Genetics

What is Genetics? •  Gene$cs'is#the#study#of#heredity.#•  Heredity'is#the#passing#of#traits#from#parents#to#offspring.#•  A#trait'is#a#physical#or#physiological#characteris7c.##

•  For#example,#height#and#colorblindness#•  Some#traits#are#inherited,#while#others#are#not.#

•  For#example,#the#shape#of#ears#vs.#pierced#ears#

What makes a trait inheritable? •  The#phenotype#is#the#physical#appearance#of#a#trait.#•  The#phenotype#for#any#inherited#trait#is#determined#by#the#proteins'that#your#cells#

produce.#•  Your#cells#know#which#proteins#to#make#based#on#what#your#genes'tell#them.#•  Genes#are#sequence#of#bases#in#the#DNA#found#within#your#chromosomes.#•  You#inherit#50%'of#your#chromosomes#from#each#parent.#•  Your#own#unique#combina7on#of#genes#are#referred#to#as#your#genotype.#•  It'is'your'genotype'that'determines'your'phenotype'of'an'inherited'trait.'

Protein'

For#example,#eyelash#length#is#a#trait#that#is#decided#by#a#gene#found#on#chromosome##1.#If#you#inherited…#

Long Long

Chromosome #1 from mom

Chromosome #1 from dad

Your Phenotype: long eyelashes

Short Short Your Phenotype: short eyelashes

Chromosome #1 from mom

Chromosome #1 from dad

There are two versions of the gene: long eyelashes and short eyelashes. A version of a gene is called an allele. If you inherit two identical alleles for a trait, then you have a homozygous genotype and are considered to be purebred or true-breeding.

If#you#inherit#two#different#alleles#for#a#trait,…#

Long

Chromosome #1 from mom

Chromosome #1 from dad

Short Your Phenotype: Long eyelashes

…then you have a heterozygous genotype, and are considered a hybrid or a carrier. Since long eyelashes are phenotypically expressed when only one allele is present, it is considered dominant. The dominant allele is represented by a capital letter (A). Short eyelashes are considered recessive, because there must be two alleles present in order for the recessive phenotype to be expressed. The recessive allele is represented by a lower-case letter (a).

PRACTICE TERMINOLOGY 1.  Circle the dominant allele.

A aa Aa AA a 2.  Circle the heterozygous genotype.

A aa Aa AA a 3.  Circle the homozygous dominant genotype.

A aa Aa AA a 4.  Circle the recessive genotype.

A aa Aa AA a 5.  Circle the recessive allele.

A aa Aa AA a 6.  Circle the phenotype.

AA Aa long eyelashes 7.  Circle the trait.

purple flowers round peas plant height

How#do#we#represent#these#alleles#on#paper?#We use CAPITAL LETTERS to represent the dominant allele. (A = Long) We use lowercase letters to represent the recessive allele. (a = short)

DESCRIPTION: GENOTYPE? PHENOTYPE? Homozygous dominant ____________ _____________ Heterozygous ____________ _____________ Pure recessive ____________ _____________ Hybrid ____________ _____________ True-breeding dominant ____________ _____________ Carrier ____________ _____________

What is dominance? •  It is important to understand that an allele is called “dominant” because it is seen in

the phenotype, not because it somehow subdues a recessive allele. •  Alleles are simply variations in a gene’s nucleotide sequence. •  When a dominant allele coexists with a recessive allele in a heterozygote, they do

not actually interact. •  It is the pathway from genotype to phenotype that dominance and recessiveness

comes into play.

Phenotype

Genotype

COMPLETE dominance •  In pea plants, round seed shape is

dominant to a wrinkled seed shape.

•  The dominant allele (E) codes for the synthesis of an enzyme that helps convert sugar to starch in the seed.

•  The recessive allele (e) codes for a defective form of this enzyme. In a recessive homozygote, sugar accumulates in the seed because it is not converted to starch. As the seed develops, the high sugar concentration causes the osmotic uptake of water and the seed swells. When the mature seed dries, it develops wrinkles.

•  In contrast, if a dominant allele is present, sugar is converted to starch, and the seeds do not wrinkle when they dry. One dominant allele results in enough of the enzyme to convert sugar to starch, and thus dominant homozygotes and heterozygotes result in the same phenotype: round seeds.

Round Seeds

Round Seeds

Wrinkled Seeds

INCOMPLETE dominance •  Neither allele is completely dominant.

•  When two purebred varieties cross, the F1 hybrids have an appearance somewhere in between the phenotypes of the two parental varieties.

•  For example, when purebred red snapdragons cross with purebred white snapdragons, the resulting F1 hybrids have pink flowers.

•  This pink phenotype results from flowers of heterozygotes having less red pigment than the red homozygotes (unlike Mendel’s pea plants, where heterozygotes make enough pigment for the flowers to be a purple color indistinguishable from purple flowers of homozygotes).

CO-dominance •  Both alleles are dominant.

•  When two purebred varieties cross, the F1 hybrids have an appearance that expresses both phenotypes of the two parental varieties.

•  For example, when pure white-feathered chickens cross with pure black feathered chickens, the resulting F1 hybrids have both white and black feathers.

•  This speckled phenotype results from the feathers of heterozygotes having both white and black pigments. Gene expression plays a large role.

Gregor Mendel •  Austrian monk •  Studied the inheritance of traits in pea plants

•  His work was not recognized until the turn of the 20th C

•  Known today as the “Father of Genetics” •  Developed the three basic laws of inheritance

#1 - Law of Dominance •  In a cross between parents that are pure for contrasting

phenotypes, only one form of the trait will appear in the next generation.

