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1、Genetics NotesWho is Gregor Mendel? Principle of Independent Assortment Inheritance of one trait has no effect on the inheritance of another trait“Father of Genetics”TraitsGenetics study of how traits are passed from parent to offspringTraits are determined by the genes on the chromosomes. A gene is
2、 a segment of DNA that determines a trait.Chromosomes come in homologous pairs, thus genes come in pairs.Homologous pairs matching genes one from female parent and one from male parentExample: Humans have 46 chromosomes or 23 pairs. One set from dad 23 in sperm One set from mom 23 in eggGene for eye
3、 color (blue eyes)Gene for eye color (brown eyes)Homologous pair of chromosomes One pair of Homologous Chromosomes: Alleles different genes (possibilities) for the same trait ex: blue eyes or brown eyesDominant and Recessive GenesGene that prevents the other gene from “showing” dominantGene that doe
4、s NOT “show” even though it is present recessiveSymbol Dominant gene upper case letter T Recessive gene lower case letter tDominant colorRecessive colorExample: Straight thumb is dominant to hitchhiker thumb T = straight thumb t = hitchhikers thumb(Always use the same letter for the same allelesNo S
5、 = straight, h = hitchhikers)Straight thumb = TTStraight thumb = TtHitchhikers thumb = tt* Must have 2 recessive alleles for a recessive trait to “show”Both genes of a pair are the same homozygous or purebredTT homozygous dominant tt homozygous recessiveOne dominant and one recessive gene heterozygo
6、us or hybridTt heterozygousBB BlackBb Black w/ white genebb WhiteGenotype and PhenotypeCombination of genes an organism has (actual gene makeup) genotype Ex: TT, Tt, ttPhysical appearance resulting from gene make-up phenotypeEx: hitchhikers thumb or straight thumbWhite fur (b)Punnett Square and Prob
7、abilityUsed to predict the possible gene makeup of offspring Punnett SquareExample: Black fur (B) is dominant to white fur (b) in miceCross a heterozygous male with a homozygous recessive female.Black fur (B)White fur (b)Heterozygous maleWhite fur (b)Homozygous recessive femaleBbBbbbbbWrite the rati
8、os in the following orders:Genotypic ratiohomozygous : heterozygous : homozygous dominant recessivePhenotypic ratio dominant : recessivebbb BPossible offspring 2N Male gametes - N(One gene in sperm)Female gametes N (One gene in egg)Male = Bb X Female = bbGenotypic ratio = 2 Bb : 2 bb50% Bb : 50% bbP
9、henotypic ratio = 2 black : 2 white50% black : 50% whiteBBBbBb bbBbBGenotypic ratio = 1 BB : 2 Bb : 1 bb 25% BB : 50% Bb : 25% bbPhenotypic ratio = 3 black : 1 white 75% black : 25% whiteCross 2 hybrid mice and give the genotypic ratio and phenotypic ratio. Bb X Bb bBBBbBbbbBbBbExample: A man and wo
10、man, both with brown eyes (B) marry and have a blue eyed (b) child. What are the genotypes of the man, woman and child?Bb X Bb Man = BbWoman = Bb1 brown and curlyBBHHBBHhBbHHBbHhBBHhBBhhBbHhBbhhBbHHBbHhbbHHbbHhBbHhBbhhbbHhbbhhBHBHBhBhbHbHbhbh9 black and straight3 black and curly3 brown and straightG
11、ametesCrossing involving 2 traits Dihybrid crossesExample: In rabbits black coat (B) is dominant over brown (b) and straight hair (H) is dominant to curly (h). Cross 2 hybrid rabbits and give the phenotypic ratio for the first generation of offspring.Possible gametes: BbHh X BbHhBHBHBhBhbHbHbhbhPhen
12、otypes - 9:3:3:1BBHHBBHhGametesGametesBHBHBh100% black and straightExample: In rabbits black coat (B) is dominant over brown (b) and straight hair (H) is dominant to curly (h). Cross a rabbit that is homozygous dominant for both traits with a rabbit that is homozygous dominant for black coat and het
13、erozygous for straight hair. Then give the phenotypic ratio for the first generation of offspring.BBHH X BBHh Possible gametes:BHBHBh(Hint: Only design Punnett squares to suit the number of possible gametes.)Phenotypes:Sex DeterminationPeople 46 chromosomes or 23 pairs22 pairs are homologous (look a
14、like) called autosomes determine body traits1 pair is the sex chromosomes determines sex (male or female)Females sex chromosomes are homologous (look alike) label XX Males sex chromosomes are different label XY XXXXXYXYXYWhat is the probability of a couple having a boy? Or a girl? Chance of having f
15、emale baby? 50% male baby? 50%Who determines the sex of the child? fatherXXIncomplete dominance and CodominanceWhen one allele is NOT completely dominant over another (they blend) incomplete dominance Example: In carnations the color red (R) is incompletely dominant over white (W). The hybrid color
