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MENDELIAN GENETICS What is genetics? The study of how traits are inherited or how genetic information is passed from one generation to the next. It also.

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Presentation on theme: "MENDELIAN GENETICS What is genetics? The study of how traits are inherited or how genetic information is passed from one generation to the next. It also."— Presentation transcript:

1 MENDELIAN GENETICS What is genetics? The study of how traits are inherited or how genetic information is passed from one generation to the next. It also explains biological variation

2 Gregor Mendel 1850’s Grew up in a farm wanting to garden Austrian monk ( Flunked out of college twice ) but became a mathematician Experimented with garden pea plants Using pea plants looked at seven different characters ( height of plants, seed color, texture, flower color ) and found evidence of how parents transmit genes to offspring Mendel’s statistical analysis provided a model for predicting what the next generation would be like

3 What was the prevalent believe about inheritance before Mendel? People believed in “spontaneous generation” and in the “blending of characters” Blending theory –Problem: Would expect variation to disappear Variation in traits persists Ex: Yellow and green parakeets should have all blue babies. This is not what you observe.

4 The gene theory An alternative idea is the “gene” idea. Parents pass on discrete individual heritable units: genes

5 Experimental genetics began in an abbey garden –Modern genetics Began with Gregor Mendel’s quantitative experiments with pea plants Petal Carpel Stamen Figure 9.2 BFigure 9.2 A

6 The Garden Pea Plant Mendel chose to work with the pea plant because he could control which plant mated with which. Pea plants are Self-pollinating True breeding (different alleles not normally introduced) Can be experimentally cross-pollinated

7 –Mendel crossed pea plants that differed in certain characteristics And traced traits from generation to generation Mendel started his experiments with plants that were “true breeding”. Figure 9.2 C 1 Removed stamens from purple flower 2 Transferred pollen from stamens of white flower to carpel of purple flower 3 Pollinated carpel matured into pod 4 Planted seeds from pod Offspring (F 1 ) Parents (P) Purple Carpel White Stamens

8 –Mendel hypothesized that there are alternative forms of genes The units that determine heritable traits Flower color Flower position Seed color Seed shape Pod color Pod shape Stem length Purple White Axial Terminal RoundWrinkled InflatedConstricted Tall Dwarf Green Yellow GreenYellow Figure 9.2 D

9 Mendel’s Principles of Genetics Mendel refuted the “blending theory” of heredity and provided an explanation of how inheritance works without knowing anything about chromosomes or genes. 1.He figured that traits must be coded for by some kind of inheritable particle which he called “factors” and now we call “genes”. 2.He said that those genes were transmitted as independent entities from one generation to the next.

10 Mendel’s insight continued… 3. He figured that there must be different versions of these “genes” ( we call them now “alleles”)and that every individual has two genes for each trait. ( Or we can say that: For each characteristic an organism inherits two alleles, one from each parent) He identified one as dominant, the other as recessive.

11 4. He figured that the two alleles a parent has are separated into different cells when gametes (sex cells) are formed. This actually happens during metaphase of meiosisI ( no one knew about meiosis in those days). This is known as the Law of Segregation What are alleles? Different versions of the same gene

12 Mendel’s Theory of Segregation An individual inherits a unit of information (allele) about a trait from each parent During gamete formation, the alleles segregate from each other

13 –Mendel’s law of segregation Predicts that allele pairs separate from each other during the production of gametes Figure 9.3 B P plants Gametes Genetic makeup (alleles) Gametes F 1 plants (hybrids) F 2 plants PP pp All P All p All Pp Sperm 1212 P P P p p PP Pp pp Eggs Genotypic ratio 1 PP : 2 Pp: 1 pp Phenotypic ratio 3 purple : 1 white 1212 p

14 Mendel’s law of segregation describes the inheritance of a single characteristic –From his experimental data Mendel deduced that an organism has two genes (alleles) for each inherited characteristic Figure 9.3 A P generation (true-breeding parents) F 1 generation F 2 generation Purple flowersWhite flowers  All plants have purple flowers Fertilization among F 1 plants (F 1  F 1 ) of plants have purple flowers 3434 of plants have white flowers 1414

15 What is a dominant trait? The trait that shows, the allele that is fully expressed What is a recessive trait? The alleles that is masked, the gene is there but it doesn’t show What is the phenotype? The observable traits What is the genotype? The genetic make up

16 –If the two alleles of an inherited pair differ Then one determines the organism’s appearance and is called the dominant allele ( use capital letters) –The other allele Has no noticeable effect on the organism’s appearance and is called the recessive allele

17 Vocabulary When you mate two contrasting true breeding plants you get a Hybrid. The true breeding parents are called the “P” (parent) generation The hybrid offspring of the P generation are called the F1 generation When two F1 individuals self pollinate you get the F2 generation

18 F 1 Results of One Monohybrid Cross

19 F 2 Results of Monohybrid Cross

20 Mendel’s Monohybrid Cross Results 787 tall277 dwarf 651 long stem207 at tip 705 purple224 white 152 yellow428 green 299 wrinkled882 inflated 6,022 yellow2,001 green 5,474 round1,850 wrinkled F 2 plants showed dominant-to- recessive ratio that averaged 3:1

21 Punnett Square of a Monohybrid Cross Female gametes Male gametes A a A a AAAa aa Dominant phenotype can arise 3 ways, recessive only one

22 A Test cross In a pea plant with purple flowers the genotype is not obvious. Could be homozygous or heterozygous Why do a test cross? It allows us to determine the genotype of an organism with a dominant phenotype but unknown genotype

23 Test Cross You cross an individual that shows the dominant phenotype with an individual with recessive phenotype ( one who is homozygous recessive for that trait) Examining offspring allows you to determine the genotype of the dominant individual

24 Punnett Squares of Test Crosses Homozygous recessive a A aaa Aa aa Homozygous recessive a A AAa Two phenotypes All dominant phenotype

25 Geneticists use the testcross to determine unknown genotypes –The offspring of a testcross, a mating between an individual of unknown genotype and a homozygous recessive individual Can reveal the unknown’s genotype Testcross: Genotypes Gametes Offspring  B_ bb Two possibilities for the black dog: BB or Bb B B b b Bb b bb All black 1 black : 1 chocolate Figure 9.6

26 Homologous chromosomes bear the two alleles for each characteristic –Alternative forms of a gene Reside at the same locus on homologous chromosomes Figure 9.4 Genotype: PPaaBb Heterozygous P a b P a B Gene loci Recessive allele Dominant allele Homozygous for the dominant allele Homozygous for the recessive allele

27 Web sites to check r/inheritance.swfhttp://gslc.genetics.utah.edu/units/basics/tou r/inheritance.swf mlhttp://library.thinkquest.org/20465/games.ht ml

28 Mendel’s two Laws 1. Law of segregation The two alleles for a trait segregate during gamete formation and only one allele for a trait is carried in a gamete. The gametes combine at random ( In other words : A cell contains two copies of a particular gene, they separate when a gamete is made). 2. Law of Independent Assortment Alleles from one trait behave independently from alleles for another trait. Traits are inherited independently from one another

29 Independent Assortment Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

30 The law of independent assortment is revealed by tracking two characteristics at once –By looking at two characteristics at once Mendel tried to determine how two characteristics were inherited

31 –Mendel’s law of independent assortment States that alleles of a pair segregate independently of other allele pairs during gamete formation Figure 9.5 A Hypothesis: Dependent assortment Hypothesis: Independent assortment RRYY rryy Gametes RRYY rryy RrYy RY ry RY Sperm RY ry RY ry Ry ry RY RRYY RrYY RRYy RrYy RrYY rrYY RrYy rrYy RRYy RrYy RRyy Rryy RrYy rrYy Rryy rryy RY ry RY Actual results contradict hypothesis Actual results support hypothesis Yellow round Green round Yellow wrinkled Green wrinkled Eggs P generation F 1 generation F 2 generation Eggs 

32 –An example of independent assortment Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn Blind 9 black coat, normal vision 3 black coat, blind (PRA) 3 chocolate coat, normal vision 1 chocolate coat, blind (PRA) BbNn  BbNn Phenotypes Genotypes Mating of heterozygotes (black, normal vision) Phenotypic ratio of offspring Figure 9.5 B

33 A Dihybrid Cross - F 1 Results AABBaabbx AaBb AB ab TRUE- BREEDING PARENTS: GAMETES : F 1 HYBRID OFFSPRING: purple flowers, tall white flowers, dwarf All purple-flowered, tall

