Patterns of Inheritance By observing how traits are passed to the next generation, how can the inheritance patterns be used to understand the principles of heredity?

Use of Garden Pea for Genetics Experiments Stamens (male) produce pollen Carpel (female) produces eggs In the intact pea flower (left), the lower petals form a container enclosing the reproductive structures—the stamens (male) and carpel (female). Pollen normally cannot enter the flower from outside, so peas normally self-fertilize. Flower dissected to show reproductive structures Intact pea flower

Mendel’s Experiment With Peas Differing in a Single Trait Parental: Smooth seed x Wrinkled seed F1: All smooth seed coats F1 smooth plants x F1 smooth plants F2: 5474 smooth: 1850 wrinkled (3/4 smooth to 1/4 wrinkled)

Patterns of Inheritance Mendel needed to explain Why one trait seemed to disappear in the first generation. 2. Why the same trait reappeared in the second generation in one-fourth of the offspring.

Mendel’s Proposal Each trait is governed by two factors – now called genes. 2. Genes are found in alternative forms called alleles. 3. Some alleles are dominant and mask alleles that are recessive.

Mendel’s Experiment With Peas Differing in a Single Trait Parental: Smooth seed x Wrinkled seed SS ss Homozygous Dominant Homozygous Recessive F1: All smooth seed coats Ss Heterozygous F1 smooth plants x F1 smooth plants Ss Heterozygous Ss Heterozygous F2

Homozygous parents can only pass one form of an allele to their offspring.

Heterozygous parents can pass either of two forms of an allele to their offspring. Locus: Area on the chromosome where a gene is located. For a heterozygote, homologous chromosomes will have different alleles at the same locus.

Additional Genetic Terms Definition Example Genotype Alleles carried by an individual SS, Ss, ss Phenotype Physical characteristic or appearance of an individual smooth or wrinkled

Mendel’s Principle of Genetic Segregation In the formation of gametes, the members of a pair of alleles separate (or segregate) cleanly from each other so that only one member is included in each gamete. Each gamete has an equal probability of containing either member of the allele pair.

Genetic Segregation Parentals: SS x ss F1 x F1: Ss x Ss

Traits Studied by Mendel Seed shape Seed color Pod shape Pod color Flower color Traits of pea plants that Mendel studied Flower location Plant size

Mendel’s Experiment With Peas Differing in Two Traits Parental: Smooth Yellow x Wrinkled Green F1: All smooth yellow seed coats F1 plants x F1 plants 1/16 32 wrinkled, green 3/16 101 wrinkled, yellow 108 smooth, green 9/16 315 smooth, yellow F2

Patterns of Inheritance Mendel needed to explain Why non-parental combinations appeared in the F2 offspring. 2. Why the ratio of phenotypes in the F2 generation was 9:3:3:1.

Mendel’s Principle of Independent Assortment When gametes are formed, the alleles of one gene segregate independently of the alleles of another gene producing equal proportions of all possible gamete types.

Genetic Segregation + Independent Assortment Parentals: SSYY x s s y y SY SY SY SY sy sy sy sy F1:

Genetic Segregation + Independent Assortment F1 x F1 : S s Y y x S s Y y SY Sy sY sy SY Sy sY sy Four different types of gametes are formed in equal proportions.

F1 x F1 SsYy X SsYy Eggs SY Sy sY sy SY Sy Pollen sY sy 1 4 1 4 1 4 Figure: FIGURE 12.6 Title: Predicting genotypes and phenotypes for a cross between gametes that are heterozygous for two traits Caption: Here we are working with both seed color and shape, with yellow (Y) dominant to green (y), and smooth (S) dominant to wrinkled (s). (a) Punnett square analysis. In this cross, both parents are heterozygous for each trait (or a single individual heterozygous for both traits self-fertilize). There are now 16 boxes in the Punnett square. In addition to predicting all the genotypic combinations, the Punnett square predicts 3/4 yellow seeds, 1/4 green seeds, 3/4 smooth seeds, and 1/4 wrinkled seeds, just as we would expect from crosses made of each trait separately. (b) Probability theory can be used to predict phenotypes that result from a cross between gametes that are heterozygous for two traits. The fraction of genotypes from each sperm and egg combination is illustrated within each box of the Punnett square. Adding the fractions for the same genotypes will give the genotypic ratios. Converting each genotype to a phenotype and then adding their numbers reveals that 3/4 of the offspring will be smooth and 1/4 will be wrinkled and that 3/4 will be yellow and 1/4 will be green. Multiplying these independent probabilities produces predictions for the phenotype of offspring. These ratios are identical to those generated by the Punnett square. sY 1 4 sy 1 4

