CHAPTER 9 GENETICS

MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc.

Flower color White Axial Purple Flower positionTerminal Yellow Seed color Green Round Seed shapeWrinkled Inflated Pod shape Constricted Green Pod colorYellow Tall Stem lengthDwarf

Mendel’s law of segregation describes the inheritance of a single character  Example of a monohybrid (HETEROZYGOUS) cross Parental generation (HOMOZYGOUS): purple flowers  white flowers F 1 generation: all plants with purple flowers F 2 generation: ¾ of plants with purple flowers ¼ of plants with white flowers  Mendel needed to explain Why one trait seemed to disappear in the F 1 generation Why that trait reappeared in one quarter of the F 2 offspring Copyright © 2009 Pearson Education, Inc.

P generation (true-breeding parents) Purple flowers White flowers F 1 generation All plants have purple flowers F 2 generation Fertilization among F 1 plants (F 1  F 1 ) of plants have purple flowers 3–43–4 of plants have white flowers 1–41–4

Mendel’s law of segregation describes the inheritance of a single character  Four Hypotheses 1.Genes are found in alternative versions called ALLELES; a GENOTYPE is the listing of alleles an individual carries for a specific gene 2.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 – A heterozygous genotype has two different alleles Copyright © 2009 Pearson Education, Inc.

Mendel’s law of segregation describes the inheritance of a single character  Four Hypotheses 3.If the alleles differ, the DOMINANT allele determines the organism’s appearance, and the RECESSIVE allele has no noticeable effect – The PHENOTYPE is the appearance or expression of a trait – The same phenotype may be determined by more than one GENOTYPE (the alleles carried by the organism) 4.Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes (MEIOSIS) so that a sperm or egg carries only one allele for each gene Copyright © 2009 Pearson Education, Inc.

P plants 1–21–2 1–21–2 Genotypic ratio 1 PP : 2 Pp : 1 pp Phenotypic ratio 3 purple : 1 white F 1 plants (hybrids) Gametes Genetic makeup (alleles) All All Pp Sperm Eggs PP p ppPp P p P p P P p PP pp All Gametes F 2 plants

F 1 genotypes 1–21–2 1–21–2 1–21–2 1–21–2 1–41–4 1–41–4 1–41–4 1–41–4 Formation of eggs Bb female F 2 genotypes Formation of sperm Bb male B B B B B B b b b bb b

COIN TOSS DEMONSTRATION 4 students toss coin once…RESULTS? 4 students toss coin 3 times…RESULTS? All students toss coin once…RESULTS? All students toss coin 3 times…RESULTS?

Homologous chromosomes bear the alleles for each character  For a pair of homologous chromosomes, alleles of a gene reside at the same locus Homozygous individuals have the same allele on both homologous chromosome Heterozygous individuals have a different allele on each homologous chromosome Copyright © 2009 Pearson Education, Inc.

Gene loci Homozygous for the dominant allele Dominant allele Homozygous for the recessive allele Heterozygous Recessive allele Genotype: P B a P PP a aa b Bb

HUMAN EXAMPLES of Dominant and Recessive Genes

Freckles Widow’s peak Free earlobe No freckles Straight hairline Attached earlobe Dominant Traits Recessive Traits

CLASSROOM EXAMPLE How many students have free earlobes? How many students have attached earlobes? Which trait (phenotype) is caused by the DOMINANT gene?

CLASSROOM EXAMPLE (cont.) Free earlobe gene = F Attached earlobe gene = f What are the possible GENOTYPES for earlobe shape in the human population? What PHENOTYPES are produced by the different genotypes?

CLASSROOM EXAMPLE (cont.) A genetics problem: If Mary has attached earlobes and John has free earlobes (John’s mother has attached earlobes) what are the chances that their children will have attached earlobes? Free earlobes? Use a PUNNETT SQUARE to prove your answer!

CLASSROOM EXAMPLE 1.) Determine the possible genotypes for Mary and John. 2.) Create a PUNNETT SQUARE. 3.) Determine the PHENOTYPES of the children.

CONNECTION: Genetic traits in humans can be tracked through family pedigrees  A pedigree Shows the inheritance of a trait in a family through multiple generations Demonstrates dominant or recessive inheritance Can also be used to deduce genotypes of family members Copyright © 2009 Pearson Education, Inc.

Ff FemaleMale Attached Free First generation (grandparents) Second generation (parents, aunts, and uncles) Third generation (two sisters) Ff ff FF or

MORE GENETICS PROBLEMS See LAB MANUAL…

The law of independent assortment is revealed by tracking two characters at once  Example of a dihybrid (HETEROZYGOUS) cross Parental generation: round yellow seeds  wrinkled green seeds F 1 generation: all plants with round yellow seeds F 2 generation: 9/16 of plants with round yellow seeds 3/16 of plants with round green seeds 3/16 of plants with wrinkled yellow seeds 1/16 of plants with wrinkled green seeds  Mendel needed to explain Why nonparental combinations were observed Why a 9:3:3:1 ratio was observed among the F 2 offspring Copyright © 2009 Pearson Education, Inc.

The law of independent assortment is revealed by tracking two characters at once  Law of independent assortment Each pair of alleles segregates independently of the other pairs of alleles during gamete formation R = round; r = wrinkled Y = yellow; y = green For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry Copyright © 2009 Pearson Education, Inc.

P generation 1–21–2 Hypothesis: Dependent assortment Hypothesis: Independent assortment 1–21–2 1–21–2 1–21–2 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 9 –– 16 3 –– 16 3 –– 16 1 –– 16 RRYY Gametes Eggs F 1 generation Sperm F 2 generation Eggs Gametes rryy RrYy ry RY ry RY ry RY Hypothesized (not actually seen) Actual results (support hypothesis) RRYY rryy RrYy ry RY RRYY rryy RrYy ry RY RrYy rrYYRrYY RRYyRrYY RRYy rrYy Rryy RRyy rY Ry ry Yellow round Green round Green wrinkled Yellow wrinkled RY rY Ry

F 1 generation R Metaphase I of meiosis (alternative arrangements) r Y y R r Y y R r Y y All round yellow seeds (RrYy)

F 1 generation R Metaphase I of meiosis (alternative arrangements) r Y y R r Y y R r Y y All round yellow seeds (RrYy) Anaphase I of meiosis Metaphase II of meiosis R y r Y r y R Y R r Y y R r Y y

F 1 generation R Metaphase I of meiosis (alternative arrangements) r Y y R r Y y R r Y y All round yellow seeds (RrYy) Anaphase I of meiosis Metaphase II of meiosis R y r Y r y R Y R r Y y R r Y y 1–41–4 R y Ry R y r Y 1–41–4 rY r Y 1–41–4 ry r y 1–41–4 RY R Y R Y Gametes Fertilization among the F 1 plants :3 9 :1 F 2 generation r y

QUESTION… Why would genes independently assort even if they are on the same chromosome??? HINT: Think Prophase I (meiosis)…

Gametes Tetrad Crossing over Yryr r y R Y R Y R y

CONNECTION: Many inherited disorders in humans are controlled by a single gene  Inherited human disorders show Recessive inheritance – Two recessive alleles are needed to show disease – Heterozygous parents are carriers of the disease- causing allele – Probability of inheritance increases with inbreeding, mating between close relatives Dominant inheritance – One dominant allele is needed to show disease – Dominant lethal alleles are usually eliminated from the population Copyright © 2009 Pearson Education, Inc.

EXAMPLES of HUMAN DISORDERS The next slide lists a number of Recessive and Dominant AUTOSOMAL disorders… AUTOSOMAL means that the defective gene is found on one of the autosomes. AUTOSOMES = Chromosomes that are NOT one of the sex chromosomes SEX Chromosomes = The X or Y chromosomes that determine the sex of the human XX = Female XY = Male

Genetics Problem Cystic Fibrosis (Autosomal Recessive): If Mary and John are both heterozygous (carriers) for the cystic fibrosis gene, what are the chances that their children will have cystic fibrosis???

Parents Normal Carrier Cc Offspring Sperm Eggs cc Cystic Fibrosis c Cc Normal (carrier) CC Normal C C c Cc Normal (carrier) Normal Carrier Cc 

Genetics Problem Huntington’s (Autosomal Dominant): Only Genotypes = Hh (Huntington’s) or hh (normal) If Mary is normal and John will eventually develop Huntington’s disease, what are the chances that their children will develop Huntington’s disease???

Parents Mary hh Offspring Sperm Eggs hh Normal h hh Normal Hh Huntington’s h H h Hh Huntington’s John Hh 

VARIATIONS ON MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc.

