Genetics_Ottolini_Biology

Genetics

Mendel &

Simple Patterns of Inheritance

Human Traits

• Your physical traits resemble those of your parents.

• Heredity = the passing of traits from parents to offspring

Gregor Johann Mendel• Mendel was an Austrian

monk who conducted breeding experiments with garden peas.

• Developed rules which accurately predict patterns of heredity

• Genetics = the branch of biology that focuses on heredity

Useful Features in Peas 1. Many traits that have two clearly different forms (no

intermediate forms) 2. Mating can be easily controlled

– Self-fertilization– Cross-fertilization

3. Small, grows easily, matures quickly & produces many offspring

Self-Pollination

Cross-Pollination Cross= mate or breed two

individuals

Nature’s Pollinators

• Insects (bees)

• Vertebrates (birds & bats)

• Wind

• Water

Mendel’s Observations: Traits are Expressed as Simple Ratios

• First experiments involved monohybrid crosses

• Monohybrid cross = a cross that involves one pair of contrasting traits

Mendel’s Experiments: Step #1• Self-pollinate each pea

plant for several generations

• True-breeding= all offspring display only one form of a particular trait

• P generation = parental generation, the first two individuals crossed in a breeding experiment

Mendel’s Experiment: Step #2• Cross-pollinate the P

generation plants with contrasting forms of a trait

• F1 generation = the first filial generation, the offspring of the P generation

• Characterize and count plants

Mendel’s Experiment: Step #3• Allow the F1 generation

to self-pollinate

• F2 generation = the second filial generation, the offspring of the F1 generation plants

• Characterize and count plants

Mendel’s Results• F1 generation showed only one form of the trait

(e.g. purple flowers) – The other form of the trait disappeared (e.g. white

flowers) – Reappeared in the F2 generation

• 3:1 ratio of the plants in the F2 generation

Expressing Ratios: Mendel’s F2 Generation

705 purple-flowered plants; 224 white-flowered plants

• Reduce the ratio to its simplest form: 705/224 = 3.15 Purple 224/224= 1 White– Ratio can be written in a few different ways:

• 3:1• 3 to 1• 3/1

Are offspring simply a blend of their parents’ characteristics?

• Before Mendel’s experiment: this was the theory (blending hypothesis) – Ex. Tall x Short = Medium Mendel’s Conclusion– Offspring have two genes for each trait (one gene

from each gamete)

Mendel’s Hypotheses:Hypothesis #1

• For each inherited trait, an individual has two copies of the gene – One gene from each parent

Mendel’s Hypotheses:Hypothesis #2

• There are alternative versions of genes

• Alleles = different versions of genes; an individual receives one allele from each parent

Mendel’s Hypotheses:Hypothesis #3

• When two different alleles occur together, one may be completely expressed, while the other may have no observable effect on the organism’s appearance.

• Dominant = the expressed form of a trait

• Recessive = the trait which is not expressed when the dominant form of the trait is present

Mendel’s Hypotheses: Hypothesis #4

• When gametes are formed, the alleles for each gene in an individual separate independently of one another. – Gametes = one

allele for each inherited trait

– Fertilization – each gamete contributes one allele

Representing Alleles

• Letters are often used to represent alleles

• Dominant Alleles = capitalize the first letter of the trait – Purple flowers = P

• Recessive Alleles = first letter of the dominant trait, in lowercase – White flowers = p

Combinations of Alleles• Homozygous = if two alleles of a particular

gene present in an individual are the same

• Heterozygous = if two alleles of a particular gene present in an individual are different

• Example: yellow peas (Y) are dominant to green peas (y) – Homozygous for yellow peas = YY– Heterozygous for yellow peas = Yy

yy YY Yy

Heterozygous Alleles - Ff• Only the dominant allele

is expressed – Recessive allele is

present but not expressed

• Example: Freckles – Freckles, F = Dominant

allele – No Freckles, f =

Recessive allele – Individuals who are

heterozygous for freckles (Ff) have freckles

Genotype vs. Phenotype• Genotype = the set

of alleles that an individual has – Uppercase letter is

always written first

• Phenotype = the physical appearance of a trait (determined by which alleles are present)

Laws of Heredity: The Law of Segregation

• Law of Segregation= the two alleles for a trait segregate (separate) when gametes are formed

Laws of Heredity:The Law of Independent Assortment

• Law of Independent Assortment = the alleles of different genes separate independently of one another during gamete formation– Example: alleles for plant height separate independently of

the alleles for flower color – Applies to:

• Genes on different chromosomes• Genes that are far apart on the same chromosome

http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

Punnett Squares & Probabilities

Punnett Squares: Predicting Expected Results in Crosses

• Punnett Square = a diagram that predicts the expected outcome of a genetic cross by considering all possible combinations of gametes in the cross

Punnett Squares• Possible gametes from one parent are written along the top of

the square• Possible gametes from the other parent are written along the

left side of the square • Letters inside the boxes = possible genotypes of the offspring

Monohybrid Crosses

• Homozygous for yellow seeds (YY) x Homozygous for green seeds (yy) =– Only yellow

heterozygous offspring (Yy)

Monohybrid Crosses • Heterozygous for

yellow seeds (Yy) x heterozygous for yellow seeds (Yy) =– ¼ YY (Homozygous

dominant) – 2/4 Yy

(Heterozygous) – ¼ yy (Homozygous

recessive)

• 1 YY: 2 Yy: 1 yy genotypic ratio

Let’s Solve a Punnett Square!!!

