In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation.

In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals self-pollinate, the F2 generation is produced

The Law of Segregation When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
Mendel called the purple flower color a dominant trait and white flower color a recessive trait Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits What Mendel called a “heritable factor” is what we now call a gene

Mendel’s Model Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring Four related concepts make up this model These concepts can be related to what we now know about genes and chromosomes

Allele for purple flowers
Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers

The second concept is that for each character an organism inherits two alleles, one from each parent
Mendel made this deduction without knowing about the role of chromosomes The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

The third concept is that 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

The fourth concept, now known as the law of segregation, states that 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 This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

3 : 1 P Generation Appearance: Purple flowers PP White flowers pp
Genetic makeup: Gametes P p F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1 2 P 1 2 p F1 sperm P p F2 Generation P PP Pp F1 eggs p Pp pp 3 : 1

The Testcross How can we tell the genotype of an individual with the dominant phenotype? Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous

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 p p P P Pp Pp Pp Pp P P Pp Pp pp pp

P Generation YYRR yyrr Gametes YR yr YyRr F1 Generation Hypothesis of
dependent assortment Hypothesis of independent assortment Sperm 1 YR Yr yR yr Sperm 4 1 4 1 4 1 4 Eggs 1 YR 1 yr 2 2 1 YR Eggs 4 YYRR YYRr YyRR YyRr 1 YR F2 Generation (predicted offspring) 2 YYRR YyRr 1 Yr 4 YYRr YYrr YyRr Yyrr 1 yr 2 YyRr yyrr 1 yR 4 YyRR YyRr yyRR yyRr 3 4 1 4 1 yr 4 Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 16 3 16 3 16 3 16 Phenotypic ratio 9:3:3:1

The Law of Independent Assortment
Using a dihybrid cross, Mendel developed the law of independent assortment The law of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes Genes located near each other on the same chromosome tend to be inherited together

The laws of probability govern Mendelian inheritance
Mendel’s laws of segregation and independent assortment reflect the rules of probability 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 and Addition Rules Applied to Monohybrid Crosses
The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities Probability in an F1 monohybrid cross can be determined using the multiplication rule Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele

Rr Rr Segregation of alleles into eggs Segregation of
alleles into sperm Sperm 1 R 1 r 2 2 R R R r 1 R 2 1 1 4 4 Eggs r r R r 1 r 2 1 1 4 4

Extending Mendelian Genetics for a Single Gene
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces multiple phenotypes

The Spectrum of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation
2 CR 1 2 CW Sperm 1 2 CR 1 2 CW Eggs F2 Generation 1 CR 2 CRCR CRCW 1 2 CW CRCW CWCW

Frequency of Dominant Alleles
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 The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage In this example, the recessive allele is far more prevalent than the dominant allele in the population

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

Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus For example, in mice and many other mammals, coat color depends on two genes One gene determines the pigment color (with alleles B for black and b for brown) The other gene (with alleles C for pigment color and c for no pigment color ) determines whether the pigment will be deposited in the hair

BbCc BbCc Sperm BC bC Bc bc BC BBCC BbCC BBCc BbCc bC BbCC bbCC BbCc
1 BC 1 bC 1 Bc 1 4 4 4 4 bc 1 BC 4 BBCC BbCC BBCc BbCc 1 bC BbCC 4 bbCC BbCc bbCc 1 BBCc BbCc BBcc Bbcc 4 Bc 1 bbcc bc BbCc bbCc Bbcc 4 9 3 4 16 16 16

Polygenic Inheritance
Quantitative characters are those that vary in the population along a continuum Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype Skin color in humans is an example of polygenic inheritance

20/64 15/64 6/64 1/64 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc
Fraction of progeny 6/64 1/64

Nature and Nurture: The Environmental Impact on Phenotype
Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The norm of reaction is the phenotypic range of a genotype influenced by the environment For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

Integrating a Mendelian View of Heredity and Variation
An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior An organism’s phenotype reflects its overall genotype and unique environmental history

Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees

Dominant trait (widow’s peak)
First generation (grandparents) Ww ww ww Ww Second generation (parents plus aunts and uncles) Ww ww ww Ww Ww ww Third generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak Dominant trait (widow’s peak)

First generation (grandparents) Ff Ff ff Ff Second generation (parents plus aunts and uncles) FF or Ff ff ff Ff Ff ff Third generation (two sisters) ff FF or Ff Attached earlobe Free earlobe Recessive trait (attached earlobe)

Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive manner Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal

Cystic Fibrosis Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent The cystic fibrosis 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 Sickle-cell disease affects one out of 400 African-Americans 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 due to dominant alleles One example is achondroplasia, a form of dwarfism that is lethal when homozygous for the dominant allele

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

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

Counseling Based on Mendelian Genetics and Probability Rules
Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders

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, ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

Amniocentesis Amniotic fluid withdrawn A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. Fetus Centrifugation Placenta Uterus Cervix Fluid Fetal cells Biochemical tests can be performed immediately on the amniotic fluid or later on the cultured cells. Biochemical tests Several weeks Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Karyotyping

Chorionic villus sampling (CVS)
A sample of chorionic villus tissue can be taken as early as the 8th to 10th week of pregnancy. Fetus Suction tube inserted through cervix Placenta Chorionic villi Fetal cells Biochemical tests Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Several hours Karyotyping

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

X-linked red-green colorblindness

Dosage compensation in female mammals

Multiple alleles in rabbits

Gene interaction

Epistasis in Labrador retrievers

Tay-Sachs disease It is caused by a dysfunctional enzyme that fails to break down specific brain lipids. The symptoms begin with seizures, blindness, and degeneration of motor and mental performance a few months after birth. Inevitably, the child dies after a few years. Among Ashkenazic Jews (those from central Europe), this disease occurs in one of 3,600 births, about 100 times greater than the incidence among non-Jews or Mediterranean (Sephardic) Jews.

Humans with Tay-Sachs disease lack a functioning enzyme to metabolize certain lipids. These lipids accumulate in the brain, harming brain cells, and ultimately leading to death. Children with two Tay-Sachs alleles (homozygotes) have the disease. Both heterozygotes with one working allele and homozygotes with two working alleles are healthy and normal at the organismal level.

The activity level of the lipid-metabolizing enzyme is reduced in heterozygotes. At the biochemical level, the alleles show incomplete dominance. Heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. At the molecular level, the Tay-Sachs and functional alleles are codominant.

Huntington’s disease Degenerative disease of the nervous system. The dominant lethal allele has no obvious phenotypic effect until an individual is about 35 to 45 years old. The deterioration of the nervous system is irreversible and inevitably fatal. Any child born to a parent who has the allele for Huntington’s disease has a 50% chance of inheriting the disease and the disorder.

Achondroplasia Heterozygous individuals have the dwarf phenotype. Those who are not achondroplastic dwarfs, 99.99% of the population, are homozygous recessive for this trait. This provides another example of a trait for which the recessive allele is far more prevalent than the dominant allele.

Sickle-cell disease Sickle-cell disease is caused by the substitution of a single amino acid in hemoglobin. When oxygen levels in the blood of an affected individual are low, sickle-cell hemoglobin aggregate into long rods that deform red blood cells into a sickle shape. This sickling creates a cascade of symptoms, demonstrating the pleiotropic effects of this allele, as sickled cells clump and clog capillaries throughout the body.

Carriers are said to have sickle-cell trait.
These individuals are usually healthy, although some suffer some symptoms of sickle-cell disease under blood oxygen stress. Homozygous normal individuals die of malaria and homozygous recessive individuals die of sickle-cell disease, while carriers are relatively free of both.

Animations and Videos Mendel's Experiments
Bozeman - Mendelian Genetics The Punnett Square Bozeman - Genotypes and Phenotypes Bozeman - Genotype Expression Blood Types Animated Blood Typing Animation Bozeman - Blood Types

Animations and Videos Bozeman - Punnett Squares Bozeman - Linked Genes
Bozeman - Genetics Review Bozeman - Mechanisms that Increase Genetic Variation Bozeman - Advanced Genetics Bozeman - Chi-squared Test Bozeman – Epigenetics

Animations and Videos Chapter Quiz Questions – 1
Bozeman - Review – Genetics Genetic Problems with Solutions

Pea plants were particularly well suited for use in Mendel’s breeding experiments for all of the following reasons except that peas show easily observed variations in a number of characters, such as pea shape and flower color. it is possible to control matings between different pea plants. it is possible to obtain large numbers of progeny from any given cross. peas have an unusually long generation time. many of the observable characters that vary in pea plants are controlled by single genes. Answer: D 61

Pea plants were particularly well suited for use in Mendel’s breeding experiments for all of the following reasons except that peas show easily observed variations in a number of characters, such as pea shape and flower color. it is possible to control matings between different pea plants. it is possible to obtain large numbers of progeny from any given cross. peas have an unusually long generation time. many of the observable characters that vary in pea plants are controlled by single genes. 62

A pea plant is heterozygous at the independent loci for flower color (white versus purple) and seed color (yellow versus green). What types of gametes can it produce? two gamete types: white/white and purple/purple two gamete types: white/yellow and purple/green four gamete types: white/yellow, white/green, purple/yellow, and purple/green four gamete types: white/purple, yellow/green, white/white, and purple/purple one gamete type: white/purple/yellow/green Answer: C The purpose of this question is to help students figure out gamete types—a step they often rush past. Students need to realize that gametes are haploid and that each gamete contains one, and only one, allele for each of the traits being studied. They also need to know that one allele of one gene will be present with one allele of the other gene. In this case, we are emphasizing that two alleles and two genes are present, and students would use a Punnett square to pair up two alleles (each from a different gene). The presence of the second pea in the question stem is a distracter. Answer A is incorrect because there are four gamete types, and it shows gametes with two alleles for flower color and no alleles for seed color. Answer B is incorrect because it shows only two possible gamete types. Answer C is correct because it shows all four possible gamete types. Answer D is incorrect because it shows gametes that are diploid for one trait and containing an allele for only one locus: white and purple is not possible, and so on. Answer E is incorrect because it shows only one gamete type and a gamete diploid for both loci.

A cross between homozygous purple-flowered and homozygous white-flowered pea plants results in offspring with purple flowers. This demonstrates the blending model of genetics. true breeding. dominance. a dihybrid cross. the mistakes made by Mendel. Answer: C 65

A cross between homozygous purple-flowered and homozygous white-flowered pea plants results in offspring with purple flowers. This demonstrates the blending model of genetics. true breeding. dominance. a dihybrid cross. the mistakes made by Mendel. 66

Imagine a genetic counselor working with a couple who have just had a child who is suffering from Tay-Sachs disease. Neither parent has Tay-Sachs, nor does anyone in their families. Which of the following statements should this counselor make to this couple? “Because no one in either of your families has Tay-Sachs, you are not likely to have another baby with Tay-Sachs. You can safely have another child.” “Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 50% chance of having the disease.” “Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 25% chance of having the disease.” “Because you have had one child with Tay-Sachs, you must both carry the allele. However, since the chance of having an affected child is 25%, you may safely have thee more children without worrying about having another child with Tay-Sachs.” “You must both be tested to see who is a carrier of the Tay-Sachs allele.” Answer: C The intent of the question is to help students understand that Tay-Sachs is a genetic disease caused by a deleterious recessive allele. It also should help students realize that, with independent events, prior events have no effect on subsequent events. The parents are both heterozygotes and students can construct a Punnett square. Answer A is likely incorrect because the fact that the couple had one child with Tay-Sachs indicates that each parent has an allele for Tay-Sachs (the one exception is if a new mutation, a rare event, occurred during the meiotic events leading up to the production of one of the gametes that formed the first child). In answer B, the first sentence is correct, but this answer is incorrect because each child has a 25% probability of having the disease (it would be 50% if the allele were dominant). Answer C is correct. Answer D is incorrect because each conception must be viewed as an independent event. If this rationale were true, every family that had a girl would next have a boy—and we’ve all seen examples of families with two girls or two boys. Answer E is incorrect because Tay-Sachs is recessive and a person must have two copies of the allele to have the disease. 67