Incomplete dominance results in intermediate phenotypes  Incomplete dominance Neither allele is dominant over the other Expression of both alleles is observed as an intermediate phenotype in the heterozygous individual Copyright © 2009 Pearson Education, Inc.

Flower Color in Plants…

P generation 1–21–2 1–21–2 1–21–2 1–21–2 1–21–2 1–21–2 F 1 generation F 2 generation Red RR Gametes Eggs Sperm RR rR Rrrr R r R r R r Pink Rr R r White rr

Hypercholesterolemia in Humans…

HH Homozygous for ability to make LDL receptors hh Homozygous for inability to make LDL receptors Hh Heterozygous LDL receptor LDL Cell Normal Mild disease Severe disease Genotypes: Phenotypes:

Sickle Cell Anemia in Humans HH = Normal Hemoglobin Hh = Sickle Cell Trait hh = Sickle Cell Anemia

The sickle cell gene also affects many phenotypic characters  Pleiotropy One gene influencing many characteristics The gene for sickle cell disease – Affects the type of hemoglobin produced – Affects the shape of red blood cells – Causes anemia – Causes organ damage – Is related to susceptibility to malaria Copyright © 2009 Pearson Education, Inc.

Clumping of cells and clogging of small blood vessels Pneumonia and other infections Accumulation of sickled cells in spleen Pain and fever Rheumatism Heart failure Damage to other organs Brain damage Spleen damage Kidney failure Anemia Paralysis Impaired mental function Physical weakness Breakdown of red blood cells Individual homozygous for sickle-cell allele Sickle cells Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped

Genetics Problem: Sickle Cell Anemia If Mary has normal hemoglobin and John has sickle cell anemia, what are the chances that their children will have sickle cell anemia? Will have sickle cell trait? Will have normal hemoglobin?

Sickle Cell Anemia Problem (cont.) What is Mary’s genotype? What is John’s genotype? Create the Punnett Square…

Many genes have more than two alleles in the population: MULTIPLE ALLELES  Multiple alleles More than two alleles are found in the population A diploid individual can carry any two of these alleles The ABO blood group has three alleles, leading to four phenotypes: type A, type B, type AB, and type O blood Copyright © 2009 Pearson Education, Inc.

CODOMINANCE  Codominance Neither allele is dominant over the other Expression of both alleles is observed as a distinct phenotype in the heterozygous individual Observed for type AB blood Copyright © 2009 Pearson Education, Inc.

Blood Group (Phenotype) Genotypes O A ii I A or I A i Red Blood Cells Carbohydrate A Antibodies Present in Blood Anti-A Anti-B Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Anti-B O AB AB B I B or I B i Carbohydrate B AB IAIBIAIB — Anti-A

GENETICS PROBLEMS ABO BLOOD TYPE…See Lab Manual

A single character may be influenced by many genes: POLYGENIC INHERITANCE  Polygenic inheritance Many genes influence one trait Skin color is affected by at least three genes Copyright © 2009 Pearson Education, Inc.

P generation 1–81–8 F 1 generation F 2 generation Fraction of population Skin color Eggs Sperm 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 aabbcc (very light) AABBCC (very dark) AaBbCc 1 –– –– 64 6 –– 64 1 –– –– 64 6 –– –– 64 1 –– –– 64 6 –– –– 64

The environment affects many characters  Phenotypic variations are influenced by the environment Skin color is affected by exposure to sunlight Susceptibility to diseases, such as cancer, has hereditary and environmental components Copyright © 2009 Pearson Education, Inc.

SEX CHROMOSOMES AND SEX-LINKED GENES Copyright © 2009 Pearson Education, Inc.

Chromosomes determine sex in many species  X-Y system in mammals (HUMANS), fruit flies XX = female; XY = male  X-O system in grasshoppers and roaches XX = female; XO = male  Z-W in system in birds, butterflies, and some fishes ZW = female, ZZ = male  Chromosome number in ants and bees Diploid = female; haploid = male Copyright © 2009 Pearson Education, Inc.

X Y

GENETICS PROBLEM What are the chances that Mary and John’s children will be Boys? Girls? Mary = XX John = XY Create a PUNNETT SQUARE…

CONNECTION: Sex-linked disorders affect mostly males  Males express X-linked disorders such as the following when recessive alleles are present in one copy Hemophilia Colorblindness Duchenne muscular dystrophy Copyright © 2009 Pearson Education, Inc.

Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis

GENETICS PROBLEMS SEX LINKED TRAITS…See Lab Manual