Genetic Inheritance Problems Exercise 9 Objectives Know how

Genetic Inheritance Problems - Exercise 9 Objectives -Know how to apply basic genetic terms. -Know how to do compute Punnett squares of monohybrid and dihybrid crosses. -Know how to do sex-linked crosses. -Be able to apply Incomplete Dominance and Codominance.

Genetic Inheritance Problems Genetics is the study of the hereditary material of life. The hereditary material (known as genes) is encoded as molecules of DNA on chromosomes. Genes can also be symbolized as letters, called alleles. Alleles are alternate forms of genes found at a particular sport on a chromosome. The place where a gene or allele is found is called the locus. Hereditary material (genes) in DNA on chromosomes.

In diploid animals, alleles exist in pairs. Before alleles and chromosomes are passed from parents (P 1 generation) to offspring (F 1 generation), the allelic and chromosomal pairs are separated by the process of meiosis. The result of meiosis in animals is the production of haploid gametes – egg and sperm. The alleles of the haploid gametes are then combined during reutilization to produce the diploid offspring (zygote).

Basic Terms in Genetics

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)

Alleles in an Individual • Homozygote - same two alleles (AA or aa) • Heterozygote - two different alleles (Aa) • Genotype - the genetic makeup (Is its genetic makeup) • Phenotype - observable characteristics (Is its physical appearance)

Useful Genetic Vocabulary • An organism that is homozygous for a particular gene – Has a pair of identical alleles for that gene – Exhibits true-breeding • An organism that is heterozygous for a particular gene – Has a pair of alleles that are different for that gene

Genetic Cross • • When two individuals are mated P generation - parent generation F 1 generation - first generation F 2 generation - second generation • The hybrid offspring of the P generation – Are called the F 1 generation • When F 1 individuals self-pollinate – The F 2 generation is produced

Character - a heritable feature, such as flower color Trait - a variant of a character, such as purple or white flowers • In a typical breeding experiment – Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents – Are called the P generation

Extensions of Mendel’s Laws • Many alleles do not show complete dominance – Incomplete dominance – Codominance – Epistasis – Environmental effects – Polygenic traits

• 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 The phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties P Generation Red CR CR White C WC W � Gametes CR CW Pink CR CW F 1 Generation Gametes Eggs F 2 Generation 1⁄ 2 CR 1⁄ 2 Cw 1⁄ 2 CR CR 1⁄2 CR CR CW CW CW Sperm • Heterozygote is an intermediate • Horses • Snapdragons

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 1: 2: 1 genotypic ¼ RR [R 1 R 1] red 1: 2: 1 phenotypic ½ Rr [R When 2 heterozygotes 1 R 2] pink are crossed ¼ rr [R 2 R 2] white

IDENTIFYING CHARACTERISTICS OF INCOMPLETE DOMINANCE 1. 2. Traits are blended. 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 • In codominance – Two dominant alleles affect the phenotype in separate, distinguishable ways • The human blood group MN – Is an example of codominance • Also ABO blood groups

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 IAIA or IAIO ANTI-B IBIB or IBIO B ANTI-A O IOIO NONE BOTH AB IAIB A and B NONE

A ANTIGEN BLOOD TYPE A B ANTIGEN BLOOD TYPE B BOTH A +B ANTIGENS NO ANTIGENS BLOOD TYPE AB BLOOD TYPE O

Codominance • Both alleles are expressed • Seen in blood types – IAIA or IAi = type A – IBIB or IBi = type B – ii = type O – IAIB = type AB The ABO blood group in humans Is determined by multiple alleles. ABO Blood Groups

GENES • Genes are discrete units of heredity determining biological characteristics of living things. • Genes exist in pairs in diploid organisms. • Alleles are alternate forms of the same gene, each is on a different homologous chromosome.

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).

MENDEL’S FIRST LAW SEGREGATION • Governs the behavior of alleles. • 3 important observations from Mendel’s crosses. • [1] Hybridization between two traits showed only one trait in the offspring. white flowers X purple flowers

Basic Patterns of Inheritance • Mendel started with true breeding plants • Recessive trait skipped a generation

Mendel’s First Law: Law of Segregation

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

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]

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] Genotypic Ratio=1: 2: 1 ¼ YY ½ Yy ¼ yy Y YY y Yy yellow Yy yy yellow green Phenotypic Ratio=3: 1 ¾ yellow seeds ¼ green seeds

MENDEL’S SECOND LAW INDEPENDENT ASSORTMENT • Governs the behavior of different genes. • Mendel started with two hypotheses. [1] All traits from 1 parent would be transmitted together and only two types of offspring would result. [2] Traits would be inherited independently and there would be more than two types of offspring.

Independent assortment • Using the information from a dihybrid cross, Mendel developed the law of independent assortment – Each pair of alleles segregates independently during gamete formation • Mendel identified his second law of inheritance – By following two characters at the same time • Crossing two, true-breeding parents differing in two characters – Produces dihybrids in the F 1 generation, heterozygous for both characters

Mendel’s Second Law: Law of Independent Assortment

Characteristics Studied

Dihybrid Cross – two characters EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with greenwrinkled 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. P Generation YYRR yyrr Gametes F 1 Generation YR Hypothesis of dependent assortment � yr Yy. Rr Hypothesis of independent assortment Sperm RESULTS 1 ⁄ Sperm 1⁄ CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. F 2 Generation (predicted offspring) 2 Eggs 1 ⁄ 2 YR 1⁄ YYRR yr yyrr 4 1 ⁄ YR 4 Yr 1 ⁄ 4 y. R 1 ⁄ 4 yr Eggs yr Yy. Rr 3 ⁄ 2 4 1 ⁄ 4 YR 4 Yr 4 y. R 4 yr 1 ⁄ YYRR YYRr Yy. RR Yy. Rr YYrr Yy. Rr Yyrr Yy. RR Yy. Rr yy. RR yy. Rr Yyrr yy. Rr yyrr 4 Phenotypic ratio 3: 1 1 ⁄ 9 ⁄ 16 3 ⁄ 16 1 ⁄ 16 Phenotypic ratio 9: 3: 3: 1 315 108 101 32 Phenotypic ratio approximately 9: 3: 3: 1

DIHYBRID CROSS WITH GENOTYPES • A cross involving two traits. R=round r=wrinkled Y=yellow y=green round, yellow seeds X wrinkled, green RRYY rryy All round, yellow [F 1] Rr. Yy

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

Recessively Inherited Disorders • 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

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

Questions - Page 7 - Lab Book • 1. State Mendel’s First Law. What part of meiosis is the basis for this law? • 2. State Mendel’s Second Law. What part of the meiotic process is the basis for this law? • 3. Why do we use a Punnett squares to solve genetic problems? -Can Punnett squares give us precise outcomes of an offspring?

Questions - Page 7 - Lab Book • 1. State Mendel’s First Law. What part of meiosis is the basis for this law? Two alleles from a heritable character separate during gamete formation and end up in different gametes, during Anaphase I. • 2. State Mendel’s Second Law. What part of the meiotic process is the basis for this law? Each pair of alleles segregates independently of other pairs of alleles during gamete formation, during metaphase I. • 3. Why do we use a Punnett squares to solve genetic problems? Shows all the possibilities of the combination of alleles in an offspring that results from a cross whether its monohybrid or dihybrid. • -Can Punnett squares give us precise outcomes of an offspring? No, it gives you possibilities of what could happen not actual outcomes.

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. FBFB individuals are brown, FBFW individuals are cream, FWFW individuals are white. What is the expected ratio of a FBFW x FWFW cross? A. 3 white: 1 brown B. 3 white: 1 cream C. 2 white: 2 cream

Questions - Monohybrid Cross • 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 • Note that blood type genotypes may be written using an "I" before the A and B, such as IAIA and IBi, 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 - Dihybrid Cross • 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 - Page 7 - Lab Book