Mendel and the Gene Idea 11 Mendel and the Gene Idea

Figure 11.1 Figure 11.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants? 4

Concept 11.1: Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the basic principles of heredity by breeding garden peas 5

Technique Parental generation (P) Stamens Carpel Results First filial Figure 11.2 Technique 1 2 Parental generation (P) 3 Stamens Carpel 4 Figure 11.2 Research method: crossing pea plants Results 5 First filial generation offspring (F1) 6

Mendel chose characters that occurred in two distinct forms and created true-breeding lineages 7

Mendel mated two contrasting, true-breeding varieties, (hybridization) parents are the P generation offspring of the P generation are called the F1 generation F1 individuals self-pollinate or cross- pollinate with other F1, producing the F2 generation 8

(true-breeding parents) Figure 11.3-1 Experiment P Generation (true-breeding parents) Purple flowers White flowers Figure 11.3-1 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? (step 1) 9

(true-breeding parents) Figure 11.3-2 Experiment P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 11.3-2 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? (step 2) 10

All plants had purple flowers Figure 11.3-3 Experiment P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 11.3-3 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? (step 3) F2 Generation 705 purple-flowered plants 224 white-flowered plants 11

Table 11.1 Table 11.1 The results of Mendel’s F1 crosses for seven characters in pea plants 12

Table 11.1a Table 11.1a The results of Mendel’s F1 crosses for seven characters in pea plants (part 1) 13

Table 11.1b Table 11.1b The results of Mendel’s F1 crosses for seven characters in pea plants (part 2) 14

Mendel’s Model Mendel’s model to explain the 3:1 F2ratio 1, alternative alleles account for variations in inherited characters 2, for each character an organism inherits two alleles, one from each parent 3, dominant alleles may mask recessive alleles 4, (law of segregation), the two alleles segregate during gamete formation (and end up in different gametes) 15

P Generation Appearance: Genetic makeup: Purple flowers PP Figure 11.5-1 P Generation Appearance: Genetic makeup: Purple flowers PP White flowers pp Gametes: P p Figure 11.5-1 Mendel’s law of segregation (step 1) 16

P Generation F1 Generation Appearance: Genetic makeup: Purple flowers Figure 11.5-2 P Generation Appearance: Genetic makeup: Purple flowers PP White flowers pp Gametes: P p F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: ½ ½ P p Figure 11.5-2 Mendel’s law of segregation (step 2) 17

P Generation F1 Generation F2 Generation Appearance: Genetic makeup: Figure 11.5-3 P Generation Appearance: Genetic makeup: Purple flowers PP White flowers pp Gametes: P p F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: ½ ½ P p Sperm from F1 (Pp) plant Figure 11.5-3 Mendel’s law of segregation (step 3) F2 Generation P p P Eggs from F1 (Pp) plant PP Pp p Pp pp 3 : 1 18

Phenotype - physical appearance genotype - genetic makeup 19

PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Figure 11.6 Phenotype Genotype PP (homozygous) Purple 1 3 Pp (heterozygous) Purple 2 Pp (heterozygous) Purple Figure 11.6 Phenotype versus genotype pp (homozygous) 1 White 1 Ratio 3:1 Ratio 1:2:1 20

Test Cross Used to tell genotype of individual with dominant phenotype dominant phenotype crossed with recessive phenotype Examining offspring determines genotype of dominant individual

Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, Figure 11.7 Technique Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If purple-flowered parent is PP or If purple-flowered parent is Pp Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs Figure 11.7 Research method: the testcross P p Pp Pp pp pp Results or All offspring purple ½ offspring purple and ½ offspring white 22

independent assortment Figure 11.8 Experiment P Generation YYRR yyrr Gametes YR yr F1 Generation YyRr Predictions Hypothesis of dependent assortment Hypothesis of independent assortment Sperm or Predicted offspring in F2 generation ¼ YR ¼ Yr ¼ yR ¼ yr Sperm ½ YR ½ yr ¼ YR YYRR YYRr YyRR YyRr ½ YR YYRR YyRr ¼ Yr Eggs YYRr YYrr YyRr Yyrr Figure 11.8 Inquiry: Do the alleles for one character segregate into gametes dependently or independently of the alleles for a different character? Eggs ½ yr YyRr yyrr ¼ yR YyRR YyRr yyRR yyRr ¾ ¼ ¼ yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 16 3 16 3 16 1 16 Phenotypic ratio 9:3:3:1 Results 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 23

Mendel’s law of independent assortment each pair of alleles segregates independently of each other pair of alleles during gamete formation applies to genes on different, nonhomologous chromosomes or those far apart on the same chromosome 24

Degrees of Dominance Complete dominance incomplete dominance codominance 25

P Generation Red CRCR White CWCW Gametes CR CW Figure 11.10-1 Figure 11.10-1 Incomplete dominance in snapdragon color (step 1) 26

