Chapter 8 Introduction to Genetics

The Work of Gregor Mendel What is Genetics? the

study of heredity

Gregor Mendel’s Peas Pollen: plant’s sperm Egg Cells: plants

reproductive cells Fertilization: joining of

pollen + egg cells develops intos embryo in a seed

Born in 1822.His work with pea

plants laid the foundation for

Genetics.

Working with pea plants… Self-pollinating: pollen fertilizes egg cells in the

SAME flower (single parent reproduction) True-breeding: offspring genetically identical to

parents due to self-pollination Cross-pollination: combining reproductive cells

from 2 DIFFERENT parent plants

Mendel could cross- breed a purple flower with a white flower…

What do you think is the color of the offspring?

Genes and Dominance TRAIT: specific characteristic (seed

color, plant height, etc)

What did Mendel do in his pea plant experiments?

Studied 7 different traits each with contrasting characters (ex) Height, short or tall

He crossed the plants (with contrasting characters) and looked at their offspring

P = parental generation = original pair of plantsF1 = first filial generation= first generationHybrids: offspring from parents with different traits

Tracking Generations

Parental generation P

mates to produce

First-generation offspring F1

mate to produce

Second-generation offspring F2

CROSS-POLLINATION: Mendel cut the male parts of one flower (ouch!) and dusted the female parts with pollen from another flower.

P GENERATION: purple x white flowers

F1 GENERATION: all purple flowers

HYBRID PLANTS

What happened in Mendel’s crosses?

All the offspring only had one of the parent’s characters…the other parent’s character disappeared!!

Mendel’s Conclusions:

1. Inheritance is determined by factors that are passed down

2. GENES: the factors that determine traits

3. Contrasting characters are different forms of a gene called ALLELES

some alleles are dominant, some are recessive DOMINANT ALLELE: form of trait that will

always be exhibited; usually expressed in capitals

RECESSIVE ALLELE: form of trait is only exhibited when the dominant allele is NOT present

(ex) Allele for tall is dominant for and the allele for short is recessive

Mendel’s Principle of Dominance

What happened to the recessive allele?

Mendel wanted to know if the recessive allele disappeared from the F1 plants.

F1 CROSS: He self-crossed the F1 generations to make F2 offspring

THE F2 GENERATION…

The recessive traits reappeared!!

~¼ plants had white flowers, the recessive trait

Summary of Crosses:

tall plants X short plants tall plants

P P F1

tall plants self-pollinating ¼ short, ¾ tall

F1 Cross F2

Explaining the F1 Cross

Why did the recessive allele reappear? At some point, the recessive allele had to separate from the dominante allele. This is called…

SEGREGATION: separation of alleles occurs during formation of gametes (eggs & sperm) in anaphase II of meiosis

F1 plants inherited 1 tall allele & 1 short allele from parents When gametes are formed, the two alleles segregate from

each other each gamete has 1 copy of each gene So, 2 different types of gametes are formed (one w/ tall allele,

one w/ short allele)

SEGREGATION

Probability & Punnett Squares

Mendel realized that the principles of probability can explain the results of genetic crosses.

PROBABILITY: likelihood an event will occur (ex) Flip coin 3x in a row, 1/8 chance it will be heads all 3 times ( ½ x ½ x ½ )

The pattern in which alleles segregate is random…just like a coin flip! So which ever allele gametes receive is also random.

Punnett SquaresWhat is a Punnett Square? A diagram showing the possible genetic

combinations from a particular cross Can be used to predict and compare the genetic

variations that will result from a cross

What do the letters represent in a punnett square? Letters represent alleles: capital = dominant

lowercase = recessive

Homozygous: has two identical alleles for a trait (ex) TT or tt

Heterozygous: has two different alleles for the same trait (ex) Tt

Punnett Squarefor TT x Tt

Punnett Squarefor YY x yy

Genotype vs Phenotype

GENOTYPE: the genetic makeup of an organism (ex) TT

PHENOTYPE: the physical characteristics exhibitied (ex) tall plant

In the Punnett Square shown

What is the genotype of the offspring?

What is the phenotype?

