Genetics You know how no two snowflakes are

Genetics You know how no two snowflakes are alike? People are kind of like that too. Thanks to genetics. . . Chapter 14 & 15

Historically, heredity was believed to be a result of “Blending”: Wrong! Gregor Mendel proposed a Particulate model: Parents pass discreet inheritable factors (genes) to offspring • Genes: retain their identity from one generation to the next.

Terms used to study genetics: Character (or characteristic): Heritable feature (such as flower color) Trait: Specific variation (yellow vs. red flowers) Generations: P 1 = parental F 1 = first filial (offspring of P 1) F 2= second filial (offspring of F 1)

FILL IN THE TERMS WHICH ARE DEFINED BELOW: DEFINITION TERM Allele One of several varieties of a gene Dominant The type of allele which is always expressed Recessive The type of allele which is not expressed in a heterozygote Homozygous dominant Presence of two dominant alleles (AA) Homozygous recessive Presence of two recessive alleles (aa) Heterozygous (hybrid) The pair of alleles are different (Aa) True-breeding, “pure” Strains which produce the same type of offspring every generation. Phenotype Observable expression of a gene (purple flowers) Genotype Alleles present (AA, Aa, or aa)

Mendel’s Laws of Heredity: Mendel observed the transmission of selected traits in pea plants for at least 3 generations and came up with 2 principles of heredity: • Segregation • Independent Assortment

Law of Segregation: Mendel crossed 2 different true-breeding strain of peas. The traits did NOT blend. He concluded: (alleles) are responsible for variations in Alternative forms of genes ______ characters. For each character: an organism inherits 2 genes (1 from each parent) If the 2 alleles differ: 1 is expressed (dominant) and 1 is masked (recessive) The 2 genes for each character segregate during: meiosis

PUNNETT SQUARES: Used to predict the results of a genetic cross. Step 1: Write the parent genotypes Step 2: Put parent gametes on outside of Punnett Square Step 3: fill in squares Example: Cross a heterozygote with a homozygous recessive plant: Aa X aa A Genotypic ratio: Phenotypic ratio: 1: 1 a a a Aa aa

TESTCROSS: Used to determine the genotype of an organism with the dominant phenotype. What are the possible genotypes? AA or Aa Testcross = Breed the unknown (dominant) organism with a homozygous recessive. WHY? If ANY offspring appear recessive, the unknown is Aa. If NO offspring appear recessive the unknown PROBABLY is AA. A? X =

INHERITANCE IS A GAME OF CHANCE: The law of segregation is a specific example of probability. Probability scale: ranges from 0 to 1 What’s the chance of a coin landing tailsto up? An eventheads that is OR certain occur has a probability = 1 An event that is certain NOT to occur has a probability = 0 1 The probabilities of all possible outcomes for an event must add up to _____ Rule of addition: Probability of this OR that = add separate probabilities Example: getting heads or tails on a coin toss = ½ + ½ = 1 Rule of Multiplication: Probability of this AND that = multiply separate probabilities Example: getting heads & heads on 2 coins tossed = ½ x ½ = ¼ Sample Size: Actual results confirm predicted results when sample size is Large ______

Law of Independent Assortment: (monohybrid) • One trait _________crosses revealed the law of segregation • Next Mendel studied the inheritance of 2 traits together: Dihybrid cross • Each character is assigned a different letter. For example, in peas: Y = yellow R= round y= green r = wrinkled Mendel proved: alleles for color segregated into gametes independently from shape alleles

How many gametes can Yy. Rr make? 4 (22) What are they? YR, Yr, y. R, yr Remember, these gametes go outside the Punnett square! Crossing Yy. Rr x Yy. Rr will yield how many genotype combinations? 4 x 4 = 16 Exception: Mendel did not deal with any linked genes (on the same chromosome)

Dihybrid problem: Cross a hybrid yellow round x hybrid yellow wrinkled: Step 1: Write the parent genotypes : Yy. Rr x Yyrr Step 2: Put parent gametes on outside of Punnett Square Step 3: fill in squares YR Yr y. R yr Yr yr YYRr YYrr Yy. Rr Yyrr Yy. Rr yyrr Genotypic Ratio: 1: 1: 2: 2: 1: 1 Phenotypic Ratio: 1: 3: 3: 1 Yyrr

Mathematical Option: Use Rules of probability. For example, find the phenotypic ratio for a cross between two Yy. Rr plants: Yellow & Round (Y_R_)= ¾ x ¾ = 9/16 Yellow & wrinkled (Y_rr) = ¾ x ¼ = 3/16 green & Round (yy. R_) = ¼ x ¾ = 3/16 green & wrinkled (yyrr) = ¼ x ¼ = 1/16 Phenotypic ratio = 9: 3: 3: 1 * Some people prefer doing it this way because it never requires a Punnett Square that is more than 4 squares big!

You will never have to draw a 16 square Punnett Square (or do a ton of math calculations) if you remember that Dihydrids (Aa. Bb x Aa. Bb) will always give a 9: 3: 3: 1 phenotypic ratio!

* Some questions require you to use math probabilities! What is the chance that parents AABb. CCDd x Aa. Bb. CCDd will have offspring AABBCCDD? ½ x ¼ x 1 x ¼ = 1/32

The Mojave Ball Python coloration is a co-dominant trait. Stuff Mendel Never Knew: Many traits do not follow Mendel’s rules of inheritance. Incomplete Dominance: heterozygotes have intermediate phenotype Human hair: Straight (HH), curly (hh), wavy (Hh) Cross 2 wavy people: Hh x Hh Genotypic Ratio: 1: 2: 1 Phenotypic Ratio: 1: 2: 1 Codominance: both alleles are fully expressed Human MN Blood groups: MM = M protein NN = N protein MN (heterozygous) = Both M & N proteins

HETEROZYGOTE PHENOTYPES CAN RANGE BETWEEN COMPLETE DOMINANCE, CODOMINANCE, AND DIFFERENT DEGREES OF INCOMPLETE DOMINANCE Tay-Sachs Disease: Defective enzyme fails to metabolize gangliosides (lipids) which accumulate in the brain. Brain functions gradually stop, leading to death. • Organism Level: Tay-Sachs seems recessive. Heterozygotes do NOT have the disease • Biochemical Level: Tay-Sachs seems codominant. Heterozygotes produce ½ normal enzyme and ½ dysfunctional enzyme

CONCLUSIONS: Dominant / Recessive relationships are rarely as straightforward as Mendel thought!!! Dominant Allele: does NOT subdue or repress recessive allele Dominant: NOT always more common! Relative frequencies of alleles in a population are mostly due to: Natural selection Achondroplasia (the type of dwarfism the family from “Little People” carries) is a dominant trait.

Multiple Alleles: More than 2 different alleles exist for some genes. Example: Human ABO blood groups 3 different alleles: IA, IB, i Blood Group Phenotype A B AB O Genotype(s) IAIA or IAi IBIB or IBi IAIB only ii only

Example: Human ABO blood group Cross: Cross a heterozygous “B” woman with a man having type “AB” blood I B i X I AI B IB Genotypic ratio: Phenotypic ratio: 1: 1: 1: 1 1: 2: 1 i IA I AI B I Ai IB IB i

Pleiotropy: Most genes have many phenotypic effects. • In tigers: 1 allele = cross-eyed & abnormal pigmentation • Most hereditary diseases have multiple effects: sickle cell anemia. . . • Enzyme encoding genes: may have cascading effects

Epistasis: Gene at one locus controls the expression of a gene at a second locus Example: mammal coat color B = black and b = brown but a separate gene, C = pigment deposition and c = no deposition If “cc”: No pigment (albino) no matter what “B’s” are there Bb. Cc x Bb. Cc: would not give Mendel’s predicted 9: 3: 3: 1 Albino Gorilla Brown Gorilla Black Gorilla

Polygenic Inheritance: 2 or more gene pairs affect a single phenotype (opposite of pleiotropy) Example: human skin color • Controlled by: at least 3 gene pairs • Graph: bell-shaped curve NATURE vs. NURTURE: Environment influences phenotype • Identical twins: show effect of nutrition, experiences, exercise. . . • Norm of Reaction = range of phenotypic variation: • Can be rigid (ABO) • Can fluctuate (white blood cell count) genetic environmental Multifactorial Characters: Many factors, ______ and ________ determine phenotype. Although genetically identical, these twins look very different at birth due to twin-to-twin transfusion, a problem that occurs when blood flows from one twin to another in the womb.

• Eye color -- determined by two genes -- appears to vary on an almost continuous scale from brown to green to gray to blue. • One gene controls texture of the iris which refracts light to make blue. A second determines relative abundance of melanin. • Less melanin = green eyes • More melanin = brown and (with relatively increasing amounts of melanin) black eyes

Mendelian Genetics in Humans: Difficult to study due to • 20 year generation time • few offspring • breeding experiments impossible Human Pedigrees: Family “tree” which shows inheritance patterns for a specific phenotype • Square = male • Circle = female • Shaded = affected (not necessarily recessive!)

Recessive Disorders: Albinism, Cystic Fibrosis • Typically caused by mating between two carriers: Aa x Aa • Consanguineous Mating: “same blood”, increases risk Dominant Disorders: Much less common (eliminated by Natural Selection) Achondroplasia: homoz. = fatal, heteroz. = can reproduce Huntington’s Disease: lethal dominant gene is expressed AFTER child-bearing years

Genetic Screening & Counseling • Consider family history, predict chances • Blood tests: Can detect carriers of sickle-cell, Tay-Sachs, etc • Fetal Testing: amniocentesis, chorionic villus sampling • Newborn Screening: PKU (treatable)

Chapter 15: “Chromosomes” 1902: It is established that Mendelian genes are located on chromosomes and it is the chromosomes that undergo segregation and independent assortment (during meiosis). Thomas Hunt Morgan: • First to associate a specific gene with a specific chromosome • Used Drosophilia melangaster (“fruit fly”): Prolific Only 4 pairs of chromosomes sex chromosomes autosomes • 3 pairs ___________, 1 pair _____________ • Like humans, XX = female, XY = male • Trait common in nature (wild type) = red eyes • Morgan found white-eyed flies: usually male, must be on X chromosome, (sex-linked)

Example X-linked cross: Cross a normal male with a female who is a carrier for hemophilia X AY x X AX a XA Genotypic ratio: Phenotypic ratio: 1: 1: 1: 1 3: 1 Y XA X AX A X AY Xa X AX a X a. Y

Genes on the Y chromosome

Genetic Recombination: Production of offspring with new combinations of traits inherited from 2 parents. • Unlinked genes: show Independent Assortment • Offspring can be parental type or recombinants • Any 2 unlinked genes = 50% frequency of recombination • Linked genes: only form recombinants if crossing over occurs • Genetic maps are created based on the crossover rates (more on this in Ch. 20)

Example: 4 genes: J, K, L, M are linked. What is their order on the chromosome if crossover frequencies between K and J = 3 K and L = 8 J and M = 12 L and M = 7 Jxxxxxx. M K x x? L? Kxx. Jxxxx. Lxxxxxx. M

Sex-linked traits: Genes on X chromosome. • If recessive, more common in: Males (only one recessive needed) Hemizygous (XAY or Xa. Y) • Examples: color blindness, hemophilia, Lesch-Nyhan syndrome, Duchenne muscular dystrophy Sex-limited traits: Genes expressed in only one sex. • Example: beard growth Sex-influenced traits: Allele is dominant in one sex and recessive in the other. (hormonal influence) • Example: pattern baldness X-inactivation: In female mammals, one X in every cell is randomly inactivated (Barr body) • Causes: mosaicism for certain heterozygote traits

CHROMOSOMAL ALTERATIONS: Physical and chemical disturbances, as well as meiotic errors, can damage chromosomes or alter their number in a cell. Aneuploidy: Abnormal chromosome number • Caused by: non-disjunction (homologous chromosomes fail to separate) • Can occur during mitosis (with sister chromatids) • Results in: • Trisomy (2 n +1): 1 extra chromosome • Monosomy (2 n - 1): 1 • Polyploidy: missing chromosome More than 2 whole sets of chromosomes • Triploidy = 3 n, Tetraploidy = 4 n • Fatal in humans • Common in plant kingdom

Alterations in Chromosome Structure (See Figure 15. 14, p. 286): Deletion: missing fragments. ABCDE becomes ABCE Duplication: (insertion) doubled section (from homologue). ABCDE becomes ABCDCDE Inversion: chromosome fragment is reattached in reverse orientation: ABCDE becomes ABDCE Translocation: fragment switched to non-homologous chromosome. May cause leukemia. ABCDE and WXYZ become ABCYZ and WXDE

HUMAN SYNDROMES RESULTING FROM CHROMOSOMAL ABNORMALITIES: • Down Syndrome (1/ 700 live births): Trisomy 21 • Patau Syndrome (1/ 5000 live births): Trisomy 13 (survive < 1 year) • Klinefelter’s Syndrome (1/ 2000 live births): XXY (may be sterile) • Triple X Syndrome (1/ 1000 live births): healthy – can only ID through karyotype • Turner’s Syndrome (1/ 5000 live births): XO sterile (ONLY viable human monosomy) • XYY Syndrome (1/ 2000 live births): phenotypically normal • cri du chat Syndrome: deletion in chromosome #5; retardation, often die in early infancy

Two final Notes: (1)Genomic Imprinting refers to the autosomal genes which are only expressed if they came from a certain parent (maternal or paternal) – RARE! (24 -36 mammal genes known) (2) DNA located in mitochondria and plasmids is not inherited in Mendelian fashion. What type of inheritance will these genes show? MATERNAL!

Gene expression – not all genes are expressed

These identical twins, separated at birth, were reunited at age 31. Although raised apart, they had a great deal in common. Both had become firefighters, wore mustaches, were balding, had poor vision, enjoyed hunting and fishing, and drank the same brand of beer. Comparisons of twins reared apart are used by scientists to study the relative influence of heredity and upbringing on particular traits.

Old explanations for disorders • Evil spirits • Impregnation by animals • Angering the gods • Witchcraft • Frightened during pregnancy Homonculus theory

Hemophilia (Sex-linked Recessive)