chapter 9: fundamentals of genetics

Chapter 9: Fundamentals of Genetics

9-1 Mendel’s Legacy9-2 Genetic Crosses

I. Gregor Mendel (1842, Austrian monk)• Experimented with a garden of PEA PLANTS unveiling laws of HEREDITY.

9-1 Mendel’s Legacy

(1) Heredity

• TRANSMISSION of inheritable TRAITS from parents to offspring.

(1) Plant Height (short vs. tall)

(2) Flower Position (axial vs. terminal)

(3) Seed Texture (smooth vs. wrinkled)

(4) Seed Color (yellow vs. green)

(A) Mendel’s Garden Peas

• Pea plants hold seven TRAITS ( observable characteristics) that exist in contrasting PAIRS:

(5) Pod Appearance (inflated vs. constricted)

(6) Pod Color (yellow vs. green)

(7) Flower Color (purple or white)

(B) Mendel’s Methods• Mendel SELECTED plants to be crossed (mated) by MIMICKING flowering reproduction.

(A) Self-Pollination• He obtained PURE plants for a TRAIT by allowing plants to SELF-POLLINATE for several GENERATIONS.

(B) Cross-Pollination (used forceps, burlap sacs, and a paintbrush)• NATURAL-pollination was INTERRUPTED by Mendel to PREVENT unwanted crosses (pollination).

II. Mendel’s Experiments (several STRAINS used)• Data collected on OFFSPRING of several generations of plants.

(NOTE: He attempted to predict the OFFSPRING of a cross)

(1) Pure

(2) Strain (e.g., PURE strain, HYBRID strain)• TYPE of plant used in cross are of a SPECIFIC TYPE.

• Plants that ALWAYS produce offspring with THAT trait.(a.k.a. homozygous, TT or tt)

(3) The Generations (of pea plants)• P1 Parental (1st Generation)• F1 Offspring of Parental (2nd Generation, P1 x P1 F1)• F2 Offspring of Offspring (3rd Generation, F1 x F1 F2)

III. Mendel’s Results and Conclusions• Each trait was inherited by a SEPARATE FACTOR Traits occurred in PAIRS; a PAIR of FACTORS control the EXPRESSION.

(A) Recessive and Dominant Traits• Data proved 1 FACTOR COULD prevent the OTHER from being EXPRESSED (some traits SKIPPED generations MASKED)

(1) Dominant ( represented by a CAPTIAL letter)• A “factor” that can PREVENT another factor from being EXPRESSED.

(i.e., TT or Tt).

(2) Recessive (represented by a lowercase letter)• A “factor” that CAN ONLY be expressed when NOT MASKED BY a dominant factor. (i.e., when it comes in a PAIR, tt)

(B) The Law of Segregation (Mendel Conclusion #1)• PAIRED FACTORS (Pp and Yy) SEPARATE during formation of SEX CELLS. (gametes UNITE, and pair is RESTORED).

(1) What happens during meiosis that would allow genes located on the same chromosome to separate independently of one another?

Critical Thinking

(C) The Law of Independent Assortment (Mendel Conclusion #2)

(NOTE: Observing TWO traits instead of ONE, allowed Mendel to record this Law during crosses)

• When “factors” for DIFFERENT TRAITS are on SEPARATE chromosomes, they are sent to gametes INDEPENDENTLY of one another.

(2) How might Mendel’s conclusions have differed if he had studied two traits determined by alleles carried on the same chromosomes?

Critical Thinking

IV. Chromosomes and Genes• During MEIOSIS, sex cells receive 1 chromosome from each HOMOLOGOUS PAIR.

NOTE: Therefore, when gametes COMBINE in fertilization, offspring receives one ALLELE (for a trait) from EACH PARENT.

(1) Molecular Genetics

• Field of BIOLOGY that examines the relationship of GENES to TRAITS.

(2) Allele (Mendel’s “FACTOR”)

• Alternative FORMS of a GENE (A or a);

(Ex: For the TRAIT of plant HEIGHT, there are TWO alleles: tall (T) and short (t); Both T and t represent ALLELES of HEIGHT.)

I. Genotype and Phenotype• Descriptions of offspring; RATIOS between types.

9-2 Genetic Crosses

• GENETIC makeup of organism.

(1) Genotype (Homozygous AA, aa, or Heterozygous Aa)

• OBSERVABLE trait of organism.

