Mendelian Genetics

Why the garden pea? “Model Organism” “Model Organism” Easy to grow Easy to grow Easy to self or cross-fertilize Easy to self or cross-fertilize Cross-fertilization may be prevented by bagging the flower Cross-fertilization may be prevented by bagging the flower Grows to maturity in one season Grows to maturity in one season

Genetic terms Homozygous (true breeding) Homozygous (true breeding) 2 identical alleles for a gene 2 identical alleles for a gene Heterozygous (hybrid) Heterozygous (hybrid) 2 different alleles for a gene 2 different alleles for a gene Dominant Dominant allele that is expressed in a heterozygote allele that is expressed in a heterozygote Recessive Recessive allele that is masked in a heterozygote allele that is masked in a heterozygote

Chapter 13 Genetic Terms: Genetic Terms: P (parental) P (parental) parental generation of a cross parental generation of a cross F1 (first filial) F1 (first filial) the first generation after the parental (the results of the first cross) the first generation after the parental (the results of the first cross) F2 (second filial) F2 (second filial) a cross between F1 individuals yields F2 a cross between F1 individuals yields F2

Mendel’s Law of Segregation Alleles Alleles alternative forms of genes alternative forms of genes 2 alleles for the same gene 2 alleles for the same gene Each parent passes one allele for each gene to his/her offspring Each parent passes one allele for each gene to his/her offspring What process does this law represent? What process does this law represent? Meiosis Meiosis What stage of meiosis? What stage of meiosis? Anaphase I Anaphase I

The test cross How can you determine the genotype of a dominant individual? How can you determine the genotype of a dominant individual? Sheep - white is dominant to black Sheep - white is dominant to black W = white and w = black (use dominance to assign letter) W = white and w = black (use dominance to assign letter) How can you tell if white sheep is homozygous (WW) or heterozygous (Ww)? How can you tell if white sheep is homozygous (WW) or heterozygous (Ww)?

Conduct a test cross, where the unknown dominant individual is crossed with the known recessive. Conduct a test cross, where the unknown dominant individual is crossed with the known recessive. W ___ x ww W ___ x ww W ? w w

Punnett Squares Monohybrid cross (Ss x Ss) Monohybrid cross (Ss x Ss) RATIO?

Ratios Genetic crosses occur in predictable ratios. Genetic crosses occur in predictable ratios. A homoz. dominant and a homoz. recessive always produce 100% heterozygotes A homoz. dominant and a homoz. recessive always produce 100% heterozygotes A heterozygote cross always produces a 1:2:1 GENOTYPIC ratio, 3:1 PHENOTYPIC ratio A heterozygote cross always produces a 1:2:1 GENOTYPIC ratio, 3:1 PHENOTYPIC ratio

Probability & Product Rule Probability Probability In a heterozygous cross for flower color (Pp), what is the probability of the offspring being heterozygous? Homozygous dominant? Homo recess? In a heterozygous cross for flower color (Pp), what is the probability of the offspring being heterozygous? Homozygous dominant? Homo recess? FOIL FOIL Pp x Pp ¼ ¼ + ¼ = ½

Law of Independent Assortment The odds of two different traits occurring together in the same offspring is the product of each independent trait occurring in gametogenesis The odds of two different traits occurring together in the same offspring is the product of each independent trait occurring in gametogenesis In which phase of meiosis does this event occur? In which phase of meiosis does this event occur? Metaphase I Metaphase I What assumption did Mendel make for this law to work? What assumption did Mendel make for this law to work? The traits occur on different chromosomes! The traits occur on different chromosomes!

Dihybrid Cross R = redr = white R = redr = white T = tallt = short T = tallt = short What is the F 2 phenotypic ratio of offspring? What is the F 2 phenotypic ratio of offspring? Punnett Square Approach Punnett Square Approach Fork Lined Approach Fork Lined Approach

What is the probability that a cross between 2 organisms with genotypes AaBbCc and AaBbCc will produce an offspring with aabbcc? What is the probability that a cross between 2 organisms with genotypes AaBbCc and AaBbCc will produce an offspring with aabbcc? Take each trait’s individual probability Take each trait’s individual probability 1/4 x 1/4 x 1/4 = 1/64 1/4 x 1/4 x 1/4 = 1/64

Incomplete Dominance Not all traits show the typical “dominant/recessive” patterns of alleles Not all traits show the typical “dominant/recessive” patterns of alleles Incomplete dominance – heterozygotes show a distinct intermediate phenotype different from homozygous genotypes Incomplete dominance – heterozygotes show a distinct intermediate phenotype different from homozygous genotypes In snapdragons, red snapdragons crossed with white and produce all pink flowers. In snapdragons, red snapdragons crossed with white and produce all pink flowers.

Flower color of snapdragons A cross between a C W C W (white) plant and a C R C R (red) plant will produce all pink F 1 offspring A cross between a C W C W (white) plant and a C R C R (red) plant will produce all pink F 1 offspring Self-pollination of the F 1 offspring produces 25% C W C W, 25% C R C R, and 50% pink offspring Self-pollination of the F 1 offspring produces 25% C W C W, 25% C R C R, and 50% pink offspring

Dominant Alleles Organismal level Organismal level Biochemial level Biochemial level Molecular level Molecular level Complete dominance  Incomplete Dominance  Codominance Complete dominance  Incomplete Dominance  Codominance

Codominance Codominance – 2 alleles affect the phenotype in separate, distinguishable ways Codominance – 2 alleles affect the phenotype in separate, distinguishable ways The "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms The "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms Curly hair = homozygous for curly hair alleles Curly hair = homozygous for curly hair alleles Straight hair = homozygous for straight hair alleles Straight hair = homozygous for straight hair alleles Heterozygous individuals have wavy hair Heterozygous individuals have wavy hair

Multiple Alleles

Terms Plietropy = multiple effects from a single gene Plietropy = multiple effects from a single gene Epistasis = one locus alters the phenotypic expression of a second gene Epistasis = one locus alters the phenotypic expression of a second gene Polygenic inheritance = additive efffect of 2 or more genes on a single trait Polygenic inheritance = additive efffect of 2 or more genes on a single trait

Pleiotropy

Epistasis

Polygenic Inheritance

Environment

Genetic Screening Carrier recognition Carrier recognition Fetal testing: Fetal testing: Amniocentesis Amniocentesis CVS CVS Ultrasound Ultrasound Fetoscopy Fetoscopy

Genetic Disorders Tay Sachs Tay Sachs Cystic Fibrosis Cystic Fibrosis Huntington’s Disease Huntington’s Disease Sickle Cell Anemia Sickle Cell Anemia 1/400 African Americans 1/400 African Americans Cause? Cause? Single sub of AA in hemoglobin Single sub of AA in hemoglobin