Mendel and the Gene Idea Chapter 14 Mendel and the Gene Idea

14.1 Blending vs. particulate ideas Anecdotally many thought children inherited “_______” of their parents Mendel showed that some traits are _____

Vocab Character- varieties with distinct heritable features (such as flower color) Trait- character variants (such as purple or white flowers) **sometimes used interchangeably**

Why Peas? .

Fig. 14-2a TECHNIQUE 1 2 Parental generation (P) Stamens Carpel 3 4

Mendel chose to track only those characters that varied in an either-or manner What does that mean? He also used varieties that were true-breeding

Vocab!! In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization

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 F2 Generation 705 purple-flowered plants 224 white-flowered plants

What Mendel called a “heritable factor” is what we now call a gene Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits

Reflect What did Mendel discover? What’s the typical ratio for F2 generation?

Law of Segregation 1. Alternative versions of genes account for variations in inherited characters Ex.

2 Organisms inherits two alleles for every gene, one from each parent Those two alleles: May be

3 If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

4. Law of Segregation The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism Meiosis anyone??

Vocab Alleles- alternative versions of a gene Locus- location on a specific chromosome Each gene resides at a specific locus Dominant Recessive

Allele for purple flowers Fig. 14-4 Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers

Punnett square- a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

homozygous – having two _______alleles for a gene heterozygous having two ________alleles for a gene Not ___________

Genotype v Phenotype Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition Genotype- genetic makeup . Phenotype- physical appearance/expression of the gene

Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous) Fig. 14-6 Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous) 2 Purple Pp (heterozygous) pp 1 White 1 (homozygous) Ratio 3:1 Ratio 1:2:1

Testcross How can we tell the genotype of an individual with the dominant phenotype? testcross:

TECHNIQUE RESULTS Dominant phenotype, unknown genotype: PP or Pp? Fig. 14-7 TECHNIQUE  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If PP If Pp or Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p Pp Pp pp pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white

Monohybrid A cross between heterozygotes is called a monohybrid cross

Dihybrid dihybrids - heterozygous for two genes Mendel used dihybrid crosses to figure out the law of independent assortment

EXPERIMENT RESULTS Fig. 14-8 P Generation F1 Generation Hypothesis of 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 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 315 108 101 32 Phenotypic ratio approximately 9:3:3:1

Law of independent assortment Each pair of alleles segregates independently of each other pair of alleles during gamete formation Ex. This law applies only to genes on different, nonhomologous chromosomes

14.2 When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles

The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

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 1/4 1/4 Eggs r r 1/2 R r r 1/4 1/4

The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous

Fig. 14-UN1

14.3 Inheritance patterns are often more complex than predicted by simple Mendelian genetics Many heritable characters are not determined by only one gene with two alleles However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

Non-Mendelian situations include When alleles are not ______________ When a gene has more than ___________ When a gene produces _________________

Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Incomplete dominance, the phenotype of F1 hybrids Codominance-two dominant alleles affect the phenotype ________________________

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 Sperm 1/2 CR 1/2 CW F2 Generation 1/2 CR CRCR CRCW Eggs 1/2 CW CRCW CWCW

Tay-Sachs Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain At the organismal level, the allele is recessive At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant At the molecular level, the alleles are codominant

Dominance is NOT PREdominance! Dominant alleles are not necessarily more common in populations than recessive alleles For example, one baby out of 400 in the United States is born with extra fingers or toes

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 for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

(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 IAIB AB ii O (b) Blood group genotypes and phenotypes

Pleiotropy pleiotropy – genes that have multiple phenotypic effects Ex. cystic fibrosis and sickle-cell disease have MULTIPLE SYMPTOMS

Epistasis Epistasis-a gene at one locus alters the phenotypic expression of a gene at a second locus Ex.

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 BBCc BbCc BBcc Bbcc 1/4 bc BbCc bbCc Bbcc bbcc 9 : 3 : 4

Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Indicate polygenic inheritance (an additive effect) of two or more genes on a single phenotype Ex.

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 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

Nature and Nurture: The Environmental Impact on Phenotype For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity Sea Turtles? Obesity/Heart Disease/ Diabetes

14.4 Human Mendelian Traits Humans are not good subjects for genetic research . However, basic Mendelian genetics endures as the foundation of human genetics

Pedigree A family tree that describes the interrelationships of parents and children across generations

(a) Is a widow’s peak a dominant or recessive trait? Fig. 14-15b 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait?

(b) Is an attached earlobe a dominant or recessive trait? Fig. 14-15c 1st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?

Recessively Inherited Disorders Recessively inherited disorders show up only in individuals homozygous for the allele Carriers -

Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

Science and Society Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele Most societies and cultures have laws or taboos against marriages between close relatives

Cystic Fibrosis The most common lethal genetic disease in the United States The allele results in defective or absent chloride transport channels in plasma membranes Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Sickle-Cell Disease The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms include physical weakness, pain, organ damage, and even paralysis

Dominantly Inherited Disorders Some human disorders are caused by dominant alleles Achondroplasia is a form of dwarfism caused by a rare dominant allele

Huntington’s A degenerative disease of the nervous system The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age

Multifactorial Disorders Many diseases, such as heart disease and cancer, have both genetic and environmental components Little is understood about the genetic contribution to most multifactorial diseases

Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease Use probability, Mendel to give families probability of having certain traits in an offspring

Fetal Testing In amniocentesis, the liquid that bathes the fetus is removed and tested In chorionic villus sampling (CVS), a sample of the placenta is removed and tested Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

Newborn Screening Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States Do you remember PKU?