X and Y • XX genotype for females • XY genotype for males • Many sex-linked traits carried on X chromosome copyright cmassengale 33

Sex-linked Traits Example: Eye color in fruit flies Sex Chromosomes fruit fly eye color XX chromosome - female Xy chromosome - male copyright cmassengale 34

Sex-linked Trait Problem • Example: Eye color in fruit flies • (red-eyes male) x (white-eyes female) XRY x X r • Remember: the Y chromosome in males does not carry traits. • RR = red eyes Xr Xr • Rr = red eyes • rr = white eyes XR • XY = male • XX = female Y copyright cmassengale 35

Sex-linked Trait Solution: Xr XR XR Xr Y Xr XR Xr Xr Y 50% red eyes female 50% white eyes male copyright cmassengale 36

Female Carriers copyright cmassengale 37

Genetic Practice Problems Breed the P 1 generation • tall (TT) x dwarf (tt) pea plants t t T T copyright cmassengale 38

Solution: tall (TT) vs. dwarf (tt) pea plants t t T Tt Tt produces the F 1 generation T Tt Tt All Tt = tall (heterozygous tall) copyright cmassengale 39

Breed the F 1 generation • tall (Tt) vs. tall (Tt) pea plants T t copyright cmassengale 40

Solution: tall (Tt) x tall (Tt) pea plants T t T TT Tt tt produces the F 2 generation 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt 1: 2: 1 genotype 3: 1 phenotype copyright cmassengale 41

Genetics and the Environment • The characteristics of any organism, is not only determined by the genes it inherits • Characteristics are determined by interactions between genes and the environment • Ex. genes may affect a plants height but the same characteristic is influenced by climate, soil conditions and availability of water.

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