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Development and Inheritance Muse s12 2440 lecture # 13 7/19/12 Gestation First Trimester Period of embryological and early fetal development Rudiments of all major organ systems appear Second Trimester Development of organs and organ systems Body shape and proportions change By end, fetus looks distinctively human Third Trimester Rapid fetal growth and deposition of adipose tissue Most major organ systems are fully functional The First Trimester Figure 29–7a The First Trimester. The First Trimester Figure 29–7b The First Trimester. What will I be when I grow up? What will I be when I grow up? What will I be when I grow up? The First Trimester Figure 29–7c The First Trimester. The First Trimester Figure 29–7d The First Trimester. The Second and Third Trimesters Second Trimester Fetus grows faster than surrounding placenta Third Trimester Most of the organ systems become ready Growth rate starts to slow Largest weight gain Fetus and enlarged uterus displace many of mother’s abdominal organs The Second and Third Trimesters Figure 29–8a The Second and Third Trimesters: A Four-Month-Old Fetus As Seen through a Fiber-Optic Endoscope. The Second and Third Trimesters Figure 29–8b The Second and Third Trimesters: Head of a Six-MonthOld Fetus As Seen through Ultrasound. The Second and Third Trimesters Figure 29–9c, d Growth of the Uterus and Fetus. Inheritance Nucleated Somatic Cells Carry copies of original 46 chromosomes present in zygote Genotype Chromosomes and their component genes Contain unique instructions that determine anatomical and physiological characteristics Derived from genotypes of parents Phenotype Physical expression of genotype Anatomical and physiological characteristics Inheritance Homologous Chromosomes Members of each pair of chromosomes 23 pairs carried in every somatic cell At amphimixis, one member of each pair is contributed by spermatozoon, other by ovum Inheritance Autosomal Chromosomes 22 pairs of homologous chromosomes Most affect somatic characteristics Each chromosome in pair has same structure and carries genes that affect same traits Inheritance Sex Chromosomes Last pair of chromosomes Determine whether individual is genetically male or female Karyotype Entire set of chromosomes Locus Gene’s position on chromosome Inheritance Figure 29–14 A Human Karyotype. Inheritance Alleles are various forms of given gene Alternate forms determine precise effect of gene on phenotype Homozygous Both homologous chromosomes carry same allele of particular gene Simple Inheritance Phenotype determined by interactions between single pair of alleles Inheritance Heterozygous Homologous chromosomes carry different allele of particular gene Resulting phenotype depends on nature of interaction between alleles Strict Dominance Dominant allele expressed in phenotype, regardless of conflicting instructions carried by other allele Inheritance Recessive Allele Expressed in phenotype only if same allele is present on both chromosomes of homologous pair Incomplete Dominance Heterozygous alleles produce unique phenotype Codominance Exhibits both dominant and recessive phenotypes for traits Inheritance Penetrance Percentage of individuals with particular genotype that show “expected” phenotype Expressivity Extent to which particular allele is expressed Teratogens Factors that result in abnormal development Punnett Square Simple box diagram used to predict characteristics of offspring Mutation - change in normal form of gene Inheritance Figure 29–15 Predicting Phenotypic Characters by Using Punnett Squares. Inheritance Polygenic Inheritance Involves interactions among alleles on several genes Cannot predict phenotypic characteristics using Punnett square Linked to risks of developing several important adult disorders Suppression One gene suppresses other Second gene has no effect on phenotype Inheritance Inheritance Complementary Gene Action Dominant alleles on two genes interact to produce phenotype different from that seen when one gene contains recessive alleles Sources of Individual Variation During meiosis, maternal and paternal chromosomes are randomly distributed Each gamete has unique combination of maternal and paternal chromosomes Inheritance Genetic Recombination During meiosis, various changes can occur in chromosome structure, producing gametes with chromosomes that differ from those of each parent Greatly increases range of possible variation among gametes Can complicate tracing of inheritance of genetic disorders Inheritance Crossing Over Parts of chromosomes become rearranged during synapsis When tetrads form, adjacent chromatids may overlap Translocation Reshuffling process Chromatids may break, overlapping segments trade places Inheritance Figure 29–17 Crossing Over and Translocation. Inheritance Genomic Imprinting During recombination, portions of chromosomes may break away and be deleted Effects depend on whether abnormal gamete is produced through oogenesis or spermatogenesis Inheritance Chromosomal Abnormalities Damaged, broken, missing, or extra copies of chromosomes Few survive to full term Produce variety of serious clinical conditions Humans are poorly tolerant of changes in gene copy number (to few or too many = lethal or bad news) Mutation Changes in nucleotide sequence of allele Inheritance Spontaneous Mutations Result of random errors in DNA replication Errors relatively common, but in most cases error is detected and repaired by enzymes in nucleus Errors that go undetected and unrepaired have potential to change phenotype Can produce gametes that contain abnormal alleles Inheritance Carriers Individuals who are heterozygous for abnormal allele but do not show effects of mutation Inheritance Sex Chromosomes X Chromosome Considerably larger Have more genes than do Y chromosomes Carried by all oocytes Y Chromosome Includes dominant alleles specifying that the individual will be male Not present in females Autosomes, sex chromosomes and sex determination Karyotype shows 46 chromosomes arranged in pairs by size and centromere position 22 pairs are autosomes – same appearance in males and females 23rd pair are sex chromosomes XX = female XY = male Inheritance Sperm Carry either X or Y chromosome Because males have one of each, can pass along either 50% chance of each Inheritance X-Linked Genes that affect somatic structures Carried by X chromosome Inheritance does not follow pattern of alleles on autosomal chromosomes Sex determination Males produce sperm carrying an X or Y Females only produce eggs carrying an X Individual’s sex determined by father’s sperm carrying X or Y Male and female embryos develop identically until about 7 weeks Y initiates male pattern of development SRY on Y chromosome Absence of Y determines female pattern of development Inheritance Figure 29–18 Inheritance of an X-Linked Trait Inheritance of red-green color blindness Sex-linked inheritance Genes for these traits on the X but not the Y Genotype XCXC Red-green colorblindness Most common type of XCXc XcXc color blindness Red and green are seen as same color Males have only one X – They express XCY XcY Phenotype Normal Normal female female Color blind (carrier) female Normal male Color blind male Inheritance Human Genome Project Goal was to transcribe entire human genome Has mapped thousands of human genes Genome Full complement of genetic material Inheritance Figure 29–19 A Map of Human Chromosomes. Inheritance Passage of hereditary traits from one generation to the next Genotype and phenotype Nuclei of all human cells except gametes contain 23 pairs of chromosomes – diploid or 2n One chromosome from each pair came from father, other member from mother Each chromosome contains homologous genes for same traits Allele – alternative forms of a gene that code for the same trait Mutation – permanent heritable change in allele that produces a different variant Inheritance Phenylketonuria or PKU example Unable to manufacture enzyme phenylalanine hydroxylase Allele for function enzyme = P Allele that fails to produce functional enzyme = p Punnet square show possible combinations of alleles between 2 parents Genotype – different combinations of genes Phenotype – expression of genetic makeup PP – homozygous dominant – normal phenotype Pp – heterozygous – normal phenotype – 1 dominant allele codes for enough enzyme – Can pass recessive allele on to offspring – carrier pp - homozygous recessive – PKU – 2 recessive alleles make no functional enzyme Inheritance Alleles that code for normal traits are not always dominant Huntington disease caused by dominant allele Both homozygous dominant and heterozygous individuals get HD Nondisjunction Error in cell division resulting in abnormal number of chromosomes Aneuploid – chromosomes added or missing Monosomic cell missing 1 chromosome (2n-1) Trisomic cell has additional chromosome (2n +1) – Down Syndrome – trisomy 21 – 3 21st chromosomes Variations of Dominant-recessive inheritance Simple dominance-recessive Just described where dominant allele covers effect of recessive allele Incomplete dominance Neither allele dominant over other Heterozygote has intermediate phenotype Sickle-cell disease Sickle-cell disease Sickle-cell disease HbAHbA – normal hemoglobin HbSHbS – sickle-cell disease HbAHbS – ½ normal and ½ abnormal hemoglobin Minor problems, are carriers for disease Incomplete Dominance Heterozygous individuals have an intermediate phenotype Example: Sickling gene SS = normal Hb is made Ss = sickle-cell trait (both aberrant and normal Hb are made); can suffer a sickle-cell crisis under prolonged reduction in blood O2) ss = sickle-cell anemia (only aberrant Hb is made; more susceptible to sickle-cell crisis) (b) Sickled erythrocyte results from a single amino acid change in the beta chain of hemoglobin. 