Earlobes Widows Peak MENDELIAN GENETICS Principles of Heredity

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) • F 1 generation (first filial) - offspring of P generation • F 2 generation (second filial) offspring from F 1 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. XX B 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’s Experiments • 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 (F 1) plants had purple flowers! What did this tell Mendel?

Mendel’s Experiments • Mendel then took purple flowered plants from the F 1 generation and crossed them. • • ¾ of the offspring (F 2) had purple flowers and ¼ of the offspring had white flowers. What did this tell Mendel?

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 #1. • Brown eyes (B) are dominant to blue eyes (b) • Cross a homozygous brown-eyed person with a blue-eyed person • B B b Bb Bb 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 2 X= Σ (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)

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 # stripes 4 50 spots 3 41 stripes/spots 9 85 Total Exp # O-E (O-E)2/E

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 of Freedom 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 Rr. Tt 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? Tt. Ww - _________ 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 5 th 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: