MENDELIAN GENETICS

WHAT IS INHERITANCE?

Inheritance is the transfer of genetic material from parent to offspring.

-Each offspring receives ½ of their genetic material/DNA from each parent.

A trait is an inherited distinguishing feature or characteristic…what you see.

-Can you name a common trait of a dog?

WHAT IS HEREDITY?

Heredity is the passing of traits from parent to offspring

Genetics is the study of heredity

HUMAN TRAITS

Little fingers: straight or bent

Ears: free or attached

Tongue: roll or not

HUMAN TRAITS

Hairline: widow peak or not

GENES AND ALLELES

Allele: is the section on the chromosome that represents a trait (these will be written as letters)

Gene: the two alleles work together to express a trait.

allele

gene

allele

GENETICS

Genotype: the code that determines the trait.

Phenotype: the physical characteristic expressed…how it looks.

Dominant: the alleles that are more influential (written in upper case letters).

Recessive: the alleles that are less influential (written in lower case letters).

GENETICS

Homozygous (purebred): Alleles are the same. (AA, aa).

Heterozygous (hybrid): Alleles are different. (Aa)

Genotypical ratio: Prediction of genotype

Phenotypical ratio: Prediction of phenotype

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MEIOSIS: TWO PART CELL DIVISION

Meiosis I

Meiosis II

Gametes

A HISTORY LESSON

In the 19th century, the leading theory on inheritance was the “blended theory” where the traits from each parent were blended from generation to generation A red rose with a white rose gives you a pink rose

+ =

GREGOR MENDEL THE FATHER OF GENETICS

Central Europe (Czech) scientist turned monk

Conducted research on pea plants

Developed basic principles of heredity for all complex organisms

Traits are passed down from generation to generation; not blended

Work published in 1866; died in 1884; recognized in 1900

MENDEL’S PEA PLANT EXPERIMENTS

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WHY DID MENDEL USE PEAS PISUM SATIVUM?

Peas can be grown in a small area

Peas produce lots of offspring

Peas will produce pure plants when allowed to self-pollinate over several generations

Peas can be artificially cross-pollinated

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REPRODUCTION IN FLOWERING PLANTS

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Pollen carries sperm to the ovary for fertilization

Self-fertilization can occur in the same flower

Cross-fertilization can occur between different flowers

pollen

MENDEL’S EXPERIMENTAL METHODS

Mendel hand-pollinated flowers using a paintbrush then he would snip the stamens to prevent self-pollination

Then he covered each flower with a cloth bag

He traced traits through the several generations

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THE PEA EXPERIMENTS

Eight traits observed by Mendel and experimented on

21,000 hybridized plants:

THE EIGHT PEA PLANT TRAITS MENDEL TRACED

Seed shape --- Round (R) or Wrinkled (r)

Seed Color ---- Yellow (Y) or Green (y)

Pod Shape --- Smooth (S) or wrinkled (s)

Pod Color --- Green (G) or Yellow (g)

Seed Coat Color ---Gray (G) or White (g)

Flower position---Axial (A) or Terminal (a)

Plant Height --- Tall (T) or Short (t)

Flower color --- Purple (P) or white (p)

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THE PEA EXPERIMENTS

Let’s take a look at the yellow vs. green

seeds:

3:1 ratio

showed up

in later

generations!

WHAT MENDEL HYPOTHESIZED…..

The inheritance of each trait is determined by “elementen” (or genes) that are passed onto offspring unchanged and not blended

An individual inherits one elementen from each parent for each trait

The alleles of the gene are the different version or forms of the gene and determine the traits that are expressed.

Example: “men have facial hair (gene); some have black hair (allele)”

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Mendel stated that physical traits are inherited as “factors” or “particles”

Mendel did not know that the “particles” were actually chromosomes & DNA

PARTICULATE INHERITANCE

DOMINANT VS. RECESSIVE

Mendel noticed that one form of the traits dominated over the other A trait may not show up in an individual but can still

be passed on to the next generation

Dominate doesn’t mean “better” but only that it masks the expression of the other trait.

Dominant allele (or gene)-Denoted with capitalized letter ( ex. B)

Recessive allele (or gene)- Denoted with lowercase letter (ex. b)

THINGS MENDEL ALSO CONSIDERED….

The parent plants were homozygous for pea seed color=each had identical alleles for the trait 2 yellows or 2 greens

True or pure breeding

The f1 generation were heterozygous = 2 different alleles from each parent Hybrid

GENOTYPE VS. PHENOTYPE

Genotype is the genetic set for the trait Alleles and denoted with

letters representing the trait (example eye color would be letter “E”

Phenotype is the physical expression of the genotype Trait or characteristic

GENOTYPES

Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. PP or pp) also called pure

Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Pp); also called hybrid

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GENOTYPE & PHENOTYPE IN FLOWERS

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Genotype of alleles: P = purple flower p = white flower

All genes occur in pairs, so 2 alleles affect a characteristic

Possible combinations are:

Genotypes PP Pp pp

Phenotypes Purple Purple white

Parental P1 generation = the parental generation in a breeding experiment.

