Observable Patterns of Inheritance and Human Genetics Chapter 10 and 12
Gregor Mendel: Father of Genetics Ch. 10 Strong background in plant breeding and mathematics Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring
Ch. 10 Genes Units of information about specific traits Passed from parents to offspring Each has a specific location (locus) on a chromosome
Ch. 10 Alleles Different molecular forms of a gene Arise by mutation Dominant allele masks a recessive allele that is paired with it
Ch. 10 Genetic Terms A pair of homologous chromosomes A gene locus A pair of alleles Three pairs of genes
Ch. 10 Allele Combinations Homozygous having two identical alleles at a locus AA or aa Heterozygous having two different alleles at a locus Aa
Ch. 10 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. WHY????
Ch. 10 The Garden Pea Plant a good subject for research Self-pollinating True breeding (different alleles not normally introduced) Can be experimentally crosspollinated
Ch. 10 Tracking Generations Parental generation mates to produce P First-generation offspring F 1 mate to produce Second-generation offspring F 2
Ch. 10 F 1 Results of One Monohybrid Cross
Ch. 10 F 2 Results of Monohybrid Cross
Mendel s Monohybrid Ch. 10 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green Cross Results 882 inflated 299 wrinkled 428 green 152 yellow F 2 plants showed dominant-torecessive ratio that averaged 3:1 705 purple 224 white 651 axial 207 at tip 787 tall 277 dwarf
Law of Dominance If one allele is dominant to another, then only the dominant allele will show in the phenotype of a heterozygous individual. Ex. If black fur is dominant to white fur, and B = black, and b = white, an animal with genotype Bb will have the phenotype of black fur.
Ch. 10 Punnett Square of a Monohybrid Cross Male gametes A a Female gametes A a AA Aa Aa aa Dominant phenotype can arise 3 ways, recessive only one
Law of Segregation Pairs of alleles separate (segregate) when gametes are formed, so that ½ of the alleles for your traits come from one parent, and the other ½ of the alleles come from the other parent. This is why our Punnett squares have 2 columns and 2 rows!
Ch. 12 Genetic Abnormality A rare, uncommon version of a trait Polydactyly Unusual number of toes or fingers Does not cause any health problems View of trait as disfiguring is subjective Genetic Disorder
Genetic Disorder Inherited conditions that cause mild to Ch. 12 severe medical problems Why don t they disappear? Mutation introduces new rare alleles In heterozygotes, harmful recessive allele is masked, so it can still be passed on to offspring
Autosomal Recessive Ch. 12 Inheritance Patterns parents both heterozygous, child will have a 25% chance being affected b. Examples:PKU, cystic fibrosis, albinism
Ch. 12 a.trait typically appears in every generation b. Examples: Huntington disorder, polydactyly, achondroplasia, progeria Autosomal Dominant Inheritance
Huntington Disorder Ch. 12 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
Test Crosses Mate the organism with unknown genotype with an organism of known genotype. See if the resulting offspring show only dominant trait or some show the recessive trait. Results from there will tell you what your mystery genotype is. Ch. 10 + = + =
Ch. 10 Punnett Squares of Test Crosses Homozygous recessive a a Homozygous recessive a a A Aa Aa A Aa Aa a aa aa A Aa Aa Two phenotypes All dominant phenotype
Ch. 10 Dominance Relations Complete dominance Incomplete dominance Heterozygote phenotype is a blend somewhere between that of two homozygotes Codominance Non-identical alleles specify two different phenotypes that are both expressed in heterozygotes
Ch. 10 Flower Color in Snapdragons: Incomplete Dominance Red-flowered plant X White-flowered plant (homozygote) (homozygote) Pink-flowered F 1 plants (heterozygotes)
Incomplete Dominance both genetic traits combine and form a median. Ch. 10 + =
Ch. 10 Flower Color in Snapdragons: Incomplete Dominance Pink-flowered plant X Pink-flowered plant (heterozygote) (heterozygote) Red, pink-, and white-flowered plants in a 1:2:1 ratio
Incomplete dominance in Cats Tail Length Ch. 10
Ch. 10 Codominance There is a single gene with two allelle variations, but both equally strong so in heterozygous both traits show + =
Sickle-Cell Anemia: Human example of Codominance Two alleles: Normal hemoglobin; and sickle cell hemoglobin 1) N = Normal hemoglobin chain, so normal Red blood cells 2) S = Mutant allele for defective hemoglobin, so sickle shaped red blood cells NN = NS = SS =
Ch. 10 Multiple Alleles: Having More than 2 possible alleles for a trait: Ex. ABO Blood Types: Three Alleles (*Note: you still inherit only 2 alleles, it is just more phenotypes are possible.). Gene that controls ABO blood type codes for a particular protein (Immunoglobulin Antigen) on blood cells Two alleles (I A and I B ) are codominant when paired Third allele (i) is recessive to others
Ch. 10 Type A - I A I A or I A i Type B - I B I B or I B i ABO Blood Type: Allele Combinations Type AB - I A I B Type O - ii
Ch. 10
Ch. 10 Genetics Problem A man with type A blood marries a woman with type B blood. Their child has type O blood. What are the genotypes of these individuals? What other genotypes, and in what frequencies, would you expect in offspring from this marriage?
