Chapter 9 MENDEL S LAWS. Patterns of Inheritance. History of Genetics. Experimental genetics began in an abbey garden. Genetics = science of heredity

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1 Chapter 9 Figure 9.0_ Chapter 9: Big Ideas Patterns of Inheritance Mendel s Laws Variations on Mendel s Laws The Chromosomal Basis of Inheritance Sex Chromosomes and Sex-Linked Genes History of Genetics MENDEL S LAWS Genetics = science of heredity How inherited characteristics are passed along from one generation to next Modern genetics began in 860 with monk named Gregor Mendel Bred garden peas, watched how characteristics were passed down through generations define-sweet-pea.htm Experimental genetics began in an abbey garden Review of Flower Anatomy & Reproduction In 866, Mendel postulated: parents pass on to offspring discrete heritable factors heritable factors (today called genes), retain their individuality through generations. Heritable feature that varies among individuals (e.g. flower color) = character. Each variant for a character (e.g. purple or white ) = trait. Usually self-fertilize if left alone. To self-fertilize (for sure), bag to prevent cross. To cross-fertilize, cut stamens, sprinkle carpels w/ foreign pollen, and plant resulting peas for next generation.

2 Figure 9.C_s3 This way, Mendel always knew the parentage. Parents (P) Carpel Removal of stamens Transfer Purple of pollen 3 Stamens Carpel matures into pea pod 4 Seeds from pod planted Mendel studied 7 characteristics, each w/ distinct forms (traits) Character Traits Dominant Recessive Flower color Purple Flower position Axial Terminal Seed color Yellow Green Seed shape Round Wrinkled Pod shape Inflated Constricted Pod color Green Yellow Offspring (F ) Stem length Tall Dwarf Some Vocabulary Mendel always started with True-breeding varieties: selffertilization produced offspring all identical to the parent. Mendel asked: The Experiment (true-breeding parents) Purple Like purebred dogs (you know what you re gonna get) Offspring of two different varieties = hybrid. Cross-fertilization = hybridization, or genetic cross. True-breeding parental plants = P ( Parental ) generation. Hybrid offspring = F ( Filial Latin for son ) generation. A cross of F plants produces an F ( Filial ) generation. Figure 9.3A_s The Experiment (true-breeding parents) Figure 9.3A_s3 The Experiment (true-breeding parents) Purple Purple F generation All plants have purple F generation All plants have purple Was the heritability of white lost? Don t know do another cross! F generation Fertilization among F plants (F F ) 3 of plants 4 have purple of plants 4 have white

3 Experimental Results disproved Blending Hypothesis The all-purple F generation did not produce light purple, as predicted by the blending hypothesis. Mendel needed to explain why: white color seemed to disappear in the F generation and white color reappeared in one quarter of the F offspring. Mendel observed the same patterns of inheritance for six other pea plant characters. Experimental Results Led to 4 New Hypotheses Mendel developed four hypotheses (described below using modern terminology).. Alleles = alternative versions of genes.. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different. A homozygous genotype has identical alleles (AA). A heterozygous genotype has two different alleles (Aa). Experimental Results Led to 4 New Hypotheses Experimental Results Led to 4 New Hypotheses 3. If the alleles of an inherited pair differ, then one determines the organism s appearance and is called the dominant allele. The other has no noticeable effect on the organism s appearance and is called the recessive allele. The phenotype is the appearance or expression of a trait (i.e. purple or white). The genotype is the genetic makeup of a trait (i.e. PP, Pp, or pp). The same phenotype may be determined by more than one genotype (i.e. purple can be either PP or Pp). 4. A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes law of segregation. Mendel s hypotheses also explain the 3: ratio in the F generation. The F hybrids all must have a Pp genotype. A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine. Figure 9.3B_s The Explanation Figure 9.3B_s The Explanation Genetic makeup (alleles) Purple PP pp Genetic makeup (alleles) Purple PP pp Gametes All P All p Gametes All P All p F generation (hybrids) All Pp Gametes P p

4 Punnett Square to Predict F Generation results Mendel predicted basic events of Meiosis! It s all about probability! F generation Phenotypic ratio 3 purple : white Genotypic ratio PP : Pp : pp P Eggs from F plant p Sperm from F plant P PP Pp p Pp pp Always place GAMETES on outside of square inside squares yield possible GENOTYPES Homologous chromosomes P P Gene loci Genotype: PP aa Bb Homozygous Homozygous for the for the dominant recessive allele allele Homologous chromosomes bear the alleles for each character at the same locus a a B b Dominant allele Recessive allele Heterozygous, with one dominant and one recessive allele Genetic traits in humans can be tracked Genetic traits in humans can be tracked through family pedigrees In a simple dominant-recessive inheritance of dominant allele A and recessive allele a, a recessive phenotype always results from a homozygous recessive genotype (aa) but a dominant phenotype can result from either the homozygous dominant genotype (AA) or a heterozygous genotype (Aa). f F A pedigree (a.k.a. family tree) Shows inheritance of a trait in a family through multiple generations, demonstrates dominant or recessive inheritance, and can also be used to deduce genotypes of family members. Many inherited disorders in humans are controlled by a single gene Deafness = Recessive Inheritance Inherited human disorders show either. recessive inheritance (more common) two recessive alleles are needed to show disease heterozygous parents are carriers of the disease-causing allele the probability of inheritance increases with inbreeding, mating between close relatives. dominant inheritance Only one dominant allele is needed to show disease dominant lethal alleles are usually eliminated from the population with early (or preterm) death of individual. Homozygous Recessive = deaf

5 Many inherited disorders in humans are controlled by a single gene Table 9.9 The most common fatal genetic disease in the United States is cystic fibrosis (CF). The CF allele is: recessive and carried by about in 3 Americans. Dominant human disorders include: achondroplasia, resulting in dwarfism (heterozygotes have disorder, homozygous dominant causes death in embryo) Huntington s disease, degenerative disorder of nervous system (develops in mid-life, after breeding age usually pass to offspring before knowledge of disease) New technologies can provide insight into one s genetic legacy New technologies offer ways to obtain genetic information Figure 9.0A Amniocentesis Amniotic fluid extracted Ultrasound transducer Fetus during pregnancy, and after birth. Genetic testing can identify potential parents who are heterozygous carriers for certain diseases. Fetus Placenta Chorionic villi Placenta Uterus Cervix before conception, Chorionic Villus Sampling (CVS) Tissue extracted from the chorionic villi Ultrasound transducer Centrifugation Amniotic fluid Fetal cells Several hours Cultured cells Week 4-6 Several weeks Several weeks Biochemical and genetics tests Cervix Uterus Fetal cells Several hours Several hours Karyotyping New technologies can provide insight into one s genetic legacy VARIATIONS ON MENDEL S LAWS Ultrasound imaging: fetus examined for anatomical deformities Newborn screening can detect diseases that can be prevented by special care and precautions but sometimes not. Brings up major ethical questions for parents and society Week 8

6 Patterns of Inheritance not usually so Simple Incomplete Dominance Mendel s pea plants always looked like one of the parental varieties complete dominance Red RR Gametes R r rr For some characters, F hybrids look like a blend of the phenotypes of the two parental varieties incomplete dominance neither allele is dominant over the other expression of both alleles occurs F generation F generation Gametes R Pink hybrid Rr Sperm R r r Eggs R r RR Rr rr rr Some genes have more than alleles in population Some genes have more than alleles in population Although an individual can carry only alleles for a particular gene, more than two alleles often exist in the wider population. Human ABO blood group phenotypes involve three alleles for a single gene. In codominance, neither allele is dominant over the other expression of both alleles is observed as a distinct phenotype in the heterozygous individual. AB blood type is an example of codominance. The four human blood groups, A, B, AB, and O, result from combinations of these three alleles. The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance. Figure 9. One Gene Many Characters Blood Group (Phenotype) A B AB Genotypes I A I A or I A i I B I B or I B i I A I B Carbohydrates Present on Red Blood Cells Carbohydrate A Carbohydrate B Carbohydrate A and Carbohydrate B Antibodies Present in Blood Anti-B Anti-A None Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB In some cases, one gene influences many physical characteristics. i.e. Sickle-cell disease Affects type of hemoglobin and shape of red blood cells causes anemia, organ/brain damage, infections O ii Neither Anti-A Anti-B No reaction Clumping reaction Sickle-cell and nonsickle alleles are codominant. Carriers of sickle-cell disease are resistant to malaria.

7 One Character Many Genes The environment also affects many characters Sometimes a single phenotypic character results from the additive effects of two or more genes. Many characters result from a combination of heredity and the environment. For example, skin color is affected by exposure to sunlight, susceptibility to diseases, such as cancer, has hereditary and environmental components, and e.g. Human skin color, or height identical twins show some differences. Only genetic influences are inherited. THE CHROMOSOMAL BASIS OF INHERITANCE Exception to the rules : Genes on the same chromosome tend to be inherited together Linked genes located close together on the same chromosome tend to be inherited together Crossing over produces new combinations of alleles Linked alleles can be separated by crossing over, forming recombinant gametes. SEX CHROMOSOMES AND SEX-LINKED GENES

8 Chromosomes determine sex in many species Egg = always X Sperm = X or Y Sex chromosomes, designated X and Y, determine an individual s sex. In mammals, Parents (diploid) Male 44 XY Female 44 XX males have XY sex chromosomes, females have XX sex chromosomes, Gametes (haploid) X Sperm Y X Egg the Y chromosome has genes for the development of testes, and an absence of the Y allows ovaries to develop. Offspring (diploid) 44 XX Female 44 XY Male Chromosomes determine sex in many species Chromosomes determine sex in many species Grasshoppers, roaches, and some other insects have an X-O system, in which O stands for the absence of a sex chromosome, females are XX, and males are XO. In some reptiles, environmental temperature during egg incubation determines the sex. Global climate change may therefore impact the sex ratio of such species. In bees, sex is determined by chromosome number. Females are diploid. Males are haploid. zoology/reptiles-amphibians/ alligator4.htm Sex-linked genes exhibit a unique pattern of inheritance Sex-linked genes are located on either of the sex chromosomes (but almost always on X). The X chromosome (big) carries many genes unrelated to sex The Y chromosome (small): all genes related to maleness The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait.

9 Figure 9.C Female Male Figure 9.D Female Male X R X r X R Y X R X r X r Y Sperm Y x R X r Sperm Y Eggs X R X R X R X R Y Eggs X R X R X r X R Y X r X r X R X r Y X r X r X r X r Y R = red-eye allele r = white-eye allele R = red-eye allele r = white-eye allele Human sex-linked disorders affect mostly males Human sex-linked disorders affect mostly males Most sex-linked human disorders are due to recessive alleles and seen mostly in males. A male receiving a single X-linked recessive allele from his mother will have the disorder. A female must receive the allele from both parents to be affected. Recessive sex-linked human disorders include hemophilia: excessive bleeding -- hemophiliacs lack proteins required for blood clotting red-green color blindness: a malfunction of lightsensitive cells in the eyes Duchenne muscular dystrophy: progressive weakening of the muscles and loss of coordination Question: Can a male with the disorder have received his recessive allele from his dad? Queen Victoria Royal Families of Old Europe Afflicted with Hemophilia Alice Albert Alexandra Louis Czar Nicholas II of Russia Alexis Female Male Hemophilia Carrier Normal Red-Green Color Blindness = malfunction of light sensitive cells. Normal=50 colors CB=only 5 (Red looks grey) XcY shows up in males with only one allele XcXc must be homozygous recessive to have trait

10 Practice what you ve Learned Genes chromosomes located on alternative versions called (a) at specific locations called (b) if both are the same, the genotype is called (c) if different, the genotype is called heterozygous the expressed allele is called the unexpressed allele is called (d) (e) inheritance when the phenotype is in between is called (f)