Imagine a genetic counselor working with a couple who have just had a child who is suffering from Tay-Sachs disease. Neither parent has Tay-Sachs, nor does anyone in their families. Which of the following statements should this counselor make to this couple? “Because no one in either of your families has Tay-Sachs, you are not likely to have another baby with Tay-Sachs. You can safely have another child.” “Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 50% chance of having the disease.” “Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 25% chance of having the disease.” “Because you have had one child with Tay-Sachs, you must both carry the allele. However, since the chance of having an affected child is 25%, you may safely have thee more children without worrying about having another child with Tay-Sachs.” “You must both be tested to see who is a carrier of the Tay-Sachs allele.” 68

25% DaDc, 25% DaDd, 25% DbDc, 25% DbDd 50% DaDb, 50% DcDd
Imagine a locus with four different alleles for fur color in an animal. The alleles are named Da, Db, Dc, and Dd. If you crossed two heterozygotes, DaDb and DcDd, what genotype proportions would you expect in the offspring? 25% DaDc, 25% DaDd, 25% DbDc, 25% DbDd 50% DaDb, 50% DcDd 25% DaDa, 25% DbDb, 25% DcDc, 25% DdDdDcDd 50% DaDc, 50% DbDd 25% DaDb, 25% DcDd, 25% DcDc, 25% DdDd Answer: A This question is designed to reinforce the idea that diploid organisms have two, but only two, copies of alleles for each locus. The existence of four alleles will confuse some students but is not an uncommon situation. To answer this question, students need to figure out gametes produced by the parents (Da or Db for the first parent and Dc or Dd for the second parent). Then using a Punnett square, they will get 25% each of four genotypes, DaDc, DaDd, DbDc, and DbDd. The correct answer is A. All offspring should have two alleles, and there cannot be any homozygotes from this set of parents. 69

25% DaDc, 25% DaDd, 25% DbDc, 25% DbDd 50% DaDb, 50% DcDd
Imagine a locus with four different alleles for fur color in an animal. The alleles are named Da, Db, Dc, and Dd. If you crossed two heterozygotes, DaDb and DcDd, what genotype proportions would you expect in the offspring? 25% DaDc, 25% DaDd, 25% DbDc, 25% DbDd 50% DaDb, 50% DcDd 25% DaDa, 25% DbDb, 25% DcDc, 25% DdDdDcDd 50% DaDc, 50% DbDd 25% DaDb, 25% DcDd, 25% DcDc, 25% DdDd 70

John, age 47, has just been diagnosed with Huntington’s disease, which is caused by a dominant allele. His daughter, age 25, now has a 2-year-old son. No one else in the family has the disease. What is the probability that the daughter will contract the disease? 0% 25% 50% 75% 100% Answer: C This question begins a series of three questions about Huntington’s disease and seeks to make sure that students understand that the daughter has a 50% probability of inheriting the normal allele (normal phenotype) and a 50% probability of inheriting the Huntington allele (Huntington phenotype). The question will also help students understand the implications of the fact that the Huntington allele is dominant. When the students construct a Punnett square, because no one in the family has the disease, they will note that the mother is probably homozygous recessive, the grandfather is heterozygous with the dominant Huntington’s allele, and the father of the boy is probably homozygous recessive. 71

John, age 47, has just been diagnosed with Huntington’s disease, which is caused by a dominant allele. His daughter, age 25, now has a 2-year-old son. No one else in the family has the disease. What is the probability that the daughter will contract the disease? 0% 25% 50% 75% 100% 72

That genotype would be impossible.
An individual with the genotype AaBbEeHH is crossed with an individual who is aaBbEehh. What is the likelihood of having offspring with the genotype AabbEEHh? 1/8 1/16 1/32 1/64 That genotype would be impossible. Answer: C

That genotype would be impossible.
An individual with the genotype AaBbEeHH is crossed with an individual who is aaBbEehh. What is the likelihood of having offspring with the genotype AabbEEHh? 1/8 1/16 1/32 1/64 That genotype would be impossible.

ABO blood type in humans exhibits codominance and multiple alleles
ABO blood type in humans exhibits codominance and multiple alleles. What is the likelihood of a type A father and a type A mother having a type O child? It is impossible. 25% if both parents are heterozygous 50% if both parent are heterozygous 25% if only the father is heterozygous 25% if only the mother is heterozygous Answer: B

ABO blood type in humans exhibits codominance and multiple alleles
ABO blood type in humans exhibits codominance and multiple alleles. What is the likelihood of a type A father and a type A mother having a type O child? It is impossible. 25% if both parents are heterozygous 50% if both parent are heterozygous 25% if only the father is heterozygous 25% if only the mother is heterozygous

If roan cattle (incomplete dominance) are allowed to breed, what ratio of phenotypes is expected in the offspring? 1:1 red:white all roan 1:2:1 red:roan:white 3:1 red:white 1:1:1 red:roan:white Answer: C

If roan cattle (incomplete dominance) are allowed to breed, what ratio of phenotypes is expected in the offspring? 1:1 red:white all roan 1:2:1 red:roan:white 3:1 red:white 1:1:1 red:roan:white

The phenotype will probably be yellow but cannot be red.
Imagine that the last step in a biochemical pathway to the red skin pigment of an apple is catalyzed by enzyme X, which changes compound C to compound D. If an effective enzyme is present, compound D is formed and the apple skin is red. However, if the enzyme is not effective, only compound C is present and the skin is yellow. What can you accurately say about a heterozygote with one allele for an effective enzyme X and one allele for an ineffective enzyme X? The phenotype will probably be yellow but cannot be red. The phenotype will probably be red but cannot be yellow. The phenotype will be a yellowish red. The phenotype will be either yellow or red. The phenotype will be either yellowish red or red. Answer: E This question again tries to connect enzyme action to phenotype. There is a yellow/red polymorphism in apple skin that appears to be controlled by one locus. Answers A and D are wrong because a heterozygote will produce some red pigmentation, so it cannot be yellow. Answers B and C are possible, but there is not enough information given to know which is more likely. Answer E is the best answer because yellowish red or red are possible—yellowish red if a heterozygote produces less pigment than the red homozygote, or red if a heterozygote produces as much pigment as a purple homozygote. Students with more sophistication may suggest that the phenotype would be a patchwork of yellow and red cells—red where the purple allele is “turned on” and yellow where the yellow allele is “turned on,” but this is not what happens in the apples. You can read about one gene involved in the pathway at 79

The phenotype will probably be yellow but cannot be red.
Imagine that the last step in a biochemical pathway to the red skin pigment of an apple is catalyzed by enzyme X, which changes compound C to compound D. If an effective enzyme is present, compound D is formed and the apple skin is red. However, if the enzyme is not effective, only compound C is present and the skin is yellow. What can you accurately say about a heterozygote with one allele for an effective enzyme X and one allele for an ineffective enzyme X? The phenotype will probably be yellow but cannot be red. The phenotype will probably be red but cannot be yellow. The phenotype will be a yellowish red. The phenotype will be either yellow or red. The phenotype will be either yellowish red or red. 80

Examine this genetic pedigree
Examine this genetic pedigree. What mode of inheritance does this trait follow? autosomal dominant autosomal recessive sex-linked recessive mitochondrial Answer: A

Examine this genetic pedigree. What mode of inheritance does this trait follow? autosomal dominant autosomal recessive sex-linked recessive mitochondrial

Examine this genetic pedigree. What mode of inheritance does this trait follow? autosomal dominant autosomal recessive sex-linked recessive mitochondrial Answer: B

Examine this genetic pedigree. What mode of inheritance does this trait follow? autosomal dominant autosomal recessive sex-linked recessive mitochondrial

PpAa x PpAA PpAa x ppAA PPAA x ppaa PpAa x PpAa PPaa x ppAA
The following offspring were observed from many crossings of the same pea plants. What genotypes were the parents? 465 purple axial flowers 152 purple terminal flowers 140 white axial flowers 53 white terminal flowers PpAa x PpAA PpAa x ppAA PPAA x ppaa PpAa x PpAa PPaa x ppAA Answer: D

PpAa x PpAA PpAa x ppAA PPAA x ppaa PpAa x PpAa PPaa x ppAA
The following offspring were observed from many crossings of the same pea plants. What genotypes were the parents? purple axial flowers 152 purple terminal flowers white axial flowers 53 white terminal flowers PpAa x PpAA PpAa x ppAA PPAA x ppaa PpAa x PpAa PPaa x ppAA

In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation.

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In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation.

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Presentation on theme: "In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation."— Presentation transcript:

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