34 16 Allele Combinations in F 2 aBaB AB abAbAb AbAb aBaB 1/4 AaBbAaBbaabbAabbaaBb AABBAABbAaBBAaBb AABbAAbbAaBbAaBbAabb AaBbAaBbaaBBaaBbAaBB 1/16

35 Phenotypic Ratios in F 2 Four Phenotypes: –Tall, purple-flowered (9/16) –Tall, white-flowered (3/16) –Dwarf, purple-flowered (3/16) –Dwarf, white-flowered (1/16) AaBb X AaBbAaBb

36 Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible aBaB AB abAbAb AbAb aBaB 1/4 AaBbAaBbaabbAabbaaBb AABBAABbAaBBAaBb AABbAAbbAaBbAaBbAabb AaBbAaBbaaBBaaBbAaBB 1/16

37 Mendel’s laws reflect the rules of probability –Inheritance follows the rules of probability

38 –The rule of multiplication Calculates the probability of two independent events –The rule of addition Calculates the probability of an event that can occur in alternate ways Figure 9.7 F 1 genotypes Bb female Formation of eggs F 2 genotypes Bb male Formation of sperm B b B B B B b b b B b b

39 Genetic traits in humans can be tracked through family pedigrees –The inheritance of many human traits Follows Mendel’s laws Dominant TraitsRecessive Traits FrecklesNo freckles Widow’s peakStraight hairline Free earlobeAttached earlobe Figure 9.8 A

40 –Family pedigrees Can be used to determine individual genotypes Dd Joshua Lambert Dd Abigail Linnell D ? John Eddy D ? Hepzibah Daggett D ? Abigail Lambert dd Jonathan Lambert Dd Elizabeth Eddy Dd Dd dd Dd Dd Dd dd Female Male Deaf Hearing Figure 9.8 B

41 Parents Offspring Sperm Normal Dd Normal Dd  Eggs D d DD Normal Dd Normal (carrier) Dd Normal (carrier) dd Deaf Figure 9.9 A Recessive Disorders –Most human genetic disorders are recessive

42 VARIATIONS ON MENDEL’S LAWS The relationship of genotype to phenotype is rarely simple –Mendel’s principles are valid for all sexually reproducing species But genotype often does not dictate phenotype in the simple way his laws describe

43 Genetics is not as simple as Gregor Mendel concluded, (one gene, one trait). We know now that there is a range of dominance and that genes can work together and interact. Incomplete dominance: When the F1 generation have an appearance in between the phenotypes of the parents. Ex: pink snapdragons offspring of red and white ones. Another way to say it is In incomplete dominance Heterozygote phenotype is somewhere between that of two homozygotes

44 Flower Color in Snapdragons: Incomplete Dominance Red-flowered plant X White-flowered plant Pink-flowered F 1 plants (homozygote) (heterozygotes)

45 Incomplete dominance in snapdragon color

46 Flower Color in Snapdragons: Incomplete Dominance Red flowers - two alleles allow them to make a red pigment White flowers - two mutant alleles; can’t make red pigment Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

47 Flower Color in Snapdragons: Incomplete Dominance Pink-flowered plant X Pink-flowered plant White-, pink-, and red-flowered plants in a 1:2:1 ratio (heterozygote)

48 Incomplete dominance in carnations

49 Co-Dominance or multiple alleles: Codominance –Non-identical alleles specify two phenotypes that are both expressed in heterozygotes Having more than 2 alleles for a given trait and both alleles show in the phenotype. No single one is dominant over the other. Example: ABO blood types

50 Genetics of ABO Blood Types: Three Alleles Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells Two alleles (I A and I B ) are codominant when paired Third allele (i) is recessive to others

51 ABO blood types

52 –The ABO blood type in humans Involves three alleles of a single gene –The alleles for A and B blood types are codominant And both are expressed in the phenotype Figure 9.13 Blood Group (Phenotype) Genotypes Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB O A B AB ii I A or I A i I B or I B i IAIBIAIB Anti-A Anti-B Anti-A —

53 Multiple alleles for the ABO blood groups

54 More exceptions to the dominant/recessive rule Pleiotropy: One genes having many effects. Only one gene affects an organism in many ways. Ex: sickle cell anemia and cystic fibrosis

