Chapter 12 – Patterns of Inheritance What is inheritance? What is the relationship between genes, alleles, phenotype and genotype? How does inheritance of genes influence an organism’s characteristics?

Need genetics help? Try online! http://www.sickkids.ca/childphysiology/cpwp/genetics/Geneticsintro.htm Understanding genetics requires understanding eukaryotic chromosome structure and the process of meiosis!

What is inheritance? (p. 222) The genetic transmission of characteristics from parents to offspring

What is the relationship between genes, alleles and genes? Alleles are differences (variation) in a gene within a population of a species (p. 222)

What is the relationship between genes, alleles, and genotype? Diploid species possess two alleles for each gene (p. 222) The two alleles constitute the genotype for a particular gene (p. 225) The M locus (p. 222)contains the M gene (leaf color). This plant is homozygous (p. 222) for the M gene (Both chromosomes carry the same allele of the M gene) FIGURE 12-1 The relationships among genes, alleles, and chromosomes Each homologous chromosome carries the same set of genes. Each gene is located at the same relative position, or locus, on its chromosome. Differences in nucleotide sequences at the same gene locus produce different alleles of the gene. Diploid organisms have two alleles of each gene. chromosome 1 (pair of homologous chromosomes) from tomato The Bk locus contains the Bk gene, (fruit shape).. This plant is heterozygous (p. 222) for the Bk gene (Each chromosome carries a different allele of the Bk gene) Figure 12-1 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

What is the relationship between genes, alleles, and phenotype? Several different alleles, each with a different phenotype (p. 225), may exist for a given gene

A single gene can have multiple alleles (p. 233) Table 12-1 Human Blood Group Characteristics Table 12-1 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Why are there alleles? Human blood type alleles (p. 235)

Alleles of a gene have nucleotide differences (due to point mutations)

Alleles of genes produce variations of the ‘wild type’ protein

Alleles & the genetics of odor perception The OR7D4 gene Hypothesis: Polymorphisms in odorant receptors contribute to variability in human odor perception

The genetics of odor perception Alleles include wild type (typical), and mutant types (super-functioning, partial functioning, non-functioning) The genetics of odor perception RT allele (R88W & T133M impair receptor function) WM allele S84N gene variation (allele) - increased sensitivity to androstenone (1% of population)

Phenotypes (traits) are genetically influenced in different ways* Types of genetic inheritance Single gene – complete dominance Single gene – incomplete dominance Single gene – codominance Single gene – multiple traits (pleiotropy) Multiple genes – polygenic inheritance *Environment influences gene expression, therefore environment influences phenotypes

Single gene – complete dominance For single gene inheritance, the type of interaction between the pair of inherited alleles for a gene determines the phenotype Single gene – complete dominance Dominant allele – expressed, regardless of the other allele present Recessive allele – not expressed in the presence of a dominant allele

Genotypes describe the pair of alleles present for a gene (p. 225) Genotype representations: Homozygous dominant AA Homozygous recessive aa Heterozygous Aa Rr rr RR

Gregor Mendel laid the foundations for understanding inheritance of single gene traits (p. 223-227)

Observing phenotypic outcomes of specific “crosses” helps to identify mode of inheritance Parental generation (P) cross-fertilize First-generation offspring (F1) Figure 12-4 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Crosses, meiosis & alleles Crossing infers sexual reproduction Sexual reproduction requires meiosis Meiosis separates pairs of alleles for any given gene (Mendel’s Law of Segregation, p. 224) gametes heterozygous parent A a

Outcomes of crosses are symbolically represented utilizing Punnett squares (p. 226) Pp 1/2 P eggs 1/2 p 1/2 P 1/4 PP 1/4 Pp sperm Pp 1/2 p 1/4 pP 1/4 pp

Male parent genotype: Rr Female parent genotype: Rr

Outcomes of crosses are symbolically represented utilizing Punnett squares (p. 226) Pp 1/2 P eggs 1/2 p 1/2 P 1/4 PP 1/4 Pp sperm Pp 1/2 p 1/4 pP 1/4 pp

