PowerLecture: Chapter 11 Observing Patterns in Inherited Traits
Early Ideas about Heredity Blending theory Problem: Variation in traits persists
Gregor Mendel Fig. 11-2, p.170
Genes Fig. 12-2, p.188
Allele Combinations Homozygous Heterozygous
A pair of homologous chromosomes, each in the unduplicated state (most often, one from a male parent and its partner from a female parent) A gene locus (plural, loci), the location for a specific gene on a chromosome. Alleles are at corresponding loci on a pair of homologous chromosomes A pair of alleles may be identical or nonidentical. They are represented in the text by letters such as D or d Three pairs of genes (at three loci on this pair of homologous chromosomes); same thing as three pairs of alleles Fig. 11-4, p.171
Genotype & Phenotype Genotype - the particular genes an individual carries Phenotype - an individual’s observable traits
Probability The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached
Tracking Generations Parental generation P mates to produce First filial F1 mate to produce Second filial F2
Experimental intercross between Monohybrid Crosses Experimental intercross between two F1 heterozygotes AA X aa Aa (F1 monohybrids) Aa X Aa ?
Monohybrid Cross Illustrated True-breeding homozygous recessive parent plant homozygous dominant An F1 plant self-fertilizes and produces gametes: F1 PHENOTYPES F2 PHENOTYPES aa Aa AA A a Monohybrid Cross Illustrated Figure 11.7 Page 173
Mendel’s Monohybrid Cross Results 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow F2 plants showed dominant-to-recessive ratio that averaged 3:1 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf Fig. 11-6, p. 172
F2 Dominant-to- Recessive Ratio Trait Studied Dominant Form Recessive Form F2 Dominant-to- Recessive Ratio SEED SHAPE 5,474 round 1,850 wrinkled 2.96:1 SEED COLOR 6,022 yellow 2,001 green 3.01:1 POD SHAPE 882 inflated 299 wrinkled 2.95:1 POD COLOR 428 green 152 yellow 2.82:1 FLOWER COLOR 705 purple 224 white 3.15:1 FLOWER POSITION 651 long stem 207 at tip 3.14:1 STEM LENGTH 787 tall 277 dwarf 2.84:1 Fig. 11-6, p.172
Mendel’s Theory of Segregation An individual inherits a unit of information (allele) about a trait from each parent During gamete formation, the alleles segregate from each other
Mendel’s Theory of Segregation Homozygous dominant parent Homozygous recessive parent Mendel’s Theory of Segregation (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Fig. 11-5, p.172
Test Cross dominant phenotype crossed with recessive phenotype Examining offspring determines genotype of dominant individual
Punnett Squares of Test Crosses True-breeding homozygous recessive parent plant F1 PHENOTYPES aa True-breeding homozygous dominant parent plant Aa Aa a a A Aa Aa AA A Aa Aa Aa Aa Fig. 11-7b1, p.173
Punnett Squares of Test Crosses An F1 plant self-fertilizes and produces gametes: F2 PHENOTYPES Aa AA Aa A a A AA Aa a Aa aa Aa aa Fig. 11-7b2, p.173
Dihybrid Cross: F1 Results purple flowers, tall white flowers, dwarf TRUE- BREEDING PARENTS: AABB x aabb GAMETES: AB AB ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall
Dihybrid Cross: F2 Results AaBb X AaBb 1/4 AB 1/4 Ab 1/4 aB 1/4 ab 9/16 purple-flowered, tall 1/4 AB 1/16 AABB 1/16 AABb 1/16 AaBB 1/16 AaBb 3/16 purple-flowered, dwarf 3/16 white-flowered, tall 1/4 Ab 1/16 AABb 1/16 AAbb 1/16 AaBb 1/16 Aabb 1/16 white-flowered, dwarf 1/4 aB 1/16 AaBB 1/16 AaBb 1/16 aaBB 1/16 aaBb 1/16 AaBb 1/16 Aabb 1/16 aaBb 1/16 aabb 1/4 ab
Independent Assortment Mendel concluded that “units” one trait assorted independently of the “units” for the other trait homologous chromosomes are sorted at random during meiosis
Independent Assortment Metaphase I: OR A A a a A A a a B B b b b b B B Metaphase II: A A a a A A a a B B b b b b B B Gametes: B B b b b b B B A A a a A A a a 1/4 AB 1/4 ab 1/4 Ab 1/4 aB
(n is the number of gene loci at which the parents differ) Tremendous Variation Number of genotypes possible in offspring due to independent assortment and hybrid crossing is 3n (n is the number of gene loci at which the parents differ)
Impact of Mendel’s Work Mendel presented his results in 1865 Paper received little notice Mendel discontinued his experiments in 1871 Paper rediscovered in 1900
