It also explains biological variation

It also explains biological variation
MENDELIAN GENETICS What is genetics? The study of how traits are inherited or how genetic information is passed from one generation to the next. It also explains biological variation

Gregor Mendel 1850’s Grew up in a farm wanting to garden
Austrian monk (Flunked out of college twice) but became a mathematician Experimented with garden pea plants Using pea plants looked at seven different characters (height of plants, seed color, texture, flower color) and found evidence of how parents transmit genes to offspring Mendel’s statistical analysis provided a model for predicting what the next generation would be like

What was the prevalent believe about inheritance before Mendel?
People believed in “spontaneous generation” and in the “blending of characters” Blending theory Problem: Would expect variation to disappear Variation in traits persists Ex: Yellow and green parakeets should have all blue babies. This is not what you observe.

The gene theory An alternative idea is the “gene” idea. Parents pass on discrete individual heritable units: genes

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

The Garden Pea Plant Mendel chose to work with the pea plant because he could control which plant mated with which. Pea plants are Self-pollinating True breeding (different alleles not normally introduced) Can be experimentally cross-pollinated

Mendel crossed pea plants that differed in certain characteristics
And traced traits from generation to generation Mendel started his experiments with plants that were “true breeding”. 1 Removed stamens from purple flower White 2 Transferred pollen from stamens of white flower to carpel of purple flower Stamens Carpel 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

Mendel’s Principles of Genetics
Mendel refuted the “blending theory” of heredity and provided an explanation of how inheritance works without knowing anything about chromosomes or genes. He figured that traits must be coded for by some kind of inheritable particle which he called “factors” and now we call “genes”. He said that those genes were transmitted as independent entities from one generation to the next.

Mendel’s insight continued… 3
Mendel’s insight continued… 3. He figured that there must be different versions of these “genes” ( we call them now “alleles”)and that every individual has two genes for each trait. (Or we can say that: For each characteristic an organism inherits two alleles, one from each parent) He identified one as dominant, the other as recessive.

4. He figured that the two alleles a parent has are separated into different cells when gametes (sex cells) are formed. This actually happens during metaphase of meiosisI ( no one knew about meiosis in those days). This is known as the Law of Segregation What are alleles? Different versions of the same gene

Mendel’s Theory of Segregation
An individual inherits a unit of information (allele) about a trait from each parent During gamete formation, the alleles segregate from each other

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

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

What is a dominant trait
What is a dominant trait? The trait that shows, the allele that is fully expressed What is a recessive trait? The alleles that is masked, the gene is there but it doesn’t show What is the phenotype? The observable traits What is the genotype? The genetic make up

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

Vocabulary When you mate two contrasting true breeding plants you get a Hybrid. The true breeding parents are called the “P” (parent) generation The hybrid offspring of the P generation are called the F1 generation When two F1 individuals self pollinate you get the F2 generation

F1 Results of One Monohybrid Cross

F2 Results of Monohybrid Cross

Mendel’s Monohybrid Cross Results
5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow F2 plants showed dominant-to-recessive ratio that averaged 3:1 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf

Punnett Square of a Monohybrid Cross
Female gametes Male gametes A a A a Aa AA aa Dominant phenotype can arise 3 ways, recessive only one

A Test cross In a pea plant with purple flowers the genotype is not obvious. Could be homozygous or heterozygous Why do a test cross? It allows us to determine the genotype of an organism with a dominant phenotype but unknown genotype

Test Cross You cross an individual that shows the dominant phenotype with an individual with recessive phenotype ( one who is homozygous recessive for that trait) Examining offspring allows you to determine the genotype of the dominant individual

Punnett Squares of Test Crosses
Homozygous recessive a a A a aa Aa Homozygous recessive a a A Aa Two phenotypes All dominant phenotype

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

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

Web sites to check

Mendel’s two Laws 1. Law of segregation
The two alleles for a trait segregate during gamete formation and only one allele for a trait is carried in a gamete. The gametes combine at random (In other words:A cell contains two copies of a particular gene, they separate when a gamete is made). 2. Law of Independent Assortment Alleles from one trait behave independently from alleles for another trait. Traits are inherited independently from one another