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

•  For example in seed color: P: Pure Yellow (YY) x Pure Green (yy) F1: Hybrid Yellow (Yy)

y y

Y

Y

Yy Yy

Yy Yy

#2 - Law of Segregation •  During meiosis, the two alleles responsible for a single trait

separate from each other. •  Alleles for a trait are then “recombined” at fertilization,

producing the genotype for the traits of the offspring.

#3 - Law of Independent Assortment

•  Alleles for different traits are distributed to gametes (and offspring) independently of one another*.

*Unless the loci are near

each other on the same chromosome - some genes are “linked.”

Monohybrid Cross A breeding experiment that tracks the inheritance of ONE trait

•  Illustrates Mendel’s Law of Segregation –  Each allele for a given trait separates independently during gamete formation (meiosis).

Punnett Squares •  Tool used to predict the genotype and

phenotype combinations in a genetic cross.

•  NOT Actual results •  Each offspring produced is a separate event

EXAMPLE PROBLEM Purple flower color is dominant over white flower color in pea plants. If a gardener cross-pollinates a homozygous purple flowering plant with a heterozygous purple flowering plant, then what is the genotypic and phenotypic ratios of the F1 offspring?

STEP 1: Identify the trait and type. Flower Color, Complete Dominance STEP 2: Create a Key. A – purple a – white STEP 3: Write the genotypes of the Parents. P: _A A_ x _A a_ STEP 4: Fill in the Punnett Square. STEP 5: Determine the genotypic and phenotypic ratios of the offspring.

A A

A

a

A A A A

A a A a

Genotypic Ratio: 2 A A : 2 A a : 0 a a Phenotypic Ratio: 100% purple flowers

PRACTICE PROBLEM Tall pea plants are dominant over short pea plants. If a gardener cross-pollinated two hybrid pea plants, then what is the genotypic and phenotypic ratios of the F1 offspring?

STEP 1: Identify the trait and type. STEP 2: Create a Key. A – __________________________________ a – __________________________________ STEP 3: Write the genotypes of the Parents. P: _______ x ________ STEP 4: Fill in the Punnett Square. STEP 5: Determine the genotypic and phenotypic ratios of the offspring.

Genotypic Ratio: Phenotypic Ratio:

PRACTICE ANSWER Tall pea plants are dominant over short pea plants. If a gardener cross-pollinated two hybrid pea plants, then what is the genotypic and phenotypic ratios of the F1 offspring?

STEP 1: Identify the trait and type. Plant Height, Complete dominance STEP 2: Create a Key. A – tall a – short STEP 3: Write the genotypes of the Parents. P: Aa x Aa STEP 4: Fill in the Punnett Square. STEP 5: Determine the genotypic and phenotypic ratios of the offspring.

Genotypic Ratio: 1 AA : 2 Aa : 1 aa Phenotypic Ratio: 3 tall : 1 short

A a

A

a

A A A a

A a a a

Dihybrid Cross A breeding experiment that tracks the inheritance of TWO traits

•  Illustrates Mendel’s Law of Independent Assortment

–  Each pair of alleles segregates independently during gamete formation (meiosis).

Determining Gametes for a Dihybrid Cross

•  Trait #1: Seed shape •  Alleles:

R - round seed shape r - wrinkled seed shape

•  Trait #2: Seed color •  Alleles:

Y - yellow seed color y - green seed color

RrYy x RrYy

RY Ry rY ry RY Ry rY ry All possible gamete combinations from each parent.

P: hybrid (RrYy) x hybrid (RrYy)

Setting up & Filling In a Dihybrid

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

RY Ry rY ry

RY

Ry

rY

ry 9:3:3:1 phenotypic ratio

PRACTICE PROBLEM In pea plants, tall plants are dominant over short plants and yellow pea color is dominant over green pea color. If a gardener cross-pollinates a hybrid pea plant with a pea plant that is short with green peas, then what is the phenotypic ratio of the F1 offspring?

STEP #1: Identify Traits. STEP #2: Make a Key. STEP #3: Parent�s Genotypes. P: ___________ x ___________ STEP #4: Fill in Punnett Square STEP#5: Find the Ratio. ______ Tall, Yellow ______ Tall, Green ______ Short, Yellow ______ Short, Green

PRACTICE ANSWER In pea plants, tall plants are dominant over short plants and yellow pea color is dominant over green pea color. If a gardener cross-pollinates a hybrid pea plant with a pea plant that is short with green peas, then what is the phenotypic ratio of the F1 offspring?

STEP #1: Identify Traits. Height (CD) & Color (CD) STEP #2: Make a Key. A – Tall a – short B – yellow b – green STEP #3: Parent�s Genotypes. P: AaBb x aabb STEP #4: Fill in Punnett Square STEP#5: Find the Ratio. ¼ Tall, Yellow ¼ Tall, Green ¼ Short, Yellow ¼ Short, Green

AB Ab aB ab

ab

ab

ab

ab

AaBb Aabb aaBb aabb

AaBb

AaBb

AaBb

Aabb

Aabb

Aabb

aaBb

aaBb

aaBb

aabb

aabb

aabb

Challenge Question: How many different gametes will be produced for the

following genotypes? Remember:

2n = # of possible gametes, n = # of heterozygotes

1.  RrYy

2.  AaBbCCDd

3.  MmNnOoPPQQRrssTtQq

Challenge ANSWERS: How many different gametes will be produced for the

following genotypes? Remember:

2n = # of possible gametes, n = # of heterozygotes

1.  RrYy = 22 = 4 gametes

RY Ry rY ry

2.  AaBbCCDd = 23 = 8 gametes

ABCD ABCd AbCD AbCd

aBCD aBCd abCD abCD

3.  MmNnOoPPQQRrssTtQq = 26 = 64 gametes