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Jay Phelan What Is Life? A Guide To Biology First Edition CHAPTER 7 Mendelian Inheritance © 2010 W. H. Freeman and Company INHERITANCE OF GENES Maternal chromosome pair Paternal chromosome pair Gene Maternal gamete: egg Paternal gamete: sperm Humans have 23 pairs of chromosomes (46 individual chromosomes) and, thus, two copies of each gene. Each human gamete has just one copy of each chromosome and, thus, one copy of each gene. Gametes unite during fertilization. Child inherits one set of chromosomes from each parent and, thus, two copies of each gene. FISH ODOR SYNDROME Egg Sperm Normal version of FMO3 gene Defective version of the FMO3 gene, responsible for fish odor syndrome Gametes unite during fertilization. Genes for FMO3 Child inherits one copy of the defective version of the FMO3 gene. The child is a “silent carrier” and will not have fish odor syndrome. If two copies of the defective version of FMO3 were present, the child would develop fish odor syndrome. Once breeders recognized the existence of heredity, selective breeding—such as for increased body size in cattle—became possible. EXAMPLES OF SINGLE-GENE TRAITS Cleft chin Non-cleft chin EXAMPLES OF SINGLE-GENE TRAITS Unattached earlobes Attached earlobes EXAMPLES OF SINGLE-GENE TRAITS Widow’s peak Straight hairline The mistaken idea that a tiny, pre-made human existed in every sperm cell was introduced in the 1600s. This theory remained popular through the 1800s. MENDEL’S RESEARCH APPROACH Gregor Mendel (1822–1884) Three features of Mendel’s methodical research were critical to its success. DOMINANT AND RECESSIVE TRAITS 1 Mendel crossed true-breeding purple-flower plants with true-breeding white-flower plants. True-breeding purple-flower plant True-breeding white-flower plant DOMINANT AND RECESSIVE TRAITS 1 Mendel crossed true-breeding purple-flower plants with truebreeding whiteflower plants. True-breeding purple-flower plant The purple-colored flower is the dominant trait, while the whitecolored flower is a recessive trait. 2 Then, Mendel crossed two of the purple-flower offspring. True-breeding white-flower plant All offspring have purple flowers. DOMINANT AND RECESSIVE TRAITS The purple-colored flower is the dominant trait, while the whitecolored flower is a recessive trait. All offspring have purple flowers. 2 Then, Mendel crossed two of the purpleflower offspring. Most offspring have purple flowers, but some have white flowers. The recessive trait for the white-colored flower must have been lurking in the previous generation, even though it is not visible. MENDEL’S LAW OF SEGREGATION According to Mendel’s law of segregation, only one of the two alleles for a gene is put into a gamete. At fertilization, offspring receive from each parent one allele for each gene. Heterozygous pea plant Heterozygous pea plant Two different alleles (white, purple) for the same gene (flower color) MEIOSIS Each gamete gets one copy of each gene. MENDEL’S LAW OF SEGREGATION FERTILIZATION Each fertilized egg gets two copies of each gene. Homozygous recessive Heterozygous Heterozygous Homozygous dominant PHENOTYPE: Little or no pigment in the eyes, hair, and skin GENOTYPE: Homozygous for the recessive allele for albinism PUNNETT SQUARE: ALBINISM A Punnett square is a useful tool for determining the possible outcomes of a cross between two individuals. Cross 1 MOTHER albino homozygous aa a FATHER pigmented homozygous AA GAMETES a GAMETES A Aa Aa Aa Aa A OFFSPRING Genotype All heterozygous Aa Phenotype All pigmented MOTHER pigmented heterozygous Aa Cross 2 A FATHER pigmented heterozygous Aa GAMETES a GAMETES A AA Aa Aa aa a OFFSPRING Genotype Phenotype 1/4 homozygous dominant AA 2/4 heterozygous Aa 1/4 homozygous recessive aa 3/4 pigmented 1/4 albino GENETICS AND PROBABILITY IF… The mother is albino, and the father is heterozygous aa Aa 100% a 50% a 1.0 A × THEN… There is