•Inflated pod shape is DOMINANT

• Constricted pod shape is RECESSIVE

Step #1: Choose a Letter to Represent your Alleles

• Inflated pea pod = I (dominant)

• Constricted pea pod = i (recessive)

Step #2: Write the Genotypes of the Parents

• Parent #1 = Ii = Heterozygous Inflated

• Parent #2 = Ii = Heterozygous Inflated

Step #3: Determine the Possible Gametes

• Parent #1 = Ii = I or i

• Parent #2 = Ii = I or i

Step #4: Enter the possible gametes at the top and side of the

Punnett Square

I i

I

i

Step #5: Write the alleles from the gametes in the appropriate boxes

I i

I

i

II Ii

Ii ii

Step #6: Determine the phenotypes of the offspring

I i

I

i

II Ii

Ii ii

Inflated Pods

Inflated Pods Constricted Pods

Inflated Pods

Step #7: Determine the genotype and phenotype ratios

I i

I

i

II Ii

Ii ii

Inflated Pods

Inflated Pods Constricted Pods

Inflated Pods

Genotype Ratio: 1 II : 2 Ii : 1 ii

Phenotype Ratio: 3 Inflated : 1 Constricted

Probabilities Can Also Predict the Expected Results of Crosses

• Probability = the likelihood that a specific event will occur – Words – 1 out of 1– Decimal – 1– Percentage – 100%– Fraction – 1/1

Probability = number of one kind of possible outcome ÷ total number of all possible outcomes

Probability in Pea Plants: Gamete

• Parent = Yy– Can either contribute a yellow allele (Y) or a

green allele (y)

• ½ chance that the gamete will have Y

• ½ chance that the gamete will have y

Probability of the Outcome of a Cross• Consider the offspring of two parents who

are heterozygous for freckles:– Mom = Ff

• Possible gametes from mom = F or f• ½ probability of mom contributing F• ½ probability of mom contributing f

– Dad = Ff• Possible gametes from dad = F or f• ½ probability of dad contributing F • ½ probability of dad contributing f

Probability of the Outcome of a Cross

– Mom F + Dad F = FF (1/2) x (1/2) = ¼ probability of FF (freckles)

– Mom F + Dad f = Ff (1/2) x (1/2) = ¼ probability of Ff (freckles)

– Mom f + Dad F = Ff (1/2) x (1/2) = ¼ probability of Ff (freckles)

– Mom f + Dad f = ff (1/2) x (1/2) = ¼ probability of ff (no freckles)

Probability of the Outcome of a Cross

• Genotype Probabilities:– ¼ FF (freckles)– ¼ Ff + ¼ Ff = ½ Ff (freckles)– ¼ ff (no freckles)

• Phenotype Probabilities: – ¼ FF + ½ Ff = ¾ freckles– ¼ ff = ¼ no freckles

Pedigrees

Studying Pedigrees: How are traits inherited???

• Pedigree= a family history that shows how a trait is inherited over several generations

Why are pedigrees helpful/useful?

• Pedigrees are helpful if couples are concerned that they might be carriers of genetic disorders

Common Genetic Disorders: Angelman SyndromeCanavan DiseaseCharcot-Marie-Tooth DiseaseCri du Chat SyndromeDuchenne Muscular DystrophyFragile X SyndromeGilbert's SyndromeJoubert SyndromeKlinefelter SyndromeKrabbe DiseaseLesch–Nyhan SyndromeMyotonic DystrophyNeurofibromatosisNoonan SyndromePelizaeus-Merzbacher DiseasePhenylketonuriaPorphyriaPrader-Willi SyndromeRetinoblastomaRubinstein-Taybi SyndromeSpina bifidiaSmith-Magenis SyndromeStickler SyndromeTurner SyndromeVariegate PorphyriaVon Hippel-Lindau SyndromeWilson's DiseaseWolf-Hirschhorn SyndromeXXXX SyndromeYY Syndrome

Pedigree Symbols

Pedigree Numbers

• Roman numerals (I, II, III, IV) represent = Generations

•Regular numbers (1,2,3,4) represent = Individuals in each generation

Pedigree Symbols – Male and Female

= Normal Male

= Normal Female

= Male with trait

= Female with trait

Horizontal line between a male and female indicates

MATING/MARRIAGE

Branching vertical lines point to OFFSPRING

Autosomal vs. Sex-linked Traits

• Autosomal Trait = appears in both sexes equally, alleles appear on the autosomal chromosomes

• Sex-linked Trait = a trait whose allele is located on the X chromosome– Appears mostly in males – Mostly recessive – Female only exhibits the condition if she

inherits two recessive alleles

Human Chromosomes

• Humans have 46 chromosomes: – 1 pair of sex chromosomes (X and Y)– 22 pairs of autosomes

• Females have 2 X chromsomes (XX).

• Males have an X chromsome and a Y chromosome (XY)

Karyotypes• A Karyotype is a test to identify and

evaluate the size, shape, and number of chromosomes in a sample of body cells.

Dominant vs. Recessive Trait

• Autosomal Dominant Traits = each individual with the trait will have a parent with the trait

• Autosomal Recessive Traits = an individual with the trait can have one, two, or neither parent who exhibit the trait

Recessive Disorder: Albinism• Albinism = a genetic disorder in which the

body is unable to produce an enzyme necessary for the production of melanin (dark color to hair, skin, scales, eyes, and feathers)

Genetic Disorders = Carriers • Carriers = individuals

who are heterozygous for a recessive inherited disorder, but do not show symptoms of the disorder – Can pass the recessive

allele for the disorder to their offspring

Red-Green Color Blindness:A Sex-Linked Recessive Disorder

• X-linked recessive disorder

• Among Caucasian individuals:– 8% of males – 0.5% of females

• Difficulty distinguishing between shades of green and red

Red-Green Color Blindness:A Sex-Linked Recessive Disorder

• Unaffected female = XRXR

• Affected female = XrXr

• Carrier female = XRXr

• Unaffected male = XRY• Affected male = XrY

Males have the disorder more often than females because they only have one X chromosome.

Red-Green Color Blindness:A Sex-Linked Disorder

Heterozygous vs. Homozygous

• Homozygous Dominant or Heterozygous individuals = phenotype will show the dominant characteristic

• Homozygous Recessive individuals = phenotype will show the recessive characteristic

***Heterozygous carriers of a recessive mutation will not show the mutation, can produce children who are homozygous for the recessive allele

Let’s look at a pedigree for polydactyly: a dominant trait

Let’s look at a pedigree for phenylketonuria (PKU): a recessive

disorder

The trait skips a generation!!

Complex Patterns of Inheritance

Traits Influenced by Several Genes• Polygenic Trait = when several

genes influence a trait – Genes can be:

• Scattered along the same chromosome

• Located on different chromosomes

– Independent Assortment = many different combinations in offspring

• Polygenic traits = – Eye color, height, weight, hair

and skin color

Polygenic Trait – Skin Color

Intermediate Traits • Incomplete Dominance =

an individual displays a trait that is intermediate between the two parents

• Examples: – Red snapdragon + White

snapdragon = Pink snapdragon offspring

– Curly hair + Straight hair = Wavy haired offspring

• Neither allele is dominant to the other

Pink Snapdragon - Heterozygous

Traits With Two Forms Displayed At The Same Time

• Codominance = when two dominant alleles are expressed at the same time, both forms of the trait are displayed

• Example: – Red Horse + White Horse =

Roan Horse

Roan Horse - Heterozygous

Traits Controlled By Genes With Three or More Alleles

• Multiple Alleles = Genes with three or more alleles

• Example: ABO blood groups in humans– IA and IB = Dominant to i;

• A & B are two carbohydrates on the surface of red blood cells

– i = Recessive – When IA and IB are present together =

Codominant

Traits Controlled By Genes With Three or More Alleles

• An individual can only have two of the possible alleles for the gene

Genotypes of Each Blood Type

Traits Influenced By The Environment

• Phenotype is affected by environmental conditions

• Hydrangeas (flowers) range from blue to pink based upon the pH of the soil– Acidic soil = blue

flowers – Basic soil = pink

flowers

Traits Influenced By The Environment• Siamese Cat

– Fur on ears, nose, paws, and tail is darker than the rest of the body

– Dark fur at locations which are cooler than normal body temperature

Traits Influenced by the Environment: Identical Twins

Human Examples: Traits Influenced By The Environment

• Height – What can influence

height besides genes?

• Skin Color

• Human Personality

Traits Caused By Mutation

• Damaged genes/genes which are copied incorrectly – result in faulty proteins

• Mutations are RARE

• Inherited Mutations cause Genetic Disorders

• Many mutations are carried in recessive alleles – Carrier = heterozygous individual

• Carry the recessive allele but do not exhibit the disorder

Sickle Cell Anemia • Recessive

• Defective hemoglobin – Red Blood Cells – Binds and transports

oxygen

• Sickle-Cell Shape – Rupture-prone– Clotting in blood vessels

• Heterozygote Advantage = protection from malaria

Hemophilia • Recessive

• Sex-linked – X chromosome – More males afflicted

than females

• Impairs blood clotting

• English royal family

Hemophilia: The Royal Family

Huntington’s Disease • Dominant • First symptoms

appear in thirties or forties: – Mild forgetfulness – Irritability

• Long-term symptoms: – Loss of muscle control– Chorea (physical

spasms) – Severe Mental Illness – Death

Detecting and Treating Genetic Disorders

• Genetic Counseling = a form of medical guidance that informs people about genetic problems that could affect them or their offspring

• Phenylketonuria (PKU) – Lacks enzyme to convert phenylalanine into tyrosine – Can cause mental retardation – Early intervention involves low-phenylalanine diet

• Gene Therapy = replacing defective genes with healthy ones