1 2 3 4 5 6 7 146 Figure 17.8b Multiple-allele inheritance Phenotype Some genes have more than 2 alleles ABO blood group IA produces A antigen IB produces B antigen Genotype (blood IA IA or IA i type) A IB IB or IB i B IA IB AB Ii O i produces neither A and B are codominant – Both genes expressed equally in heterozygote Blood type inheritance Complex inheritance Polygenic inheritance – most inherited traits not controlled by one gene Complex inheritance – combined effects of many genes and environmental factors Skin color, hair color, height, metabolism rate, body build Even if a person inherits several genes for tallness, full height can only be reached with adequate nutrition Skin color is a complex trait Depends on environmental conditions like sun exposure and nutrition and several genes Additive effects of 3 genes plus environmental affect produces actual skin color Polygene Inheritance of Skin Color Alleles for dark skin (ABC) are incompletely dominant over those for light skin (abc) The first-generation offspring each have three “units” of darkness (intermediate pigmentation) The second-generation offspring have a Hair color genes Eye color genes Homologous chromosomes synapse during prophase of meiosis I. Each chromosome consists of two sister chromatids. H Allele for brown hair h Allele for blond hair E Allele for brown eyes e Allele for blue eyes Paternal chromosome Homologous pair Maternal chromosome Figure 29.3 (1 of 4) Chiasma One chromatid segment exchanges positions with a homologous chromatid segment—in other words, crossing over occurs, forming a chiasma. H Allele for brown hair E Allele for brown eyes h e Allele for blue eyes Allele for blond hair Paternal chromosome Homologous pair Maternal chromosome Figure 29.3 (2 of 4) The chromatids forming the chiasma break, and the broken-off ends join their corresponding homologues. H Allele for brown hair E Allele for brown eyes h e Allele for blue eyes Allele for blond hair Paternal chromosome Homologous pair Maternal chromosome Figure 29.3 (3 of 4) Random Fertilization A single egg is fertilized by a single sperm in a random manner Because of independent assortment and random fertilization, an offspring represents one out of 72 trillion (8.5 million 8.5 million) zygote possibilities Gamete 1 Gamete 2 Gamete 3 Gamete 4 At the conclusion of meiosis, each haploid gamete has one of the four chromosomes shown. Two of the chromosomes are recombinant (they carry new combinations of genes). H Allele for brown hair E Allele for brown eyes h e Allele for blue eyes Allele for blond hair Paternal chromosome Homologous pair Maternal chromosome Figure 29.3 (4 of 4) Environmental Factors in Gene Expression Phenocopies: environmentally produced phenotypes that mimic conditions caused by genetic mutations Environmental factors can influence genetic expression after birth Poor nutrition can affect brain growth, body development, and height Childhood hormonal deficits can lead to abnormal skeletal growth and proportions Nontraditional Inheritance Influences due to Genes of small RNAs Epigenetic marks (chemical groups attached to DNA) Extranuclear (mitochondrial) inheritance Small RNAs MicroRNAs (miRNAs) and short interfering RNAs (siRNAs) Act directly on DNA, other RNAs, or proteins Inactivate transposons, genes that tend to replicate themselves and disable or hyperactivate other genes Control timing of apoptosis during development In future, RNA-interfering drugs may treat Epigenetic Marks Genomic imprinting tags genes as maternal or paternal and is essential for normal development Allows the embryo to express only the mother’s gene or the father’s gene Epigenetic Marks Information stored in the proteins and chemical groups bound to DNA Determine whether DNA is available for transcription or silenced May predispose a cell to cancer or other devastating illness Epigenetic Marks The same allele can have different effects depending on which parent it comes from For example, deletions in chromosome 15 result in Prader-Willi syndrome if inherited from the father Angelman syndrome if inherited from the Extranuclear (Mitochondrial) Inheritance Some genes (37) are in the mitochondrial DNA (mtDNA) Transmitted by the mother in the cytoplasm of the egg Errors in mtDNA are linked to rare disorders: muscle disorders and neurological problems, possibly Sins of the father? Epigenetics at work Scientists at Australia’s University of New South Wales fed healthy, svelte, male rats a high-fat diet (43 percent of calories from fat—a typical American diet). Not surprisingly, the rats put on weight and fat, and developed insulin resistance and glucose intolerance—basically, type 2 diabetes, the scientists reported last month in Nature. None of that was surprising. What made the scientists take notice was the daughters these rats sired: although their mothers were of normal weight and ate a healthy diet while pregnant, daughters of the highfat-diet dads developed insulin resistance and glucose resistance as adults—even though they never ate a high-fat diet themselves. Mothers’ diet while pregnant affects their children’s health as adults because of how nutrients and toxic compounds pass through the placenta. But fathers have no contact with their daughters except through the sperm that created them. These rat fathers were not genetically diabetic. The conclusion is therefore inescapable: the fathers’ high-fat diet altered their sperm in a way that induced adultonset disease in their daughters.