Alleles Dominant - expressed when paired with a different allele – Represented by an uppercase letter (RR) or (Rr) Recessive - no effect when paired with a dominant allele – Represented by a lowercase letter (rr)

GENOTYPE AND PHENOTYPE The genotype is the genetic constitution of the individual, in other words, the genes (and alternate forms) that are carried. Alternate forms of the same gene are called alleles. The phenotype is the observable trait (characteristic) produced by the genotype (gene).

GENERATION DESIGNATIONS The parental generation (P 1 ) is the first generation of the controlled cross. The first filial generation (F 1 ) is the result of crossing the parental generation. The second filial generation (F 2 ) is produced from the crossing of the F 1 progeny.

PUNNETT SQUARE Graphical means of visualizing a monohybrid cross and applying probability to the outcome. E.G. cross 2 heterozygous individuals [Yy] Yy Y y YYYy yy yellow green Genotypic Ratio=1:2:1 ¼ YY ½ Yy ¼ yy Phenotypic Ratio=3:1 ¾ yellow seeds ¼ green seeds

Alleles in an Individual Homozygote - same two alleles (AA or aa) Heterozygote - two different alleles (Aa)

Phenotype versus Genotype Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1

Monohybrid cross

yellow seeds X green seeds [parental generation] YY yy [P 1 ] yellow seeds [ first filial generation] Yy [F 1 ] ¾ yellow seeds [F 2 ] ¼ green seeds [second filial generation]

YYRR P Generation GametesYRyr  yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment F 2 Generation (predicted offspring) 1⁄21⁄2 YR yr 1 ⁄ 2 1⁄21⁄2 yr YYRRYyRr yyrr YyRr 3 ⁄ 4 1 ⁄ 4 Sperm Eggs Phenotypic ratio 3:1 YR 1 ⁄ 4 Yr 1 ⁄ 4 yR 1 ⁄ 4 yr 1 ⁄ 4 9 ⁄ 16 3 ⁄ 16 1 ⁄ 16 YYRR YYRr YyRR YyRr YyrrYyRr YYrr YyRR YyRr yyRRyyRr yyrr yyRr Yyrr YyRr Phenotypic ratio 9:3:3: Phenotypic ratio approximately 9:3:3:1 F 1 Generation Eggs YR Yr yRyr 1 ⁄ 4 Sperm RESULTS CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green- wrinkled seeds—were crossed, producing dihybrid F 1 plants. Self-pollination of the F 1 dihybrids, which are heterozygous for both characters, produced the F 2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. Dihybrid Cross – two characters

Dihybrid cross

DIHYBRID CROSS WITH GENOTYPES A cross involving two traits. round,yellow seeds X wrinkled, green All round, yellow [F 1 ] R=round r=wrinkled Y=yellow y=green RRYYrryy RrYy

MENDEL’S EXPERIMENT THE DIHYBRID CROSS The dihybrid cross, a cross involving two traits. round,yellow seeds X wrinkled, green All round, yellow [F 1 ] 9/16 round, yellow 3/16 wrinkled, yellow 3/16 round, green 1/16 wrinkled, green Phenotypic ratio=9:3:3:1

Incomplete Dominance: One does not completely cover the other. Halfway between two extremes, so blending of one another. -Classic example: A red and white flower is cross, so you end up with a pink flower. Codominance: There are equally strong, so nothing over powers the other. Equal in strength, you see both phenotypes. -Classic example: Cow having brown and white spots (Roam).

INCOMPLETE DOMINANCE Characterized by an absence of complete dominance in one allele. This manifests as a “blending” of traits, or a “hybrid” phenotype. Common in flower color genes.

4 O’ CLOCKS red flowers X white flowers RR X rr [ R 1 R 1 ] [R 2 R 2 ] Rr [R 1 R 2 ] F 1 pink ¼ RR [R 1 R 1 ] red ½ Rr [R 1 R 2 ] pink ¼ rr [R 2 R 2 ] white 1:2:1 genotypic 1:2:1 phenotypic When 2 heterozygotes are crossed

IDENTIFYING CHARACTERISTICS OF INCOMPLETE DOMINANCE 1.Traits are blended. 2.Crossing two heterozygous individuals in a monohybrid cross produces a 1:2:1 genotypic ratio and 1:2:1 phenotypic ratio. Incomplete dominance is an apparent exception to Mendel’s First Law because a different phenotypic ratio is obtained. The alleles are in fact segregating according to Mendel’s first law, the mechanism by which the phenotype is produced is different than in pea plants.

CODOMINANCE A codominant gene in a heterozygous individual will express the phenotype of both alleles. The phenotype of both alleles are expressed independently. ABO blood groups in humans are an example. The I gene (isoagglutinogen) has three alleles (A, B, and O). The A and B alleles are dominant to O and codominant to each other.

ABO GENOTYPES PHENOTYPE GENOTYPE ANTIGEN ANTIBODY A I A I A or I A I O A ANTI-B B I B I B or I B I O B ANTI-A O I O I O NONE BOTH AB I A I B A and B NONE

Codominance Both alleles are expressed Seen in blood types – I A I A or I A i = type A – I B I B or I B i = type B – ii = type O – I A I B = type AB ABO Blood Groups The ABO blood group in humans Is determined by multiple alleles.

Many genetic disorders – Are inherited in a recessive manner Recessively inherited disorders – Show up only in individuals homozygous for the allele Carriers – Are heterozygous individuals who carry the recessive allele but are phenotypically normal Recessively Inherited Disorders

Inheritance of Sex-Linked Genes The sex chromosomes – Have genes for many characters unrelated to sex A gene located on either sex chromosome – Is called a sex-linked gene Other sex-linked conditions Some recessive alleles found on the X chromosome in humans cause certain types of disorders – Color blindness – Duchenne muscular dystrophy – Hemophilia

Sex-linked genes follow specific patterns of inheritance

If an allele for tall plants (T) is dominant to short plants (t), what offspring would you expect from a TT x Tt cross? A. ½ tall; ½ short B. ¾ tall; ¼ short C. All tall Questions

If an allele for tall plants (T) is dominant to short plants (t), what offspring would you expect from a TT x Tt cross? A. ½ tall; ½ short B. ¾ tall; ¼ short C. All tall Questions

Fur color in rabbits shows incomplete dominance. F B F B individuals are brown, F B F W individuals are cream, F W F W individuals are white. What is the expected ratio of a F B F W x F W F W cross? A.3 white: 1 brown B.3 white: 1 cream C.2 white: 2 cream Questions

Fur color in rabbits shows incomplete dominance. F B F B individuals are brown, F B F W individuals are cream, F W F W individuals are white. What is the expected ratio of a F B F W x F W F W cross? A.3 white: 1 brown B.3 white: 1 cream C.2 white: 2 cream Questions

Height in pea plants is determined by the genes T (dominant) and t (recessive). Cross a homozygous tall pea plant with a dwarf pea plant and determine the probability of producing a tall plant. Questions - Monohybrid Cross

Height in pea plants is determined by the genes T and t. Cross two heterozygous tall plants and determine the probability of producing a dwarf plant. Questions - Monohybrid Cross

Note that blood type genotypes may be written using an "I" before the A and B, such as I A I A and I B i, etc. In this problem I’m not using "I". Hazel has type B blood (genotype BO) and Elijah has type O blood (genotype OO). If they have children, what is the probability that they will have a type B child? What is the probability they will have a type A child? In this problem you are given the genotypes so you know both genes for each blood type. Questions

When a genetic cross involves the consideration of two factors (such as shape and colour in pea seeds), the cross is called a "dihybrid". Cross a completely heterozygous round/yellow seeded plant with a completely homozygous round/green seeded plant. Then determine the probability of obtaining a round/yellow seeded plant in the offspring. R = round seeds, r = wrinkled seeds Y = yellow seeds, y = green seeds Questions - Dihybrid Cross

Questions - Page 7 - Lab Book