Chapter 15 PDF

it’s an organelle gene inheritance know also as cy...

it’s an organelle gene inheritance know also as cytoplasmic genes organelles in the cytoplasm found in mitochondria, chloroplasts, and plant plastids carry small circular DNA molecules extranuclear genes inheritance maternally due to egg's cytoplasm inheritance of yellow or white patches on leaves of otherwise green plant observed that offspring coloration was determined by the maternal parent, not the paternal parent in plastid genes study Karl Correns prevent cells from making enough ATP diseases affect muscular and nervous systems result in defects in mitochondrial genes mitochondrial myopathy ex. leber's hereditary optic neuropathy is variation in phenotype few mammalian traits, phenotype depends on which parent passed along alleles for those traits involves silencing of certain genes that are "stamped" with imprint during gamete production methylation (addition of -CH3) of cytosine nucleotides result of Mendelian genes have specific loci on chromosomes only affect small fraction of mammalian genes chromosome theory of inheritance states segregation Chromosomes undergo it’s critical for embryonic development independent assortment genomic imprinting independent assortment explains Mendel's laws of segregation involves genes located in the nucleus exceptions involves genes located outside the nucleus Mendelian genetics’s in both cases, sex of parent contributing allele is a factor in pattern of inheritance number and structure are associated with some serious disorders in humans and other mammals can result in spontaneous abortions (miscarriages) or various developmental disorders deletion in chromosome 5 syndrome cri du chat ("cry of the cat") ex. way chromosomes behave during meiosis usually die in infancy or early childhood chromosome‘s alterations experiment on fruit flies importance of Mendel's experiment convincing evidence that chromosomes are location of Mendel's heritable factors produce many offspring structure’s types reasons for choosing fruit flies generation can be bred every two weeks have only four pairs of chromosomes chronic myelogenous leukemia (CML) normal, phenotypes that were common in fly populations wild type female flies red eyes including certain cancers ex. type abnormal, traits alternative to wild type Morgan's experimental mutant phenotypes male flies white eyes F1 generation all had red eyes F2 generation showed 3:1 red white eye ratio white-eyed mutant allele located on X chromosome supported chromosome theory of inheritance pairs of homologous chromosomes do not separate normally during meiosis transmission of X chromosome correlates with eye-color trait inheritance abnormal chromosome number first solid evidence linking specific gene with specific chromosome one gamete receives two of same type of chromosome, another gamete receives no copy result his notes genes on sex chromosomes exhibit unique inheritance patterns lead to a variety of genetic disorders each chromosome has hundreds or thousands of genes except Y chromosome is condition where individual has extra or missing chromosome results from fertilization of gametes in which nondisjunction occurred offspring with this condition have abnormal number of particular chromosome disrupt genetic balance less, leading to individuals surviving to birth and beyond, with specific symptoms of type Chromosomal Basis zygote has only one copy of particular chromosome aneuploidy of Inheritance missing X chromosome monosomic monosomy X turner syndrome X0 females ex. extra X chromosome types klinefelter syndrome XXY male include zygote has three copies of particular chromosome nondisjunction trisomic trisomy 21 down syndrome ex. there is chromosomal basis or sex determination in humans and some animals have 3 copies of chromosome 21 larger X chromosome is condition in which an organism has more than two complete sets of chromosomes varieties of sex chromosomes smaller Y chromosome common in plants, but not animals Y chromosome ends only contain regions homologous to corresponding regions of X chromosome are more normal in appearance than aneuploids polyploidy are XX is three sets of chromosomes triploidy (3n) female types ovum contain X chromosome is four sets of chromosomes tetraploidy (4n) chromosomal basis of sex ex. are XY age male sperm contain either X or Y chromosome certain medications factors to increase risk other animals have different methods of sex determination genetic factors on Y chromosome codes SRY gene for protein that directs development of male anatomical features 78 genes on Y that code for 25 proteins, ½ are expressed in testes genes that are located on same chromosome and tend to be inherited together don’t assort independently because they are on same chromosome dihybrid cross following two linked genes will not produce an F2 phenotypic ratio of 9:3:3:1 tend to be inherited together because they are located near each other on same chromosome linked genes recombination data using one of Morgan's students ordered list of genetic loci along particular chromosome genetic map constructed Alfred Sturtevant predicted that farther apart two genes are, higher probability of crossover and recombination frequency used recombination frequencies to make linkage maps of fruit fly genes is genetic map of chromosome based on recombination frequencies linkage map distances between genes know also as centimorgan one map unit represents 1% recombination frequency map units mapping gene distance gene that is located on either sex chromosome indicate relative distance and order, not precise locations of genes genes on the Y chromosome excessively hairy ears Y-linked gene ex. hypertrichosis of pinna mainly encodes genes related to sex determination genes far apart on same chromosome can have recombination frequency near 50% sex-linked gene genes on the X chromosome genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes have genes for many characters unrelated to sex using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes it follow specific patterns of inheritance indicate the positions of genes with respect to chromosomal features cytogenetic maps homozygous female needs two copies of allele for recessive X-linked trait body color and wing size traits are typically inherited together in specific combinations types hemizygous male needs only one copy of allele these genes do not assort independently, originating on same chromosome recessive disorders are much more common in males than in females mostly X-linked Color blindness nonparental phenotypes were also produced X-linked gene disorders caused by recessive alleles in humans production of offspring with combinations of traits differing from either parent linkage affects inheritance occurs in linked genes duchenne muscular dystrophy Morgan discovered incomplete gene linkage hemophilia this linkage was incomplete, because some recombinant phenotypes were observed one of two X chromosomes in each cell inactivated during embryonic development proposed process breaks physical connection between genes on same chromosome inactive X condenses into a Barr body that mechanism was crossing over of homologous chromosomes heterozygous females become mosaics for specific X chromosome genes crossing over genetic recombination result involves exploring in female mammals process unlinked genes occurs in offspring's trait combinations differ from parents Mendel's observation offspring with matching parental traits parental offspring with nonparental traits new combinations of traits type genetic findings of Mendel and Morgan relate to chromosomal basis of recombination recombinant variation for normal selection independent assortment of chromosomes recombinant chromosomes combine alleles in new gametes random fertilization increases even further new combinations of alleles number of variant combinations that produced abundance of genetic variation supports natural selection 50% frequency of recombination observed for two genes on different chromosomes