Punnett squares predict expected outcomes Predictions require an assumption about the type of genetic inheritance The typical assumption is single gene inheritance with complete dominance If phenotypic ratios of offspring differ significantly than the expected outcome, then it is likely a different type of genetic inheritance is involved

Predicting expected outcomes Cross Aa x Aa Expect 2 phenotypes in a 3:1 ratio (assuming single gene/complete dominance) Outcome of cross: 3 phenotypes in a 2:1:1 ratio Does inheritance of this trait involve a single gene with complete dominance?

Not all genes/alleles display dominant/recessive interactions (p Codominance – both alleles for a gene are expressed (neither functions as a recessive allele) Incomplete dominance – heterozygous phenotype is different from either the dominant or recessive phenotype

Codominance = heterozygote expresses both alleles (p. 235) A and B are dominant to O (i); A & B are codominant

Human hair texture alleles exhibit incomplete dominance (p. 233) mother C1 C2 C1 eggs C2 Figure 12-24 Incomplete dominance The inheritance of hair texture in humans is an example of incomplete dominance. In such cases, we use capital letters for both alleles, here C1 and C2. Homozygotes may have curly hair (C1C1) or straight hair (C2C2). Heterozygotes (C1C2) have wavy hair. The children of a man and a woman, both with wavy hair, may have curly, straight, or wavy hair, in the approximate ratio of 1/4 curly : 1/2 wavy : 1/4 straight. father C1 C1 C1 C1 C2 sperm C1 C2 C2 C1 C2 C2 C2 Figure 12-24 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

A few disease-causing alleles function as dominant alleles in single gene inheritance (p. 239) Heterozygous persons have the disease/condition Forms of deafness Huntington’s disease Myotonic dystrophy

Many traits are influenced by more than one gene (p. 235-236) Figure 12-25b Polygenic inheritance of skin color in humans (b) The combination of complex polygenic inheritance and environmental effects (especially exposure to sunlight) produces an almost infinite gradation of human skin colors. Figure 12-25b Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Polygenic inheritance of human skin color involves 3 dinstinct genes (p. 236) eggs Figure 12-25a Polygenic inheritance of skin color in humans (a) At least three separate genes, each with two incompletely dominant alleles, determine human skin color (the inheritance is actually much more complex than this). The backgrounds of each box indicate the depth of skin color expected from each genotype. sperm Figure 12-25a Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Sex-linked traits are encoded by genes on the X or Y chromosome (p Genes located on X-chromosome Males possess only 1 allele for sex-linked traits -In males, recessive disease alleles cannot be masked by a dominant WT allele

Sex-linked trait example Red eyes is dominant to white eyes in fruit flies. A homozygous dominant female is crossed to a white eyed male. What will be the predicted phenotypic frequencies in the offspring?

Geneticists use pedigrees to understand inheritance of human characteristics (p. 237) (a) A pedigree for a dominant trait I III II IV ? How to read pedigrees or = cannot determine genotype from pedigree = known carrier (heterozygote) for recessive trait = does not show trait = shows trait = offspring = parents = female = male I, II, III = generations (b) A pedigree for a recessive trait Figure 12-27 Family pedigrees (a) A pedigree for a dominant trait. Note that any offspring showing a dominant trait must have at least one parent with the trait. (b) A pedigree for a recessive trait. Any individual showing a recessive trait must be homozygous recessive. If that person's parents did not show the trait, then both of the parents must be heterozygotes (carriers). Note that the genotype cannot be determined for some offspring, who may be either carriers or homozygous dominants. Figure 12-27 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Sex-linked pedigree (p. 240) maternal grandfather II mother father aunts III sister G. Audesirk T. Audesirk Figure 12-30b Color blindness, a sex-linked recessive trait (b) Pedigree of one of the authors (G. Audesirk, who sees only a 6 in the Ishihara chart), showing sex-linked inheritance of red-green color blindness. Both the author and his maternal grandfather are color deficient; his mother and her four sisters carry the trait but have normal color vision. This pattern of more-common phenotypic expression in males and transmission from affected male to carrier female to affected male is typical of sex-linked recessive traits. IV ? ? daughter or = colorblind = heterozygous carrier female, normal color vision or = normal color vision (not carrier) Figure 12-30b Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Analyzing pedigrees Are both genders affected by the disorder? Sex-linked disorders typically affect only males How frequent do affected individuals occur? Infrequent suggests the disorder is recessive to the wild type allele

