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Genetics

Genetics. The study of the mechanism of heredity Basic principles proposed by Mendel in the mid-1800s. Genetics. Diploid number of chromosomes In all cells except gametes Diploid number = 46 (23 pairs of homologous chromosomes) (For Humans!) 1 pair of sex chromosomes

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Genetics

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  1. Genetics • The study of the mechanism of heredity • Basic principles proposed by Mendel in the mid-1800s

  2. Genetics • Diploid number of chromosomes • In all cells except gametes • Diploid number = 46 (23 pairs of homologous chromosomes) (For Humans!) • 1 pair of sex chromosomes • XX = female, XY = male • 22 pairs of autosomes

  3. Chromosomes & the Genome • Karyotype: diploid chromosomal complement displayed in homologous pairs (b) (c) (a) The slide is viewed with a microscope, and the chromosomes are photographed. The photograph is entered into a computer, and the chromo- somes are electronically rearranged into homologous pairs according to size and structure. The resulting display is the karyotype, which is examined for chromosome number and structure.

  4. Genotype and Phenotype • Genome • all the chromosomes & genes of an organism • Genotype • the particular set of gene versions an organism has • Phenotype • the physical/biochemical manifestation of the genes

  5. Genetic Terms loci E locus centromeres H locus sister chromatids homologous chromosomes alleles sister chromatids

  6. Alleles • A version of a particular gene • Set of genes at the same locus on homologous chromosomes • Homozygous genotype • Both alleles at a locus are identical • Heterozygous • The alleles at a locus are different

  7. Genetic Terms • Inheritance Patterns • Dominant • A trait appears in the phenotype even if only one allele for that trait is present • Recessive • A trait that can be masked • Only appears in phenotype if it is homozygous

  8. Genetic Terms • Inheritance patterns • Simple dominance • One allele is dominant one is recessive • Thumbs, blood type • Co-dominance • Both alleles affect a trait – each give different effect • AB Blood type • Intermediate dominance • Two alleles give full effect, one allele gives half effect haploinsufficiency

  9. Sexual Sources of Genetic Variation • Independent assortment of chromosomes • Crossover between homologues

  10. Segregation and Independent Assortment • During gametogenesis, maternal and paternal chromosomes are randomly distributed to daughter cells • The two alleles of each pair are segregated during meiosis I • Alleles on different pairs of homologous chromosomes are distributed independently

  11. Segregation and Independent Assortment • The number of gamete types = 2n, where n is the number of homologous pairs • In a man’s testes, 2n = 223 = 8.5 x 106

  12. Meiotic Recombination • Genes on the same chromosome are linked • Chromosomes can cross over and exchange segments • Recombinant chromosomes have mixed contributions from each parent

  13. Hair color genes Eye color genes Homologous chromosomes synapse in prophase of meiosis I H E Allele for brown hair Allele for brown eyes h e Allele for blond hair Allele for blue eyes Paternal chromosome Homologous pair Maternal chromosome Figure 29.3 (1 of 4)

  14. Chiasma Non-sister chromatids undergo recombination (crossing over) H E Allele for brown hair Allele for brown eyes h e Allele for blond hair Allele for blue eyes Paternal chromosome Homologous pair Maternal chromosome Figure 29.3 (2 of 4)

  15. The chromatids break and rejoin H E Allele for brown hair Allele for brown eyes h e Allele for blond hair Allele for blue eyes Paternal chromosome Homologous pair Maternal chromosome Figure 29.3 (3 of 4)

  16. Gamete 1 Gamete 2 Gamete 3 Gamete 4 At end of meiosis, each haploid gamete has 1 of the 4 chromosomes shown. 2 chromosomes are recombinant – new combinations of genes H E Allele for brown hair Allele for brown eyes h e Allele for blond hair Allele for blue eyes Paternal chromosome Homologous pair Maternal chromosome Figure 29.3 (4 of 4)

  17. Types of Inheritance • Most traits are determined by multiple alleles or by the interaction of several gene pairs

  18. Simple Dominance Inheritance • Reflects the interaction of dominant and recessive alleles • Punnett square • Tool to represent genetic crosses • Helps predict possible gene combinations resulting from the mating

  19. Dominant-Recessive Inheritance • Example: probability of genotypes from mating two heterozygous parents • Dominant allele—capital letter; recessive allele—lowercase letter • T = tongue roller and t = cannot roll tongue • TT and tt are homozygous; Tt is heterozygous

  20. Dominant-Recessive Inheritance • Offspring: 25% TT, 50% Tt, 25% tt • The larger the number of offspring, the greater the likelihood that the ratios will conform to the predicted values

  21. female male Heterozygous female forms two types of gametes Heterozygous male forms two types of gametes 1/2 1/2 1/2 1/2 1/4 1/4 1/4 1/4 Possible combinations in offspring Figure 29.4

  22. Simple Dominance Inheritance • Dominant traits (for example, widow’s peaks, freckles, dimples) • Polydactyly, Marfan Syndrome • Huntington’s disease is caused by a delayed-action gene

  23. Simple Dominance Inheritance • Simple dominance inheritance patterns frequently cause disease only when the genomtype is homozygous for the recessive alleles • Albinism, cystic fibrosis, and Tay-Sachs disease, sickle-cell disease • Heterozygotes have one mutant allele and one normal allele but do not have disease

  24. What is Dominant? What is Recessive? • Dominant alleles usually encode functional proteins • Recessive alleles often encode defective proteins or no protein at all.

  25. Multiple-Allele Inheritance • Genes that exhibit more than two allele forms • ABO blood grouping is an example • Three alleles (IA, IB, i) determine the ABO blood type in humans • IA and IB are codominant (both are expressed if present), and i is recessive

  26. Table 29.2

  27. Sex-Linked Inheritance • Inherited traits determined by genes on the sex chromosomes • X chromosomes > 2500 genes • Y chromosomes ~80 genes

  28. Sex-Linked Inheritance • e.g. X-linked genes • clotting factor (hemophilia) • red opsin (red-green color blindness) • Females have two X – so normal allele masks mutant • Males – 1 X so mutant is expressed if present

  29. colorblindness deuteranopia protanopia tritanopia

  30. Polygenic Inheritance • Depends on several different genes at acting in concert • Gives more fine-scale phenotypic variation between two extremes • Examples: skin color, eye color, height • Quantitative traits • QTLs

  31. Eye color variation

  32. Polygenic 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 wide variation in possible pigmentations

  33. 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

  34. Non-Mendelian Inheritance • Influences due to • Genes of small RNAs • Epigenetic marks (chemical groups attached to DNA) • Extranuclear (mitochondrial) inheritance

  35. Small RNAs • MicroRNAs (miRNAs) and short interfering RNAs (siRNAs) • Bind to mRNAs • Cause mRNA to be destroyed or not translated into proteins • In future, RNA-interfering drugs may treat diseases such as age-related macular degeneration and Parkinson’s disease

  36. Extranuclear (Mitochondrial) Inheritance • 37 genes are in human 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 Alzheimer’s and Parkinson’s diseases

  37. Genetic Screening, Counseling, and Therapy • Newborn infants are routinely screened for a number of genetic disorders: congenital hip dysplasia, imperforate anus, PKU and other metabolic disorders • Other examples: screening adult children of parents with Huntington’s disease: for testing a woman pregnant for the first time after age 35 to see if the baby has trisomy-21 (Down syndrome)

  38. Carrier Recognition • Two major avenues for identifying carriers of genes: pedigrees and DNA testing • Pedigrees trace a particular genetic trait through several generations; helps to predict the future

  39. Pedigree Symbols

  40. Dominant or Recessive

  41. Autosomal Recessive

  42. X-linked recessive

  43. Key Male Affected male Mating Offspring Female Affected female 1st generation grandparents ww ww Ww Ww 2nd generation (parents, aunts, uncles) 3rd generation (two sisters) Widow’s peak No widow’s peak Figure 29.7

  44. Carrier Recognition • DNA probes can detect the presence of unexpressed recessive mutations • Tay-Sachs and cystic fibrosis mutations can be identified by such tests

  45. Fetal Testing • Used when there is a known risk of a genetic disorder • Amniocentesis: amniotic fluid is withdrawn after the 14th week and fluid and cells are examined for genetic abnormalities • Chorionic villus sampling (CVS): chorionic villi are sampled and karyotyped for genetic abnormalities

  46. (a) Amniocentesis Amniotic fluid withdrawn A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. 1 Fetus Placenta Centrifugation Uterus Cervix Fluid Biochemical tests can be performed immediately on the amniotic fluid or later on the cultured cells. 2 Biochemical tests Fetal cells Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. 3 Several weeks Karyotyping of chromosomes of cultured cells Figure 29.8

  47. Human Gene Therapy • Genetic engineering has the potential to replace a defective gene • Defective cells can be infected with a genetically engineered virus containing a functional gene

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