F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation) From breeding individuals from the P1 generation

F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) From breeding individuals from the F1 generation

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Generations

FOLLOWING THE GENERATIONS

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Cross 2 Pure Plants

TT x tt

Results in all

Hybrids Tt

Cross 2 Hybrids get

3 Tall & 1 Short TT, Tt, tt

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LAW OF DOMINANCE

• In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.

• All the offspring will be heterozygous and express only the dominant trait.

• RR x rr yields all Rr (round seeds)

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LAW OF SEGREGATION

During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.

Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

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LAW OF INDEPENDENT ASSORTMENT

Alleles for different traits are distributed to sex cells or gametes independently of one another.

This law can be illustrated using dihybrid crosses.

MODE OF INHERITANCE

Principles state that inheritance is predictable based on the rules so how can we predict what traits will be passed and/or expressed? Answer: by determining the mode of inheritance

Pedigree

(Phenotype and genotype)

Punnett Square (Genotype)

But this isn’t Geometry!

PEDIGREE

Similar to a family tree but used to trace traits

Can be combined with genotype information

Squares for male

Circles for female

Filled/darkened means “affected” or expressing the trait

Unfilled could mean unaffected or carrier

IMPLICATIONS OF INBREEDING

The Fugate

Family

PUNNETT SQUARE OR GENETIC CROSS

- Calculate probability

of homozygosity or

heterozygosity

-Calculate probability

and frequency of

phenotypic expression

of trait

Example of red and

black coats on

domestic cattle

TYPES OF GENETIC CROSSES

Monohybrid cross - cross involving a single trait e.g. flower color

Dihybrid cross - cross involving two traits e.g. flower color & plant height

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MONOHYBRID CROSS

Female genotype listed vertically

Male genotype listed horizontally

Problem:

A widow’s peak hairline is

dominant to straight hairline.

Cross a heterozygous widow’s

peak hairline person to a

straight hairline person.

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DIHYBRID CROSS

Traits: Seed shape & Seed color

Alleles: R (round) r (wrinkled) Y (yellow) y (green)

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RrYy x RrYy

RY Ry rY ry RY Ry rY ry

All possible gamete combinations

DIHYBRID CROSS

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RY Ry rY ry

RY

Ry

rY

ry

DIHYBRID CROSS

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RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow:3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RY Ry rY ry

RY

Ry

rY

ry

DIHYBRID CROSS

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Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1

9:3:3:1

INCOMPLETE DOMINANCE

When they have an appearance somewhat in between the phenotypes of the two parental varieties.

Example: snapdragons (flower)

red (RR) x white (rr)

RR = red flower

rr = white flower

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R

R

r

r

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INCOMPLETE DOMINANCE

CODOMINANCE

Two alleles are expressed (multiple alleles) in heterozygous individuals.

Example: blood type

1. type A = IAIA or IAi

2. type B = IBIB or IBi

3. type AB = IAIB

4. type O = ii

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CODOMINANCE PROBLEM

Example: homozygous male Type B (IBIB)

x heterozygous female Type A (IAi)

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IAIB IBi

IAIB IBi

1/2 = IAIB

1/2 = IBi

IB

IA i

IB

SEX-LINKED TRAITS

Traits (genes) located on the sex chromosomes

Sex chromosomes are X and Y

XX genotype for females

XY genotype for males

Many sex-linked traits carried on X chromosome

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SEX-LINKED TRAITS

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Sex Chromosomes

XX chromosome - female Xy chromosome - male

fruit fly

eye color

Example: Eye color in fruit flies

SEX-LINKED TRAIT PROBLEM

Example: Eye color in fruit flies

(red-eyed male) x (white-eyed female) XRY x XrXr

Remember: the Y chromosome in males does not carry traits

RR = red eyed Rr = red eyed rr = white eyed XY = male XX = female

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XR

Xr Xr

Y

SEX-LINKED TRAIT SOLUTION:

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XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female 50% white eyed male

XR

Xr Xr

Y

HEMOPHILIA

XH XH = female normal

XH Xh = female carrier

Xh Xh = female hemophiliac

XH Y = male normal

Xh Y = male hemophiliac

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SEX-LINKED TRAIT PROBLEM

Example: Eye color in fruit flies

(red-eyed female) x (white-eyed male)

XR XR Xr Y

How many? Red eyed males?

White eyed males?

White eyed females?

Red eyed females?

Xr

XR XR

Y

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SEX-LINKED TRAIT PROBLEM

Example: Eye color in fruit flies

(red-eyed female heterozygous) x (red-eyed male)

XR Xr XR Y

How many ? White eyed males?

Red eyed males?

White eyed females?

Red eyed females?

XR

XR Xr

Y