16、is pink. Give the genotypic and phenotypic ratio from a cross between 2 pink flowers.RW X RWRRRWRW WWRWRWGenotypic = 1 RR : 2 RW : 1 WWPhenotypic = 1 red : 2 pink : 1 whiteWhen both alleles are expressed CodominanceExample: In certain chickens black feathers are codominant with white feathers.Hetero
17、zygous chickens have black and white speckled feathers. Sex linked TraitsGenes for these traits are located only on the X chromosome (NOT on the Y chromosome)X linked alleles always show up in males whether dominant or recessive because males have only one X chromosomeExamples of recessive sex-linke
18、d disorders:1.colorblindness inability to distinguish between certain colors Color blindness is the inability to distinguish the differences between certain colors. The most common type is red-green color blindness, where red and green are seen as the same color. You should see 58 (upper left), 18 (
19、upper right), E (lower left) and 17 (lower right).2. hemophilia blood wont clot XNXNXNXnXNYXnYXNXnXNYPhenotype: 2 normal vision females1 normal vision male1 colorblind maleExample: A female that has normal vision but is a carrier for colorblindness marries a male with normal vision. Give the expecte
20、d phenotypes of their children. N = normal visionn = colorblindness XN Xn X XN Y PedigreesGraphic representation of how a trait is passed from parents to offspringTips for making a pedigree1.Circles are for females2.Squares are for malesHorizontal lines connecting a male and a female represent a mar
21、riageVertical line and brackets connect parent to offspringA shaded circle or square indicates a person has the trait1.A circle or square NOT shaded represents an individual who does NOT have the traitPartial shade indicates a carrier someone who is heterozygous for the traitExample: Make a pedigree
22、 chart for the following couple. Dana is color blind; her husband Jeff is not. They have two boys and two girls. HINT: Colorblindness is a recessive sex-linked trait.XNYHas traitCan pass trait to offspringXnXnMultiple Alleles 3 or more alleles of the same gene that code for a single traitIn humans,
23、blood type is determined by 3 alleles A, B, and O BUT each human can only inherit 2 allelesDominant A and B (codominance)Recessive O 2. Blood type A = AA or AOB = BB or BO AB = ABO = OOABExample: What would be the possible blood types of children born to a female with type AB blood and a male with t
24、ype O blood?AB X OOAOBOAOBOOOChildren would be type A or B onlyMutationsMutation sudden genetic change (change in base pair sequence of DNA)Can be :Harmful mutations organism less able to survive: genetic disorders, cancer, deathBeneficial mutations allows organism to better survive: provides geneti
25、c variationNeutral mutations neither harmful nor helpful to organismMutations can occur in 2 ways: chromosomal mutation or gene/point mutation Chromosomal mutation:less common than a gene mutationmore drastic affects entire chromosome, so affects many genes rather than just onecaused by failure of t
26、he homologous chromosomes to separate normally during meiosischromosome pairs no longer look the same too few or too many genes, different shapeExamples:Downs syndrome (Trisomy 21) 47 chromosomes, extra chromosome at pair #21Turners syndrome only 45 chromosomes, missing a sex chromosome (X) Girls af
27、fected short, slow growth, heart problemsKlinefelters syndrome 47 chromosomes, extra X chromosomes (XXY) Boys affected low testosterone levels, underdeveloped muscles, sparse facial hairHaving an extra set of chromosomes is fatal in animals, but in plants it makes them larger and hardier.HardierGene
28、 or Point Mutationmost common and least drasticonly one gene is alteredExamples:Recessive gene mutations:Sickle cell anemia red blood cells are sickle shaped instead of round and cannot carry enough oxygen to the body tissues heterozygous condition protects people from malaria Cystic fibrosis mucous
29、 builds up in the lungsTay-Sachs Disease deterioration of the nervous system early deathMutated genes produce enzymes that are less effective than normal at breaking down fatty cell products known as gangliosides. As a result, gangliosides build up in the lysosomes and overload cells. Their buildup
30、ultimately causes damage to nerve cells.Phenylketonuria (PKU) an amino acid common in milk cannot be broken down and as it builds up it causes mental retardation newborns are tested for this Dominant gene mutations:Huntingtons disease gradual deterioration of brain tissue, shows up in middle age and is fatalDwarfism variety of skeletal abnormalitiesDetecting Genetic Disorderspicture of an individuals chromosomes karyotypeamniotic fluid surrounding the embryo is removed for analysis amniocentesis Female with Downs syndrome
