Lecture: Chapter 14 (2024) - PDF

The Welcome Library, https://wellcomecollection.org/images?query=fnvdhpva EARLY IDEAS ON INHERITANCE Particulate inheritance- heritable characters are definable units Blending inheritance- characters mix to give intermediate traits Gregor Mendel used pea plants to make observations about inh...

The Welcome Library, https://wellcomecollection.org/images?query=fnvdhpva EARLY IDEAS ON INHERITANCE Particulate inheritance- heritable characters are definable units Blending inheritance- characters mix to give intermediate traits Gregor Mendel used pea plants to make observations about inheritance, fig. 14.1 FIGURE How are traits transmitted from parents to offspring? 14.01A Each parent cell has two alleles for each Parental cells that character. will form gametes Allele for purple Allele flowers for white The two alleles Gamete Gamete flowers segregate (separate) formation formation during gamete Sperm Eggs formation. Purple-flowered offspring Offspring inherit one allele from each parent. Paternal chromosome Maternal chromosome Gamete formation When the offspring Sperm or eggs reproduces, alleles segregate GREGOR MENDEL, WHY PEAS? Short generation time, can observe several different generations in one growing season Controlled mating- peas have male and female organs, remove male organs to control fertilization, fig. 14.2 Careful recording of data, kept records of thousands of plants FIGURE Technique 14.02 1 2 Parental generation (P) Stamens 3 Carpel 4 Results First 5 filial generation offspring (F1) PLANNING Mendel was careful and thorough Mendel carried out experiments with successive generations, generated lots of data Mendel was careful in his selection of traits to study, studied only traits showing distinct forms Purple or white flowers Green or Yellow seeds TERMS Character- a heritable feature Trait- a variant of a character P generation- parental plants that were crossed F1- first filial, offspring that were scored for traits and allowed to self fertilize F2- second filial, offspring of F1 P (PARENTAL) GENERATION The parental plants were true-breeding, had been allowed to self-fertilize for several generations, had stable traits for characters under study Purple flowered plants only gave rise to offspring with purple flowers White flowered plants only gave rise to offspring with white flowers Plant with purple flowers crossed with plant with white flowers, fig. 14.3 FIGURE 14.03_1 Experiment P Generation × (true-breeding parents) Purple White flowers flowers FIGURE 14.03_2 Experiment P Generation × (true-breeding parents) Purple White flowers flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination FIRST RESULTS In F1 generation all plants had purple flowers Not light purple- argues against blending hypothesis But no white either MENDEL’S RESULTS In F2 generation, white flowers returned on some plants The white “particle” of inheritance was present but masked by the purple “particle” Ratio in F2 was 3:1 purple:white Same pattern noted with six other characters, Table 14.1 FIGURE Experiment 14.03_3 P Generation × (true-breeding parents) Purple White flowers flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination F2 Generation 705 purple-flowered 224 white-flowered plants plants FIGURE 14.T01A FIGURE 14.T01B MENDEL’S EXPLANATION Particulate inheritance supported by data- no intermediate forms found Different versions of a particle (gene) account for variations in a character Each version of the gene is an allele The modern explanation: An organism inherits two alleles for each character, one from each parent, fig. 14.4 One allele is on each of the pair of homologous chromosomes FIGURE 14.04 Enzyme C T A A A T C G G T Allele for G A T T T A G C C A purple flowers Production of CTAAATCGGT enzyme that helps synthesize Pair of purple pigment Locus for flower- homologous color gene chromosomes Allele for DNA results in One allele white flowers absence of enzyme results in A T A A A T C G G T sufficient T A T T T A G C C A pigment ATAAATCGGT for purple flowers. THE MEIOSIS EXPLANATION The two alleles segregate during gamete production, fig. 14.5 Independent assortment during meiosis I If the alleles differ, the dominant allele is seen while the recessive allele has no effect on the organism’s appearance In F2 generation, independent assortment and random fertilization can result in a portion of the offspring having 2 recessive alleles and thus the recessive appearance FIGURE 14.05_1 P Generation × Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p FIGURE 14.05_2 P Generation × Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: p P F1 Generation Appearance: Purple flowers Genetic makeup: Pp Gametes: p ½ P ½ PUNNETT SQUARE Fig. 14.5 Since alleles segregate independently, we can predict the allele combinations present in the gametes from each parent Therefore we can predict the traits seen in the offspring Monohybrid- a cross between plants differing in one character FIGURE 14.05_3 P Generation × Purple flowers White flowers Appearance: PP pp Genetic makeup: P p Gametes: F1 Generation Appearance: Purple flowers Genetic makeup: Pp Gametes: ½ P ½ p Sperm from F1 (Pp) plant F2 Generation p P P Eggs from PP Pp F1 (Pp) plant p Pp pp 3 :1 MORE TERMS Homozygous- having two identical alleles True-breeding plants are homozygous Heterozygous- having two different alleles Genotype- the alleles present in an organism, genetic composition Phenotype- an organism’s appearance, physical manifestation, fig. 14.6 FIGURE Phenotype Genotype 14.06 Purple PP 1 (homozygous) 3 Purple Pp (heterozygous) 2 Purple Pp (heterozygous) pp 1 White 1 (homozygous) Ratio 3 purple : 1 white Ratio 1 PP : 2 Pp : 1 pp TESTCROSS If organism has dominant phenotype, we can’t know it’s genotype Cross to a homozygous recessive, score progeny and compare to prediction from Punnett square, fig. 14.7 FIGURE Technique 14.07 × Dominant phenotype, Recessive phenotype, unknown genotype: known genotype: PP or Pp? pp Predictions If purple-flowered or If purple-flowered parent is PP: parent is Pp: Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p Pp Pp pp pp Results or All offspring purple 1 2 offspring purple and 1 offspring white 2 DIHYBRID CROSS Simultaneously examine the inheritance of two characters, fig. 14.8 Be certain to account for all possible gametes, independent assortment Linkage- presence of the alleles on the same chromosome, will limit the gametes formed and skew the ratio of phenotypes among the offspring FIGURE 14.08A Experiment P Generation YYRR × yyrr Gametes YR yr F1 Generation YyRr (dihybrid) INDEPENDENT ASSORTMENT The independent assortment of homologous chromosomes in meiosis I results in the independent segregation of alleles during Mendel’s experiments, fig. 14.8, 15.4 For independently segregating genes, there are characteristic phenotypic ratios among the offspring For a heterozygous dihybrid cross this ratio is 9:3:3:1, fig. 14.8 FIGURE All F1 plants produce 15.04B F1 Generation yellow round seeds (YyRr). R R y y r r Y Y LAW OF INDEPENDENT LAW OF ASSORTMENT Alleles SEGREGATION of genes on R r Meiosis r R The two alleles for nonhomologous each gene Metaphase chromosomes assort separate. Y y I Y y independently. 1 1 R r r R Anaphase I Y y Y y R r Metaphase r R II 2 Y y Y y 2 Y y Y Y y Y y y R R r r r r R R 1 4 YR 1 4 yr 1 4 Yr 1 4 yR FIGURE Hypothesis of Hypothesis of 14.08B dependent assortment independent assortment Sperm Predicted 1 1 1 1 Sperm 4 YR 4 Yr 4 yR 4 yr offspring of F2 generation 1 2 YR 1 2 yr 1 YR 4 YYRR YYRr YyRR YyRr 1 2 YR YYRR YyRr 1 Yr Eggs 4 YYRr YYrr YyRr Yyrr 1 Eggs 2 yr YyRr yyrr 1 4 yR YyRR YyRr yyRR yyRr 3 1 4 4 1 4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 3 3 1 16 16 16 16 Phenotypic ratio 9:3:3:1 Results 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 FIGURE 14.08C 3 4 14 FIGURE 14.08D 9 16 3 16 3 16 1 16 3 POINTS ABOUT DOMINANT/RECESSIVE There may be a range of interactions between dominant and recessive genes Genes aren’t inherently dominant or recessive, the phenotype reflects the mechanisms that result in a phenotype, e.g. the presence of an enzyme that creates the purple pigment The ratio of dominant and recessive phenotypes in a population do not reflect the frequency of dominant or recessive alleles in that population PROBABILITY Genetics problems can also be solved with rules of probability, fig. 14.9 Multiplication rule The chance that two independent events will occur together is the product of their individual probabilities Probability of genotype rr = probability of gamete r ½ times probability of r gamete ½ = ¼ FIGURE Rr × Rr 14.09 Segregation of Segregation of alleles into eggs alleles into sperm Sperm 1 R 1 r 2 2 R R 1 R R r 2 1 1 4 4 Eggs r r r R r 1 2 1 1 4 4 PROBABILITY II Addition rule The probability that any one of two or more mutually exclusive events will occur is the sum of their individual probabilities The probability of a pea plant having round seeds (R) Probability of RR ¼ + Probability of Rr ½ = ¾ DIHYBRIDS Cross of YyRr with YyRr What proportion of offspring with genotype YYRR Probability of gamete YR (¼) times probability of gamete YR (¼) = 1/16 FIGURE Hypothesis of Hypothesis of 14.08B dependent assortment independent assortment Sperm Predicted 1 1 1 1 Sperm 4 YR 4 Yr 4 yR 4 yr offspring of F2 generation 1 2 YR 1 2 yr 1 YR 4 YYRR YYRr YyRR YyRr 1 2 YR YYRR YyRr 1 Yr Eggs 4 YYRr YYrr YyRr Yyrr 1 Eggs 2 yr YyRr yyrr 1 4 yR YyRR YyRr yyRR yyRr 3 1 4 4 1 4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 3 3 1 16 16 16 16 Phenotypic ratio 9:3:3:1 Results 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 USE OF THE ADDITION RULE Probability of Yellow (Y-) wrinkled (rr)seeds in offspring, genotypes YYrr,Yyrr Probability of Yr gamete (parent 1) ¼ times Probability of Yr (parent 2) gamete ¼ = 1/16 PLUS probability of Yr gamete (parent 1) ¼ times probability of yr (parent 2) ¼ = 1/16 Plus probability of yr gamete (parent 1) ¼ times probability of Yr (parent 2) ¼ = 1/16 3/16 MENDELIAN INHERITANCE IN HUMANS Most of what we know about human inheritance comes from pedigree analysis, fig. 14.15, Some inherited traits follow Mendelian rules with dominant and recessive alleles FIGURE 14.15 Male Female Offspring, in Male with Female with Mating birth order the trait the trait (first-born on left) 1st generation Tt Tt tt Tt (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, TT or Tt tt tt Tt Tt tt and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) WW ww tt TT or or Tt Ww Widow’s peak No widow’s peak Cannot taste PTC Can taste PTC (a) Is a widow’s peak a dominant or recessive trait? (b) Is the inability to taste a chemical called PTC a dominant or recessive trait? RECESSIVE DISORDERS Heterozygous individuals are carriers, carry gene without showing recessive phenotype If 2 heterozygous people have a child, 25% chance of child being affected Inbreeding increases chances of recessive disorders appearing EXAMPLE RECESSIVE DISORDERS Albinism- Lack of skin pigment, inherited as recessive gene Fig 14.16 Cystic fibrosis- Defect in chloride transporter protein, causes mucus buildup in airway, increased susceptibility to infection Sickle Cell Anemia- Low oxygen concentration in blood causes RBC to deform into sickle shape, Likely evolved as resistance to malaria, Fig. 14.17 Also an example of pleiotropy, 1 gene with multiple aspects of phenotype FIGURE 14.16 Parents Normal Normal phenotype phenotype Aa × Aa Sperm A a Eggs AA Aa A Normal Carrier with phenotype normal phenotype Aa aa Carrier with Albinism a normal phenotype phenotype FIGURE Sickle-cell alleles 14.17 Low O2 Sickle- cell disease Sickle-cell Part of a fiber of Long fibers cause hemoglobin sickle-cell hemo- red blood cells to proteins globin proteins be sickle-shaped (a) Homozygote with sickle-cell disease: Weakness, anemia, pain and fever, organ damage Sickle-cell allele Normal allele Very low O2 Sickle- cell trait Sickle-cell Part of a sickle-cell Sickled and normal fiber and normal and normal hemoglobin hemoglobin proteins red blood cells proteins (b) Heterozygote with sickle-cell trait: Some symptoms when blood oxygen is very low; reduction of malaria symptoms https://sites.ualberta.ca/~pletendr/tm-modules/genetics/70gen-hemophil.html DOMINANT DISORDERS Less common than recessive disorders Achondroplastic dwarfism, fig 14.18 Huntington’s disease Degenerative disease of nervous system, does not become apparent until late 30’s, irreversible and fatal FIGURE 14.18 Parents Dwarf Normal phenotype phenotype Dd × dd Sperm D d Eggs Dd dd d Dwarf Normal phenotype phenotype Dd dd d Dwarf Normal phenotype phenotype SUMMARY Mendel’s experiments provided a key insight into inheritance Particles of inheritance that we call genes Different version of genes we all alleles Ability to predict outcomes once we know the genotype Mendel’s model of dominant/recessive genes is useful in understanding genes But there are genes that show different patterns of interaction INCOMPLETE DOMINANCE F1 hybrids have intermediate appearance, fig. 14.10 Heterozygous individuals have distinct phenotype from either homozygous dom. or rec. FIGURE P Generation 14.10_3 Red White CRCR × CWCW Gametes CR CW F1 Generation Pink CRCW Gametes 1/ 2 CR 1/ 2 CW F2 Generation Sperm 1/ 1/ 2 CR 2 CW 1/ 2 CR CRCR CRCW Eggs 1/ 2 CW CRCW CWCW CODOMINANCE Both alleles expressed equally, Fig. 14.11 Human ABO blood types 3 alleles, A B O (A and B co-dom, O rec.) 4 phenotypes This is also an example of a character with multiple alleles  Figure 14.11 (a) The three alleles for the ABO blood groups and their carbohydrates Allele IA IB i Carbohydrate A B none (b) Blood group genotypes and phenotypes Genotype IAIA or IAi IBIB or IBi IAIB ii Red blood cell with surface carbohydrates Phenotype (blood group) A B AB O © 2021 Pearson Education, Inc. EPISTASIS A gene at one locus affects the expression of a gene at another locus, fig. 14.12 Genes for coat color in labradors B for Black, bb for brown A separate gene, C, controls the ability of pigment to be deposited in the hair, C- coat color is expressed, cc fails to deposit pigment and is albino FIGURE 14.12 × BbEe BbEe Sperm 1 1 1 1 4 BE 4 bE 4 Be 4 be Eggs 1 BE 4 BBEE BbEE BBEe BbEe 1 bE 4 BbEE bbEE BbEe bbEe 1 Be 4 BBEe BbEe BBee Bbee 1 4 be BbEe bbEe Bbee bbee 9 : 3 : 4 POLYGENIC INHERITANCE Two or more genes influence a phenotype. Human height, eye color, skin pigmentation, fig. 14.13 Interaction of multiple alleles creates a broad spectrum of possible phenotypes FIGURE 14.13 × AaBbCc AaBbCc Sperm 1 1 1 1 1 1 1 1 8 8 8 8 8 8 8 8 1 8 1 8 1 8 1 8 Eggs 1 8 1 8 1 8 1 8 1 6 15 20 15 Phenotypes: 64 64 64 64 64 6 64 1 64 Number of dark-skin alleles: 0 1 2 3 4 5 6 MULTIFACTORIAL TRAITS Genes may predispose us to certain conditions, but environmental factors also play a critical role There is a genetic component to many diseases Alcoholism Heart Disease Diabetes Manic Depressive Disorder NATURE VS. NURTURE We are not simply a product of our genes, but our genes play a tremendous role in our lives In some cases the environment determines how a given gene is expressed, fig 14.4 FIGUR E 14.14 (a) Hydrangeas grown in basic soil (b) Hydrangeas of the same genetic variety grown in acidic soil with free aluminum FIGURE 14.UN05 Relationship among alleles of a single gene Description Example Complete dominance Heterozygous of one allele phenotype same as PP Pp that of homozygous dominant Incomplete dominance Heterozygous of either allele phenotype intermediate between the two homozygous phenotypes CRCR CRCW CWCW Codominance Both phenotypes expressed in IAIB heterozygotes Multiple alleles In the population, ABO blood group some genes have more alleles than two alleles IA, IB, i Pleiotropy One gene affects Sickle-cell disease multiple phenotypic characters FIGURE 14.UN06 Relationship among Description Example two or more genes Epistasis The phenotypic BbEe × BbEe expression of one gene affects the BE bE Be be expression of BE another gene bE Be be 9 :3 :4 Polygenic inheritance A single phenotypic AaBbCc × AaBbCc character is affected by two or more genes