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Mendelian Genetics Using a six-sided die, what is the probability of rolling either a 5 or a 6? A) 1/6 x 1/6 = 1/36 B) 1/6 + 1/6 = 1/3 C) 1/6 + 1/6 = 2/3 D) 1/6 + 1/6 = 1/12 E) 1/6 People knew that traits were inherited • Selective breeding is based on empirical knowledge that offspring resemble their parents • “Like begets like” • Mechansim unknown • Possible blending of traits, possible environmental factors Mendel studied the mechanism of inheritance • Particulate theory of inheritance • Mendel got low scores in biology Mendel’s model organism- the pea plant • Monoecious plant- both male + female reproductive parts on same flower • Pistil- contains unfertilized seeds • Stamen- hold pollen Mendel studied characters for which 2 traits existed • • • • Purple vs. White flowers Axial vs. Apical flowers Yellow vs. green peas Round vs. wrinkled seeds • Etc. Mendel crossed true-breeding pea plant strains • True-breeding plants were generated from plants by repeatedly inbreeding like traits • When purple flowers only begot purple flowers, and white only begot white, they were said to be true-breeding F1 generation showed dominance of one of the traits over the other • But when the F1 plants were allowed to selfpollinate, the lost trait returned • In the F2 generation, 25% of the plants had the recessive trait (3:1 ratio) Mendel’s Model • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring • Four related concepts make up this model • These concepts can be related to what we now know about genes and chromosomes 1. There are different versions of a gene determining a trait (alleles) 2. Alleles are inherited in pairs- each parent donates one allele of a gene 3. When individuals inherit different alleles from each parent, some will be manifest (dominant) and others will hide (recessive) 4. Law of segregation- gamete formation involves placing a single allele for each trait into a gamete What Mendel did not know • The genes are on DNA • The DNA is on chromosomes • Alleles exist at specific loci on chromosomes • Chromosomes are segregated into gametes LE 14-4 Allele for purple flowers Locus for flower-color gene Allele for white flowers Homologous pair of chromosomes Useful Genetic Vocabulary • An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character • An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character • Unlike homozygotes, heterozygotes are not true-breeding • Physical apperance is called the phenotype • Genes which influence that appearance is called the genotype LE 14-6 3 Phenotype Genotype Purple PP (homozygous Purple Pp (heterozygous 1 2 1 Purple Pp (heterozygous White pp (homozygous Ratio 3:1 Ratio 1:2:1 1 LE 14-5_2 P Generation Appearance: Genetic makeup: Purple flowers PP White flowers pp P p Gametes F1 Generation Appearance: Genetic makeup: Purple flowers Pp 1 Gametes: 2 1 P 2 p F1 sperm P p PP Pp Pp pp F2 Generation P F1 eggs p 3 :1 Gamete formation and fertilization are influenced by chance • For heterozygotes, half of gametes will have a given allele • 50% chance that a fertilizing gamete will contain a particular gene Making predictions about offspring with a Punnett square • If genotypes of parents are known, predictions can be made about offspring • The Punnett square is used to calculate odds • Notation: Capitals for dominant alleles, lowercase for recessive Like algebra, doing Punnett squares correctly depends on setting up the problem correctly • Setting up the problem correctly depends on generating gametes correctly • Recall, gametes are generated by meiosis Generating gametes are generated only for the genes being considered • Living things have thousands of genes • Punnett squares only look at one or two (maybe three) • Aa • AA • A, a • ___? • Aa • AA • AaBB • A, a • A • ____? • • • • Aa AA AaBB AaBb • • • • A, a A AB, aB _____? • • • • Aa AA AaBB AaBb • • • • A, a A AB, aB AB, Ab, aB, ab • • • • Aa AA AaBB AaBb • • • • A, a A AB, aB AB, Ab, aB, ab • ≠ AA, BB, aa, bb • Why are these gametes not formed? Gamete formation and fertilization are influenced by chance • For heterozygotes, half of gametes will have a given allele • 50% chance that a fertilizing gamete will contain a particular gene The Test Cross • For determining Genotype of a dominant phenotype • Crossed with a recessive, a homozygote yields all domiant phenotype, with a heterozygote, a 1:1 ratio The Testcross • How can we tell the genotype of an individual with