Reebop Ratios

Part 1: Introduction

Part 2: Mendelian Genetics and Beyond

Part 1: Introduction

Remember your baby Reebops?

• Let’s think about the tail trait.

• What were the possible phenotypes?curly tail

straight tail• What were the possible genotypes?

TT = curly tailTt = curly tail

tt = straight tail

Important Words

• Homozygous: Having two alleles (forms of a gene) that are identical (TT or tt).

• Heterozygous: Having two alleles (forms of the gene) that are different (Tt).

• Dominant: The dominant allele is expressed when homozygous (TT) and when heterozygous (Tt).

• Recessive: The recessive allele is only expressed when homozygous (tt).

• Which of the genotypes below are heterozygous? Homozygous?

Tt TT tt

• Based on these genotypes and phenotypes, which allele is dominant, T or t? How do you know?

TT = curly tail

Tt = curly tail

tt = straight tail

Quick-Think Time• If your original Reebop parents had 100

offspring, how many of them would you expect to have a curly tail?

• How many of them would you expect to have a straight tail?

• Why do you think so?

• Why would someone want to predict the numbers of offspring to have a particular trait?

Punnett’s Square

• We can find out the expected number of curly-tailed and straight-tailed babies with a tool called Punnett’s Square.

• We will call it Punnett’s “Fertilization Probability” Square.

Who was Punnettand what is his square?

• REGINALD CRUNDALL PUNNETT (1875-1967)was among the first English geneticists. He created the “Punnett Square” to work out what the possible offspring of two parents will be.

T

t

T

t

tt

TtTT

Tt

Curly-tailed Mom

Curly-tailed Dad

tT

tTpossible gametes

po

ssib

le g

amet

es

T

t

T

t

tt

TtTT

Tt

TT

tT

tt

Quick-Think Time

How many out of 100?

T

t

T

t

tt

TtTT

Tt

TT

tT

tt

Expected Genotype

50%

25%

25%

T

t

T

t

tt

TtTT

Tt

TT

tT

tt

Expected Phenotype

50% + 25% = 75% curly-tailed

25% straight-tailed

From Percentages to Ratios

75% curly-tailed to 25% straight-tailed =

25% TT to 50% Tt to 25% tt =

Genotypic ratio of 1 to 2 to 1, or 1:2:1

Phenotypic ratio of 3 to 1 or 3:1

How can we test this prediction?

Expected phenotypic ratio: 3:1

Expected genotypic ratio: 1:2:1

STOP

to complete

Reebop Ratios

activity

Part 2: Mendilian Genetics and Beyond

You and Gregor Mendel…

• In the first Reebop activity, you looked at the variety of offspring produced by two parents.

• You looked at 7 traits, the gene for each residing on a different chromosome.

• You learned about how meiosis and fertilization contribute to variation in organisms.

…You and Gregor Mendel

• In the second Reebop activity, you looked at the expected and experimental ratios of offspring with curly and straight tails.

• You, and a nineteenth-century monk named Gregor Mendel have a lot in common.

• Let’s find out why.

Mendel & the Garden Pea

• Gregor Mendel, Austrian Monk

• In 1866, Mendel performed breeding experiments with garden pea plants and observed consistent, predictable patterns of inheritance.

• From his observations, Mendel developed a number of principles, today referred to as Mendel's Laws of Inheritance.

Before Mendel…

• In the 19th century, when both Gregor Mendel and Charles Darwin were alive, “blending inheritance” was a popular idea.

• According to blending inheritance, traits from parents “blended” in offspring producing intermediate traits.

• Example: The offspring of one short parent and one tall parent would be of medium height. This offspring would pass the new “medium sized” trait to its own offspring.

Quick-Think Time

• Imagine that blending inheritance occurs in Reebops.

• What type of tail would the offspring of curly-tailed and straight-tailed parents have?

• What would happen to variation in this population over time?

Darwin and Mendel…

1809–1882 1822–1884

Darwin and Mendel…

• Darwin observed that variation, in the form of individual differences, exists in every species, and is not reduced from one generation to the next.

• He proposed that natural selection acts on this variation.

• Darwin knew that in order for his idea to be valid, a mechanism for preserving and inheriting variation had to exist.

Darwin and Mendel…

THE BIG IDEA:• Mendel’s work answered Darwin’s

difficult question.