Independent Assortment
Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

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

A Dihybrid Cross - F1 Results
purple flowers, tall white flowers, dwarf TRUE- BREEDING PARENTS: AABB x aabb GAMETES: AB AB ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall

16 Allele Combinations in F2
1/4 1/4 1/4 1/4 AB Ab aB ab 1/4 1/16 1/16 1/16 1/16 AB AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 Ab AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 aB AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 ab AaBb Aabb aaBb aabb

Phenotypic Ratios in F2 Four Phenotypes: Tall, purple-flowered (9/16)
AaBb X AaBb Four Phenotypes: Tall, purple-flowered (9/16) Tall, white-flowered (3/16) Dwarf, purple-flowered (3/16) Dwarf, white-flowered (1/16)

Explanation of Mendel’s Dihybrid Results
If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible 1/4 1/4 1/4 1/4 AB Ab aB ab 1/4 1/16 1/16 1/16 1/16 AB AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 Ab AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 aB AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 ab AaBb Aabb aaBb aabb

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

The rule of multiplication The rule of addition
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

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

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

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

VARIATIONS ON MENDEL’S LAWS
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

that genes can work together and interact.
Genetics is not as simple as Gregor Mendel concluded, (one gene, one trait). We know now that there is a range of dominance and that genes can work together and interact. Incomplete dominance: When the F1 generation have an appearance in between the phenotypes of the parents. Ex: pink snapdragons offspring of red and white ones. Another way to say it is In incomplete dominance Heterozygote phenotype is somewhere between that of two homozygotes

Flower Color in Snapdragons: Incomplete Dominance
Red-flowered plant X White-flowered plant Pink-flowered F1 plants (homozygote) (homozygote) (heterozygotes)

Incomplete dominance in snapdragon color

Red flowers - two alleles allow them to make a red pigment White flowers - two mutant alleles; can’t make red pigment Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

Pink-flowered plant X Pink-flowered plant White-, pink-, and red-flowered plants in a 1:2:1 ratio (heterozygote) (heterozygote)

Incomplete dominance in carnations

Co-Dominance or multiple alleles:
Non-identical alleles specify two phenotypes that are both expressed in heterozygotes Having more than 2 alleles for a given trait and both alleles show in the phenotype. No single one is dominant over the other. Example: ABO blood types

Genetics of ABO Blood Types: Three Alleles
Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells Two alleles (IA and IB) are codominant when paired Third allele (i) is recessive to others

ABO blood types

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

Multiple alleles for the ABO blood groups

More exceptions to the dominant/recessive rule
Pleiotropy: One genes having many effects. Only one gene affects an organism in many ways. Ex: sickle cell anemia and cystic fibrosis

Pleiotropy Alleles at a single locus may have effects on two or more traits Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

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

Genetics of Sickle-Cell Anemia
Two alleles 1) HbA Encodes normal beta hemoglobin chain 2) HbS Mutant allele encodes defective chain HbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia

Pleiotropic effects of the sickle-cell allele in a homozygote

Epistasis: Interaction between the products of gene pairs
Interaction between two genes in which one of the genes modifies the expression of the other. Ex: fur /hair color in mammals and albinism

Albinism Phenotype results when pathway for melanin production is completely blocked Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway

Genetics of Coat Color in Labrador Retrievers
Two genes involved - One gene influences melanin production Two alleles - B (black) is dominant over b (brown) - Other gene influences melanin deposition Two alleles - E promotes pigment deposition and is dominant over e

Allele Combinations and Coat Color
Black coat - Must have at least one dominant allele at both loci BBEE, BbEe, BBEe, or BbEE Brown coat - bbEE, bbEe Yellow coat - Bbee, BbEE, bbee

An example of epistasis

Human Variation Some human traits occur as a few discrete types
Attached or detached earlobes Many genetic disorders Other traits show continuous variation Height Weight Eye color

More modifications to Mendel’s rule
Polygenic Inheritance: In this case many genes have an additive effect. The characteristic or trait is the result of the combined effect of several genes. Ex: human skin color, height. Controlled by more than one pair of genes

