Earlobes

Widow’s Peak

MENDELIAN GENETICS

Principles of Heredity • There are two factors which determine

physical features and behavior of an organism:– HEREDITY- the genetic make-up – ENVIRONMENT- conditions during

development

• Is it “NATURE” (Heredity) OR “NURTURE” (Environment) that determines the ultimate product?– Studies on identical TWINS are inconclusive

Hereditary Terms• Gene – sequence of DNA that codes for a

protein (hundreds to thousands of genes per chromosome).

• Allele – different forms of a gene (B= brown eyes or b= blue eyes)

• Phenotype- observable feature (eye color, hair color, blood type)

• Genotype- arrangement of alleles (Aa, tt, or BB )

• Dominant- allele that masks the presence of the other allele (capital)

• Recessive- allele that is masked by the dominant allele (lower case)

• Homozygous (pure)– when the 2 alleles of a gene are the same (tt or TT)

• Heterozygous (hybrid or carrier)- when the 2 alleles of a gene are different (Tt)

• P generation (parental) - Pure (is not a carrier of another allele form)

• F1 generation (first filial) - offspring of P generation

• F2 generation (second filial) - offspring from F1 cross

Why does every trait have 2 alleles?

• Because each chromosome of a homologous pair carries 1 allele of a trait. The “paired” alleles are in the same position on homologous chromosomes.

X XB b

Father of Genetics• Gregor Mendel:

– Austrian Monk (1822-1884)– Published the results of scientific research on

Garden Peas (Pisium sativum) in 1865.Why study peas?– Great natural variation- stem length, seed

color, pod shape, pod color, and flower color. – They are small, easy to grow, and have both

male and female reproductive parts (easy to cross fertilize).

Floral Anatomy

• Mendel started with pure (homozygous) pea plants and selectively crossed them.

• These plants are the parental (P) generation.

• He crossed a pure purple flowered plant with a pure white flowered plant.• All offspring (F1) plants had purple

flowers!• What did this tell Mendel?

Mendel’s Experiments

• Mendel then took purple flowered plants from the F1 generation and crossed them.• ¾ of the offspring (F2) had purple flowers

and ¼ of the offspring had white flowers.• What did this tell Mendel?

Mendel’s Experiments

Mendel’s Laws of Inheritance• From his experiments, Mendel came up with 2 laws

of inheritance.

1. The Law of Segregation- Alleles separate from each other when gametes are formed during meiosis. Remember that homologous chromosomes separate in Meiosis I.

2. The Law of Independent Assortment- Genes for different traits are sorted independently of one another. For example, brown haired people don’t always have brown eyes!

Who was Punnett?

• REGINALD CRUNDALL PUNNETT (1875-1967)was among the first English geneticists. He created the “Punnett Square” – a diagram to work out the possible allele combinations of the offspring of two parents.

Monohybrid vs. Dihybrid

• Monohybrid cross- A genetic cross between 2 individuals involving 1 trait; 4 squares.

• Dihybrid cross- A genetic cross between 2 individuals involving 2 traits; 16 squares

Setting up a Punnett Square

B B

b

b

Bb Bb

Bb Bb

#1.

•Brown eyes (B) are dominant to blue eyes (b)

•Cross a homozygous brown-eyed person with a blue-eyed person

• BB vs. bb

•What are the genotypic and phenotypic ratios of the offspring?

Dihybrid Cross

• Step 1: Figure out the different gametes for each parent

• Step 2: Set-up the cross and complete• Step 3: Calculate the genotypic and phenotypic

ratios

Dihybrid Cross#1. Cross a pea plant that is homozygous purple and wrinkled with one that is homozygous white and smooth.

P= purple p= white

N= non-wrinkled (smooth) n= wrinkled

#2. Cross 2 pea plants that are both heterozygous purple flowered and heterozygous for smooth seeds.

P= purple p= white

N= non-wrinkled (smooth) n= wrinkled

The Chi-Square Test

• An important question to answer in any genetic experiment is “How can we decide if our data fits the Mendelian ratios we have discussed”? A statistical test that can answer this is the Chi-Square or “Goodness of Fit” test. The calculations from this test determine if the data is acceptable.

Chi-Square Formula

(observed value - expected value)2

(expected value)

•Σ (sigma) means “the sum of”

•Degrees of freedom (df) = n-1 where n is the number of classes (different phenotypes)

X2= Σ

Let's test the following data to determine if it “fits” the expected ratio.

A genetics engineer was attempting to cross a tiger and a cheetah. She predicted a phenotypic outcome of the traits to be:4/16 stripes only: 3/16 spots only: 9/16 both stripes and spots. When the cross was performed she found 50 with stripes only, 41 with spots only and 85 with both. According to the Chi-square test, does her data fit the expected outcome?

Complete the chart below:

Exp ratio Obs # Exp # O-E (O-E)2 (O-E)2/E

stripes 4 50

spots 3 41

stripes/spots 9 85

Total

What next?

• Number of classes (phenotypes) (n) =

• Degrees of freedom = n-1 =

• Chi-square value (look up on the table)– By statistical convention, we use the 0.05

probability level as our critical value. If the calculated chi-square value is less than the 0.05 value, we accept the hypothesis. If the value is greater than the value, we reject the hypothesis.

A Chi-Square Table

Probability

Degrees ofFreedom

0.9 0.5 0.1 0.05 0.01

1 0.02 0.46 2.71 3.84 6.64

2 0.21 1.39 4.61 5.99 9.21

3 0.58 2.37 6.25 7.82 11.35

4 1.06 3.36 7.78 9.49 13.28

5 1.61 4.35 9.24 11.07 15.09

Review Questions

• RRTT

• RrTt

• RRtt

• Rrtt

• List gametes possible from these parents

• (T) is the allele for tall humans and is dominant over the allele (t) for short humans. The allele for widows peak (W) is dominant over (w) non-widows peak hairlines.

What are the phenotypes for the following parents?

TtWw - _________________

TTww - _________________

What are the possible gametes formed by the parents listed above?

AND

• 1. A woman with free ear lobes marries a man with attached ear lobes. Two of their children have attached ear lobes and two have free ear lobes. The alleles are (E) - free (e) - attached

• A. what is the genotypes of the woman?

• B. what is the genotype of the man?

• C. What are the chances of the 5th child having free ear lobes?

• (F) striped fur is dominant over (f) white fur in Australian cats.

What is the result of a cross between a homozygous recessive parent and a heterozygous parent? Give phenotypic ratios and genotypic ratios.

Genotypic Ratio:

Phenotypic Ratio: