Patterns of Inheritance Chapter 9 Patterns of Inheritance

Purebreds and Mutts–A Difference of Heredity Purebred dogs Are very similar on a genetic level due to selective breeding

Mutts, or mixed breed dogs on the other hand Show considerably more genetic variation

9.1 The science of genetics has ancient roots MENDEL’S LAWS 9.1 The science of genetics has ancient roots The historical roots of genetics, the science of heredity Date back to ancient attempts at selective breeding

9.2 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 B Figure 9.2 A

Mendel crossed pea plants that differed in certain characteristics And traced traits from generation to generation 1 Removed stamens from purple flower White Stamens Carpel 2 Transferred pollen from stamens of white flower to carpel of purple flower Parents (P) Purple 3 Pollinated carpel matured into pod 4 Planted seeds from pod Offspring (F1) Figure 9.2 C

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 Round Wrinkled Inflated Constricted Tall Dwarf Green Yellow Figure 9.2 D

9.3 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 P generation (true-breeding parents) F1 generation F2 generation Purple flowers White flowers  All plants have purple flowers Fertilization among F1 plants (F1  F1) of plants have purple flowers 3 4 of plants have white flowers 1 Figure 9.3 A

For each characteristic An organism inherits two alleles, one from each parent

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

Mendel’s law of segregation Predicts that allele pairs separate from each other during the production of gametes P plants Gametes Genetic makeup (alleles) F1 plants (hybrids) F2 plants PP pp All P All p All Pp Sperm 1 2 P p Pp Eggs Genotypic ratio 1 PP : 2 Pp: 1 pp Phenotypic ratio 3 purple : 1 white Figure 9.3 B

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

9.5 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

Actual results support hypothesis Mendel’s law of independent assortment States that alleles of a pair segregate independently of other allele pairs during gamete formation Hypothesis: Dependent assortment Hypothesis: Independent assortment RRYY rryy Gametes RrYy RY ry Sperm Ry RrYY RRYy rrYY rrYy RRyy Rryy Actual results contradict hypothesis Actual results support hypothesis Yellow round Green round Yellow wrinkled Green wrinkled Eggs P generation F1 generation F2 generation 1 2 4 9 16 3  Figure 9.5 A

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, 1 chocolate coat, BbNn  BbNn Phenotypes Genotypes Mating of heterozygotes (black, normal vision) Phenotypic ratio of offspring Figure 9.5 B

9.6 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 Bb All black 1 black : 1 chocolate Figure 9.6

9.7 Mendel’s laws reflect the rules of probability Inheritance follows the rules of probability

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 F1 genotypes Bb female Formation of eggs F2 genotypes Bb male Formation of sperm B b 1 2 4 Figure 9.7

9.8 Genetic traits in humans can be tracked through family pedigrees CONNECTION 9.8 Genetic traits in humans can be tracked through family pedigrees The inheritance of many human traits Follows Mendel’s laws Dominant Traits Recessive Traits Freckles No freckles Widow’s peak Straight hairline Free earlobe Attached earlobe Figure 9.8 A

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

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

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

Dominant Disorders Some human genetic disorders are dominant Figure 9.9 B

CONNECTION 9.10 New technologies can provide insight into one’s genetic legacy New technologies Can provide insight for reproductive decisions

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

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

Fetal Imaging Ultrasound imaging Uses sound waves to produce a picture of the fetus Figure 9.10 B

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

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

VARIATIONS ON MENDEL’S LAWS 9.11 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

9.12 Incomplete dominance results in intermediate phenotypes When an offspring’s phenotype is in between the phenotypes of its parents It exhibits incomplete dominance P generation F1 generation F2 generation Red RR Gametes  White rr Sperm Eggs Pink Rr R r rR 1 2 Genotypes: HH Homozygous for ability to make LDL receptors Hh Heterozygous hh for inability to make Phenotypes: LDL receptor Cell Normal Mild disease Severe disease Figure 9.12 A Figure 9.12 B

9.13 Many genes have more than two alleles in the population In a population Multiple alleles often exist for a characteristic

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 Blood Group (Phenotype) Genotypes Antibodies Present in Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB O A B AB ii IAIA or IAi IBIB IBi IAIB Anti-A Anti-B — Figure 9.13

