Mendelian Genetics Chapter Four

Theories of Inheritance Homunculus (Ancient Greeks – 17th ce) sperm caries a miniature human that uses egg as a growth medium (spermists) Problem: Doesn’t explain why kids sometimes look like their mom 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 Pangenesis Heredity units (pangenes) are formed in all organs, spread through blood to genitals Problem: Blood transfusions into experimental animals did not change characteristics of progeny.

Moravian Sheep Breeders Association (1837) Why do valued traits sometimes disappear and then reappear in some offspring? 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?

Gregor J. Mendel (1822-1884) Monastery of St. Thomas Brno, Czech Republic Versuche über Pflanzen-Hybriden "Experiments in Plant Hybridization" Society for the Study of the Natural Sciences Proceedings (1866)

Pisum sativum Mendel 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 • Used pure-breeding lines (8 generations) to form hybrid lines: offspring of dissimilar parents • Can follow discrete traits – no intermediate forms

Mendel’s Experiments Studied 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

Mendel’s Experiments Cross fertilization

emasculation

Mendel was careful: • many controls • reciprocal crosses

Mendel’s Experiments Monohybrid Cross P parental

x

F1

first filial

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

Mendel’s Experiments Monohybrid Cross P parental

F1

first filial

F2 5474 smooth, 1850 wrinkled smooth : wrinkled 2.96 : 1

x

Mendel’s Experiments Monohybrid Cross Seed shape Seed color Flower color Pod color Pod shape Stem length Flower position

5474 smooth, 1850 wrinkled 6022 yellow, 2001 green 705 purple, 224 white 428 green, 152 yellow 882 round, 299 pinched 787 long, 277 short 858 stem, 651 tip

3:1 Dominant : Recessive

2.96 : 1 3.01 : 1 3.15 : 1 2.82 : 1 2.95 : 1 2.84 : 1 3.14 : 1

Mendel’s Deductions Proposed that “unit factors” exist in pairs to explain these results Each parent has two unit factors but contributes only one to every progeny in the form of gametes Designated upper-case as Dominant and lower-case as Recessive

Mendel’s Deductions Seed coat color

YY

yy

Dominant

Recessive

Yy All offspring will be yellow and will be heterozygotes

Genetic Language: Gene - Discrete “unit factors” of inheritance Allele - Different forms of a gene (e.g. Y or y) Genotype - Allelic composition of a trait (e.g. YY, Yy, or yy) Phenotype - Physical manifestation of a trait (e.g. Yellow or green seed)

Genetic Language: 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: Parental Y YY “Pure-breeding Homozygous Lines” F1 “Hybrid”

Yy Heterozygous

yy Homozygous

Punnett Squares: Yy

F1 monohybrid self-fertilization

Y

Y

y

YY

Yy

F2

Yy y

Yy

Phenotype

yy

3:1

BUT Genotype 1 : 2 : 1

Genotype vs. Phenotype YY

Yy

yy

Homozygous dominant

Heterozygous hybrid

Homozygous recessive

Same phenotype How do you distinguish between the two?

Test Cross: yy

Y

yy

y

y

Yy

Yy

YY

y

y

Y

Yy

Yy

y

yy

yy

Yy Y

Yy

Yy

If homozygous, all progeny are Yellow

If heterozygous, progeny 1 : 1 Yellow : Green

Mendel’s 1st Law: • Principle

of segregation

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

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

Mendel Understood Probability 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 two 2’s with a pair of dice:

The probability of rolling a 2 = 1/6 So rolling two 2’s = 1/6 x 1/6 =1/36

Mendel Understood Probability 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 by one parent p(y) = ½ The probability is ½ that a y will be contributed by the other parent p(y) = ½ The probability of having one yy offspring ½x½=¼ p(yy) = ¼ The probability of having three yy offspring ¼ x ¼ x ¼ = 1/64

Mendel Understood Probability 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

Mendel Understood Probability 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

Probability One more thing to remember: p(a mutually exclusive event) = 1 – p(all the other events)

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 Dominant Also 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 Parental Pure-breeding lines for two traits Homozygous

yyrr

Homozygous

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

Mendel’s Next Experiment: Dihybrid cross yyrr

YYRR P

X Let’s check the F2 Yy R r

F1 Were the two traits transmitted together or independently?

Mendel’s Next Experiment: Dihybrid cross YyRr

YyRr

YR (½) yr (½)

YR (½)

yr (½)

YYRR

YyRr

¼

Two Phenotypes ¼

YyRr

¼

yyrr ¼

3 : 1 yellow green round wrinkled

Traits transmitted together

Mendel’s Next Experiment: Dihybrid cross In Reality F2 Looks Like: Four Phenotypes:

315

9 :

108

3

101

:

3

32

:

1

Mendel’s Next Experiment: Dihybrid cross F2 offspring of Dihybrid cross Four Phenotypes:

new phenotypes recombinants original phenotypes parental or non-recombinant

Mendel’s Next Experiment: Dihybrid cross YyRr

4 different possible gametes

YR (¼)

Yr +

yR

(¼) + (¼)

yr +

(¼) = 1

Mendel’s Next Experiment: Dihybrid cross YR(¼) Yr (¼) yR (¼) yr (¼) YR (¼) YYRR

YYRr

YyRR

YyRr

YYRr

YYrr

YyRr

Yyrr

Yr (¼) yR (¼)

YyRR

YyRr

yyRR

YyRr

Yyrr

yyRr

yyRr

12 4 : round wrinkled

9:3:3:1 12 : yellow

4 green

yr (¼) yyrr

3:1

Therefore traits must be transmitted independently

Mendel’s 2nd Law: Independent Assortment 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

Mendel’s 3rd Experiment: Trihybrid cross yyrrtt

YYRRTT Parental

X Tall plants

Short plants YyRrTt

Independent Assortment

F1 Tall plants

F2

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

Mendel’s Laws 1. Principle of Segregation Two alleles segregate randomly during formation of gametes

2. Independent Assortment Two genes will assort independently and randomly from each other

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 Analysis Pedigrees 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