Probability and Segregation

F2 generation from Tall F1 plants ¾ tall, ¼ short

3:1 ratio of tall to short plants

Punnet squares work to predict outcomes, so Mendel’s ideas about segregation are accurate!

Exploring Mendelian Genetics

Does the gene that determines flower color have anything to do with the gene for height?

(ex) Do all tall plants have purple flowers?

Mendel performed TWO-FACTOR CROSSES: crossing 2 different genes and following traits as they pass from one generation to the next

Two-Factor Cross: F1

Two Genes: shape of pea & color of pea The Cross: Round yellow peas x wrinkled green peas

RRYY x rryy

What are the possible alleles parent 1 can pass? RY What are the possible alleles parent 2 can pass? ry Draw a Punnett Square for this cross. All F1 were RrYy (round and yellow) or HYBRIDS

This cross does not answer question, but provides hybrids for next cross

Two-Factor Cross: F2

F1 Generation = RrYy

How would these alleles segregate when F1 self-pollinated? RrYy x RrYy

Do the two dominant alleles stay together?

What are the possible alleles each parent can pass on?

There are 4 possible combinations: Ry, RY, rY, ry

Draw a Punnett Square for this cross.

The F1 Hybrid cross produces a 9:3:3:1 phenotype ratio

Mendel found that the 2 alleles (seed shape & seed color) don’t influence each other’s inheritance

This is called the principle of Independent Assortment: genes for different traits can segregate independently during the formation of gametes

INDEPENDENT ASSORTMENT

Independent Assortment

Metaphase I:

Metaphase II:

Gametes:

1/4 AB 1/4 ab 1/4 Ab 1/4 aB

A A A A

A A A A

AAAA

B B

B B

BB

B B

BBBB

a a a a

aa aa

aaaa

bb b b

bb b b

b b b b

OR

Summary of Mendel’s Principles

A. Inheritance of characteristics is determined by genes which are passed to offspring

B. If 2+ alleles of a trait exist, some alleles may be dominant, others may be recessive

C. Sexually reproducing organisms have 2 copies of each gene which segregate during gamete formation

D. Alleles for different genes segregate independently

Beyond Dominant and Recessive AllelesGenetics is more complicated

Some alleles are neither dominant nor recessive

Many traits are controlled by multiple alleles or multiple genes

Other Inheritance Patterns…

1. Incomplete Dominance

2. Codominance

3. Multiple Alleles

4. Polygenic Traits

XHomozygous parent (RR)

Homozygous Parent (rr)

All F1 are heterozygous

X

F2 shows three phenotypes in 1:2:1 ratio

Incomplete Dominance When one allele is not

completely dominant; recessive allele is not totally masked

Heterozygous phenotype is in between the two homozygous phenotypes

(ex) Red snapdragon flowers (RR) X snapdragon white (rr) flowers pink hybrid flowers (Rr)

Xhomozygous parent homozygous parent

All F1 offspring heterozygous for flower color:

Cross two of the F1 plants and the F2

offspring will show three phenotypes in a 1:2:1 ratio:

Incomplete Incomplete DominanceDominance

Codominance Both alleles contribute to the phenotype Heterozygous genotype expresses both

phenotypes (ex) Feather colors in chickens: white feathers X

black feathers speckled chicken (ex) Horse coats: red X white roan coat

Codominance: ABO Blood Types

Alleles that controls blood type are codominant

Two alleles A & B are both exhibited when paired, a third allele (i) is recessive to others AA or Ai = Type A Blood BB or Bi = Type B Blood AB = Type AB Blood ii = Type O Blood

Multiple Alleles

> 2 possible alleles for a gene Individuals can still only have 2 alleles each

but more than 2 alleles exist in a population

(ex) coat color in rabbits lots of options due to 4 different alleles

(ex) blood type is determined by multiple alleles

Polygenic Traits

Traits controlled by the interaction of 2+ genes

(ex) Fruit fly eye color (3+ different genes)

(ex) Skin color in humans (4+ different genes), eye color, height, weight

Applying Mendel’s Principles Early 1900’s Morgan used Mendel’s

principles to study fruit flies advanced study of genetics

Mendel’s Principles also apply to study the inheritance of human traits and to calculate the probability of traits appearing in the next generation.