9.14 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 Impaired mental function Paralysis Pneumonia and other infections Rheumatism Kidney 5,555 Figure 9.14

9.15 A single characteristic may be influenced by many genes Polygenic inheritance Creates a continuum of phenotypes P generation F1 generation F2 generation Sperm Eggs aabbcc (very light) AABBCC (very dark) AaBbCc  1 8 64 6 15 20 Skin color Fraction of population Figure 9.15

9.16 The environmental affects many characteristics Many traits are affected, in varying degrees By both genetic and environmental factors Figure 9.16

9.17 Genetic testing can detect disease-causing alleles CONNECTION 9.17 Genetic testing can detect disease-causing alleles Predictive genetic testing May inform people of their risk for developing genetic diseases

THE CHROMOSOMAL BASIS OF INHERITANCE 9.18 Chromosome behavior accounts for Mendel’s laws Genes are located on chromosomes Whose behavior during meiosis and fertilization accounts for inheritance patterns

Fertilization among the F1 plants The chromosomal basis of Mendel’s laws All round yellow seeds (RrYy) Metaphase I of meiosis (alternative arrangements) Anaphase I of meiosis Metaphase II of meiosis Gametes F1 generation F2 generation Fertilization among the F1 plants (See Figure 9.5A) 1 4 RY ry R y Y r rY Ry 9 : 3 : 1 Figure 9.18

Not accounted for: purple round and red long 9.19 Genes on the same chromosome tend to be inherited together Certain genes are linked They tend to be inherited together because they reside close together on the same chromosome Experiment Explanation: linked genes PpLI  PpLI Long pollen Observed Prediction Phenotypes offspring (9:3:3:1) Purple long Purple round Red long Red round Parental diploid cell PpLI Most gametes offspring Eggs 3 purple long : 1 red round Not accounted for: purple round and red long Meiosis Fertilization Sperm 284 21 55 215 71 24 P I P L Purple flower Figure 9.19

9.20 Crossing over produces new combinations of alleles Crossing over can separate linked alleles Producing gametes with recombinant chromosomes A B a b Tetrad Crossing over Gametes Figure 9.20 A

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

Demonstrated the role of crossing over in inheritance Morgan’s experiments Demonstrated the role of crossing over in inheritance Experiment Gray body, long wings (wild type) GgLI Female  Black body, vestigial wings ggll Male Offspring Gray long 965 944 206 185 Black vestigial Gray vestigial Black long Parental phenotypes Recombinant Recombination frequency = = 0.17 or 17% 391 recombinants 2,300 total offspring Explanation (female) (male) G L g l Eggs Sperm Figure 9.20 C

9.21 Geneticists use crossover data to map genes Morgan and his students Used crossover data to map genes in Drosophila Figure 9.21 A

Recombination frequencies Can be used to map the relative positions of genes on chromosomes. Mutant phenotypes Short aristae Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown Long aristae (appendages on head) Gray (G) Red (C) Normal (L) Wild-type phenotypes Chromosome g c l 9% 9.5% 17% Recombination frequencies Figure 9.21 B Figure 9.21 C

SEX CHROMOSOMES AND SEX-LINKED GENES 9.22 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 XX 22 X Y Figure 9.22 A

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

Other systems of sex determination exist in other animals and plants 22 + XX X Figure 9.22 B 76 + ZW ZZ Figure 9.22 C 32 16 Figure 9.22 D

9.23 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

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

The inheritance pattern of sex-linked genes Is reflected in females and males Female Male Sperm Xr Y XR Xr Y  XR XR XR Xr XR Y Eggs R = red-eye allele r = white-eye allele Xr XR Xr Xr Figure 9.23 B Figure 9.23 C Figure 9.23 D

9.24 Sex-linked disorders affect mostly males CONNECTION 9.24 Sex-linked disorders affect mostly males Most sex-linked human disorders Are due to recessive alleles Are mostly seen in males Queen victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Figure 9.24 A Figure 9.24 B

A male receiving a single X-linked allele from his mother Will have the disorder A female Has to receive the allele from both parents to be affected