Genetics Part I: Introduction

Nestor T. Hilvano, M.D., M.P.H.

(Images Copyright Discover Biology, 5 th ed., Singh-Cundy and Cain, Textbook, 2012.)

Learning Objectives

1.

Define key terminologies. - alleles, dominant, recessive, homozygous, heterozygous, genotype, phenotype, P generation, F1 generation, and F2 generation 2.

Give the translation of the following word stems.

– co-, di-, pleio-, poly-,gen-, and gamet 3. Define monohybrid cross and illustrate an example. 4. Explain how Mendel’s law of segregation describes the inheritance of a single characteristic.

5. Explain how Mendel’s law of independent assortment applies to dihybrid cross. Illustrate an example of dihybrid cross. 6. Discuss how a testcross is performed to determine the genotype of an organism.

Wordstems

• co- together • di- two • pleio-more • poly- many • gen- produce • gamet- wife or husband

Science of Genetics

• Blending hypothesis – characteristics from both parents blend in the offspring (like blue and yellow paint blend to make green) • Particulate hypothesis - is the idea that parents pass on discrete heritable units (genes) - Gregor Mendel documented this through experiments with garden peas; Father of modern genetics • Genetics: - Trait (physical appearance specified by genes); Ex: eye color, shape of thumb, height - Types of Trait (set of genetic trait for each form); Ex: Eye color= brown eyes(B) and blue eyes (b); Thumb’s shape= straight thumb (H) and hitchhiker’s thumb (h) - Heritable trait= passed from parent to offspring

• Self-fertilization

Mendel’s Genetics

developed true breeding varieties • Cross-fertilization between 2 different plants produced hybrids (genetic cross) • Hypothesized that there are alternative forms of genes for specific trait= Alleles • Each of us has 2 alleles- ex: (BB, bb, Bb);(HH, hh, Hh) • P generation= True-breeding (parents) • F1 generation= hybrid offspring • F2 generation= product of F1 cross • Monohybrid cross -1 trait (ex: flower color: purple and white) • Dihybrid cross and green) 2 traits (flower color: purple and white, and seed color: yellow

Monohybrid Cross: Law of Segregation

• Pair of genes segregate during formation of gametes; the genes pairs once again at fertilization - Purple (PP)= (P) (P), white (pp)= (p) (p) • Homozygous genotype - identical alleles; Ex: PP (homozygous dominant); pp (homozygous recessive) • Heterozygous genotype - two different alleles (Ex: Pp) • Dominant allele (trait) capital letter; determines appearance; ex: (P) • Recessive allele (trait) -lower case; no noticeable effect on appearance; ex: (p) • Phenotype – expression of trait or appearance; ex: purple, white • Genotype – actual genetic make-up; ex: PP, Pp, or pp • Punnett square – shows the

possible combinations of alleles

(genetic cross)

♂ possible gametes ♀ possible gametes • Monohybrid cross (1 trait) Ex: Flower color, P (purple); p (white) • Possible combinations: PP (purple); Pp (purple); pp (white) • Phenotype ratio: 3:1 (3 purple:1 white) • Genotype ratio: 1:2:1 (1PP:2Pp:1pp)

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Monohybrid Cross

Ex. 1 Trait – eye color of fruitfly

dominant trait – Red (R) recessive trait – white (r) • P generation : Red (RR) x white (rr) • F1 generation? All _______________ • F2 generation? __________________ • Fill in

punnett square

(diagram for predicting the results of genetic cross). ♂ possible gametes ♀ possible gametes Genotype ratio? ___________________ Phenotype ratio? __________________

Law of Independent Assortment

• Dihybrid cross : one pair segregate independently from the second pair during gamete formation (independent assortment) • F1 produce 4 gamete genotypes • F2 generation : – 9/16 had round yellow seeds – 3/16 had wrinkled yellow seeds – 3/16 had round green seeds – 1/16 had wrinkled green seeds • Genotype ratio : 1:2:1:2:4:2:1:2:1 • Phenotype ratio: 9:3:3:1

Dihybrid Cross

• Ex. 2 traits – eye color and straight hair dominant traits – Brown eye (B) - Straight hair (S) recessive traits – blue eye (b) - curly hair (s) P generation: BBSS x bbss F1= BbSs x BbSs F1 produce 4 genotypes: BS, Bs, bS, bs F2=Phenotype ratio: 9:3:3:1 Give the phenotype of: BS= _____; Bs= _____; bS= _____; bs= _____

Testcross

• Use of the test is to identify the genotypes responsible for the phenotypes of the progenies after a cross • Crossing of unknown genotype (expressing dominant phenotype) and a homozygous recessive individual • Ex. Tall plant(T_) and dwarf plant(tt) 2 possibilities for Tall plant: TT or Tt • Do a cross of each possibility. t t T T t T t

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Homework

Define terms: genotype; phenotype; alleles; monohybrid cross; dihybrid cross; test cross; law of segregation Differentiate homozygous alleles and heterozygous alleles, and dominant trait and recessive trait.

The allele for straight hair (S) is dominant to the allele for curly hair (s). A father with straight hair who is heterozygous (Ss) has 4 children with his wife who is also heterozygous (Ss). Construct a punnett square showing the location of gametes and offspring. Identify offspring as to genotype ratio and phenotype ratio. What is the phenotypic trait (expressed appearance) of child with ss alleles? with Ss alleles?