Mendelian Genetics

Genetics

• The study of heredity:

Passing genetic traits from one generation to next.

Two basic questions about heredity:

1. What determines the traits that an organism has?

Examples: hair color, behavior

2. How are traits transmitted from one generation tothe next? (Why do we resemble our parents?)

Answer: Genes (of course)

Gregor Mendel

• “Father of Genetics”• Austrian Monk• Studied heredity in

1850s• Used garden peas

• Mendel studied peas because their phenotypes were easy to identify.

• Phenotype - the outward, physical appearance of a particular trait

Mendel's pea plants exhibited the following phenotypes: - round or wrinkled seeds- yellow or green seeds- red or white flowers- tall or short plants

Mendel’s Peas

Mendel’s Experiment• He cross pollinated a tall plant with a short

plant

ResultsIn the 1st filial (F1) generation all of the

offspring showed the same trait – all tall.

F1 generation – children of the parents.

Next, he let the F1 generation self-pollinate.

Results: ¾ of the F2 generation showed the dominant trait & ¼ showed the recessive trait.

F2 generation – grandchildren of the parents.

P (parent) generation were “pure-bred” for traits…they only carried the trait they showed!

Mendel did not know about chromosomes, genes or DNA!

He suggested that some “factors” were passed from parents to offspring.

“Factors” that Mendel talked about were actually ALLELES (versions of a gene)

• “Factors” (alleles) are inherited from your parents.

• You get 1 allele for each trait from each parent.

Ex – You have 2 genes for tongue rolling. You can have 2 alleles for the ability to roll the tongue, 2 alleles for inability to roll, or 1 of each allele.

• Genotype Tongue Rolling (Dominant) roll/not roll    • Widow's Peak (D) - just like Eddie Munster yes/no     • Wet ear wax (D) - stick your finger in to check wet/dry     • L/R interlocking finger (D) - without thinking, clasp your hands together, is

the right thumb over the left, or vice versa? L/R or R/L     • Attached earlobes (D) - ask a neighbor or check out the mirror yes/no     • Hitchhiker Thumb (r) - does it bend back at a 90 angle yes/no     • PTC tasting (D) - I'll have some test paper in class yes/no     • Chin fissure (D) - like actor Michael Douglas yes/no     • Darwin tubercle (D) - little bump on the inside of the ear yes/no     • S-methylthioester detection (Recessive) - can you smell asparagus odor

in urine? yes/no     • Pigmented iris (D) - any color but blue yes/no     • Freckles (D) yes/no     • Polydactyl (D) - more than five fingers and/or toes yes/no     • Dimples (D) yes/no     • Wooly hair (R) yes/no     • Long eyelashes (> 1cm; D) yes/no     • Short big toe (D) - the big toe is shorter than your second one yes/no     •  

Mendel’s laws

1) Law of Dominance – some alleles show and some alleles are “hidden”. The traits that mask other traits are dominant traits.

EX: Tall plant X Short plant

all offspring are Tall

Tall is the dominant trait!

• Use capital letter to represent dominant allele

• Use lower case letter to represent recessive allele

EX: T = tall t = short

• REMEMBER…

Each organism has a pair of alleles (genes) for each trait (diploid)

TT = homozygous dominant (tall)

Tt = heterozygous (tall, but carries an allele for short)

tt = homozygous recessive (short)

2) Law of Segregation – during meiosis, one member of a gene pair separates into different gametes.

Therefore, each gamete only carries one member of the gene pair.

3) Law of Independent Assortment – Gametes unite at random and irrespective of the other gene pairs involved.

• Inheritance follows the rules of probability– The rule of

multiplication and the rule of addition can be used to determine the probability of certain events occurring

Mendel’s principles reflect the rules of probability

F1 GENOTYPES

Bb female

F2 GENOTYPES

Formation of eggs

Bb male

Formation of sperm

1/2

1/2

1/2

1/21/4

1/41/4

1/4

B B

B B

B B

b

b b

b

b b

Figure 9.7

• Using symbols we can depict the cross of tall and short pea plants in the following manner:

• Allele - one alternative form of a given gene pair.

• Tall and dwarf are the alleles for the height of a pea plant.

• More than two alleles can exist for any specific gene, but only two of them will be found within any individual.

PP11 Monohybrid Cross Monohybrid Cross

R

R

rr

Rr

RrRr

Rr

Genotype:Genotype: RrRr

PhenotypePhenotype: RoundRound

• phenotypic ratios?

Dihybrid CrossDihybrid CrossRYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

• A human male has one X chromosome and one Y chromosome

• A human female has two X chromosomes

• Whether a sperm cell has an X or Y chromosome determines the sex of the offspring

SEX CHROMOSOMES AND SEX-LINKED GENES

Chromosomes determine sex in many species

Figure 9.21A

X Y

Male

(male)

Parents’diploidcells

(female)

Sperm

Offspring(diploid)

Egg

• All genes on the sex chromosomes are said to be sex-linked– In many organisms, the X chromosome carries

many genes unrelated to sex– Fruit fly eye

color is a sex-linked characteristic

Sex-linked genes exhibit a unique pattern of inheritance

Figure 9.22A

–Their inheritance pattern reflects the fact that males have one X chromosome and females have two

• Most sex-linked human disorders are due to recessive alleles– Examples: hemophilia,

red-green color blindness

– A male receives a single

X-linked allele from his mother, and will have the disorder

A female has to receive the allele from both parents to be affected

Sex-linked disorders affect mostly males

Figure 9.23A

• Hemophilia - sex linked trait

• A high incidence of hemophilia has plagued the royal families of Europe

Figure 9.23B

QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

• In a population, multiple alleles often exist for a characteristic– The three alleles for ABO blood type in humans

is an example

Many genes have more than two alleles in the population

Figure 9.13

– The alleles for A and B blood types are codominant, and both are expressed in the phenotype

BloodGroup(Phenotype)

O

Genotypes

AntibodiesPresent inBlood

Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left

O A B AB

A

B

AB

ii

IA IA

orIA i

IB IB

orIB i

IA IB

Anti-AAnti-B

Anti-B

Anti-A

• ABO blood types

Figure 9.13x