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CHAPTER 9 Patterns of Inheritance Overview: Mendel’s Laws Variations of Mendel’s Laws Chromosomes Sex linked genes Purebreds and Mutts — A Difference of Heredity • Genetics is the science of heredity • These black Labrador puppies are purebred— their parents and grandparents were black Labs with very similar genetic makeups – Purebreds often suffer from serious genetic defects • The parents of these puppies were a mixture of different breeds – Their behavior and appearance is more varied as a result of their diverse genetic inheritance MENDEL’S LAWS The science of genetics has ancient roots • The science of heredity dates back to ancient attempts at selective breeding • Until the 20th century, however, many biologists erroneously believed that – characteristics acquired during lifetime could be passed on – characteristics of both parents blended irreversibly in their offspring Experimental genetics began in an abbey garden • Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants – Was the first person to analyze patterns of inheritance – Deduced the fundamental principles of genetics • Mendel studied garden peas – These plant are easily manipulated – These plants can self-fertilize • Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation • This illustration shows his technique for cross-fertilization • He also created true-breeding varieties of plants • Mendel then crossed two different true-breeding varieties, creating hybrids • Mendel studied seven pea characteristics • He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity Mendel’s principle of segregation describes the inheritance of a single characteristic • From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic – One characteristic comes from each parent • A monohybrid cross is a cross between parent plants that differ in only one characteristic • Mendel’s principle of segregation – Pairs of alleles segregate (separate) during gamete formation; the fusion of gametes at fertilization creates allele pairs again Allele: Any one of the alternative forms of a given gene (e.g. the ABO gene has three major alleles: A, B and O alleles). Alternative forms of a gene (alleles). • A sperm or egg carries only one allele of each pair – The pairs of alleles separate when gametes form – This process describes Mendel’s law of segregation – Alleles can be dominant or recessive • An explanation of Mendel’s results, including a Punnett square Homologous chromosomes bear the two alleles for each characteristic • Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes Genetic Alleles and Homologous Chromosomes • Homologous chromosomes – Have genes at specific loci – Have alleles of a gene at the same locus • Homozygous – When an organism has identical alleles for a gene • Heterozygous – When an organism has different alleles for a gene The principle of independent assortment is revealed by tracking two characteristics at once • By looking at two characteristics at once, Mendel found that the alleles of a pair segregate independently of other allele pairs during gamete formation – This is known as the principle of independent assortment Mendel’s Principle of Independent Assortment • Two hypotheses for gene assortment in a dihybrid cross – Dependent assortment – Independent assortment • Mendel’s principle of independent assortment – Each pair of alleles segregates independently of the other pairs during gamete formation Using a Testcross to Determine an Unknown Genotype • A testcross is a mating between – An individual of unknown genotype and – A homozygous recessive individual Mendel’s principles reflect the rules of probability • Inheritance follows the rules of probability – The rule of multiplication and the rule of addition can be used to determine the probability of certain events occurring Connection: Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits follows Mendel’s principles and the rules of probability Connection: Many inherited disorders in humans are controlled by a single gene • Most such disorders are caused by autosomal recessive alleles – Examples: cystic fibrosis, sickle-cell disease • A few are caused by dominant alleles – Examples: achondroplasia, Huntington’s disease Connection: Fetal testing can spot many inherited disorders early in pregnancy • Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions – Fetal cells can be obtained through amniocentesis VARIATIONS ON MENDEL’S PRINCIPLES The relationship of genotype to phenotype is rarely simple • Mendel’s principles are valid for all sexually reproducing species – However, often the genotype does not dictate the phenotype in the simple way his principles describe • Phenotype – An organism’s physical traits • Genotype – An organism’s genetic makeup BEYOND MENDEL • Some patterns of genetic inheritance are not explained by Mendel’s principles Incomplete Dominance in Plants and People • In incomplete dominance F1 hybrids have an appearance in between the phenotypes of the two parents Many genes have more than two alleles in the population • In a population, multiple alleles often exist for a characteristic – The three alleles for ABO blood type in humans is an example A single gene may affect many phenotypic characteristics • A single gene may affect phenotype in many ways – This is called pleiotropy – The allele for sickle-cell disease is an example Connection: Genetic testing can detect diseasecausing alleles • Genetic testing can be of value to those at risk of developing a genetic disorder or of passing it on to offspring A single characteristic may be influenced by many genes • This situation creates a continuum of phenotypes – Example: skin color Polygenic Inheritance • Polygenic inheritance is the additive effects of two or more genes on a single phenotype THE CHROMOSOMAL BASIS OF INHERITANCE Chromosome behavior accounts for Mendel’s principles • Genes are located on chromosomes – Their behavior during meiosis accounts for inheritance patterns Genes on the same chromosome tend to be inherited together • Certain genes are linked – They tend to be inherited together because they reside close together on the same chromosome • This inheritance pattern was later explained by linked genes, which are – Genes located on the same chromosome – Genes that are typically inherited together Crossing over produces new combinations of alleles • This produces gametes with recombinant chromosomes • The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over Geneticists use crossover data to map genes • Crossing over is more likely to occur between genes that are farther apart – Recombination frequencies can be used to map the relative positions of genes on chromosomes SEX CHROMOSOMES AND SEXLINKED GENES Chromosomes determine sex in many species • A human male has one X chromosome and one Y chromosome • A human female has two X chromosomes • Whether a sperm cell has an X or Y chromosome determines the sex of the offspring Sex-linked genes exhibit a unique pattern of inheritance • All genes on the sex chromosomes are said to be sex-linked – In many organisms, the X chromosome carries many genes unrelated to sex – Fruit fly eye color is a sex-linked characteristic – Their inheritance pattern reflects the fact that males have one X chromosome and females have two – These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait Connection: Sex-linked disorders affect mostly males • Most sex-linked human disorders are due to recessive alleles – Examples: hemophilia, red-green color blindness – These are mostly seen in males – A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected