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Chapter 11 MENDELIAN PATTERNS OF INHERITANCE Early Ideas about Heredity People knew that sperm and eggs transmitted information about traits Blending theory Problem: Would expect variation to disappear Variation in traits persists Gregor Mendel 1st to apply mathematics to biology Careful, delibrate Well prepared Garden Pea Easy to grow Self/cross pollination Many varieties Used discrete traits, easily observed Tracking Generations Parental generation P mates to produce First-generation offspring F1 mate to produce Second-generation offspring F2 Monohybrid Crosses Experimental intercross between two F1 heterozygotes AA X aa Aa (F1 monohybrids) Aa X Aa ? Mendel’s Monohybrid Cross Results F2 plants showed dominant-torecessive ratio that averaged 3:1 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf Fig. 11-6, p. 172 Genes Units of information about specific traits Passed from parents to offspring Each has a specific location (locus) on a chromosome Allele Different molecular forms of a gene Arise by mutation Dominant allele masks a recessive allele that is paired with it Allele Combinations Homozygous having two identical alleles at a locus AA or aa true-breeding Heterozygous having two different alleles at a locus Aa hybrids Genotype & Phenotype Genotype refers to particular genes an individual carries Phenotype refers to an individual’s observable traits Cannot always determine genotype by observing phenotype Check Progress B = black, b= white If a heterozygous rabbit reproduces with one of its own kind, what phenotypic ratio do you expect among the offspring? If there are 120 rabbits, how many expected to be white? 11.2 Mendel’s laws Law of Segregation Law of Independent Assortment Each indv has 2 alleles Each pair of alleles for each trait Alleles separate when gametes form Gametes have 1 allele for each trait Fertilization gives new indv 2 alleles assorts indepently All possible combo of alleles can occur Applies to alleles on diff c-somes Expected ratio 9:3:3:1 Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits In fruit flies, L = long wings and l = short wings; G = gray body and g = black body. List all possible gametes for a heterozygote Dihybrid Cross: F1 Results purple flowers, tall TRUEBREEDING PARENTS: AABB GAMETES: AB x white flowers, dwarf aabb AB ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall meiosis, gamete formation 1/4 AB 1/4 Ab aB 1/4 ab 1/4 AB 1/4 aB 1/4 Ab 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 1/16 AaBB 1/16 AaBb 1/16 aaBB 1/16 aaBb 1/16 AaBb 1/16 Aabb 1/16 aaBb 1/16 aabb Fig. 11-9b, p.175 Laws and Meiosis Doesn’t matter which way the homologous pair faces during metaphase I (2nd law) Metaphase II cell has one chromosome (1st law) Gametes can have all possible combinations of alleles (both laws) OR See p. 196 Test Cross Individual that shows dominant phenotype is crossed with individual with recessive phenotype Examining offspring allows you to determine the genotype of the dominant individual Autosomal Recessive Inheritance Patterns If parents are both heterozygous, child will have a 25% chance of being affected Fig. 12-10b, p. 191 Examples Methemoglobinemia Cystic Fibrous Niemann-Pick Autosomal Dominant Inheritance Trait typically appears in every generation Fig. 12-10a, p. 190 Achondroplasia Autosomal dominant allele In homozygous form usually leads to stillbirth Heterozygotes display a type of dwarfism Have short arms and legs relative to other body parts Huntington Disorder Autosomal dominant allele Causes involuntary movements, nervous system deterioration, death Symptoms don’t usually show up until person is past age 30 People often pass allele on before they know they have it ABO Blood Type Range of genotypes: IAIA IBIB or or IAi Blood Types: A IAIB AB IBi ii B O Fig. 11-10a, p.176 Codominance: ABO Blood Types Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells Two alleles (IA and IB) are codominant when paired Third allele (i) is recessive to others Also known as Multiple Alleles Incomplete Dominance X Incomplete Homozygous parent Dominance Homozygous parent •Intermediate phenotype All F1 are heterozygous •Familial hypercholesterolemia •CF heterozygous •Incomplete pentrance •polydactyly X F2 shows three phenotypes in 1:2:1 ratio The allele for albinism (c) is recessive to allele for normal pignmentation (C). A normally pigmented woman whose father is an albino marries a man whose parents are normal. They have 3 children, 2 normal and 1 albino. Give the genotypes for each person. In garden peas, one pair of alleles controls the height of the plant and a second pair of alleles controls flower. The allele for tall (D) is dominant to allele for dwarf (d), and the allele for purple (P) is dominant to the allele for white (p). A homozygous tall purple plant is crossed with a dwarf white plant. What is the appearance of F1 and F2? Epistasis Interaction between the products of gene pairs Common among genes for hair color in mammals Examples Antocyanin pigments Start molecule colorless Intermediate colorless Purple Lab coat color Start molecule melanin deposition Eye color humans Blue 1 Blue 2 Green Brown Coat Color in Retrievers BBEE X bbee F1 puppies are all BbEe F2 puppies BE Be bE be BE BBEE BBEe BbEE BbEe Be BBEe BBee BbEe Bbee bE BbEE BbEe bbEE bbEe be BbEe Bbee bbEe bbee black brown yellow 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 Polygenic A more or less continuous range of small differences in a given trait among individuals The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait Continuous Variation Variation in human eye color Eye color Calculator Fig. 11-18, p.180 Temperature Effects on Phenotype Rabbit is homozygous for an allele that specifies a heatsensitive version of an enzyme in melanin-producing pathway Melanin is produced in cooler areas of body Figure 11.16 Page 179 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 Elevation and the yarrow Fig. 11-17b, p.179 X-Linked Recessive Inheritance Males show disorder more than females Son cannot inherit disorder from his father Fig. 12-10, p.194 Thomas Morgan 1900’s Red female x white male = all red F2 = 3 red: 1 white male Supported chromosome theory of inheritance Males = hemizygous Fig. 12-9, p.193 Examples of X-Linked Traits Color blindness Hemophilia Menkes syndrome Muscular Dystrophy Adrenoleukodystrohy (ALD) Color Blindness Fig. 12-12, p.195 Color Blindness Fig. 12-12, p.195 1 XrXr 2 XRY 3 XRY 4 XRXr 5XrY 6 XRXr 7 XrY 8 XrXr 9 XrY 10XRX 11 XRY 12 XRX 13 XrY 14 XrXr 15 XrY Hemophilia Fig. 12-11, p.194