F2 Genotypes and Phenotypes Smooth Yellow Green Wrinkled

Meiotic Segregation Explains Independent Assortment Two possible orientations

Additional Genetic Patterns Mendel’s peas Alternative Pattern Complete Dominance Incomplete Dominance Incomplete dominance: neither allele masks the other and both are observed as a blending in the heterozygote

Incomplete Dominance Four o’clock flowers R = red, R’ = white Red x White RR R’R’ Four o’clock flowers R = red, R’ = white Pink RR’

Incomplete Dominance F1 x F1 Pink x Pink RR’ x RR’ Genotypic Ratio: Phenotypic Ratio:

Additional Genetic Patterns Mendel’s peas Alternative Patterns Complete Dominance Codominance Two alleles per gene Multiple Alleles Codominance: Neither allele masks the other so that effects of both alleles are observed in heterozygotes without blending Multiple Alleles: Three or more alleles exist for one trait Note: A diploid individual can only carry any two of these alleles at once.

Multiple Alleles and Codominance ABO Blood Type in Humans Blood Type Allele Type A A Type B B Type O o A = B > o A and B are codominant. A and B are completely dominant over o.

Human ABO Blood Types Type Genotype Antigen on RBCs Anti- bodies Re- ceives Donates Freq A AA or Ao Type A B A or O A or AB 40% B BB or Bo Type B A B or O B or AB 10% AB AB A and B Neither AB, A, B, O (universal) AB (universal) 4% Figure: TABLE 12.1 Title: Human blood group characteristics Caption: O oo Neither Both O O,AB, A,B (universal) 46% Codominance is observed for Type AB Blood since the products of both the A and B alleles are found on the cells.

Inheritance of Rh Factor Phenotype Genotype* Gene Product Antibodies Present Rh Positive RR or Rr Rhesus Protein None Rh Negative rr None unless exposed *Although there are multiple R alleles, R1, R2, R3, etc. all are completely dominant over all of the r alleles, r1, r2, r3, etc. ABO Blood Type and Rh Factor are controlled by separate genes. They show independent assortment.

Multiple Alleles and Codominance Type A, Rh positive x Type B, Rh negative Phenotypic Ratio of Offspring

Additional Genetic Patterns Mendel’s peas Alternative Patterns One gene affects one trait Polygenic Inheritance Polygenic Inheritance: Many genes affect one trait

Example of Polygenic Inheritance Two genes affecting one trait Number of Dominant Alleles Skin Color* (Phenotype) Genotypes % Pigmentation* White aabb 0-11% 1 Light Black Aabb or aaBb 12-25% 2 Medium Black AAbb or AaBb or aaBB 26-40% 3 Dark Black AABb or AaBB 41-55% 4 Darkest Black AABB 56-78% *Based on a study conducted in Jamaica.

Example of Polygenic Inheritance Medium Black Woman X Darkest Black Man (her mother is white)

Additional Genetic Patterns Mendel’s peas Alternative Patterns One gene affects one trait Pleiotropy Pleiotropy: One gene affects many traits

Sickle-Cell Anemia One gene affects many phenotypic characteristics Gene Product Cell Shape Disease Conditions SS Hemoglobin A Spherical, slightly concave No anemia SS’ Hemoglobin S Some sickling under extreme conditions Sickle Cell Trait Resistance to Malaria S’S’ Sickled under low O2 tension Sickle Cell Anemia