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Observing Patterns in Inherited Traits

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1 Observing Patterns in Inherited Traits
Chapter 10

2 10.1 Mendel, Pea Plants, and Inheritance Patterns
By experimenting with pea plants, Mendel was the first to gather evidence of patterns by which parents transmit genes to offspring

3 Mendel’s Experiments

4 carpel stamen a Garden pea flower, cut in half. Sperm form in pollen grains, which originate in male floral parts (stamens). Eggs develop, fertilization takes place, and seeds mature in female floral parts (carpels). b Pollen from a plant that breeds true for purple flowers is brushed onto a floral bud of a plant that breeds true for white flowers. The white flower had its stamens snipped off. This is one way to assure cross-fertilization of plants. c Later, seeds develop inside pods of the cross-fertilized plant. An embryo within each seed develops into a mature pea plant. d Each new plant’s flower color is indirect but observable evidence that hereditary material has been transmitted from the parent plants. Fig. 10.3, p.154

5 Animation: Crossing garden pea plants

6 Producing Hybrids Hybrids
Offspring of a cross between two individuals that breed true for different forms of a trait Each inherits nonidentical alleles for a trait being studied

7 Producing Hybrid Offspring

8 Homozygous dominant parent Homozygous recessive parent
(chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Fig. 10.5, p.156

9 Heritable Units of Information
Genes Heritable units of information about traits Each has its own locus on the chromosome Alleles Different molecular forms of the same gene Mutation Permanent change in a gene’s information

10 Heritable Units of Information

11 a A pair of homologous chromosomes,
both unduplicated. In most species, one is inherited from a female parent and its partner from a male parent. b A gene locus (plural, loci), the location for a specific gene on a chromosome. Alleles are at corresponding loci on a pair of homologous chromosomes c A pair of alleles may be identical or not. Alleles are represented in the text by letters such as D or d. d Three pairs of genes (at three loci on this pair of homologous chromosomes); same thing as three pairs of alleles. Fig. 10.4, p.155

12 Modern Genetic Terms Homozygous dominant Homozygous recessive
Has two dominant alleles for a trait (AA) Homozygous recessive Has two recessive alleles (aa) Heterozygote Has two nonidentical alleles (Aa)

13 Modern Genetic Terms Dominant allele may mask effect of recessive allele on the homologous chromosome Genotype An individual’s alleles at any or all gene loci Phenotype An individual’s observable traits

14 Animation: Genetic terms

15 Key Concepts: MODERN GENETICS
Gregor Mendel gathered the first indirect, experimental evidence of the genetic basis of inheritance His meticulous work tracking traits in many generations of pea plants gave him clues that heritable traits are specified in units The units, distributed into gametes in predictable patterns, were later identified as genes

16 10.2 Mendel’s Theory of Segregation
Diploid organisms have pairs of genes, on pairs of homologous chromosomes Based on monohybrid experiments During meiosis Genes of each pair separate Each gamete gets one or the other gene

17 Producing Hybrid Offspring
Crossing two true-breeding parents of different genotypes yields hybrid offspring All F1 offspring are heterozygous for a gene, and can be used in monohybrid experiments All F1 offspring of parental cross AA x aa are Aa

18 A Monohybrid Cross Crosses between F1 monohybrids resulted in these allelic combinations among F2 offspring Phenotype ratio 3:1 Evidence of dominant and recessive traits

19 F2 Offspring: Dominant and Recessive Traits

20 F2 Dominant-to-Recessive Ratio
Trait Studied Dominant Form Recessive Form F2 Dominant-to-Recessive Ratio Seed shape 5,474 round 1,850 wrinkled 2.98:1 Seed color 6,022 yellow 2,001 green 3.01:1 Pod shape 2.95:1 882 inflated 299 wrinkled Pod color 428 green 152 yellow 2.82:1 Flower color 705 purple 224 white 3.15:1 Flower position 651 long stem 3.14:1 207 at tip Stem length 2.84:1 787 tall 277 dwarf Fig. 10.6, p.156

21 Predicting Probability: Punnett Squares

22 female gametes A a A a A a A a A A A Aa A AA Aa male gametes a aa a Aa
a From left to right, step-by-step construction of a Punnett square. Circles signify gametes. A stands for a dominant allele and a for a recessive allele at the same gene locus. Offspring genotypes are indicated inside the squares. Fig. 10.7a, p.157

23 a A aa female gametes male gametes a A aa Aa a A aa Aa a A aa Aa AA
Stepped Art Fig. 10-7a, p.157

24 Predicting F1 Offspring

25 True-breeding homozygous recessive parent plant
F1 offspring aa True-breeding homozygous recessive parent plant a a Aa Aa A Aa Aa AA A Aa Aa True-breeding homozygous dominant parent plant Aa Aa b Cross between two plants that breed true for different forms of a trait. Fig. 10.7b, p.157

26 Predicting F2 Offspring

27 c Cross between heterozygous F1 offspring.
Aa Heterozygous F1 offspring A a AA Aa A AA Aa Aa a Aa aa Heterozygous F1 offspring Aa aa c Cross between heterozygous F1 offspring. Fig. 10.7c, p.157

Some experiments yielded evidence of gene segregation When one chromosome separates from its homologous partner during meiosis, the pairs of alleles on those chromosomes also separate and end up in different gametes

29 Animation: Monohybrid cross

30 Animation: F2 ratios interaction

31 Animation: Testcross CLICK HERE TO PLAY

32 10.3 Mendel’s Theory of Independent Assortment
Meiosis assorts gene pairs of homologous chromosomes independently of gene pairs on all other chromosomes Based on dihybrid experiments Pairs of homologous chromosomes align randomly at metaphase I

33 Independent Assortment in Meiosis I

34 A a a A A a a B B b b b b B B A A a a A A a a B B b b b b B B B A A B
One of two possible alignments The only other possible alignment a Chromosome alignments at metaphase I: A a a A A a a B B b b b b B B b The resulting alignments at metaphase II: A A a a A A a a B B b b b b B B B A A B b a a b b A A b B a a B c Possible combinations of alleles in gametes: AB ab Ab aB Fig. 10.8, p.158

35 Dihybrid Experiments Start with a cross between true-breeding heterozygous parents that differ for alleles of two genes (AABB x aabb) All F1 offspring are heterozygous for both genes (AaBb)

36 Mendel’s Dihybrid Experiments
AaBb x AaBb Phenotypes of the F2 offspring of F1 hybrids were close to a 9:3:3:1 ratio 9 dominant for both traits 3 dominant for A, recessive for b 3 dominant for B, recessive for a 1 recessive for both traits

37 Results of Mendel’s Dihybrid Experiments

38 Meiosis, gamete formation in true-breeding parent plants
parent homozygous dominant for purple flowers, tall stems parent homozygous recessive for white flowers, short stems Gametes at fertilization Possible genotypes resulting from a cross between two F1 plants: meiosis, gamete formation meiosis, gamete formation All F1 plants are AaBb heterozygotes with purple flowers and tall stems. Fig. 10.9, p.159

Some experiments yielded evidence of independent assortment During meiosis, the members of a pair of homologous chromosomes are distributed into gametes independently of all other pairs

40 Animation: Dihybrid cross

41 10.4 Beyond Simple Dominance
Other types of gene expression Codominant alleles Incomplete dominance Epistasis Pleiotropy

42 Codominant Alleles Both expressed at the same time in heterozygotes
Example: Multiple alleles in ABO blood typing

43 AA BB or or Genotypes: AO AB BO OO Phenotypes (Blood type): A AB B O
Fig , p.160

44 Animation: Codominance: ABO blood types

45 Incomplete Dominance An allele is not fully dominant over its partner on a homologous chromosome Both are expressed Produces a phenotype between the two homozygous conditions

46 Incomplete Dominance

47 homozygous parent (RR) homozygous parent (rr) heterozygous
F1 offspring (Rr) x Cross two of the F1 plants, and the F2 offspring will show three phenotypes in a 1:2:1 ratio: RR Rr Rr rr Fig , p.160

48 Animation: Incomplete dominance

49 Epistasis Interacting products of one or more genes affect the same trait

50 RRpp (rose comb) X rrPP (pea comb) F1 of spring: RrPp
(all walnut comb) F2 offspring: RrPp X RrPp RRPP, RRPp, RrPP, or RrPp RRpp or Rrpp rrPP or rrPp rrpp 9/16 walnut 3/16 rose 3/16 pea 1/16 single comb Fig , p.161

51 Animation: Comb shape in chickens

52 More Epistasis

53 EB Eb eB eb EEBB black EEBb black EeBB black EeBb black EB EEBb black
chocolate EeBb black Eebb chocolate Eb EeBB black EeBb black eeBB yellow eeBb yellow eB eeBb yellow eebb yellow EeBb black Eebb chocolate eb Fig , p.161

54 Animation: Coat color in Labrador retrievers

55 Pleiotropy A single gene may affects two or more traits
Example: Marfan syndrome

56 Animation: Pleiotropic effects of Marfan syndrome

57 10.5 Linkage Groups All genes on the same chromosome are part of one linkage group Crossing over between homologous chromosomes disrupts gene linkages

58 Linkage Groups and Meiosis
During meiosis, genes relatively close together on a chromosome tend to stay together Few crossover events occur between them Genes that are relatively far apart tend to assort independently into gametes Greater frequency of crossing over between them

59 Linkage and Crossing Over

60 × AC ac Parental generation A a C c A a C c F1 offspring All AaCc
meiosis, gamete formation Gametes A a A a c c C C Most gametes have parental genotypes A smaller number have recombinant genotypes Fig , p.162

61 Animation: Crossover review

62 10.6 Genes and Environment Environmental factors may affect gene expression in individuals Example: Temperature and fur color

63 Animation: Coat color in the Himalayan rabbit

64 Elevation and Plant Height

65 60 a Mature cutting at high elevation (3,060 meters above sea level)
Height (centimeters) 60 b Mature cutting at mid-elevation (1,400 meters above sea level) Height (centimeters) 60 Height (centimeters) c Mature cutting at low elevation (30 meters above sea level) Fig , p.163

66 Predation and Body Form

67 10.7 Complex Variations in Traits
Polygenic Inheritance When products of many genes influence a trait, individuals of a population show a range of continuous variation for the trait

68 Continuous Variation

69 This red graph line of the range of variation for a trait
in a population plots out as a bell-shaped curve. Such curves indicate continuous variation in a population. Number of individuals with a measurable value for the trait Range of values for the trait Fig a, p.164

70 Animation: Continuous variation in height

71 Variations in Gene Expression
Gene interactions and environmental factors affect most phenotypes Gene products control metabolic pathways Mutations may alter or block pathways

Not all traits have clearly dominant or recessive forms One allele of a pair may be fully or partially dominant over its nonidentical partner, or codominant with it

Two or more gene pairs often influence the same trait, and some single genes influence many traits The environment also influences variation in gene expression

74 Video: One Bad Transporter and Cystic Fibrosis

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