the dominant phenotype? • Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous • The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous Mendel studied two traits at a time • Punnett squares can be used in crosses of two genes • Mendel found that the two genes were inherited independently of each other • The law of independent assortment LE 14-8 P Generation YYRR yyrr Gametes YR yr YyRr F1 Generation Hypothesis of dependent assortment Hypothesis of independent assortment Sperm 1 Sperm 1 2 YR 1 2 yr 1 1 2 2 1 4 Yr 1 4 yR 1 4 yr 4 YR 4 Yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr YR YYRR 1 YR Eggs Eggs F2 Generation (predicted offspring) 4 YyRr 1 yr YyRr 3 4 yyrr 1 1 yR 4 4 1 Phenotypic ratio 3:1 yr 4 9 16 3 16 3 16 3 16 Phenotypic ratio 9:3:3:1 Meiosis accounts for Independent Assortment of genes • The probablility of one chromosome being passed to a gamete is 50% • Different genes are on different chromosomes Mendel’s data was extremely good • Improbably good, in fact • R.A. Fisher analyzed the odds of his dataextremely remote! • Did Mendel fudge his data? Mendel’s data was extremely good • Improbably good, in fact • R.A. Fisher analyzed the odds of his dataextremely remote! • Did Mendel fudge his data? • Did Mendel suffer from confirmation bias? Other patterns of inheritance • • • • • • Incomplete dominance Codominance Multiple alleles Pleiotropy Polygenic inheritance Epistasis Incomplete dominance • Neither gene is dominant of the other • Result is a blending of phenotypes • Dominance: Genotype ≠ Phenotype • Incomplete dominance: Genotype = phenotype Codominance • Both genes expressed simultaneously Codominance in rhododendrons • Red and white genes expressed simultaneously Many human diseases exhibit Mendelian (and non-Mendelian) inheritance Mendelian Ineritance in Man • Inheritance of certain alleles can be analyzed using pedigrees • Many genetic diseases exhibit Mendelian inheritance LE 14-14a Ww ww ww Ww ww ww Ww WW or Ww Widow’s peak Dominant trait (widow’s peak) Ww Ww First generation (grandparents) Second generation (parents plus aunts and uncles) ww Third generation (two sisters) ww No widow’s peak Sickle cell anemia • Round cells- normal • Crescent-shaped cellssickle cell • Caused by a single mutation in the human gene for hemoglobin Cystic Fibrosis and Sickle Cell Anemia exhibit Mendelian Inheritance • Both are recessive disorders • Both exhibit multiple phenotypic traits for a single gene defect (pleiotropy) Pleiotropy Individuals heterozygous for sickle cell exhibit resistance to malaria • Incidence of sickle cell correlates to the prevalence of malria-carrying mosquitoes in the environment • What is the inheritance pattern of sickle celldominance? INHERITANCE OF HD Completely Genetic Autosomal Dominant 100% Lethal Why has Huntington’s not been totally eradicated from the population? Genetic testing raises many ethical questions • Alro Guthrie’s dad, Woody, had HD • What were the chances Arlo would have HD? • What if he could have been tested? • What about insurance? • Should he be allowed to have children? Polygenic inheritance • One trait is affected by many genes • Human height is an example Many traits have multiple alleles Sex Linkage • Genes on sex chromosomes are not necessarily involved in sex determination • Genes for color blindness and hemophilia are found on human X-chromosomes • Sex-linkage (or X-linkage) shows different patterns of inheritance • In males, Recessive traits cannot be masked by domiannt ones • Sex is not always determined by presence of a Y chromosome LE 15-9 Parents Ova Sperm Zygotes (offspring) The X-Y system The X-0 system The Z-W system The haplo-diploid system Epistasis • One gene nullifies the effects of another • Bombay O blood type is an example of recessive epistasis Alleles of a gene are found at __________ chromosomes. A) the same locus on homologous mitochondrial B) the same locus on heterologous C) different loci on homologous D) different loci on heterologous E) the same locus on homologous MENDEL’S THREE LAWS The Law of Dominance Dominant alleles can phenotypically mask the effect of recessive alleles. The Law of Segregation of Alleles The two alleles in a gene pair segregate into gametes so that half of the gametes carry one allele and the other half carry the other allele. The Law of Independent Assortment Allelic segregation at one gene during gamete formation is independent of allelic segregation at another gene. Exceptions to Mendel’s