(2) Phenotype (tall or short, round or wrinkled)

• BOTH alleles of a PAIR are ALIKE (AA or aa).

(3) Homozygous (i.e., PURE)

• Alleles of a PAIR are UNLIKE (Aa).

(4) Heterozygous (i.e., HYBRID)

II. Probability (parts OVER whole)• Likelihood an event WILL occur.

EX: What is the PROBABILITY that TWO heterozygous tall (Tt) pea plants will produce a short (tt) offspring?

• About 1 in 4 or ¼ or 25% probability of producing a short individual (Tt x Tt 1 TT, 2 Tt, and 1 tt)

Answer…

(3) One rule of probability can be expressed as the following: The probability of two independent events occurring simultaneously is the product of the probability of their occurring separately. If, for example, you had a pair of dice and rolled each die one at a time, what would be the probability that you would get two 4s? On the first roll, you would have a 1/6 chance. On the second roll, you would have a 1/6 chance. The probability of obtaining two 4s would be 1/6 x 1/6 or 1/36. Suppose you were playing a game with five dice. What is the chance of rolling a 6 on all five dice?

Critical Thinking

III. Predicting Results of Monohybrid Crosses

• Approach a GENETIC CROSS in a 3-STEP fashion:

(1) Set up a key with letters (e.g., T = Tall, t = short)

(2) Set up P1 cross between two parents

(3) Determine possible gametes donated from each parent and circle each gamete

(4) Set up a correct Punnett Square to determine offspring possibilities

(5) Solve Punnett Square and record offspring data

(1) Homozygous x Homozygous (PP x pp)

• ALL F1 in 1st cross expressed DOMINANT phenotype, Purple Flowers (with genotype, Pp).

(2) Homozygous x Heterozygous (BB x Bb)• Offspring ALL are BLACK, BUT genotypically are in a 1:1 ratio (BB : Bb).

(3) Heterozygous x Heterozygous (Bb x Bb)

• 75% of offspring come out BLACK, 25% come out BROWN.

(4) Testcross (T_ x tt) Offspring types tells you about __ • Used to find out genotype of an UNKNOWN dominant individual

(testing to see what it is, TT or Tt ?);

NOTE: All testcrosses cross unknown with a HOMOZYGOUS RECESSIVE.

(5) Genotypic and Phenotypic Ratio

• If OFFSPRING of a Bb x Bb cross IS 1 BB, 2 Bb, and 1 bb, then…

The PHENOTYPIC ratio is…

3 Black and 1 Brown (ratio is 3:1) The GENOTYPIC ratio is…

1 homozygous Black, 2 heterozygous Black, 1 homozygous Brown (ratio 1:2:1)

(6) Incomplete Dominance (BLENDING Pattern of Inheritance)• Snapdragon flower color NEITHER allele DOMINATES the other.

(4) The offspring of two-short tailed cats have a 25 percent chance of having no tail, a 25 percent chance of having a long tail, and a 50 percent chance of having a short tail. Based on this information, what can you hypothesize about the genotypes of the parents?

Critical Thinking

(7) Codominance (Both alleles expressed but NO blending)• Human BLOOD TYPES a CO-dominant pattern of inheritance.

• Type A (AA, AO), Type B (BB, BO), Type AB (AB), Type O (OO)

IV. Predicting Results of Dihybrid Crosses• 2 letters in EACH GAMETE (RED), and 4 letters in each BOX (BLACK):

(1) Homozygous x Homozygous (rryy x RRYY)

• All F1 will turn out…• Round and Yellow (PHENOTYPE), as RrYy (GENOTYPE).

(2) Heterozygous x Heterozygous (RrYy x RrYy)

• F1 will turn out…

• A ratio of 9:3:3:1 with…

9 Round and Yellow

3 Round and Green

3 Wrinkled and Yellow

1 Wrinkled and Green

Extra Slides AND Answers for Critical Thinking Questions(1) During crossing-over, homologous chromosomes exchange pieces

of DNA, enabling alleles to move from one chromosome to a homologous chromosome.

(2) Each short-tailed parent has two incompletely dominant alleles, one for a long tail and one for no tail.

(3) Mendel would not have observed independent assortment occurring for all traits, so he probably would not have formulated his hypothesis of independent assortment.

(4) The chance of rolling a 6 on all five dice would be

1/6 x 1/6 x 1/6 x 1/6 x1/6 or 1/7,776.