Genetics and environment (p. 237) Some characteristics are fully genetic (example: human eye color) Many characteristics are influenced by both genes and environment (example: hydrangea flower color and soil pH) The same allele in different environments can result in different outcomes Different alleles in the same environment can result in different outcomes

Role of Genotype in the Cycle of Violence in Maltreated children

Role of Genotype in the Cycle of Violence in Maltreated children What are the social & legal implications of this research? If a violent criminal offender (with a known family history of maltreatment) is shown to have the low expression MAOA allele by a genetic test, can/should he be held responsible for his violent criminal behavior?

What is genetic linkage? (p. 229) Genes located close together on the same chromosome are genetically linked (i.e they tend to get inherited together) Is there any difference concerning inheritance for genes located closer together or further apart?

Linkage between genes varies with distance between genes A B C D a b c d Tightly linked genes

Some genes are so tightly linked that they are rarely/never separated by crossing over Observing phenotypic ratios of offspring produced in a dihybrid cross allows one to determine of linkage or independent assortment is occurring between two distinct genes a A b B a A B b Individual with genotype AaBb

Gametes, assuming complete linkage AaBb a b A B a A B b AB ab

Complete linkage AB ab AABB AaBb aabb AB ab AaBb x AaBb B A b a What is the phenotypic ratio?

Gametes, assuming independent assortment B b or AaBb Ab aB AB ab 25% a A b B

Two genes exhibiting independent assortment (p. 228) AB Ab aB ab AABB AABb aabb AB Ab aB ab

Genetic linkage can be broken by crossing over (p. 230) Genotype: AaBb Gametes produced: AB Ab aB ab

Working genetics problems Phenotypic ratios of offspring vary and depend on the genotypes of the parents and the type of inheritance (3:1 ratio in monohybrid crosses and 9:3:3:1 ratio in dihybrid crosses only apply to single gene/complete dominance traits) Practice problems: p. 246-248

Red flowers and thorns are dominant to white flowers and no thorns in roses. Which of the following can be a genotype for a red, thorny rose? 1) RT 2) RRTT 3) RrTT 4) RrTt 5) 2, 3 & 4

Red flowers and thorns are dominant to white flowers and no thorns in roses. What would be the phenotype of a rose heterozygous for both traits? 1) RrTt 2) Red, thorny 3) RRTT 4) White, thornless 5) rrtt

Which of the following represents the genotype of a gamete produced by an individual of genotype Aabb (assuming independent assortment)? Aa Bb AB Ab 5. Aabb 6. Both 1 and 2 7. Both 3 and 4 8.None of these

Assume flower color in daffodil is determined by a single gene Assume flower color in daffodil is determined by a single gene. A yellow flowered daffodil is crossed with a white flowered daffodil. All 100 of the offspring produce yellow flowers. What can you conclude? 1. The yellow flowered parent is homozygous for the flower color gene 2. The white flowered plant is homozygous for the flower color gene 3. The yellow allele is dominant to the white allele 4. All of the above 5. Both 2 and 3

Which genes are most likely to be inherited together? 1 2 3 4 5 6 7 5a 6a 7a Which genes are most likely to be inherited together? 1) 2 & 2a 2) 1 & 5 3) 1a&2a 4) 5&7 5) 1a & 4a 6) Responses 3, 4 & 5 are all correct 1a 2a 3a 4a