55 Pleiotropy Alleles at a single locus may have effects on two or more traits Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

56 A single gene may affect many phenotypic characteristics –In pleiotropy A single gene may affect phenotype in many ways Individual homozygous for sickle-cell allele Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle-cell (abnormal) hemoglobin Sickle cells Breakdown of red blood cells Clumping of cells and clogging of small blood vessels Accumulation of sickled cells in spleen Physical weakness Anemia Heart failure Pain and fever Brain damage Damage to other organs Spleen damage Impaired mental function Paralysis Pneumonia and other infections Rheumatism Kidney failure 5,555  Figure 9.14

57 Genetics of Sickle-Cell Anemia Two alleles 1) Hb A Encodes normal beta hemoglobin chain 2) Hb S Mutant allele encodes defective chain Hb S homozygotes produce only the defective hemoglobin; suffer from sickle- cell anemia

58 Pleiotropic effects of the sickle-cell allele in a homozygote

59 Epistasis: Interaction between the products of gene pairs Interaction between two genes in which one of the genes modifies the expression of the other. Ex: fur /hair color in mammals and albinism

60 Albinism Phenotype results when pathway for melanin production is completely blocked Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway

61 Genetics of Coat Color in Labrador Retrievers Two genes involved - One gene influences melanin production Two alleles - B (black) is dominant over b (brown) - Other gene influences melanin deposition Two alleles - E promotes pigment deposition and is dominant over e

62 Allele Combinations and Coat Color Black coat - Must have at least one dominant allele at both loci –BBEE, BbEe, BBEe, or BbEE Brown coat - bbEE, bbEe Yellow coat - Bbee, BbEE, bbee

63 An example of epistasis

64 Human Variation Some human traits occur as a few discrete types –Attached or detached earlobes –Many genetic disorders Other traits show continuous variation –Height –Weight –Eye color

65 More modifications to Mendel’s rule Polygenic Inheritance: In this case many genes have an additive effect. The characteristic or trait is the result of the combined effect of several genes. Ex: human skin color, height. Controlled by more than one pair of genes

66 Continuous Variation Polygenic inheritance results in a continuous range of small differences in a given trait among individuals The greater the number of genes that affect a trait, the more continuous the variation in versions of that trait

67 A simplified model for polygenic inheritance of skin color

68 Environmental effects: The degree to which an allele is expressed depends on the environment Ex: Siamese cat fur color ( enzyme for melanin production inhibited by heat), hydrangea flowers ( depends on acidity of soil), height (nutrition)

69 Temperature Effects on Phenotype Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway Melanin is produced in cooler areas of body

70 Environmental Effects on Plant Phenotype Hydrangea macrophylla Action of gene responsible for floral color is influenced by soil acidity Flower color ranges from pink to blue

71 The effect of environment of phenotype

72 Web sites to check r/inheritance.swfhttp://gslc.genetics.utah.edu/units/basics/tou r/inheritance.swf mlhttp://library.thinkquest.org/20465/games.ht ml

73 Thomas Hunt Morgan (1910) and Sex Linked Inheritance Morgan’s Experimental Evidence: Scientific Inquiry The first solid evidence associating a specific gene with a a specific chromosome came from Thomas Hunt Morgan Morgan’s experiments with fruit flies (Columbia University, 1910) provided convincing evidence that chromosomes are the location of Mendel’s heritable factors. He provided confirmation of the correctness of the chromosomal theory of inheritance.

74 –Morgan’s experiments Demonstrated the role of crossing over in inheritance Figure 9.20 C Experiment Gray body, long wings (wild type) GgLI Female  Black body, vestigial wings ggll Male Offspring Gray long Black vestigialGray vestigial Black long Parental phenotypes Recombinant phenotypes Recombination frequency = = 0.17 or 17% 391 recombinants 2,300 total offspring Explanation GgLI (female) ggll (male) GL g l g l g l GL g lGl g L g l EggsSperm GL g l g l g l g l g l L g l G Offspring

75 –Thomas Hunt Morgan Performed some of the early studies of crossing over using the fruit fly Drosophila melanogaster Figure 9.20 B

76 –In Drosophila White eye color is a sex-linked trait Figure 9.23 A

77

78 SEX LINKED INHERITANCE CHROMOSOMES Humans have 22 pairs of AUTOSOMES and one pair of SEX CHROMOSOMES : total=23 prs Thomas Morgan discovered SEX LINKED INHERITANCE studying Drosophila (fruit fly) In fruit flies red eyes is the wild type and white eyes is a mutant. He noticed the connection between gender and certain traits. Only the male flies had mutant white eyes.

