Chapter 14 158 Mendel and the Gene Idea Homologous chromosome: Heterologous chromosome: Haploid cell: Diploid cell: Allele: Dominant gene: Recessive gene: Phenotype: Genotype:

158 Gene: Homozygous: Heterozygous: Hybrid: Monohybrid: Dihybrid:

159 Mendel’s First Law--- Law of Segregation: Genes exist in pairs, and in the formation of gametes, each gene separates or segregates from the other member of the pair, and passes into a different gamete so that each gamete has only one of each kind of gene. P PP x pp | F1 Pp

Fig. 14-1 Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?

159 Pp x Pp P p P PP Pp p Pp pp Phenotypic ratio: 3 purple : 1 white Genotypic ratio: 1PP : 2Pp : 1 pp

EXPERIMENT P Generation (true-breeding parents) Purple flowers White Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Figure 14.3 When F1 hybrid pea plants are allowed to self-pollinate, which traits appear in the F2 generation? F2 Generation 705 purple-flowered plants 224 white-flowered plants

160 The Testcross: It is a cross between an organism with homozygous recessive and an organism with an unknown genotype to find out the genotype of the test organism. If the offspring of the cross yields only dominant types, the test organism must be a homozygous dominant. It the offspring are all recessive, the organism must be a homozygous recessive. It the offspring are half recessive and half dominant, the organism must be a heterozygote.

160 Mendel’s Second Law: Law of Independent Assortment (or Segregation): The members of one pair of genes separate (or segregate) at random from each other in meiosis independent of the members of other pairs of genes. YYRR x yyrr | F1 YyRr (yellow and round)

160 YyRr x YyRr F2 | YR yR Yr yr YR YYRR YyRR YYRr YyRr

EXPERIMENT P Generation F1 Generation Hypothesis of Hypothesis of Fig. 14-8a EXPERIMENT P Generation YYRR yyrr Gametes YR  yr F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predictions Sperm or Predicted offspring of F2 generation 1/4 YR 1/4 Yr 1/4 yR 1/4 yr Sperm 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1

RESULTS Phenotypic ratio approximately 9:3:3:1 Fig. 14-8b 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character?

161 Rule of Multiplication: The chance of coming up with a head or a tail in tossing a coin is ½ or 50%. So, the chance of coming up with two heads when two coins are simultaneously tossed is ½ x ½ = ¼ or 25% chance. Rule of Addition: The probability of an event that can occur in two or more alternative ways is the sum of the separate probabilities of the different ways. The probability of F2 heterozygote is 1.4 + ¼ = ½ or 50%.

1/2 1/2 1/2 1/4 1/4 1/2 1/4 1/4 Rr Rr  Segregation of Segregation of Fig. 14-9 Rr Rr  Segregation of alleles into eggs Segregation of alleles into sperm Sperm 1/2 R 1/2 r R R 1/2 R R r Figure 14.9 Segregation of alleles and fertilization as chance events 1/4 1/4 Eggs r r 1/2 R r r 1/4 1/4

161 Partial, intermediate or incomplete dominance: The traits are not completely dominant. An example is the color of the flowers of snapdragons. A cross between a homozygous red and a homozygous white yields all pink snapdragons. When these pink snapdragons are crossed among themselves, they yield 1 red, 2 pink and 1 white (Fig. 14.9, p. 256).

P Generation Red White CRCR CWCW Gametes CR CW Fig. 14-10-1 Figure 14.10 Incomplete dominance in snapdragon color

P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW Fig. 14-10-2 P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW Gametes 1/2 CR 1/2 CW Figure 14.10 Incomplete dominance in snapdragon color

P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW Fig. 14-10-3 P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW Gametes 1/2 CR 1/2 CW Figure 14.10 Incomplete dominance in snapdragon color Sperm 1/2 CR 1/2 CW F2 Generation 1/2 CR CRCR CRCW Eggs 1/2 CW CRCW CWCW

162 Multiple Alleles: Some genes exist in more than two allelic forms. An example is the human ABO blood types. The genes are said to be codominant as they both express in heterozygous individuals.

(a) The three alleles for the ABO blood groups Fig. 14-11 Allele Carbohydrate IA A IB B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Red blood cell appearance Phenotype (blood group) Genotype IAIA or IA i A IBIB or IB i B Figure 14.11 Multiple alleles for the ABO blood groups IAIB AB ii O (b) Blood group genotypes and phenotypes

162 Blood Agglutinogens Agglutinins Genotypes (Antigens) (Antibodies) A A B AA or AO B B A BB or BO AB A & B none AB O none A & B OO

162-163 Pleiotropy: The ability of a single gene to have multiple effects. An example is sickle-cell anemia. It is caused by a single gene which causes complex sets of symptoms. The genes that control fur pigmentation may also cause cross-eye in animals. Epistasis: It is a condition in which a gene at one locus alters the phenotypic expression of a gene at a second locus.

163 B: able to produce pigment b: unable to produce pigment C: able to store pigment c: unable to store pigment P B/B C/C x b/b c/c | F1 B/b C/c x B/b C/c F2 9 B/- C/- 3 B/- c/c 3 b/b C/- 1 b/b c/c (black) (brown) (white) (white)

1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4  BbCc BbCc Sperm Eggs BBCC BbCC BBCc Fig. 14-12  BbCc BbCc Sperm 1/4 1/4 1/4 1/4 BC bC Bc bc Eggs 1/4 BC BBCC BbCC BBCc BbCc 1/4 bC BbCC bbCC BbCc bbCc 1/4 Bc Figure 14.12 An example of epistasis BBCc BbCc BBcc Bbcc 1/4 bc BbCc bbCc Bbcc bbcc 9 : 3 : 4

163 Polygenic Inheritance: Polygenic traits are influenced by many genes in an additive way. They are qualitative traits as they vary in a continuous way. The examples are human height, skin pigmentation, and IQ. Phenotype is determined by genes and environmental factors. The phenotypic range is called the norm of reaction. Height, skin color and IQ show a continuous variation. Human ABO blood group does not show a continuous phenotypic expression. A person can be one of the four blood types.

Fig. 14-13  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 14.13 A simplified model for polygenic inheritance of skin color 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 15/64 6/64 1/64 Number of dark-skin alleles: 1 2 3 4 5 6

164-165 A family tree or pedigree describes the interrelationships of parents and children across the generations. Recessively Inherited Disorders: Cystic fibrosis: Tay-Sachs disease: Sickle-cell anemia: Dominantly Inherited Disorders: Achondroplasia: Huntington’s disease

Parents Dwarf Normal Sperm Eggs Dwarf Normal Dwarf Normal Dd dd D d Dd Fig. 14-17 Parents Dwarf Normal Dd  dd Sperm D d Eggs Dd dd Figure 14.17 Achondroplasia: a dominant trait d Dwarf Normal Dd dd d Dwarf Normal

166 Fetal Testing: Amniocentesis: 14th-16th weeks of pregnancy Chorionic villi sampling (CVS): 8 to 10 weeks Ultrasound: Fetoscopy: Newborn Screening: Phenylketonuria (PKU): cannot break down phenylalanine to tyrosine. The amino acid and its by-product phenylpyruvic acid accumulate in toxic levels in the blood, causing mental retardation as the brain cells cannot develop.

(b) Chorionic villus sampling (CVS) Fig. 14-18 Amniotic fluid withdrawn Centrifugation Fetus Fetus Suction tube inserted through cervix Placenta Placenta Chorionic villi Uterus Cervix Fluid Bio- chemical tests Fetal cells Several hours Fetal cells Several hours Several weeks Figure 14.18 Testing a fetus for genetic disorders Several weeks Several hours Karyotyping (a) Amniocentesis (b) Chorionic villus sampling (CVS)

Amniotic fluid withdrawn Centrifugation Fetus Placenta Uterus Cervix Fig. 14-18a Amniotic fluid withdrawn Centrifugation Fetus Placenta Uterus Cervix Fluid Bio- chemical tests Fetal cells Several hours Several weeks Figure 14.18 Testing a fetus for genetic disorders Several weeks Karyotyping (a) Amniocentesis

(b) Chorionic villus sampling (CVS) Fig. 14-18b Fetus Suction tube inserted through cervix Placenta Chorionic villi Bio- chemical tests Fetal cells Several hours Figure 14.18 Testing a fetus for genetic disorders Several hours Karyotyping (b) Chorionic villus sampling (CVS)