Laws • The Law of Dominance • The Law of Segregation • The Law of independent assortment Which one of the following is false? A) Sickle-cell disease is common in tropical Africa. B) Persons who are heterozygous for sickle-cell disease are also resistant to malaria. C) Sickle-cell disease causes white blood cells to be sickle shaped. D) All of the symptoms of sickle-cell disease result from the actions of just one allele. E) About one in 10 African Americans is a carrier of sickle-cell disease. Exceptions to Mendel’s Laws • The Law of Dominance • The Law of Segregation • The Law of independent assortment • Incomplete dominance, codominance Exceptions to Mendel’s Laws • The Law of Dominance • The Law of Segregation • The Law of independent assortment • Incomplete dominance, codominance • Nondisjunction • ______? Reginald Punnett found two traits in peas that did not assort independently What could have caused this? LINKAGE • Physical association between 2 genes on the same chromosome • Linkage is the exception to Mendel’s Law of Independent Assortment T. H. Morgan found the particles of inheritance • Mendel’s work had just been rediscovered (1900) • He found them on chromosomes through the study of Drosophila melanogaster, fruit flies • Fruit flies are an ideal model organism for a variety of reasons Morgan treated flies with mutagens • Ultimately, he obtained a fly with white eyes • Mendelian ratios were observed in the offspring • Only male flies exhibited the recessive trait in the F2 generation Male flies also had different chromosome sets • This linked individual traits with the particles that carried them- the chromosomes • It also showed a new pattern of inheritancesex linkage Two-gene crosses of linked genes were studied • Morgan generated a few more mutants • Offspring did not sort independently • Yet the genes did not sort completely together • Higher incidence of parental phenotypes What determines the rate at which linked genes cross over? LINKAGE • Physical association between 2 genes on the same chromosome •Linkage Correlates with physical proximity of genes on a chromosomes •Genes can be unlinked through recombination, or crossing over RECOMBINATION • Reassortment of genes or alleles in new combinations • Physical crossing over of two non-sister chromatids from homologous chromosomes during Prophase I **The closer the genes, the lower the recombination frequency** Since recombination correlates with physical proximity, genes can be mapped • Genes can thus be located on chromosomes relative to other genes • Map distance is measured with recombination rates in a two-point cross • Distance can be quantified in CentiMorgans RECOMBINATION FREQUENCY # of recombinants R.F. = x 100% total # of progeny 1 map unit = 1 centiMorgan (cM) = 1% crossover THEORETICAL LIMIT OF RECOMBINATION Why is the maximum recombination frequency between two genes 50%? Four haploid cells One diploid cell 25% 25% Recombination 25% 25% Drosophila labs are full of weirdos Changes in chromosomes cause aberrant phenotypes • Abnormal chromosome number is called aneuploidy • can be caused by nondisjunction of chromosomes in either Meiosis I or II • Complete chromosomal nondisjunction causes polyploidy Polyploidy is common in plants • Gigantic strawberries are polyploid • Polyploidynondisjunction of an entire genome • Double diploid genomes = tetraploid Polyploidy usually results in larger structures Chromosomes can be damaged • Deletion- a segment of a chromosome is lost • Duplication • Inversion • Translocation • Causes of disease (cri du chat) and evolution • Error often occur during crossover Chromosome alterations can drive speciation • Comparative analysis of chimpanzee and human Y chromosomes unveils complex evolutionary pathway • ..Here we report the finished sequence of the chimpanzee Y chromosome... We also identified lineage-specific changes, including deletion of a 200-kb fragment …expansion of young Alu families… Reconstruction of the common ancestral Y chromosome reflects the dynamic changes in our genomes in the 5–6 million years since speciation. • Nature, January 2006 Y-chromosome similarities help trace human migration and evolution Cytoplasmic Inheritance • Includes carriers of cytoplasmic DNA • Cytoplasm is completely maternally inherited • Also includes cytoplasmic proteins, epigenetic programming Inheritance patterns of mtDNA also help trace human migrations All human mtDNA is traceable to a single human female origin