Patterns of InheritancePatterns of Inheritance

Chapters 11.1/2/3, 14.1/2, 17.1Chapters 11.1/2/3, 14.1/2, 17.1

Gregor MendelAll Things Inheritance Begin with Him!

• Austrian Monk• During his childhood, Mendel worked as a

gardener.• Studied practical and theoretical

philosophy as well as physics.• Also bred bees in a bee house in hives that

he designed. • He also studied astronomy and

meteorology.

Gregor Mendel

The Hypothesis of InheritanceTrait – A variation of a particular character• The color of flowers

Mendel’s Experiments• 19th Century – Mendel was one of the first to apply and

experimental approach to the question of inheritance.His work led to:Genetics – The study of heredity• Mendel bred pea plants for 7 years

– He recorded inheritance patterns in offspringParticulate Hypothesis – this hypothesis states that parents pass on to their offspring separate and distinct factors (AKA Genes) that are responsible for inherited traits.

Mendel’s Experiments• Mendel stressed that these heritable

factors retain their identity generation after generation.– Genes are like marbles; They do not blend like

paint.Starry Night by Vincent van Gogh

How Mendel did it…1st – Identify true breeding pea-plants• True – Breeding plants produce offspring

identical generation after generation

Purple flowering pea plants (when self fertilized) always produced purple flowers

To make sure there was no cross fertilization he put bags over the

flowers

How Mendel did it…2nd – Tested his “particulate hypothesis”• Mendel crossed true breeding plants

+

Parents

Cross Fertilization – Sperm from the pollen of one

flower fertilizes the eggs in the

flower of a different plant.

•Fertilized eggs produced seeds; Mendel planted seeds

•Seeds grew produced flowers

Inheritance and Rules of chance:• Mendel experimented with pea plant’s

flower color and seed shape.Principle of Segregation

Hybrids – The offspring of two different true breeding varieties• Parent Plants are designated the:

– P generation (P for Parent)– F₁ generation (F for Filial; Latin for son)

• F₁ self fertilize or fertilize each other their offspring is designated F₂

Inheritance and Rules of chance:• One experiment – Mendel crossed purple-

flowered pea plants with white-flowered pea-plants

Monohybrid Cross – a pairing in which the parent plants differ in only one character• Results: The F₁ hybrids were not a “blend” of

purple and white• All flowers were purpleThen: F₁ plants self-fertilized• Results: ¼ the of F₂ plants had white flowers!• So purple + white did not make white flower

disappear.

Inheritance and Rules of chance:

• Mendel reasoned that F₁ plants carried two factors 1 for purple and 1 for white

• These factors are called gene.• To confirm his results Mendel investigated 6 other

pea plants characteristics– Each cross produced the same pattern– Results were all the same; 1 of 2 parent traits

disappeared in the F₁ generation, then reappeared in about ¼ of the F₂ offspring.

The Results Produced 4 Hypothesis

1. Alleles – Alternate form of a gene• The gene for flower color in pea plants exist in

one form for purple; one form for white2. For each inherited character, an organism has

2 alleles for the gene controlling character• One from each parentHomozygous – When two alleles are the same form that character.Heterozygous – Two alleles are different.

The Results Produced 4 Hypothesis

3. Dominant Allele – when only one of two different alleles in a heterozygous individual appears to affect the trait.• Represented by the capital letter PRecessive Allele – the trait in a heterozygous that does not appear • Represented by the lower case letter p

Mendel’s Hypothesis

4. Two alleles for a characteristic segregate (separate) during the formation of gametes (sex cells)

• So each gamete carries only one allele for each character

Mendel’s Principle of Segregation• The union of gametes during

fertilization reforms allele pairs in the offspring

Probability and Punnett Squares• In a monohybrid cross of true-breeding

(homozygous) purple-flowered and white flowered plants– He hypothesized that each: F₁ (Plant) will get

the dominant purple – flower allele (P) from one parent.• One recessive white flour allele (p) from

the other parent• Each F₁ plant will be heterozygous: Pp

– These plants make gametes on P; one p– If 2 F₁ plants fertilize each other the F₂

generation will individual pair

Probability and Punnett Squares

PP – ¼ Pp – ½ Pp – ¼

PP Pp

Pp pp

PP, pp Each with probability of ¼ each

Punnett Square – A type of diagram that shows all possible outcomes of genetic cross

Genotype and Phenotype

Phenotype – An observable trait• Ph for physicalGenotype – the genetic make-up or combination of alleles • GEN for genetics• So the phenotype for purple vs. white flowers is

3 purple : 1 white• Genotypic Ratio: 1 PP: 2 Pp: 1 pp

Testcross• Used to determine whether a plant is

homozygous or heterozygous• A testcross breeds an individual of unknown

genotype, but dominant phenotype with a homozygous recessive individual– The appearance of the offspring resulting

from the testcross will reveal the genotype of mystery plant.

Why: a recessive parent can only contribute a recessive allele to offspring– Phenotype will tell you the allele contributed

by the mystery plant

TestcrossPurple – Flowered Homozygous • Expect all offspring to be purple because

mystery plant can only contribute a P allele.– So all offspring will be Pp– But: if purple flowered plant is heterozygous

(Pp) expect both purple-flowered (Pp) and white flowered (pp)

– When you cross Pp x pp the predicted outcome 1 : 1 (1 purple to 1 white) of phenotype.