79 SEX LINKED TRAITS ARE THOSE CARRIED BY THE X CHROMOSOME Red-Green color blindness Inability to see those colors. Red and green look all the same,like gray Hemophilia Blood clotting disorder. The clotting factor VIII is not made, individual can bleed to death. Muscular dystrophy X linked recessive, gradual and progressive destruction of skeletal muscles. Faulty teeth enamel Extremely rare, X linked Dominant

80 Sex-linked genes exhibit a unique pattern of inheritance –All genes on the sex chromosomes Are said to be sex-linked –In many organisms The X chromosome carries many genes unrelated to sex

81 new technologies can provide insight into one’s genetic legacy –New technologies Can provide insight for reproductive decisions

82 Identifying Carriers –For an increasing number of genetic disorders Tests are available that can distinguish carriers of genetic disorders

83 Newborn Screening –Some genetic disorders can be detected at birth By simple tests that are now routinely performed in most hospitals in the United States

84 Figure 9.10 A Amniocentesis Chorionic villus sampling (CVS) Ultrasound monitor Fetus Uterus Amniotic fluid Fetal cells Several weeks Biochemical tests Several hours Fetal cells Uterus Cervix Suction tube inserted through cervix to extract tissue from chorionic villi Needle inserted through abdomen to extract amniotic fluid Centrifugation Ultrasound monitor Fetus Placenta Chorionic villi Karyotyping Placenta Cervix Fetal Testing –Amniocentesis and chorionic villus sampling (CVS) Allow doctors to remove fetal cells that can be tested for genetic abnormalities

85 Ethical Considerations –New technologies such as fetal imaging and testing Raise new ethical questions

86 Mutations Mutations are permanent changes in DNA Causes? Errors in DNA replication that can be spontaneous. Also caused by high energy radiation (X rays, gamma rays),toxic chemicals in the environment ( pesticides,asbestos, tar) and viruses.

87 MUTATION: A PERMANENT CHANGE IN THE DNA. When it happens in the gametes it is inheritable. Some mutations are lethal but most are harmless. Mutations are very important because it creates DIVERSITY WHAT CAUSES MUTATIONS? Most mutations are spontaneous, changes in DNA caused by errors in replication ( the DNA is copied incorrectly during cell division). The cell has mechanism to find and correct mistakes but those that get through get passed along. Some mutations can cause genetic disorders. Some environmental factors can cause molecular changes in DNA. X rays, toxic chemicals (insecticides, fertilizers, dry cleaning fluids, tar), some viruses, high energy radiation.

88 Many inherited disorders in humans are controlled by a single gene –Some autosomal disorders in humans Table 9.9

89 DISORDERS RESULTING FROM AUTOSOMAL RECESSIVE INHERITANCE These are conditions in which the gene that is defective is recessive. It is only expressed when the child receives both recessive genes for the disorder (one from each parent) If a person is heterozygous, that is it has one dominant regular gene and one recessive abnormal gene for the condition, he will be a CARRIER but not have the disorder. The dominant allele will mask the expression of the abnormal condition. EXAMPLES: ALBINISM: SICKLE CELL ANEMIA: CYSTIC FIBROSIS: TAY- SACHS DISEASE; PHENYLKETONURIA; GALACTOSEMIA:

90 DISORDERS RESULTING FROM RECESSIVE INHERITANCE Many not life threatening traits are inherited this way. widows peak, and attached earlobes. ALBINISM: No pigmentation in skin This is also an example of “EPISTASIS”(one pair of genes modifies the expression of another) SICKLE CELL ANEMIA: This is also an example of “PLEIOTROPY” Red blood cells curved shape. Decreased oxygen to brain and muscles (offers resistance to Malaria)

91 DISORDERS RESULTING FROM RECESSIVE INHERITANCE CYSTIC FIBROSIS: Excessive mucus secretions.Impaired lung function, lung infections. Protein channel that transport chloride across cell membrane does not function. Protects against cholera. This is also an example of “PLEIOTROPY” TAY –SACHS DISEASE: Nervous system degeneration in infants. Enzyme fails to breakdown lipids which accumulate in nerve cells and kills the cells. Progressive degeneration starting with the brain cells.

