Mendel and the Gene Idea Chapter 14 BCOR

Mendel and the Gene Idea Chapter 14 BCOR 012 January 20, 22, and 25, 2010

Mendel and the Gene Idea Chapter 14 I. Mendel and his contribution to biology A. A brief biography of Mendel B. Why his work was revolutionary C. Mendel’s organism D. Brief review of meiosis E. So what did Mendel do and why is it important? F. Mendel's First Law of Inheritance, the law of segregation G. Dihybrid Crosses and Independent

Gregor Mendel, 1822 -1884

Garden pea (Pisum sativum - Fabaceae)

Mendel and the Gene Idea Chapter 14 I. Mendel and his contribution to biology A. A brief biography of Mendel B. Why his work was revolutionary C. Mendel’s organism D. Brief review of meiosis E. So what did Mendel do and why is it important? F. Mendel's First Law of Inheritance, the law of segregation G. Dihybrid Crosses and Independent

Each cell in the leaf of a pea plant has seven pairs of chromosomes. How many chromosomes in the egg cell? A. B. C. D. E. 14 28 7 21 The question cannot be answered with the information given

A short review of meiosis. . .

Most organisms, including ourselves and garden peas, are diploid. That means they have two sets of chromosomes per cell. One of these sets came from the mother, via the egg, and the other came from the father, via the sperm.

Meiosis is also called reductional division because the number of chromosomes in each cell is reduced by half. During meiosis, members of the two sets separate into separate cells.

Mendel’s Explanation of his Observations (in present-day language): 1) Alternative versions of genes (that is to say, different alleles) account for variation in inherited characters. 2) For each character, an organism inherits two alleles, one from each parent. 3) If an organism is heterozygous at a particular gene locus, the organism’s appearance (phenotype) will be determined by the dominant allele. 4) The two alleles for each character segregate during gamete production.

This fourth point is also called Mendel’s Law of Segregation: Alleles segregate into separate gametes during meiosis.

Some useful Genetic Vocabulary: • Homozygous: an individual having an identical pair of alleles at a locus is homozygous at that locus. (example: PP ) • Heterozygous: an individual having different alleles at a locus is heterozygous at that locus (example: Pp)

P - purple color (dominant allele) p - white color (recessive allele) Note that the probability of a white-flowered plant in the F 2 generation is 0. 25, or 25% (This is a Punnett square. )

When solving a problem involving a cross in Mendelian genetics: • First ask yourself: what kind of gamete (genotype) can each parent produce? • And in what proportion will each be produced? • Then figure out all possible ways in which these gametes can combine at syngamy • Note that a Punnet square is one way to approach the problem, but not necessarily the most efficient way.

Here a Punnet square has been used to solve the problem Another way to approach the problem: • Half (0. 5) of the heterozygous mother’s eggs carry the P allele and half carry the p allele. • Similarly, half of the heterozygous father’s sperm cells carry the P allele and half carry the p allele. • Thus the progeny will include 0. 5 P x 0. 5 P = 0. 25 PP, (0. 5 P x 0. 5 p)+ (0. 5 P x 0. 5 p) = 0. 5 Pp, and 0. 5 p x 0. 5 p, = 0. 25 pp

More useful Genetic Vocabulary: • Genotype: The specific genetic makeup of an organism, as contrasted with the actual characteristics of an organism (its phenotype). • Phenotype: The observable characteristics of an organism, as opposed to the set of genes it possesses (its genotype).

A testcross: a pea plant showing the dominant phenotype - but whose genotype is unknown - is bred to a pea plant showing the recessive phenotype. The results permit the unknown parent’s genotype to be determined.

Mendel and the Gene Idea Chapter 14 I. Mendel and his contribution to biology A. A brief biography of Mendel B. Why his work was revolutionary C. Mendel’s organism D. Brief review of meiosis E. So what did Mendel do and why is it important? F. Mendel's First Law of Inheritance, the law of segregation G. Dihybrid Crosses and Independent

Mendel’s Law of Independent Assortment: Pairs of alleles segregate independently during meiosis.

Mendel and the Gene Idea Chapter 14 II. The Relationship Between Genotype and Phenotype A. Incomplete Dominance B. Codominance C. Pleiotropy: Epistasis D. Polygenic traits E. Environmentally Induced Variation III. Pedigree Analysis in Humans A. Recessive alleles B. Dominant alleles IV. Tools for detection of genetic disorders A. Pedigree analysis and risk assessment B. Amniocentesis C. Chorionic villus sampling

The Relationship Between Genotype and Phenotype

The inheritance of flower color in snapdragons provides a useful example of incomplete dominance. In incomplete dominance, a heterozygous genotype creates an intermediate phenotype.

Human Blood Types In the human population, there are three alleles determining blood groups: IA IB i Of course, any given individual will possess only one (if homozygous at the blood group locus) or two (if heterozygous at that locus) of these. Your genotype at the blood group locus determines your blood type.

Human Blood Types Phenotype Type A Type B Type AB Type O Genotype IA IA or IA i IB IB or IB i IA IB ii Human blood group alleles IA and IB demonstrate codominance. In codominance, neither phenotype is dominant. Instead, the individual expresses both phenotypes.

Epistasis is the condition in which the genotype at one locus affects the expression of the genotype at another locus.

Locus C determines whether pigment is deposited or not Locus B determines coat color, but its expression depends on the genotype at the C locus. Coat color in mice: an example of epistatic interaction between two loci.

< water level The submersed and emersed leaves of water marigold (Megalodonta beckii, Asteraceae) demonstrate an environmental contribution to the phenotype.

Mendel and the Gene Idea Chapter 14 II. The Relationship Between Genotype and Phenotype A. Incomplete Dominance B. Codominance C. Pleiotropy: Epistasis D. Polygenic traits E. Environmentally Induced Variation III. Pedigree Analysis in Humans A. Recessive alleles B. Dominant alleles IV. Tools for detection of genetic disorders A. Pedigree analysis and risk assessment B. Amniocentesis C. Chorionic villus sampling

Human Pedigree Analysis: the Conventions

Pedigree Analysis of a Benign Condition in Humans: the Attached Earlobe I II III Q. Is the trait inherited as a dominant or a recessive condition? A. Recessive. You can tell because the affected daughter in generation III was born to unaffected parents.

For those traits exhibiting recessive gene action: • affected individuals may be born to unaffected parents • the trait is not manifested in every generation The pedigree of a family with cystic fibrosis, a recessively inherited disorder.

The pedigree of a family with Huntington’s disease, a dominantly inherited disorder For those traits exhibiting dominant gene action: • affected individuals have at least one affected parent • the phenotype generally appears every generation • two unaffected parents only have unaffected offspring

His son, the 60 s legend Arlo Guthrie, did not inherit the disease. Woody Guthrie, American folk hero and composer of “This Land is Your Land, ” died of Huntington’s disease in 1967.

Mendel and the Gene Idea Chapter 14 II. The Relationship Between Genotype and Phenotype A. Incomplete Dominance B. Codominance C. Pleiotropy: Epistasis D. Polygenic traits E. Environmentally Induced Variation III. Pedigree Analysis in Humans A. Recessive alleles B. Dominant alleles IV. Tools for detection of genetic disorders A. Pedigree analysis and risk assessment B. Amniocentesis C. Chorionic villus sampling

Know the Difference: • • • meiosis vs. mitosis gene vs. allele genotype vs. phenotype homozygous vs. heterozygous dominant vs. recessive monohybrid vs. dihybrid

Solving Problems in Mendelian Genetics A corn plant of genotype Xx. Yy. Zz is crossed with a plant of genotype XXYyzz. Assume the genes assort independently. Of their offspring, what proportion will: a. be heterozygous at all three loci? b. be homozygous at all three loci? c. be homozygous for the dominant allele at all three loci? d. have the genotype XXYy. Zz? 1/2 X 1/2 = 1/8 1/2 X 1/4 X 0 = 0 1/2 X 1/2 = 1/8