Mendel’s Principle of Independent Assortment

Dihybrid Cross – Crossing organisms different in two characters.• When crossing true-breeding with true breeding

– Dominant Round Yellow with a dominant wrinkled green:

P generation – RRYY x rryyGamete – RY x ryF₁ generation: RrYyF₂ generation: 4 different pea phenotypesresult: 9 (Round yellow):3 (wrinkled yellow):3 (round green):1 (wrinkled green )

Mendel’s Principle of Independent Assortment

Based on these results Mendel proposed:The Principle of Independent AssortmentThe Principle of Independent Assortment

• States that gamete formation in an F₂ cross, a particular allele for 1 character can be paired with either allele of another character

The Principle of Independent The Principle of Independent AssortmentAssortment

Example:In the Round-Yellow x Wrinkled Green (Pea)• R (round) can end up with Y(yellow) or y (green) and r can end up with Y or y• Like everything else there are exceptions!

Variations of Inheritance PatternsIntermediate Inheritance• In Mendel’s pea crosses the F₁ offspring always

looked like the dominant homozygous parent.– Because one dominant allele produced one

dominant phenotype– A recessive phenotype required inheriting two

recessive alleles

• If neither allele is dominant – the heterozygotes have a phenotype that is intermediate between the phenotypes of the two homozygotes

Variations of Inheritance Patterns

Intermediate Inheritance• Inheritance in which heterozygotes have a

phenotype intermediate between the phenotypes of the two homozygotes– NOT A MIX

• Not a Blend/Mix because the parent phenotypes can reappear in in F₂ generation– F₂ Generation is 1 : 2 : 1 ratio

Multiple Alleles• This is what happens when many alleles

exist in the genes– This expands the number of genotypes and

phenotypes

Example: Blood TypesLetters represent a carbohydrate (antigen) that exists on the surface of the red blood cell• There are 6 possible genotypes

Multiple AllelesCodominance – the heterozygote expresses both traits• Shows separate traits of both alleles.

Polygenic Inheritance• When two or more genes affect a single

character– Example: Height and Skin Color– Height = A, B, C (Tall) X, Y, Z (Short)

AABBCC = Very TallAXBBCC = Less Tall

Now imagine the different combinations

Environmental Effects• A leaf on a tree will vary through-out the

year depending on varying sunlight.

Environmental Effects• Siamese Cat – are covered with creamy

white fur, except for the ears, face, feet, and tail.– An enzyme responsible for black fur is only

active at the cooler temperatures found at the extremities

Environmental EffectsIn Humans Phenotype is not rigid it can be

influenced by the environment:• Nutrition influences height, exercise build, sunlight

(darkness of skin)– Even twins have the same genotypes have

differences in phenotypes as the results of their exposure to the environment.

– In humans genes determine phenotypes like blood type with no environmental influence

– But blood count (Red vs. White) is sensitive to the environment.• Like altitude, physical activity, and infection

(white blood cell count)

Meiosis and Mendel’s PrinciplesChromosomes Theory of Inheritance

• States that genes are located on chromosomes and the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns

• Biologist worked out mitosis and meiosis in the 1800’s– 1900’s Biologist noticed similarities between

Mendel’s principles and inheritance patterns• European Biologist: Hugo de Vries, Karl Correns,

and Erich von Tschermak independently made the connection.

Meiosis and Mendel’s Principles

Remember:• Every diploid individual has two sets of

homologous chromosomes– 1 from the male– 1 from the female

Gene Locus – a specific location of a gene on a chromosome.• The alleles of a gene reside at the same location

Meiosis and Mendel’s Principles

Gene Locus

Meiosis and Mendel’s PrinciplesGenetic Linkage – the tendency for the alleles on one chromosome to be inherited together.• When genes are on separate chromosomes they

sort independently of each other during meiosis.

• During genetic linkage: the closer two genes are on a chromosome the greater the genetic linkage

• The farther apart the genes are, the more likely it is that a cross-over event will separate them.

Sex-Linked TraitsSex-Linked Gene – Is any gene located on a sex chromosome• In humans most sex-linked genes are found on

the X chromosome• They are obviously larger than the Y

chromosome

Sex-Linked TraitsDiscovered by Thomas Hunt Morgan

• Studied the inheritance of white eye color in fruit flies– White eyes are rare; normal fruit fly eye color

is red.

Sex-Linked Traits• Morgan mated white eyed male fruit fly with

red-eyed females– All F₁ (offspring) had red eyes– When Morgan bred the F₁: offspring had the

3 : 1 ratio in the F₂ generation • 3 red : 1 White

– After MANY trials Morgan realized all white eyed fly’s were male

– Morgan deduced that this trait was located on the X chromosome.

Sex-Linked TraitsExplanation: White eye color is recessive so a

female can only have white eyes if she has this chromosome on both X chromosomes

• A male only needs one.

Male: Missing part of chromosome allows the trait to show

Female: Other X chromosome “blocks” the recessive trait

Sex-Linked DisordersRed – Green Color Blindness

Sex-Linked DisordersHemophilia (a blood clotting disorder)

Queen Victoria Queen of England at the age of 18

Appeared in her children