Dominance Relations Complete dominance Incomplete dominance Codominance
Incomplete Dominance X All F1 offspring heterozygous for flower color: homozygous parent X homozygous parent All F1 offspring heterozygous for flower color: Cross two of the F1 plants and the F2 offspring will show three phenotypes in a 1:2:1 ratio: Fig. 11-11, p.176
Codominance: ABO Blood Types ABO type Gene codes for enzyme that dictates structure of a blood cell glycolipid Two alleles (IA and IB) are codominant when paired Third allele (i) is recessive to others
ABO Blood Type Range of genotypes: IAIA IBIB or or IAi IAIB IBi ii Fig. 11-10a, p.176
ABO and Transfusions Recipient’s immune system attacks blood cells with unfamiliar glycolipid on surface Type O is universal donor because it has neither type A nor type B glycolipid
Pleiotropy Alleles at a single locus may have effects on two or more traits Marfan syndrome - Mutation in gene for fibrillin affects skeleton, cardiovascular system, lungs, eyes, and skin
Epistasis Interaction between the products of gene pairs Common among genes for hair color in mammals
Epistasis Fig. 11-13, p.177
Coat Color in Retrievers (Epistasis) BBEE X bbee F1 puppies are all BbEe F2 puppies BE Be bE be BE BBEE BBEe BbEE BbEe black Be BBEe BBee BbEe Bbee brown bE BbEE BbEe bbEE bbEe yellow be BbEe Bbee bbEe bbee
Comb Shape in Poultry rose comb pea comb walnut comb single comb X RRpp pea rrPP F1 all walnut RrPp RrPp RrPp F2 9/16 walnut RRPP, RRPp,RrPP, or RrPp 3/16 rose RRpp or Rrpp 3/16 pea rrPP or rrPp 1/16 single rrpp Fig. 11-12, p.177
Crossing Over
Linkage Groups Genes on one type of chromosome travel together Degree of linkage may vary
Full Linkage A B a b x Parents: A B a b AB ab F1 offspring: All AaBb meiosis, gamete formation Equal ratios of two types of gametes: A B a b Figure 11.15 Page 178 50% AB 50% ab
Incomplete Linkage AC ac A C a c x Parents: A C a c F1 offspring: All AaCc meiosis, gamete formation A a A a Unequal ratios of four types of gametes: C c c C parental genotypes recombinant genotypes Figure 11.15 Page 178
Crossover Frequency Proportional to the distance that separates genes B C D Crossing over will disrupt linkage between A and B more often than C and D In-text figure Page 178
Linkage Mapping in Humans Linkage maps based on pedigree analysis through generations Color blindness and hemophilia are very closely linked on X chromosome
Environmental Effects on Plant Phenotype Hydrangea macrophylla Action of gene responsible for floral color is influenced by soil acidity Flower color ranges from pink to blue
Environmental Effects on Plant Phenotype Fig. 11-17a, p.179
Environmental Effects on Plant Phenotype Fig. 11-17b, p.179
Temperature Effects on Phenotype This Rabbit is homozygous for allele producing heat-sensitive version of an enzyme in melanin-producing pathway Melanin is produced in cooler areas of body Figure 11.16 Page 179
This Siamese cat, raised in a cold environment in Moscow in the late 20s, developed a relatively dark coat. An area on his shoulder was shaved, and the cat wore a warm jacket while the fur was growing back. When the shaved hair grew back in, it was white, the same color as the cat's belly, due to the increased temperature under the jacket. This was not due to scarring, as the hair grew in normally colored later.
Campodactyly: Unexpected Phenotypes Effect of allele varies: Bent fingers on both hands Bent fingers on one hand No effect Many factors affect gene expression
Continuous Variation Relatively continuous range of differences in a given trait among individuals More genes and environmental factors affect trait - more continuous variation
Human Variation Some human traits occur as a few discrete types Attached or detached earlobes Many genetic disorders Other traits show continuous variation Height Weight Eye color
Continuous Variation Variation in human eye color Fig. 11-18, p.180
Describing Continuous Variation Range of values for the trait Number of individuals with some value of the trait Range of values for the trait Number of individuals with some value of the trait (line of bell-shaped curve indicates continuous variation in population)
Describing Continuous Variation Fig. 11-20, p.181
Fig. 11-21, p.183