Fundamentals of Genetics Chapter 9 Genetics The field

Fundamentals of Genetics Chapter 9

Genetics § The field of biology dedicated to understanding how characteristics are transmitted from parent to offspring. § Studied by Gregor Mendel § Heredity- the transmission of characteristics from parents to offspring.

Gregor Mendel § Austrian monk & science teacher in the mid 1800’s § Worked with pea plants in his garden § Studied 7 characteristics of peas

§ Each characteristics had 2 different traits EX) tall/short, purple/white § Page 176

Mendel’s Experiments: 1. Started off with 2 pure plant groups for a particular characteristic 1. P 1 generation 2. Cross pollinated these plant groups (sexual reproduction) to produce the next generation of plants 1. F 1 generation

3. When the seeds matured from the F 1 generation, he counted & recorded the traits of the offspring. 4. Then he allowed the F 1 generation plants self pollinate (sexual reproduction with self) to produce the next generation of plants 1. F 2 generation

(notice that the predicted ratios & the actual ratios are not exact!)

Mendel’s Results: § The F 1 generation always showed only 1 of the 2 traits for the characteristic § Mendel named this the dominant factor § The F 2 generation always showed a 3: 1 (or a 75% to a 25%) ratio between the 2 traits § Mendel named the second the recessive factor

Example: Purple is dominant White is recessive F 1 generation: all purple F 2 generation: 75% purple 25% white

The Law of Segregation § Mendel stated that… a pair of factors is segregated, or separated, during the formation of gametes. § So, What does this mean? § Each egg and sperm receives only one factor from each parent.

The Law of Independent Assortment § Mendel also stated that … factors for different characteristics are distributed to gametes independently. § So, What does this mean? § The factors for different characteristics are not connected.

Molecular Genetics § The study of the structure & function of chromosomes & genes § Allele: alternate form of a gene § Mendel called them “factors” § Abbreviations: § Dominant allele = capital letter § Recessive allele = lower case letter (B) (b)

9. 2 Vocabulary: § Genotype: the genetic makeup of the organism (BB, Bb, bb) § Phenotype: the physical appearance of the organism (Black, white)

Genotype Vocabulary: § Homozygous: “same genes” § When the organism has the same alleles for the characteristic § BB = homozygous dominant § bb = homozygous recessive § Heterozygous: “different genes” § When the organism has different alleles for the characteristic § Bb

Genetic Probability § The likelihood (probability) of offspring of known parents can be determined by Punnett squares

Punnett Squares: diagrams biologists use to predict the probability that certain traits will be inherited by offspring. 3 Types: Monohybrid Cross – One characteristic Dihybrid Cross – Two characteristics Trihybrid Cross – Three characteristics

Monohybrid Cross A cross between individuals that involves one pair of contrasting traits. § 1 characteristic § 2 contrasting traits

Dihybrid Cross A cross between individuals that involves 2 pairs of contrasting traits. • 2 characteristics • 4 contrasting traits

How do you find an unknown genotype? Testcross Example) Is the red flower a pure (RR) or hybrid (Rr) ? • Cross the unknown flower with a pure recessive flower (rr). • This will allow the recessives to show up in the next generation.

The results: § If the flower is hybrid, the offspring will be 50% red and 50% white. Result if flower is hybrid § If the flower is pure, the offspring will be all red. Result if flower is pure

Complications! § Incomplete Dominance and Codominance 1) Incomplete Dominance: occurs when the F 1 offspring has a phenotype between that of the parents. Ø neither allele is completely dominant

Example) Incomplete Dominance § Four o’clock flowers Q- What would happen if you crossed a red four o’clock flower with a white four o’clock flower? A- a pink flower!

2) Codominance § Occurs when both alleles are expressed in a heterozygous offspring. § Neither allele is dominant or recessive Q- What would happen if you crossed a horse with a white coat with a horse with a red coat? A- A horse with a roan coat (both red and white hairs) !

Codominance

§ Question: In rabbits, the allele for black coat color (B) is dominant over the allele for brown coat color (b). Predict the results of a cross between a rabbit homozygous for black coat color and a rabbit homozygous for brown coat color. What is the genotypic ratio? What is the phenotypic ratio?

§ Brown hair color is dominant over blond hair, and brown eyes are dominant over blue eyes. § Q- If 2 heterozygous parents for both traits mate, what is the phenotypic ratio of their offspring? _____ Brown hair, Brown eyes _____ Brown hair, Blue eyes _____ Blond hair, Brown eyes _____ Blond hair, Blue eyes

Complications: Multiple alleles § ABO blood groups Dominant Codominant Recessive

Complications: Polygenic Inheritance and Quantitative Characters § One trait determined by multiple genes § Ex) skin color: at least 3 genes

Complications: Epistasis § Expression of one gene depends on another § Mouse coat color: Ø B - black coat Ø b - brown coat Ø C - pigment Ø c - no pigment

What are my chances? 1. Obtain 2 pennies. 2. Flip each coin simultaneously 50 times. Record the results of each set of flips. 3. Total the number of combinations of your results: __ Heads-Heads __ Heads-Tails __ Tails-Tails

4. Record your results on the board. 5. Calculate your percent chance of each combination. (# of combos) 50 6. Calculate the class average percent chance of each combination.