a 100% chance that the mother’s egg will carry the recessive a allele and a 50% chance that a sperm will carry the recessive a allele a 0.5 Multiply the two components together to determine the overall probability. AND… = 0.5 or 50% chance the offspring will be albino. aa TAY-SACHS AND PROBABILITY IF… Parents are heterozygous for Tay-Sachs Tt Tt 50% T t 50% T THEN… There is a 50% chance that a gamete will carry the recessive t allele t AND… × 0.5 0.5 = 0.25 or 25% chance the child will be homozygous Multiply the two components together for Tay-Sachs. to determine the overall probability. tt TEST-CROSS: WHITE ALLIGATORS TEST-CROSS: WHITE ALLIGATORS MOTHER white homozygous mm Test-cross FATHER pigmented unknown genotype M_ (MM or Mm) m GAMETES m GAMETES M M Unknown allele could be M or m. m Mm Mm Mm Mm Mm Mm OFFSPRING (if unknown genotype is MM) Genotype Phenotype All heterozygous Mm All pigmented OFFSPRING (if unknown genotype is Mm) Genotype Phenotype 2/4 heterozygous Mm 2/4 pigmented 2/4 homozygous recessive mm 2/4 white PEDIGREE A pedigree is a useful tool to document a trait of interest across multiple generations of family members. Grandfather Aunt First cousin Grandmother Uncle First cousin Grandfather Mother Father Sister Me Grandmother Uncle Aunt Sister Female exhibiting trait of interest Female not exhibiting trait Male exhibiting trait of interest Male not exhibiting trait ANURY Anury is a condition in dogs and some other animals in which they are born without a tail. The condition is inherited as a recessive trait. ANURY ? Can you figure out the genotype of the individual labeled “1”? IF… A dog has no tail, but his father was normal THEN… The normallooking mother (1) of the tailless dog must be heterozygous. PEDIGREE Female with anury 1 Female with tail Male with anury Male with tail ? ? Can you now determine the probability of this puppy having anury? INCOMPLETE DOMINANCE: SNAPDRAGONS Incomplete dominance occurs when a heterozygote exhibits an intermediate phenotype between the two homozygotes. Cross 1 The superscript W represents the allele that produces no pigment (white flower). The superscript R represents the allele that produces pigment (red flower). MOTHER white-flower homozygous CW CW CW FATHER red-flower homozygous CRCR GAMETES CW GAMETES CR CWCR CWCR CWCR CWCR CR OFFSPRING Genotype All heterozygous CWCR Phenotype All pink flowers INCOMPLETE DOMINANCE: SNAPDRAGONS Cross 2 CW GAMETES CR CW GAMETES FATHER pink-flower heterozygous CW CR MOTHER pink-flower heterozygous CW CR CWCW CWCR CWCR CRCR CR OFFSPRING Genotype Phenotype 1/4 homozygous CRCR 1/4 red flowers 2/4 heterozygous CWCR 2/4 red flowers 1/4 homozygous CWCW 1/4 white flowers CODOMINANCE: SICKLE-CELL DISEASE Codominance occurs when a heterozygote displays characteristics of both homozygous parents. CODOMINANCE: SICKLE-CELL DISEASE HbAHbA Homozygote HbSHbA Heterozygote HbSHbS Homozygote Individual does not have sicklecell disease. Individual carries a defective allele and has an intermediate condition. Some cells become sickled under extreme conditions. Individual has two copies of the defective hemoglobin allele and has sickle-cell disease. MULTIPLE ALLELISM: BLOOD TYPE Multiple allelism occurs when there are three or more alleles for a gene within a population. An individual still inherits only two alleles— one from each parent. Three alleles possible: A (dominant to O and codominant with B) B (dominant to O and codominant with A) O (recessive to A and B) Gene that determines blood type Type A A A Type B or A B B Type A Type AB A B O O or O B O Red blood cells have 6 different genotypes (AA, AO, BB, BO, AB, and OO). These genotypes result in 4 different phenotypes (type A, type B, type AB, and type O). BLOOD TYPE, ANTIGENS, AND ANTIBODIES Antigens are chemicals on the surface of some cells. They act as signposts that tell the immune system whether the cell belongs in the body. Antibodies are