Continuous Variation Polygenic inheritance results in a continuous range of small differences in a given trait among individuals The greater the number of genes that affect a trait, the more continuous the variation in versions of that trait

A simplified model for polygenic inheritance of skin color

Environmental effects:
The degree to which an allele is expressed depends on the environment Ex: Siamese cat fur color ( enzyme for melanin production inhibited by heat), hydrangea flowers ( depends on acidity of soil), height (nutrition)

Temperature Effects on Phenotype
Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway Melanin is produced in cooler areas of body

Environmental Effects on Plant Phenotype
Hydrangea macrophylla Action of gene responsible for floral color is influenced by soil acidity Flower color ranges from pink to blue

The effect of environment of phenotype

Web sites to check

Thomas Hunt Morgan (1910) and Sex Linked Inheritance Morgan’s Experimental Evidence: Scientific Inquiry The first solid evidence associating a specific gene with a a specific chromosome came from Thomas Hunt Morgan Morgan’s experiments with fruit flies (Columbia University, 1910) provided convincing evidence that chromosomes are the location of Mendel’s heritable factors. He provided confirmation of the correctness of the chromosomal theory of 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

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

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

SEX LINKED INHERITANCE
CHROMOSOMES Humans have 22 pairs of AUTOSOMES and one pair of SEX CHROMOSOMES : total=23 prs Thomas Morgan discovered SEX LINKED INHERITANCE studying Drosophila (fruit fly) In fruit flies red eyes is the wild type and white eyes is a mutant. He noticed the connection between gender and certain traits. Only the male flies had mutant white eyes.

SEX LINKED TRAITS ARE THOSE CARRIED BY THE X CHROMOSOME
Red-Green color blindness Inability to see those colors. Red and green look all the same ,like gray Hemophilia Blood clotting disorder. The clotting factor VIII is not made, individual can bleed to death. Muscular dystrophy X linked recessive, gradual and progressive destruction of skeletal muscles . Faulty teeth enamel Extremely rare, X linked Dominant

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

new technologies can provide insight into one’s genetic legacy
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

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

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

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

Mutations Mutations are permanent changes in DNA Causes?
Errors in DNA replication that can be spontaneous. Also caused by high energy radiation (X rays, gamma rays),toxic chemicals in the environment ( pesticides,asbestos, tar) and viruses.

MUTATION: A PERMANENT CHANGE IN THE DNA
MUTATION: A PERMANENT CHANGE IN THE DNA. When it happens in the gametes it is inheritable. Some mutations are lethal but most are harmless. Mutations are very important because it creates DIVERSITY WHAT CAUSES MUTATIONS? Most mutations are spontaneous, changes in DNA caused by errors in replication ( the DNA is copied incorrectly during cell division). The cell has mechanism to find and correct mistakes but those that get through get passed along. Some mutations can cause genetic disorders. Some environmental factors can cause molecular changes in DNA. X rays, toxic chemicals (insecticides, fertilizers, dry cleaning fluids, tar), some viruses, high energy radiation.

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

DISORDERS RESULTING FROM AUTOSOMAL RECESSIVE INHERITANCE
These are conditions in which the gene that is defective is recessive. It is only expressed when the child receives both recessive genes for the disorder (one from each parent) If a person is heterozygous, that is it has one dominant regular gene and one recessive abnormal gene for the condition, he will be a CARRIER but not have the disorder. The dominant allele will mask the expression of the abnormal condition. EXAMPLES: ALBINISM: SICKLE CELL ANEMIA: CYSTIC FIBROSIS: TAY- SACHS DISEASE; PHENYLKETONURIA; GALACTOSEMIA:

SICKLE CELL ANEMIA: This is also an example of “PLEIOTROPY”
DISORDERS RESULTING FROM RECESSIVE INHERITANCE Many not life threatening traits are inherited this way. widows peak, and attached earlobes. ALBINISM: No pigmentation in skin This is also an example of “EPISTASIS”(one pair of genes modifies the expression of another) SICKLE CELL ANEMIA: This is also an example of “PLEIOTROPY” Red blood cells curved shape. Decreased oxygen to brain and muscles (offers resistance to Malaria)

DISORDERS RESULTING FROM RECESSIVE INHERITANCE
CYSTIC FIBROSIS: Excessive mucus secretions.Impaired lung function, lung infections. Protein channel that transport chloride across cell membrane does not function. Protects against cholera. This is also an example of “PLEIOTROPY” TAY –SACHS DISEASE: Nervous system degeneration in infants. Enzyme fails to breakdown lipids which accumulate in nerve cells and kills the cells. Progressive degeneration starting with the brain cells.