Ch. 10 Dihybrid Cross: Studying 2 traits at the same time. Genotypes will have 4 alleles represented, 2 per trait. TRUE- BREEDING PARENTS: purple flowers, tall white flowers, dwarf AABB x aabb GAMETES: AB AB ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall
Ch. 10 F 1 Results of Mendel s Dihybrid Crosses All plants displayed the dominant form of both traits We now know: All plants inherited one allele for each trait from each parent All offspring plants were heterozygous (AaBb)
Mendel then crossed F 1 offspring to get Phenotypic Ratios in F 2 = 9:3:3:1 Ch. 10 AaBb X AaBb Four Phenotypes: Tall, purple-flowered (9/16) Tall, white-flowered (3/16) Dwarf, purple-flowered (3/16) Dwarf, white-flowered (1/16)
Ch. 10 Explanation of Mendel s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen zygote combinations are possible 1/4 AB 1/4 Ab 1/4 ab 1/4 ab 1/4 1/4 1/4 1/4 AB Ab ab ab 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/16 1/16 1/16 1/16 AaBB AaBb aabb aabb 1/16 1/16 1/16 1/16 AaBb Aabb aabb aabb
Law of Independent Assortment When gametes are formed, pairs of chromosomes (and the alleles they carry) enter the gametes independently of the other chromosomes. Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis So, the inheritance of 1 trait does not influence the inheritance of any other trait! This only applies if alleles are NOT on the SAME chromosome, and Crossing Over HAS NOT occurred. This explains why we set up dihybrid squares the way we do, with 16 sections.
Ch. 10 Independent Assortment Metaphase I Metaphase II: A A a a A A a a B B OR b b b b B B A A a a A A a a B B b b b b B B Gametes: A B A B a b b b b a A A a B a B 1/4 AB 1/4 ab 1/4 Ab 1/4 ab
Linkage maps estimate distances between genes. Alfred Sturtevant- said further apart the genes were, the more likely they were to be separated during crossover in meiosis The closer together two genes are, the more likely they will be inherited together. Cross-over frequencies are related to distances between genes. Linkage maps show the relative locations of genes.
Chromosome Linkage Maps
Polygenic Traits Ch. 10 Controlled by more than 1 pair of alleles Get a range of phenotypes, shows up as a parabola curve on a graph. Number of people 500 400 300 200 100 0 4' 5 ' 5' 6 ' 6' 6" tall 6" tall 6" tall tall Series 1 Column1 Column2 Examples: Human height, human skin color, corn cob length
Interactions Between Gene Pairs = Epistasis multiple genes that control the same trait but one gene can mask the other. The gene for color in mice is controlled by the C gene. If the mouse gets cc, it will be albino.
Albinism A phenotype that results when pathway for melanin production (skin pigment) is completely blocked Genotype - Homozygous recessive for the gene that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway
Ch. 10 Environment can affect Phenotype Ex. Temperature: Himalayan rabbits are Homozygous for an allele for a heat-sensitive version of an enzyme that helps make melanin More Melanin is produced in cooler areas of body Ex. Soil ph can cause hydrangeas to have different flower colors: blue or pink!!!
