Segregation, Assortment, and Dominance Relationships

1. Segregation, Assortment, and Dominance Relationships • Genes and alleles • Random segregation • Independent assortment • Assortment vs. Linkage • Dominance relationships

2. A. Genes and Alleles • Gene • Classical definition: • A unit of inheritance • A factor transmitted during reproduction and responsible for the appearance of a given trait • Contemporary understanding: • A segment on a DNA molecule • Usually at a specific location (locus) on a chromosome • Characterized by its nucleotide sequence

3. A. Genes and Alleles • Genes play three notable roles: • To encode the amino acid sequences of proteins • To encode the nucleotide sequences of tRNA or rRNA • To regulate the expression of other genes

4. A. Genes and Alleles • Alleles: • Variant forms of a gene found within a population • Alleles of a gene usually have small differences in their nucleotide sequences • The differences can affect the trait for which the gene is responsible • Most genes have more than one allele

5. A. Genes and Alleles • Homozygous and heterozygous: • In a diploid species, each individual carries two copies of each gene (with some exceptions) • The two copies are located on different members of a homologous chromosome pair • If the two copies of the gene are identical alleles, then the individual is homozygous for the gene • If the two copies are different alleles, then the individual is heterozygous for the gene

6. A. Genes and Alleles • Genotype: • The genetic makeup of an individual with reference to one or more specific traits • A genotype is designated by using symbols to represent the alleles of the gene

7. A. Genes and Alleles • Example: • Consider a gene for plant height in the pea plant with two alleles, “D” and “d” • Each individual pea plant will carry two copies of the plant height gene, on a homologous chromosome pair • An individual pea plant will be one of three possible genotypes: • Homozygous “DD” • Homozygous “dd” • Heterozygous “Dd”

8. A. Genes and Alleles • Dominant and recessive: • A dominant allele is expressed over a recessive allele in a heterozygous individual • This means that a heterozygous individual and a homozygous dominant individual have identical phenotypes • Often, a dominant allele encodes a functional protein, such as an enzyme • The recessive allele is a mutation that no longer has the information for the correct amino acid sequence; Therefore, its protein product in nonfunctional • In the heterozygote, the dominant allele encodes sufficient production of the protein to produce the dominant phenotype. This is also called complete dominance

9. A. Genes and Alleles • Phenotype: • The appearance or discernible characteristics of a trait in an individual • Phenotypes can be determined by a combination of genetic and environmental factors

10. A. Genes and Alleles • Example: • In the pea plant height gene, the dominant allele “D” encodes a hormone that promotes tall growth • The recessive allele “d” is a mutation that does not produce functional hormone • If an individual pea plant has at least “one good copy” of the “D” allele, then it makes enough hormone to grow tall • Otherwise, the plant is dwarf in size

11. A. Genes and Alleles • Example (continued): • Therefore, there are two possible phenotypes for plant height in peas: • Genotype “DD” produces tall plants • Genotype “Dd” produces tall plants • Genotype “dd” produces dwarf plants • Note that “D” is completely dominant over “d” • There is no observable difference in phenotype between “DD” (homozygous dominant) and “Dd” (heterozygous) plants

12. B. Random Segregation • Mendel’s law of random segregation: • Diploid germ-line cells of sexually reproducing species contain two copies of almost every chromosomal gene • The two copies are located on members of a homologous chromosome pair • During meiosis, the two copies separate, so that a gamete receives only one copy of each gene

13. B. Random Segregation • Random segregation can be demonstrated with a monohybrid cross experiment • Monohybrid cross: • A parental cross between two individuals that differ in the genotype of one gene • The offspring of the parental generation is called the F1 (first filial) generation • The F1 generation can be allowed to interbreed or self-fertilize(inter se cross, or “selfing”) to produce the F2 (second filial) generation

14. B. Random Segregation • Example of a monohybrid cross:

15. B. Random Segregation • Genotypic explanation of the monohybrid cross: • Parental generation: Pollen from a DD plant X ovules from a dd plant Pollen genotype: D Ovule genotype: d • Therefore, in the F1 generation: Genotype of all F1 plants: Dd F1 pollen: ½ D and ½ d F1 ovules: ½ D and ½ d

16. B. Random Segregation • Genotypic explanation (continued): • When the F1 plants self-fertilize:

17. B. Random Segregation • Random segregation can also be demonstrated with a testcross • Testcross: • Cross heterozygous F1 individuals with homozygous recessive

18. C. Independent Assortment • Mendel’s law of independent assortment • When the alleles of two different genes separate during meiosis • They do so independently of one another • Unless the genes are located on the same chromosome (linked)

19. C. Independent Assortment • Independent assortment is demonstrated by a dihybrid cross • Dihybrid cross: • A parental cross between two individuals that differ in the genotype of two different genes

20. C. Independent Assortment • Example: Consider genes for vestigial wing shape and ebony body color in Drosophila melanogaster • Vestigial wing shape gene: vg+ allele: normal “wild type” wing shape; dominant vg allele: vestigial wing; recessive • Ebony body color gene: e+ allele: tan-colored “wild type” body; dominant e allele: ebony body; recessive

21. C. Independent Assortment • As usual with complete dominance, there are three possible genotypes for wing shape, and three for body color: vg+ vg+ = homozygous wild type wing vg+ vg = heterozygous wild type wing vg vg = vestigial wing e+ e+ = homozygous wild type body color e+ e = heterozygous wild type body color e e = ebony body color

22. C. Independent Assortment

23. C. Independent Assortment • Genotypic explanation for the dihybrid cross • P generation: vg+ vg+ e+ e+ males X vg vg e e females • F1 generation: All heterozygous vg+ vg e+ e , males and females F1 spermF1 ova ¼ vg+ e+ ¼ vg+ e+ ¼ vg+ e¼ vg+ e¼ vg e+ ¼ vg e+ ¼ vg e ¼ vg e

24. C. Independent Assortment • How many different ways can we make wild type wing, wild type body color in the F2? F1 spermF1 ova ¼ vg+ e+ ¼ vg+ e+ ¼ vg+ e¼ vg+ e ¼ vg e+ ¼ vg e+ ¼ vg e ¼ vg e Answer: 9 different ways

25. C. Independent Assortment • How many different ways can we make wild type wing, ebony body color in the F2? F1 spermF1 ova ¼ vg+ e+ ¼ vg+ e+ ¼ vg+ e¼ vg+ e ¼ vg e+ ¼ vg e+ ¼ vg e ¼ vg e Answer: 3 different ways

26. C. Independent Assortment • How many different ways can we make vestigial wing, wild type body color in the F2? F1 spermF1 ova ¼ vg+ e+ ¼ vg+ e+ ¼ vg+ e¼ vg+ e ¼ vg e+ ¼ vg e+ ¼ vg e ¼ vg e Answer: 3 different ways

27. C. Independent Assortment • How many different ways can we make vestigial wing, ebony body color in the F2? F1 spermF1 ova ¼ vg+ e+ ¼ vg+ e+ ¼ vg+ e¼ vg+ e ¼ vg e+ ¼ vg e+ ¼ vg e ¼ vg e Answer: 1 way

29. C. Independent Assortment • Here is a “shortcut” for dihybrid cross ratios: combine the monohybrid cross ratios! F2 wing phenotypes:F2 body phenotypes: ¾ wild type wings ¾ wild type body ¼ vestigial wings ¼ ebony body  ¾ x ¾ = 9/16 wild wings, wild body ¾ x ¼ = 3/16 wild wings, ebony body ¼ x ¾ = 3/16 vestigial wings, wild body ¼ x ¼ = 1/16 vestigial wings, ebony body

30. C. Independent Assortment • The testcross can also be applied to independent assortment: vg+ vg e+ e X vg vg e e  ¼ vg+ vg e+ e (wild wing, wild body) ¼ vg+ vg e e (wild wing, ebony body) ¼ vg vg e+ e (vestigial wing, wild body) ¼ vg vg e e (vestigial wing, ebony body)

31. D. Assortment vs. Linkage • Independent assortment works because the two genes are located on separate homologous chromosomes pairs • Their alleles assort independently during meiosis

32. D. Assortment vs. Linkage

33. D. Assortment vs. Linkage • If two genes are located on the same chromosome, their alleles can recombine only when there is crossing over during meiosis • The probability that crossover will occur is proportional to the distance between the genes • Typically, there are fewer recombinant (crossover) gametes than nonrecombinant gametes

34. D. Assortment vs. Linkage

35. E. Dominance Relationships • Codominance • Two alleles are codominant if each encodes a different but functional protein product • In the heterozygote, the presence of two different functional proteins means that the phenotype of the heterozygote is different from either homozygous dominant or homozygous recessive • Example: M-N blood groups

36. E. Dominance Relationships • Example of codiminance: M-N blood group gene in humans • Two alleles, LM & LN • Each produces a “functional” blood cell antigen (capable of causing an immunological reaction) • Three possible genotypes & phenotypes • LM LM: Produces group “M” blood • LM LN: Produces group “MN” blood • LN LN: Produces group “N” blood

37. E. Dominance Relationships • Incomplete dominance • An incompletely dominant allele produces a functional protein product • However, in the heterozygote, there is insufficient protein production from the allele to produce the same phenotype as homozygous dominant • Therefore, the phenotype of the heterozygote is different from either homozygous dominant or homozygous recessive • Example: snapdragon flower color

38. E. Dominance Relationships • Example of incomplete dominance: snapdragon flower color • Two alleles, “R” and “r” • “R” produces red pigment; “r” produces no pigment • Three possible genotypes & phenotypes • RR: Red flowers • Rr: Pink flowers (One copy of “R” produces less red pigment than two copies of “R”) • rr: White flowers

39. E. Dominance Relationships • Because each genotype has a unique phenotype, the F2 phenotypic ratio in codominance or incomplete dominance is 1:2:1