92 DISORDERS RESULTING FROM RECESSIVE INHERITANCE GALACTOSEMIA: Produces brain, liver, eye damage. Enzyme that breaks down lactose is lacking. It accumulates to toxic levels. Death in infancy PHENYLKETONURIA: Results in mental retardation

93 Disorders resulting from Autosomal Dominant Inheritance Dominant genes: Many are harmless for example:freckles, dimples, cleft chin, free earlobe, short big toe, tongue rollers, left thumb on top, curly hair and dark hair Dominant traits appear in each generation since the allele shows in the heterozygous individual.

94 Figure 9.9 B Dominant Disorders –Some human genetic disorders are dominant

95 Disorders resulting from Dominant Inheritance Acondroplasia or dwarfism: A condition where the bone does not grow properly and can’t make proper cartilage. Person is less than 4 feet with short arms and legs but a regular size trunk. Cholesterolemia: High cholesterol levels in the blood causing arteries to clog and high incidence of early heart attacks. Marfan Syndrome: Abnormal connective tissue

96 Disorders resulting from Autosomal Dominant Inheritance Huntington’s Disorder: Progressive degeneration of nervous system and muscle control. Affects motor and mental abilities and it is irreversible. Late onset, usually late 30’s. Usually the person already had children. Progeria: Premature accelerated aging. Usually dead by 18. Genes that bring about growth and development are abnormal. Polydactily: Extra toes and fingers

97 Karyotype A karyotype is a visual display of an individual’s chromosomes. A man made picture of a person’s 23 pairs of chromosomes. ( the photo is taken during metaphase when the sister chromatids are lined up together) It is useful in sex determination and diagnosis of certain conditions.

98 INHERITED DISORDERS DUE TO CHROMOSOMES CHANGES Chromosome changes can cause a lot of genetic disorders as well as a lot of variety WHEN AND HOW CAN A CHROMOSOME CHANGE? Mistakes in replication. During the S phase of the cell cycle segments of a chromosome could be deleted, duplicated, inverted or moved to a new location. Also during Metaphase I (meiosis) there can be improper separation after duplication. This can change the total number of chromosomes in each gamete of the new individual.

99 If during meiosis the paired chromatids fail to separate correctly this is called NON- DISJUNCTION ANEUPLOIDY means an abnormal number of chromosomes. When an individual ends up with the wrong number of chromosomes most of the time it is miscarried ( spontaneous abortion). The wrong number of somatic chromosomes are almost always lethal. Ex: trisomy 21(three chrom. 21): Down Syndrome You can live with the wrong number of sex pair chromosomes.

100 CHANGES IN THE NUMBER OF SEX CHROMOSOMES X Turner syndrome One X instead of a pair. This happens because of non disjuction of sperm. Most are aborted spontaneously. If they live, she is very short, infertily and with reduced sex characteristics. XXY Klinefelter syndrome One in 500 live male births. Taller than average, infertile, some low intelligence, some normal. Testosterone injections help. XYY “super male” about 1 in taller, mildly retarded but normal phenotype.

101 SEX CHROMOSOMES AND SEX- LINKED GENES Chromosomes determine sex in many species –In mammals, a male has one X chromosome and one Y chromosome And a female has two X chromosomes (male)(female) Parents’ diploid cells Sperm Egg Offspring (diploid) 44 + XY 44 + XX 22 + X 22 + Y 22 + X 44 + XX 44 + XY Figure 9.22 A

102 –Other systems of sex determination exist in other animals and plants 22 + XX 22 + X 76 + ZW 76 + ZZ Figure 9.22 D Figure 9.22 C Figure 9.22 B

103 –The Y chromosome Has genes for the development of testes –The absence of a Y chromosome Allows ovaries to develop

104 Comb Shape in Poultry Alleles at two loci (R and P) interact Walnut comb - RRPP, RRPp, RrPP, RrPp Rose comb - RRpp, Rrpp Pea comb - rrPP, rrPp Single comb - rrpp

105 Campodactyly: Unexpected Phenotypes Effect of allele varies: –Bent fingers on both hands –Bent fingers on one hand –No effect Many factors affect gene expression


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