Human Genetics chapter 12 Alleles and Gametes Diploid cell has a pair of alleles for each trait because of homologous chromosomes) Haploid cell (gamete) has a single allele for each trait (no homologues after meiosis)
Sex Determination Ch. 10 X and Y chromosomes are sex chromosomes, last pair of chromosomes shown in a karyotype In humans and most animals XX female and XY male BIRDS xx male xy female
Human Karyotype 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX (or XY)
Ch. 10 Sex Determination eggs sperm X Y X Female germ cell Male germ cell X X Y X XX XY X XX XY sex chromosome combinations possible in new individual
The X Chromosome Carries more than 2,300 genes Most genes deal with nonsexual traits Genes on X chromosome can be expressed in both males and females Ch. 10, 12
The Y Chromosome Fewer than two dozen genes found One is the master gene for male sex determination SRY gene (Sex-determining region of Y) SRY present, testes form SRY absent, ovaries form Ch. 10, 12
Ch. 12 Examples of X-Linked Traits Color blindness Inability to distinguish among some of all colors Hemophilia Blood-clotting disorder 1/7,000 males has allele for hemophilia A Was common in European royal families
Pedigree a family tree that indicates the phenotype of one trait being studied for every member of a family. Females=circle, males = square Normal= unshaded, carrier=1/2 shaded, exhibits trait=shaded Ch. 12
Ch. 12 Pedigree for Polydactly I female male II 5,5 6,6 III * 5,5 6,6 6,6 5,5 6,6 5,5 IV 6 7 5,5 6,6 5,5 6,6 5,5 6,6 5,5 6,6 5,6 6,7 V *Gene not expressed in this carrier. 12 6,6 6,6
Autosomal Recessive Ch. 12 Inheritance Patterns parents both heterozygous, child will have a 25% chance being affected b. Examples:PKU, cystic fibrosis, albinism c. Trait often skips generations, shows up equally in boys and girls
Ch. 12 a.trait typically appears in every generation b. Examples: Huntington disorder, polydactyly, achondroplasia, progeria Autosomal Dominant Inheritance
Ch. 10, 12 Males show disorder more than females X-Linked Recessive Son cannot inherit disorder from his father Inheritance
Ch. 12
Ch. 12 Karyotype Definition: Photograph of chromosomes arranged to form diagram from largest pair to smallest pair of autosomes and sex chromosomes Karyotype Analysis can show chromosomal abnormalities (ie. Nondisjunction effects)
Ch. 12 Karyotype Preparation Arrested cells (cells whose mitosis is halted) are broken open Metaphase chromosomes are fixed and stained Chromosomes are photographed through microscope Photograph of chromosomes is cut up and arranged to form karyotype diagram
Chromosome Mutations Change in structure of chromosome or loss of an entire chromosome. Ch. 12 Includes: Duplication, Inversion, Translocation and Deletion
Ch. 12 Translocation A piece of one chromosome becomes attached to another nonhomologous chromosome Philadelphia chromosome arose from a reciprocal translocation between chromosomes 9 and 22
Nondisjunction Ch. 12 n + 1 n + 1 n - 1 chromosome alignments at metaphase I nondisjunction at anaphase I alignments at metaphase II anaphase II n - 1
Aneuploidy Ch. 12 Individuals have one extra or less chromosome (2n + 1 or 2n - 1) Major cause of human reproductive failure Most human miscarriages are aneuploids
Down Syndrome Trisomy of chromosome 21 Ch. 12 Mental impairment and a variety of additional defects Can be detected before birth Risk of Down syndrome increases dramatically in mothers over age 35
Ch. 12
Ch. 12
Turner Syndrome Inheritance of only one X (XO) 98% spontaneously aborted Survivors are short, infertile females May be treated with hormones, surgery Klinefelter Syndrome XXY condition Results mainly from nondisjunction in mother (67%) Phenotype is tall males Sterile or nearly so Feminized traits (sparse facial hair, somewhat enlarged breasts) Treated with testosterone injections Ch. 12
Polyploidy Individuals have three or more of each type of chromosome (3n, 4n) Common in flowering plants Lethal for humans 99% die before birth Newborns die soon after birth Ch. 12
Genetic Counseling - Assesses risk of passing on inherited disorders prior to child bearing Large-scale screening programs detect affected persons Newborns in United States routinely tested for PKU Early detection allows dietary intervention and prevents brain impairment Ch. 12
Ch. 12 Prenatal Diagnosis Amniocentesis Chorionic villus sampling Fetoscopy All methods have some risks