– Mendel showed that traits from parents do not blend. The traits remain distinct and can be passed on to later generations. Genetic variation is preserved.

– Mendel also proposed a mechanism for how this variation is inherited and preserved.

Mendel summarized his findings in three theories…

Mendel’s first law, or the principle of segregation:

During the formation of eggs and sperm, “paired factors” (now known as alleles, which reside on chromosomes) segregate, or separate.

Quick-Think Time

Explain how Mendel’s Principle of Segregation was demonstrated in the Reebop activities.

Mendel summarized his findings in three theories…

Mendel’s second law, or the principle of independent assortment:

Each pair of “factors” (now known as alleles) separate and recombine into gametes independently of each other.

Quick-Think Time

Explain how Mendel’s Principle of Independent Assortment was demonstrated in the Reebop activities.

Mendel summarized his findings in three theories.

Mendel’s third law, or the law of dominance:

Each characteristic is determined by the interaction of two hereditary factors (now called alleles), one from each parent. Mendel found that one allele was always dominant over the other.

Quick-Think Time

Explain how Mendel’s Law of Dominance was demonstrated in the Reebop activities.

Mendel & Probability

• coin (heads/tails)- probability of heads or tails = ½- probability of getting heads 3 times in a row- multiplication rule: (½ x ½ x ½ = 1/8)

Extensions of Mendelian Genetics

• Many researchers have encountered exceptions to Mendelian Principles – was he wrong?

• Majority of heritable characters not as simple as peas– Phenotypes affected by: many genes, sequential,

jointly, environment, etc.

CR CW

CR

CRCR CRCW

CW

CRCW CWCW

Extensions of Mendel…• Incomplete Dominance – appearance between

dominant and recessive

1:2:1 phenotypic ratio1:2:1 genotypic ratio

CRCR

CWCW

F1 GenerationAll CRCW Eggs

Spe

rm

F2 generation

1 : 2 : 1CRCR:CRCW:CWCW

CR

Cw

Chestnut, CC

Palomino,

CCcr

Cremello,

CcrCcr

Another example of Incomplete Dominance

Extensions of Mendel…

• Codominance – both alleles affect phenotype in separate ways– Only observed at the cellular or molecular level

– e.g., M N bloodgroups (RBC surface marker)• MM• NN• MN (both equally expressed)

Extensions of Mendel…

• Multiple alleles of a single gene– ABO blood groups

– A, B, AB, O– Combinations of 3 different alleles– IA, IB, I– A and B dominant to O, but not to each other– A and B are codominant

Extensions of Mendel…

• I – enzyme that adds sugar molecules to lipids on surface of RBC recognized by our immune system

• IB –adds galactose (IBIB or IBi) “B”• IA – adds galactosamine (IAIA or IAi) “A”• i– does not add a sugar (ii) “O”• IA IB adds both sugars “AB”

•Multiple alleles of a single gene– ABO blood groups

Multiple Alleles—ABO Blood Groups

Possible alleles from femaleP

ossi

ble

alle

les

from

mal

e

IA IB I

IA IAIA IAIA IAi

IB IAIB IBIB IBi

i IAi IBi ii

Blood Types A AB B O

Extensions of Mendel…

• Epistasis – one gene influences a second gene (gene interaction)For example in dogs:– B gene: determines whether pigment (melanin) is brown or

black. – E gene:enables or prevents melanin from being deposited in

the coat.

Epistatic Ineractions on Coat Coloree E_

No dark pigment in fur Dark pigment in fur Yellow Lab

E_bb E_B_ eebb eeB_ Chocolate Lab Black Lab

Yellow fur, Yellow fur, Brown fur, Black fur, brown nose, black nose, nose, lips, nose, lips,lips, eye rims lips, eye rims eye rims eye rims

Pedigree Analysis

• Display of family relationships to help keep track of relationships and traits– phenotypes– genotypes

– e.g., X-linked clotting disorder XX (normal) XY (normal)

XXh (carrier) XhY (hemophilia)

XhXh (hemophilia)

Hemophilia in European Royalty

XhY

XXh

XXh

Click and choose the button

Genetic Testing

• fetal testing• amniocentesis – needle inserted in uterus to remove ~10 mL of fluid from amniotic sac (14-16th week)

– chromosomal analysis

• chorionic villus sampling (CVS)– tube inserted through cervix into uterus removing sample of fetal tissue from placenta