P Generation Red CRCR White CWCW Gametes Pink CRCW F1 Generation Figure 11.10-2 P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation Gametes ½ CR ½ CW Figure 11.10-2 Incomplete dominance in snapdragon color (step 2) 27

P Generation Red CRCR White CWCW Gametes Pink CRCW F1 Generation Figure 11.10-3 P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation Gametes ½ CR ½ CW Figure 11.10-3 Incomplete dominance in snapdragon color (step 3) Sperm ½ CR ½ CW F2 Generation ½ CR CRCR CRCW Eggs ½ CW CRCW CWCW 28

Multiple Alleles Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles of the gene: IA, IB, and i. The enzyme (I) adds specific carbohydrates to the surface of blood cells The enzyme encoded by IA adds the A carbohydrate, and the enzyme encoded by IB adds the B carbohydrate; the enzyme encoded by the i allele adds neither 29

(a) The three alleles for the ABO blood groups and their carbohydrates Figure 11.11 (a) The three alleles for the ABO blood groups and their carbohydrates Allele IA IB i Carbohydrate A B none (b) Blood group genotypes and phenotypes Genotype IAIA or IAi IBIB or IBi IAIB ii Figure 11.11 Multiple alleles for the ABO blood groups Red blood cell appearance Phenotype (blood group) A B AB O 30

Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes 31

¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ BbEe BbEe Sperm Eggs BBEE BbEE BBEe BbEe BbEE bbEE Figure 11.12 BbEe BbEe Sperm ¼ BE ¼ bE ¼ Be ¼ be Eggs ¼ BE BBEE BbEE BBEe BbEe ¼ bE BbEE bbEE BbEe bbEe ¼ Be BBEe BbEe BBee Bbee Figure 11.12 An example of epistasis ¼ be BbEe bbEe Bbee bbee 9 : 3 : 4 32

Polygenic Inheritance Quantitative variation usually indicates polygenic inheritance, Skin color in humans is an example of polygenic inheritance 33

Figure 11.13 AaBbCc AaBbCc Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 Eggs 1 8 1 8 Figure 11.13 A simplified model for polygenic inheritance of skin color 1 8 1 8 1 64 6 64 15 64 20 64 15 64 6 64 1 64 1 64 Phenotypes: Number of dark-skin alleles: 1 2 3 4 5 6 34

Describing Continuous Variation Fig. 11-20, p.181

Nature and Nurture: The Environmental Impact on Phenotype Sometimes the phenotype depends on environment as well as genotype 36

Temperature Effects on Phenotype This Rabbit is homozygous for allele producing heat-sensitive version of an enzyme in melanin-producing pathway Melanin is produced in cooler areas of body Figure 11.16 Page 179

This Siamese cat, raised in a cold environment in Moscow in the late 20s, developed a relatively dark coat. An area on his shoulder was shaved, and the cat wore a warm jacket while the fur was growing back. When the shaved hair grew back in, it was white, the same color as the cat's belly, due to the increased temperature under the jacket. This was not due to scarring, as the hair grew in normally colored later.

FF or Ff FF or Ff WW or Ww Attached earlobe Free earlobe Figure 11.14 Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left) Ff Ff ff Ff 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff Ww ww ww Ww Ww ww 3rd generation (two sisters) ff FF or Ff Figure 11.14 Pedigree analysis WW or Ww ww Widow’s peak No widow’s peak Attached earlobe Free earlobe (a) Is a widow’s peak a dominant or recessive trait? (b) Is an attached earlobe a dominant or recessive trait? 40

Parents Normal Aa Normal Aa Sperm A a Eggs Aa Normal (carrier) AA Figure 11.15 Parents Normal Aa Normal Aa Sperm A a Eggs Aa Normal (carrier) AA Normal A Aa Normal (carrier) Figure 11.15 Albinism: a recessive trait aa Albino a 41

Fig. 11-21, p.183

Fig. 11-21, p.183

Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications Sickle-cell disease affects one out of 400 African-Americans Recessive trait caused by a single amino acid substitution in hemoglobin Symptoms include physical weakness, pain, organ damage, and even paralysis Heterozygotes -less susceptible to malaria parasite, 46

Autosomal Dominant Inheritance example… Achondro-plasia Fig. 12-5, p.190

Achondroplasia Autosomal dominant allele Homozygous usually leads to stillbirth Heterozygotes display a type of dwarfism (short arms and legs relative to other body parts)

Huntington Disorder Autosomal dominant allele Causes involuntary movements, nervous system deterioration, death Symptoms appear after age 30 People often pass allele on before they know they have it

Huntington Disorder

Hutchinson-Gilford Progeria Mutation causes accelerated aging No evidence of it running in families Appears dominant Seems to arise as spontaneous mutation Usually causes death in early teens

Hutchinson-Gilford Progeria Fig. 12-7, p.191

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