Mendelian Genetics - The University of · PDF fileMendelian Genetics Chapter Four. ... Trihybrid cross F 1 F 2 27:9:9:9:3:3:3:1 ... • Homework – Chapter Four Problems; – Review:

Mendelian GeneticsChapter Four

Theories of InheritanceHomunculus

(Ancient Greeks – 17th ce) sperm caries a miniature human that uses egg as a growth medium (spermists)

PangenesisHeredity units (pangenes) are formed in all organs, spread through blood to genitals

Blending Descendents possess traits that are intermediate between those of parents, become mixed and forever changed in the offspring. Problem: Over time, a population would become uniform in appearance Once blended, traits should not reappear in subsequent generations

Problem: Blood transfusions into experimental animals did not change characteristics of progeny.

Problem: Doesn’t explain why kids sometimes look like their mom

Moravian Sheep BreedersAssociation (1837)

Breeders could predict the traits of offspring if they could answer the basic questions:

What is inherited?

How is it inherited?

What is the role of chance in heredity?

Why do valued traits sometimes disappear and then reappear in some offspring?

Gregor J. Mendel(1822-1884)

Versuche über Pflanzen-Hybriden "Experiments in Plant Hybridization"Society for the Study of the Natural

Sciences Proceedings (1866)

Monastery of St. ThomasBrno, Czech Republic

Pisum sativumMendel chose a great

“model organism”• Self fertilizing plants that can be cross-fertilized

• Relatively quick generation time

• Can grow large numbers of plants in limited space

• Can follow discrete traits – no intermediate forms

• Used pure-breeding lines (8 generations) to formhybrid lines: offspring of dissimilar parents

Mendel’s ExperimentsStudied 7 characteristics of

pure-breeding lines:

• Seed color (yellow vs. green)• Seed shape (round vs. wrinkled)• Flower color (purple vs. white)• Pod color (green vs. yellow)• Pod shape (round vs. pinched)• Stem length (long vs. short)• Flower position (along stem vs. at the tip)

“either - or” phenotypes with no intermediates

Mendel’s Experiments

• Pure Breeding Lines:– Crossing two of same phenotype always

produces one phenotype• Hybrids:

– Crossing two of same phenotype can lead to offspring of two phenotypes

– Example – cross two tall plants, offspring are a combination of tall and short plants

Cross fertilization

emasculation

Mendel was careful: • many controls • reciprocal crosses

Mendel’s Experiments

xPparental

F1first filial

recessive – a trait that disappears in the hybrids (but may re-appear in subsequent generations)

dominant – a trait “unchanged” in the hybrids

Mendel’s ExperimentsMonohybrid Cross

xPparental

F1first filial

F2

smooth : wrinkled 2.96 : 15474 smooth, 1850 wrinkled


Seed shape 5474 smooth, 1850 wrinkled 2.96 : 1Seed color 6022 yellow, 2001 green 3.01 : 1 Flower color 705 purple, 224 white 3.15 : 1 Pod color 428 green, 152 yellow 2.82 : 1Pod shape 882 round, 299 pinched 2.95 : 1Stem length 787 long, 277 short 2.84 : 1 Flower position 858 stem, 651 tip 3.14 : 1

3 : 1Dominant : Recessive


Mendel’s Deductions

Proposed that “unit factors” existin pairs to explain these results

Each parent has two unit factors but contributes only one to every progenyin the form of gametes

Designated upper-case as Dominantand lower-case as Recessive

Seed coat color

Dominant Recessive

YY yy

Y y

Mendel’s Deductions

All offspring will be yellow and will be heterozygotes

Discrete “unit factors” of inheritanceGene -

Physical manifestation of a trait (e.g. Yellow or green seed)

Allelic composition of a trait (e.g. YY, Yy, or yy)

Different forms of a gene (e.g. Y or y)

Genetic Language:

Allele -

Genotype -

Phenotype -

Homozygous – Individuals with two identical copies of a gene

Same allele (yy)

Heterozygous - Individuals with two different copies of a gene

Two different alleles (Yy)

Genetic Language:

YY yyY

Homozygous Homozygous

Yy

Heterozygous

Parental “Pure-breeding

Lines”

F1“Hybrid”

Genetic Language:

YyF1 monohybridself-fertilization

Yy

YY Yy

Yy yy

Y y

Y

y

F2

Phenotype 3 : 1BUT

Genotype 1 : 2 : 1

Punnett Squares:

Same phenotypeHow do you distinguish

between the two?

YY Yy yy

Homozygous dominant

Heterozygoushybrid

Homozygous recessive

Genotype vs. Phenotype

y y

Y

Y

Yy

Yy

Yy

Yy

If homozygous, all progeny are Yellow

yy

YY

yy

Yy

y y

Y

y

Yy

yy

Yy

yy

If heterozygous,progeny 1 : 1

Yellow : Green

Test Cross:

Hereditary traits are determined by discrete factors (now called genes) that appear in pairs. During sexual development, these pairs are separated (segregated) into gametes and only one factor from each parent is passed to the offspring.

•• Principle of segregation

Discrete factors explained how a characteristic could persist through generations without blending and why it could “disappear and reappear” in subsequent generations

Mendel’s 1st Law:

Practice Your Punnetts!• Draw the punnett squares• Calculate # of each genotype and

phenotype– Yy cross yy (Y = yellow, y = green)– Yy cross Yy– Rr cross rr (R = round, r = wrinkled)– RR cross rr– BB cross Bb (B = brown, b = blue)– bb cross bb

Probability: The number of times an event is expected to occur divided by the number of trials during which that event could have happened

The probability of rolling a 2 with one roll of one die:

1 event / 6 possible outcomes =

1/6

Mendel Understood Probability

The Multiplication Rule: The probability of two or more independent events occurring simultaneously is the product of their individual probabilities.

The probability of rolling a 2 =

1/6 So rolling two 2’s =

1/6 x 1/6 =1/36

The probability of rolling two 2’s with a pair of dice:


In the cross Yy x Yy , what is the probability of yielding 3 yy offspring?

The probability is ½ that a y will be contributed byone parent p(y) = ½


The probability is ½ that a y will be contributed by theother parent p(y) = ½

The probability of having one yy offspring½ x ½ = ¼p(yy) = ¼

The probability of having three yy offspring¼ x ¼ x ¼ = 1/64

The Addition Rule: The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways.

(Used to answer “either / or” questions only)

The probability of rolling a 2 or a 5 =

1/6 + 1/6 = 1/3



In the cross Yy x Yy , what is the probability of yielding yellow seeded

offspring (Yy or YY)?

The probability of being YY p(YY) = ¼

The probability of being Yy p(Yy) = ½

The probability of being either YY or Yy:¼ + ½ = ¾

One more thing to remember:

p(a mutually exclusive event) = 1 – p(all the other events)

Probability

Practice Probability• What is the probability that you will roll

one dice and see:1. A 3?2. A odd number?3. A 3 or a 4?

• Rolling two dice what is the probability to see:

1. Two 3’s (one on each dice)?2. A 3 and a 4?

Mode of Inheritance

The pattern that the trait follows in families:Four Mendelian:• Autosomal (non-sex chromosome) Recessive• Autosomal Dominant• X-linked Recessive• X-linked DominantAlso complex inheritance• will be covered later

Autosomal Traits:

Both Males and Females affected, and both transmit to both sexes of offspring

• Recessive – usually rare in population– Skips Generations– Inbreeding increases risk of recessive traits

• Dominant – more common– Doesn’t skip generations

• Complex

X-Linked Traits:Gene on X chromosome is carrying trait.• Recessive

– Only males are affected– Passed from unaffected mothers to sons– Affected fathers will only transmit to

heterozygous, unaffected daughters • Dominant

– Males and females both affected– Can be passed to both offspring, however

often see more females affected because of male lethality

– Affected fathers to every single daughter

Two genes• Now lets examine what happens

when we look at more than one gene at a time:– Two Traits– Two different genes– Two alleles per gene– Genes are each on separate

chromosomes

Mendel’s Next Experiment:Dihybrid cross

YYRR yyrr

Homozygous Homozygous

Parental Pure-breeding

linesfor two traits

Yellow or Green Seed Color (Y or y)Round or Wrinkled Shape (R or r)

YYRR yyrr

P X

F1

Y Ry r

Were the two traits transmitted together or independently?

Let’s check the F2


YyRr

YyRr

YR yr(½) (½)

YR

yr(½)

(½) YYRR

¼

yyrr ¼

YyRr

YyRr

¼

¼

Traits transmitted together

3 : 1yellowround

greenwrinkled

Two Phenotypes


315 108 101 32

Four Phenotypes:

In Reality F2 Looks Like:

9 : 3 : 3 : 1


Four Phenotypes:

F2 offspring of Dihybrid cross

new phenotypesrecombinants

original phenotypesparental or non-recombinant


(¼) (¼) (¼) (¼)

YyRr4 different

possible gametes

= 1+ + +


Y RR Y y rr y

YR

Therefore traits must be transmitted independently

Yr yR yr(¼) (¼) (¼) (¼)

YR

Yr

yR

yr

(¼)

(¼)

(¼)

(¼)

YYRR YyRRYYRr YyRr

YYRr YYrr YyRr Yyrr

YyRR

YyRr

YyRr yyRR yyRr

Yyrr yyRr yyrr

12 round : 4

wrinkled

12 yellow : 4

green

9:3:3:1

3 : 1


Independent Assortment

Mendel’s 2nd Law:

Inheritance of a pair of factors for one trait is independent of the simultaneous inheritance of factors for another trait

Two genes will assort independently and randomly

YYRRTT yyrrtt

Tallplants

YyRrTt

Tallplants

Parental

Shortplants

X

Mendel’s 3rd Experiment:Trihybrid cross

F1

F2 27:9:9:9:3:3:3:1

IndependentAssortment

2. Independent Assortment

Mendel’s Laws

Two genes will assort independently and randomly from each other

1. Principle of SegregationTwo alleles segregate randomly during formation of gametes

Practice Your Punnetts!• Draw the punnett squares for two genes• Calculate # of each genotype and

phenotype(Y = yellow, y = green)(R = round, r = wrinkled)– YyRr x YyRr

– YYRr x Yyrr

Pedigree AnalysisPedigrees are visual ways to examine a

family’s inheritance pattern for any trait of interest

• Identify:– Relationships between family members– Who has trait of interest (phenotype)

Mode of inheritance

Pedigree Analysis

Insert Figure 4.13

Autosomal Recessive

Autosomal Dominant

Complex Inheritance

Next Class:• Read Chapter Five

• Homework – Chapter Four Problems;– Review: 1,3,5, 7– Applied: 1,2,4,5, 8, 9,10, 11,15

• Pedigree Assignment –Due October 11th

Documents

Mendelian Genetics - The University of · PDF fileMendelian Genetics Chapter Four. ... Trihybrid cross F 1 F 2 27:9:9:9:3:3:3:1 ... • Homework – Chapter Four Problems; – Review: