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Chapter 14 Mendel and the Gene Idea
Green book -p 267 Ch 14 Genetics Problems (19 ) – due Nov. 18 p Ch 15 Genetics Problems (16) - The answers are in the back of the book!!!! - I need to get the impression that you have worked out the problems & not copied the correct answer!
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Mendel – Father of Genetics – peas 1 5 4 3 2 Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Pollinated carpel matured into pod Carpel (female) Stamens (male) Planted seeds from pod Examined offspring: all purple flowers First offspring (F1) APPLICATION By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color. TECHNIQUE When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers. RESULTS P generation F1 Generation
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Chapter 14 Mendel and the Gene Idea
P Generation (true-breeding parents) Purple flowers White F1 Generation (hybrids) All plants had purple flowers F2 Generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F1 hybrids were allowed to self-pollinate or were cross- pollinated with other F1 hybrids. Flower color was then observed in the F2 generation. RESULTS Both purple-flowered plants and white- flowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Mendel – Father of Genetics – peas Allele – alternate form of a gene that gives different traits Dominant allele gives a visible trait with only 1 copy A Recessive allele gives a visible trait with 2 copies a Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Mendel – Father of Genetics – peas Allele – alternate form of a gene that gives different traits Dominant allele gives a visible trait with only 1 copy A Recessive allele gives a visible trait with 2 copies a Homozygous 2 copies of the same allele AA or aa Heterozygous 1 copy of each allele Aa True breeders parents who mate & always yield the same genetic offspring AA x AA or aa x aa
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Mendel – Father of Genetics – peas Allele – alternate form of a gene that gives different traits Dominant allele Recessive allele Homozygous Heterozygous True breeders parents who mate & always yield the same genetic offspring AA x AA or aa x aa Monohybrid – organism that’s heterozygous for 1 gene of interest Dihybrid – organism that is heterozygous for 2 genes of interest Genotype – genetic make up of an organism (its alleles) Phenotype – physical traits exhibited – based on genotype
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Figure 14.6 Phenotype versus genotype
3 1 2 Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) pp Ratio 3:1 Ratio 1:2:1
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Mendel – Father of Genetics – peas Allele – alternate form of a gene that gives different traits Dominant allele Recessive allele Homozygous Heterozygous True breeders parents who mate & always yield the same genetic offspring AA x AA or aa x aa Monohybrid – organism that’s heterozygous for 1 gene of interest Dihybrid – organism that is heterozygous for 2 genes of interest Genotype – genetic make up of an organism (its alleles) Phenotype – physical traits exhibited – based on genotype Law of Segregation – each allele in a pair separates into a different gamete during gamete formation
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Figure 14.5 Mendel’s law of segregation
P Generation F1 Generation P p Appearance: Genetic makeup: Purple flower PP White flowers pp Purple flowers Pp Gametes: Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple- flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.
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Figure 14.5 Mendel’s law of segregation
P Generation F1 Generation P p Appearance: Genetic makeup: Purple flower PP White flowers pp Purple flowers Pp Gametes: Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple- flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. P Generation F1 Generation F2 Generation P p Pp PP pp Appearance: Genetic makeup: Purple flower PP White flowers pp Purple flowers Pp Gametes: F1 sperm F1 eggs 1/2 Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple- flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 F1 (Pp Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm. Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F2 generation. 3 : 1
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Mendel – Father of Genetics – peas Allele – alternate form of a gene that gives different traits Dominant allele Recessive allele Homozygous Heterozygous True breeders parents who mate & always yield the same genetic offspring AA x AA or aa x aa Monohybrid – organism that’s heterozygous for 1 gene of interest Dihybrid – organism that is heterozygous for 2 genes of interest Genotype – genetic make up of an organism (its alleles) Phenotype – physical traits exhibited – based on genotype Law of Segregation – each allele in a pair separates into a different gamete during gamete formation Testcross – breeding an organism of unknown genotype with a homozygous recessive
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then 1⁄2 offspring purple
Figure 14.7 The Testcross Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then 1⁄2 offspring purple and 1⁄2 offspring white: p P Pp APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. RESULTS
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Mendel – Father of Genetics – peas Allele – alternate form of a gene that gives different traits Dominant allele Recessive allele Homozygous Heterozygous True breeders parents who mate & always yield the same genetic offspring AA x AA or aa x aa Monohybrid – organism that’s heterozygous for 1 gene of interest Dihybrid – organism that is heterozygous for 2 genes of interest Genotype – genetic make up of an organism (its alleles) Phenotype – physical traits exhibited – based on genotype Law of Segregation – each allele in a pair segregates into a different gamete during gamete formation Testcross – breeding an organism of unknown genotype with a homozygous recessive Law of Independent Assortment – each pair of alleles separates independently during gamete formation when genes for 2 traits are on different homologous chromosomes
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Phenotypic ratio approximately 9:3:3:1
Figure 14.8 Do the alleles for seed color and seed shape sort into gametes dependently (together) or independently? YYRR P Generation Gametes YR yr yyrr YyRr Hypothesis of dependent assortment independent F2 Generation (predicted offspring) 1 ⁄2 3 ⁄4 1 ⁄4 Sperm Eggs Phenotypic ratio 3:1 Yr yR 9 ⁄16 3 ⁄16 1 ⁄16 YYRr YyRR Yyrr YYrr yyRR yyRr Phenotypic ratio 9:3:3:1 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 F1 Generation RESULTS CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Terms to know… How does genetics reflect probabilities? 0 = never will happen 1 = always happens Consider a coin…. Probability of heads = 0.5 Probability of tails = 0.5 What is the rule of multiplication? prob. of RR offspring from Rr x Rr R (½ ) x R (½) = ¼ Prob. of rr offspring from r (½) x r (½) = ¼ What is the rule of addition? Prob. of Rr offspring from R (½) x r (½) = ¼ (1st parent gives R) R (½) x r (½) = ¼ (2nd parent gives R) ¼ + ¼ = ½ Rr offspring Only when Rr x Rr make Rr Rr Segregation of alleles into eggs alleles into sperm R r 1⁄2 1⁄4 Sperm Eggs
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Let’s practice….. PpYyRr x Ppyyrr ppyyRr = ? ppYyrr = ? Ppyyrr = ? PPyyrr = ? ppyyrr = ? 1/16 1/8 ½ · ½ · ½ · 1 · ½ · 1 = 1/16 (½ · ½) + (½ · ½) · ½ · 1 · ½ · 1 = ½ · ½ · 1 · ½ · 1 = 1/8
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Terms to know… How does genetics reflect probabilities? What is the rule of multiplication? What is the rule of addition? What is incomplete dominance? Heterozygous phenotype is in between the 2 homozygous phenotypes snapdragons 1:2:1 for phenotype = genotype P Generation F1 Generation F2 Generation Red CRCR Gametes CR CW White CWCW Pink CRCW Sperm Cw 1⁄2 Eggs CR CR CR CW CW CW
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Terms to know… How does genetics reflect probabilities? What is the rule of multiplication? What is the rule of addition? What is incomplete dominance? What is codominance? Phenotype shows both alleles being expressed Both alleles can be detected Blood typing – glycoprotein on RBC Blood typing also shows multiple alleles (more than 2 allelic variations at a gene locus)
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Chapter 14 Mendel and the Gene Idea
Tay-Sachs disease – brain cells cannot metabolize a certain lipid because of an enzyme deficiency Phenotype seen - dd (homozygous recessive) - consider Dd - appear normal – 1 copy sufficient - biochemically – incomplete dominance – less enzyme activity can be detected….in between DD and dd - molecularly – codominant – both normal enzyme & mutant enzyme can be detected
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Terms to know… How does genetics reflect probabilities? What is the rule of multiplication? What is the rule of addition? What is incomplete dominance? What is codominance? What is pleiotropy? 1 gene with multiple effects Sickle cell allele RBC form a sickle shape (“C”) when oxygen deprived causing blood flow problems leading to fatigue, pain, brain damage, What is epistasis? - Gene at 1 locus alters the phenotypic expression of a gene at a 2nd locus
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Figure 14.11 An example of epistasis
BC bC Bc bc 1⁄4 BBCc BbCc BBcc Bbcc bbcc bbCc BbCC bbCC BBCC 9⁄16 3⁄16 4⁄16 Sperm Eggs B = black coat b = brown coat C = pigment c = no pigment In order to have color, the cells must have pigment.
