Would expect variation to disappear Variation in traits persists (Example: freckles show up in unfreckled parents offspring!)

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Morgan Fox
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1 Genetics

blended together Problem: Would expect variation to disappear Variation

2 Early Ideas about Heredity People knew that sperm and eggs transmitted information about traits Blending theory mother and father s traits blended together Problem: Would expect variation to disappear Variation in traits persists (Example: freckles show up in unfreckled parents offspring!) + =

3 Mendel s Experimental Approach Mendel was a monk with training in plant breeding and mathematics considered to be the founder of modern genetics! ( ) He studied the garden pea (Pisum sativum), which breeds true for a number of traits

4 The Garden Pea Plant Self-pollinating True breeding (different alleles not normally introduced) Can be experimentally crosspollinated excellent for genetic manipulation!!! Small, easy to grow, can self-fertilize, easily distinguishable traits!

5 Terms Used in Modern Genetics Genes Heritable units of information about traits Parents transmit genes to offspring Each gene has a specific locus on a chromosome Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes

6 Terms Used in Modern Genetics A mutation is a permanent change in a gene May cause a trait to change Alleles are different molecular forms of a gene A hybrid has nonidentical alleles for a trait Offspring of a cross between two individuals that breed true for different forms of a trait are hybrids

7 Terms Used in Modern Genetics An individual with nonidentical alleles of a gene is heterozygous for that gene An individual with identical alleles of a gene is homozygous for that gene

8 Terms Used in Modern Genetics An allele is dominant if its effect masks the effect of a recessive allele paired with it Capital letters (A) signify dominant alleles; lowercase letters (a) signify recessive alleles Homozygous dominant (AA) Homozygous recessive (aa) Heterozygous (Aa)

9 Terms Used in Modern Genetics Gene expression The process by which information in a gene is converted to a structural or functional part of a cell or body Expressed genes determine traits

10 Terms Used in Modern Genetics Genotype The particular alleles an individual carries Phenotype An individual s observable traits

11 Terms Used in Modern Genetics P stands for parents, F for filial (offspring) AA x aa F1: First generation offspring of parents Aa x Aa F2: Second generation offspring of parents AA, Aa, Aa, aa

12 Key Concepts Where Modern Genetics Started Gregor Mendel gathered the first experimental evidence of the genetic basis of inheritance His meticulous work gave him clues that heritable traits are specified in units The units, which are distributed into gametes in predictable patterns, were later identified as genes

13 Mendel s Law of Segregation Garden pea plants inherit two units of information for a trait, one from each parent

14 Monohybrid Experiments Monohybrid experiments Testcrosses that check for a dominance relationship between two alleles at a single locus May be crosses between true breeding (homozygous) individuals (AA x aa), or between identical heterozygotes (Aa x Aa)

15 Mendel s Monohybrid Experiments Mendel used monohybrid experiments to find dominance relationships among pea plant traits When he crossed plants that bred true for white flowers with plants that bred true for purple flowers, all F1 plants had purple flowers When he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers

16 Segregation of Alleles at a Gene Locus

17 Mendel s Monohybrid Experiments

18 Calculating Probabilities Probability A measure of the chance that a particular outcome will occur Punnett square A grid used to calculate the probability of genotypes and phenotypes in offspring

19 Construction of a Punnett Square

20 Phenotype Ratios in a Monohybrid Experiment

21 Phenotype Ratios in a Monohybrid Experiment

22 Mendel s Law of Segregation Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses Consistent with the probability of the aa genotype in the offspring of a heterozygous cross (Aa x Aa) This is the basis of Mendel s law of segregation Diploid cells have pairs of genes on pairs of homologous chromosomes The two genes of each pair separate during meiosis, and end up in different gametes

23 Key Concepts Insights from Monohybrid Experiments Some experiments yielded evidence of gene segregation: When one chromosome separates from its homologous partner during meiosis, the alleles on those chromosomes also separate and end up in different gametes - practice with Punnett Squares!!! To be done on your own or with me in office hours!!!

24 Mendel s Law of Independent Assortment Mendel s law of independent assortment Many genes are sorted into gametes independently of other genes

25 Dihybrid Experiments Dihybrid experiments Tests for dominance relationships between alleles at two loci Individuals that breed true for two different traits are crossed (AABB x aabb) F2 phenotype ratio is 9:3:3:1 (four phenotypes) Individually, each dominant trait has an F2 ratio of 3:1 inheritance of one trait does not affect inheritance of the other

26 Independent Assortment at Meiosis

27 Mendel s Dihybrid Experiments

28 Mendel s Law of Independent Assortment Mendel s dihybrid experiments showed that units specifying one trait segregated into gametes separately from units for other traits Exception: Genes that have loci very close to one another on a chromosome tend to stay together during meiosis

29 Key Concepts Insights from Dihybrid Experiments Some experiments yielded evidence of independent assortment: Genes are typically distributed into gametes independently of other genes

30 Beyond Simple Dominance Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward

31 Continuous Variation Continuous variation Traits with a range of small differences The more factors that influence a trait, the more continuous the distribution of phenotype

32 Incomplete Dominance Incomplete dominance One allele is not fully dominant over its partner The heterozygote s phenotype is somewhere between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring Example: Snapdragon color RR is red Rr is pink rr is white

33 Incomplete Dominance in Snapdragons

34 Genes and the Environment Expression of some genes is affected by environmental factors such as temperature, altitude, or chemical exposure The result may be variation in traits

35 Effects of Temperature on Gene Expression Enzyme tyrosinase, works at low temperatures

36 Codominance in ABO Blood Types Codominance Two nonidentical alleles of a gene are both fully expressed in heterozygotes, so neither is dominant or recessive May occur in multiple allele systems Multiple allele systems Genes with three or more alleles in a population Example: ABO blood types

37 Codominance in ABO Blood Types

38 ABO Blood Type: Glycolipids on Red Cells Type A - Glycolipid A on cell surface Type B - Glycolipid B on cell surface Type AB - Both glyocolipids A & B Type O - Neither glyocolipid A nor B

!! Enzymes and other gene products control steps of most metabolic pathways

39 Regarding the Unexpected Phenotype Phenotype results from complex interactions among gene products and the environment identical twins!!! Enzymes and other gene products control steps of most metabolic pathways Mutations, interactions among genes, and environmental conditions may affect one or more steps

40 Key Concepts Variations on Mendel s Theme Not all traits appear in Mendelian inheritance patterns An allele may be partly dominant over a nonidentical partner, or codominant with it The environments also influences gene expression

characteristics and diminished mental capacity (extra X).

41 Chromosomal Abnormalities some examples Errors sometimes occur during meiosis Down Syndrome extra copy of Chromosome #21 Klinefelter Syndrome (XXY male) sterile male with female characteristics and diminished mental capacity (extra X). Normally XX and XY Turner Syndrome (XO female) sterile female with webbed neck and diminished stature. Missing all or part of 2nd X chromosome

Observing Patterns in Inherited Traits. Chapter 11

Observing Patterns in Inherited Traits Chapter 11 Impacts, Issues: The Color of Skin Like most human traits, skin color has a genetic basis; more than 100 gene products affect the synthesis and deposition