Patterns in Inheritance Chapter 10
What you absolutely need to know Punnett Square with monohybrid and dihybrid cross Heterozygous, homozygous, alleles, locus, gene Test cross, P, F1, F2 Mendel and his work Patterns of human 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
No Blending Involved We cannot say that a red flower crossed with a what flower produces a pink flower.
Name this man
Gregor Mendel Strong background in plant breeding and mathematics Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring
Genetic Terms A pair of homologous chromosomes A gene locus A pair of alleles Three pairs of genes
The Garden Pea Plant Self-pollinating True breeding (different alleles not normally introduced) Can be experimentally crosspollinated
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 and finally appreciated
Genes Units of information about specific traits Passed from parents to offspring Each has a specific location (locus) on a chromosome
Alleles 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 Heterozygous having two different alleles at a locus Aa
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. Ex. A blond haired person can produce a red-haired offspring
Tracking Generations Parental generation mates to produce P First-generation offspring F 1 mate to produce Second-generation offspring F 2
Monohybrid Crosses Use F 1 offspring of parents that breed true for different forms of a trait: (AA x aa = Aa) The experiment itself is a cross between two identical F 1 heterozygotes, which are the monohybrids (Aa x Aa)
F 1 Results of One Monohybrid Cross
Punnett Square of a Monohybrid Cross Female gametes A a Male gametes A a AA Aa Aa aa Dominant phenotype can arise 3 ways, recessive only 1
F 2 Results of Monohybrid Cross
Testcross Individual that shows dominant phenotype is crossed with individual with recessive phenotype (AA x aa) Examining offspring allows you to determine the genotype of the dominant individual
Dominance Relations Complete dominance Incomplete dominance Heterozygote phenotype is somewhere between that of two homozyotes Codominance Non-identical alleles specify two phenotypes that are both expressed in heterozygotes
F 1 Results of Mendel s Dihybrid Crosses All plants displayed the dominant form of both traits: AaBb We now know: All plants inherited one allele for each trait from each parent All plants were heterozygous (AaBb)
F 1 Results of Mendel s Dihybrid Crosses All plants displayed the dominant form of both traits: AaBb We now know: All plants inherited one allele for each trait from each parent All plants were heterozygous (AaBb)
Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits
Now we want to try two traits Purple AA Tall BB Parent with purple flowers and and long stems = AABB Purple AA Tall BB Parent with white flowers and short stems White aa and short stems bb Dihybrid cross is AABB x aabb
Phenotypic Ratios in F 2 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)
AaBb x AaBb produces the following gametes If the two traits are coded for by genes on separate chromosome s, sixteen gamete combinations are possible
Down Syndrome An extra Chromosome 21
A karyotype, performing a genetic analysis
A female with Down Syndrome
Genes Units of information about heritable traits In eukaryotes, distributed among chromosomes Each has a particular locus Location on a chromosome
Homologous Chromosomes Homologous autosomes are identical in length, size, shape, and gene sequence Sex chromosomes are nonidentical but still homologous Homologous chromosomes interact, then segregate from one another during meiosis
Homologous Chromosomes Crossing over with non sisters
Alleles Different molecular forms of a gene Arise through mutation Diploid cell has a pair of alleles at each locus Alleles on homologous chromosomes may be same or different
Sex Chromosomes Discovered in late 1800s Mammals, fruit flies XX is female, XY is male Human X and Y chromosomes function as homologues during meiosis
Sex Determination eggs sperm X Y X Female germ cell Male germ cell X X X X XX XX Y XY XY sex chromosome combinations possible in new individual
The Y Chromosome Fewer than two dozen genes identified One is the master gene for male sex determination SRY gene (sex-determining region of Y) SRY present, testes form SRY absent, ovaries form
Effect of Y Chromosome appearance of structures that will give rise to external genitalia Y pre sen t 7 weeks Y absent 10 weeks appearance of uncommitted duct system of embryo at 7 weeks Y present Y absent testes ovaries birth approaching ovary testis
The X Chromosome Carries more than 2,062 genes Most genes deal with nonsexual traits Genes on X chromosome can be expressed in both males and females
Karyotype Preparation - Stopping the Cycle Cultured cells are arrested at metaphase by adding colchicine This is when cells are most condensed and easiest to identify
Karyotype Preparation Arrested cells 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
Human Karyotype
Crossover Frequency Proportional to the distance that separates genes Crossing over will disrupt linkage between A and B more often than C and D
Human Genetic Analysis Geneticists often gather information from several generations to increase the numbers for analysis If a trait follows a simple Mendelian inheritance pattern they can be confident about predicting the probability of its showing up again
Pedigree Chart that shows genetic connections among individuals Standardized symbols Knowledge of probability and Mendelian patterns used to suggest basis of a trait Conclusions most accurate when drawn from large number of pedigrees
Pedigree for Nicholas of Russia
Genetic Disorder 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 Inherited conditions that cause mild to severe medical problems Why don t they disappear? Mutation introduces new rare alleles In heterozygotes, harmful allele is masked, so it can still be passed on to offspring
Autosomal Dominant Inheritance Trait typically appears in every generation Achhondroplasi a Huntington s
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
Woody Guthrie
Autosomal Recessive Inheritance Patterns If parents are both heterozygous, child will have a 25% chance of being affected
Galactosemia Caused by autosomal recessive allele Cannot manufacture an ezyme. Lethal Albinism, cystic fibrosis, sickle cell anemia
X-Linked Recessive Inheritance Males show disorder more than females Son cannot inherit disorder from his father
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
Aneuploidy Individuals have one extra or less chromosome (2n + 1 or 2n - 1) Major cause of human reproductive failure
n + n + Nondisjunction n - chro moso nondis junctio align ments anap chromos n -
Down Syndrome Trisomy of chromosome 21 Mental impairment and a variety of additional defects Can be detected before birth Risk of Down syndrome increases dramatically in mothers over age 35