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Terms to know… How does genetics reflect probabilities? What is the rule of multiplication? What is the rule of addition? What is incomplete dominance? What is codominance? What is pleiotropy? What is epistasis? What is polygenic inheritance? An additive effect of 2 or more genes on a single phenotype Opposite of pleiotropy Skin pigmentation
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Skin pigmentation controlled by 3 separate genes
Figure 14.12 A simplified model for polygenic inheritance of skin color Skin pigmentation controlled by 3 separate genes AaBbCc aabbcc Aabbcc AaBbcc AABbCc AABBCc AABBCC 20⁄64 15⁄64 6⁄64 1⁄64 Fraction of progeny
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Terms to know… How does genetics reflect probabilities? What is the rule of multiplication? What is the rule of addition? What is incomplete dominance? What is codominance? What is pleiotropy? What is epistasis? What is polygenic inheritance? Can the environment influence phenotype? Plants – leaf shape, size, greenness – depend on wind, sunlight, rain nutrition – Figure 50.9 Us – height, weight, exercise – depend on nutrition - Hydrangea color depends on soil acidity - Multifactorial – many factors, genetic & environmental influence phenotype
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Figure 14.14 Pedigree analysis
Pedigree analysis - follows specific traits over generations of a family Ww ww WW or First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ff FF or Ff FF Widow’s peak No Widow’s peak Attached earlobe Free earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Terms to know… How does genetics reflect probabilities? What is the rule of multiplication? What is the rule of addition? What is incomplete dominance? What is codominance? What is pleiotropy? What is epistasis? What is polygenic inheritance? Can the environment influence phenotype? What are some recessively inherited human disorders? - Only seen as dd, Dd are considered carriers Cystic fibrosis – caused by a defect in a Cl- channel protein in lungs which leads to a mucus build up because an increase in Cl- concentration 1 in 2500 Caucasians of European descent 1 in 25 (4%) carriers Sickle-cell disease
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Terms to know… How does genetics reflect probabilities? What is the rule of multiplication? What is the rule of addition? What is incomplete dominance? What is codominance? What is pleiotropy? What is epistasis? What is polygenic inheritance? Can the environment influence phenotype? What are some recessively inherited human disorders? 12. What are some dominantly inherited alleles? - Achondroplasia - form of dwarfism - 1 in 25,000 - Huntington’s disease - degenerative disease of the nervous system - lethal dominant - phenotype visible between 35 – 45 yrs old - 1 in 10,000
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Chapter 14 Mendel and the Gene Idea
What do you know about genetics? Terms to know… How does genetics reflect probabilities? What is the rule of multiplication? What is the rule of addition? What is incomplete dominance? What is codominance? What is pleiotropy? What is epistasis? What is polygenic inheritance? Can the environment influence phenotype? What are some recessively inherited human disorders? 12. What are some dominantly inherited alleles? 13. How do doctors test for genetic anomalies? - amniocentesis - chorionic villus sampling (CVS)
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Figure 14.17 Testing a fetus for genetic disorders
(a) Amniocentesis Amniotic fluid withdrawn Fetus Placenta Uterus Cervix Centrifugation A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. (b) Chorionic villus sampling (CVS) Fluid Fetal cells Biochemical tests can be Performed immediately on the amniotic fluid or later on the cultured cells. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Several weeks Biochemical tests hours Chorionic viIIi A sample of chorionic villus tissue can be taken as early as the 8th to 10th week of pregnancy. Suction tube Inserted through cervix Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Karyotyping
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