DISORDERS RESULTING FROM RECESSIVE INHERITANCE
GALACTOSEMIA: Produces brain, liver, eye damage. Enzyme that breaks down lactose is lacking. It accumulates to toxic levels. Death in infancy PHENYLKETONURIA: Results in mental retardation

Disorders resulting from Autosomal Dominant Inheritance
Dominant genes: Many are harmless for example:freckles, dimples, cleft chin, free earlobe, short big toe, tongue rollers, left thumb on top, curly hair and dark hair Dominant traits appear in each generation since the allele shows in the heterozygous individual.

Dominant Disorders Some human genetic disorders are dominant
Figure 9.9 B

Disorders resulting from Dominant Inheritance
Acondroplasia or dwarfism: A condition where the bone does not grow properly and can’t make proper cartilage. Person is less than 4 feet with short arms and legs but a regular size trunk. Cholesterolemia: High cholesterol levels in the blood causing arteries to clog and high incidence of early heart attacks. Marfan Syndrome: Abnormal connective tissue

Disorders resulting from Autosomal Dominant Inheritance
Huntington’s Disorder: Progressive degeneration of nervous system and muscle control. Affects motor and mental abilities and it is irreversible. Late onset, usually late 30’s. Usually the person already had children. Progeria: Premature accelerated aging. Usually dead by 18. Genes that bring about growth and development are abnormal. Polydactily: Extra toes and fingers

Karyotype A karyotype is a visual display of an individual’s chromosomes. A man made picture of a person’s 23 pairs of chromosomes. ( the photo is taken during metaphase when the sister chromatids are lined up together) It is useful in sex determination and diagnosis of certain conditions.

INHERITED DISORDERS DUE TO CHROMOSOMES CHANGES
Chromosome changes can cause a lot of genetic disorders as well as a lot of variety WHEN AND HOW CAN A CHROMOSOME CHANGE? Mistakes in replication. During the S phase of the cell cycle segments of a chromosome could be deleted, duplicated, inverted or moved to a new location. Also during Metaphase I (meiosis) there can be improper separation after duplication. This can change the total number of chromosomes in each gamete of the new individual.

If during meiosis the paired chromatids fail to separate correctly this is called NON-DISJUNCTION
ANEUPLOIDY means an abnormal number of chromosomes. When an individual ends up with the wrong number of chromosomes most of the time it is miscarried ( spontaneous abortion). The wrong number of somatic chromosomes are almost always lethal. Ex: trisomy 21(three chrom. 21): Down Syndrome You can live with the wrong number of sex pair chromosomes.

CHANGES IN THE NUMBER OF SEX CHROMOSOMES
X Turner syndrome One X instead of a pair. This happens because of non disjuction of sperm. Most are aborted spontaneously. If they live, she is very short, infertily and with reduced sex characteristics. XXY Klinefelter syndrome One in 500 live male births. Taller than average, infertile, some low intelligence, some normal. Testosterone injections help. XYY “super male” about 1 in taller, mildly retarded but normal phenotype.

SEX CHROMOSOMES AND SEX-LINKED GENES
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

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

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

Alleles at two loci (R and P) interact
Comb Shape in Poultry Alleles at two loci (R and P) interact Walnut comb - RRPP, RRPp, RrPP, RrPp Rose comb - RRpp, Rrpp Pea comb - rrPP, rrPp Single comb - rrpp

Campodactyly: Unexpected Phenotypes
Effect of allele varies: Bent fingers on both hands Bent fingers on one hand No effect Many factors affect gene expression

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