immune system molecules that attack cells with foreign antigens. Type A A antigens B antibodies Type B B antigens A antibodies Type AB A and B antigens Neither A nor B antibodies Type O Neither A nor B antigens A and B antibodies THE SCIENCE BEHIND BLOOD DONATION BLOOD TYPE CAN DONATE TO CAN RECEIVE FROM Type A Type A Type AB Type A Type O Type B Type B Type AB Type B Type O • Has A antigens • Produces antibodies that attack B antigens • Has B antigens • Produces antibodies that attack A antigens Type AB • Has A and B antigens • Produces no antibodies • Universal recipient Type O • Has no antigens • Produces antibodies that attack A and B antigens • Universal donor Type AB Type A Type B Type AB Type O Type A Type B Type AB Type O Type O Individuals with type O blood are universal donors. Individuals with type AB are universal recipients. POLYGENIC TRAITS PLEIOTROPY: ONE GENE, MANY EFFECTS Pleiotropy occurs when one gene influences multiple, different traits. Heterozygote for the sickle-cell trait An allele that causes cells to sickle has two effects: 1 It disrupts red blood cells’ oxygen delivery. 2 It causes red blood cells to be inhospitable to malarial parasites. PLEIOTROPY: ONE GENE, MANY EFFECTS Someone with no sickled cells does not have sickle-cell anemia but is susceptible to malaria. Malarial parasite infecting a red blood cell SEX-LINKED TRAITS: COLOR-BLINDNESS A sex-linked trait is carried on the X chromosome. Women carry two copies of the X chromosome, while men carry an X chromosome and a Y chromosome. X Gene with instructions for light-sensitive proteins within the eye Two alleles possible: R (produces functioning light-sensitive proteins) r (produces defective light-sensitive proteins) SEX-LINKED TRAITS: COLOR-BLINDNESS TO BE COLOR-BLIND X r X X Y Male must inherit colorblindness allele (r) from his mother. r r Female must inherit color-blindness allele (r) from both parents. SEX-LINKED TRAITS: COLOR-BLINDNESS TO HAVE NORMAL VISION X R X X Y Male must inherit normal color-vision allele (R) from his mother. R X X r R Female can inherit normal colorvision allele (R) from either her mother or her father. R Siamese cats and some rabbits have genes that produce light fur at warmer temperatures and dark fur at colder temperatures (such as on the tail, nose, ears, and feet). MENDEL’S LAW OF INDEPENDENT ASSORTMENT Mendel’s law of independent assortment states that one trait does not influence the inheritance of another trait. MOTHER albino homozygous aa dimpled chin homozygous DD Cross 1 a FATHER pigmented homozygous AA non-dimpled chin homozygous dd D GAMETES a D A d Aa Dd Aa Dd Aa Dd Aa Dd A d OFFSPRING Genotype All heterozygous Aa All heterozygous Dd Phenotype All pigmented All dimpled chin In this example, having a dimpled chin does not affect which alleles are inherited for skin pigmentation. MENDEL’S LAW OF INDEPENDENT ASSORTMENT Mendel’s law of independent assortment states that one trait does not influence the inheritance of another trait. MOTHER pigmented heterozygous Aa dimpled chin heterozygous Dd Cross 2 A FATHER pigmented heterozygous Aa dimpled chin heterozygous Dd D GAMETES a D A D AA DD Aa Dd Aa Dd aa dd aA d OFFSPRING Genotype 1/4 homozygous dominant AA 1/4 homozygous dominant DD 2/4 heterozygous Aa 2/4 heterozygous Dd 1/4 homozygous recessive aa 1/4 homozygous recessive dd Phenotype 3/4 pigmented 3/4 dimpled chin 1/4 albino 1/4 non-dimpled chin LINKED GENES If this gamete is fertilized, the linked genes will be passed on to the offspring together as a group. Homologous chromosomes Linked genes Maternal copy Paternal copy When crossing over occurs, linked genes usually stay together. Homologous chromosomes Each of the four chromatids after the exchange of genetic gets packaged into a gamete. information If genes are far apart on a chromosome or are on different chromosomes, the inheritance of one does not influence the inheritance of another.