Mendelian Genetics.

Mendelian Genetics

Drawing from the Deck of Genes
What genetic principles account for how traits are passed from parents to offspring? The “blending” hypothesis Says that alleles from the two parents blends together (so if mom gave you curly hair and dad gave you straight, your hair would be wavy) The “particulate” hypothesis Says that your appearance is determined by the allele from one of your parents (so you’d have curly hair like mom or straight hair like dad, but not a mix) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Gregor Mendel, a monk, studied inheritance in pea plants in the 1800s
Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?

Mendel used the scientific approach to identify two laws of inheritance
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Mendel’s Experimental Approach
Pea plants are good for genetic study because: They have lots of characters (same as genes) e.g. genes for flower color, seed texture, etc. They have easily recognizable traits (same as alleles), e.g. alleles for purple or white flowers Mating of plants is easy to control Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

TECHNIQUE Parental generation (P) Stamens Carpel 1 2 3 4 Fig. 14-2a
Figure14.2 Crossing pea plants Parental generation (P) Stamens Carpel 3 4

In genetic research, we call the first parent generation “P”
Fig. 14-2b RESULTS First filial generation offspring (F1) 5 Figure14.2 Crossing pea plants In genetic research, we call the first parent generation “P” The children of the P generation are called F1 Their children are F2, and so on.

Mendel chose to track only those characters that varied in an either-or manner (you get one or the other, not both) He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Words that mean the same thing
True-breeding Pure Purebred Homozygous = Mom and Dad both gave you the same allele for a gene e.g. PP, pp Hybrid Heterozygous = Mom gave you a different allele than Dad for that gene e.g. Pp

In most of his experiments, Mendel mated two different, true-breeding varieties, a process called hybridization When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings EXPERIMENT P Generation (true-breeding parents) Purple flowers White  F1 Generation (hybrids) All plants had purple flowers

The Law of Segregation 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

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 the white flower color a recessive trait Mendel got the same results for six other pea plant characters, each represented by two traits: When you mate two hybrids, ¾ of their offspring have the dominant trait, and ¼ have the recessive trait. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Table 14-1

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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

These alternative versions of a gene are now called alleles
The first concept is that different versions of genes cause differences between organisms For example, the gene for flower color in pea plants has two versions, one for purple flowers and the other for white flowers These alternative versions of a gene are now called alleles Each gene resides at a specific locus on a specific chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Allele for purple flowers
Fig. 14-4 Allele for purple flowers Homologous pair of chromosomes Figure 14.4 Alleles, alternative versions of a gene 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
The two alleles at a locus could be the same (like Mendel’s true breeding parent peas) Alternatively, the two alleles at a locus could be different, like Mendel’s F1 hybrids Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Pp flowers are purple, the p has no effect on appearance.
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 Pp flowers are purple, the p has no effect on appearance. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 allele from each parent Pp Parent Cell meiosis P p p P Gamete cells Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

P Generation Appearance: Purple flowers White flowers Genetic makeup:
Fig P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p Figure 14.5 Mendel’s law of segregation

Fig P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F1 Generation Appearance: Purple flowers Figure 14.5 Mendel’s law of segregation Genetic makeup: Pp Gametes: 1/2 P 1/2 p

Fig P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F1 Generation Appearance: Purple flowers Figure 14.5 Mendel’s law of segregation Genetic makeup: Pp Gametes: 1/2 P 1/2 p Sperm F2 Generation P p P PP Pp Eggs p Pp pp 3 1

We distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous)
Fig. 14-6 Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous) Figure 14.6 Phenotype versus genotype 2 Purple Pp (heterozygous) pp 1 White 1 (homozygous) Ratio 3:1 Ratio 1:2:1

The Test cross How can we figure out whether our plant is true breeding or not? If the flower is dominant (purple), it could be PP or Pp. How do we know which? The answer is to carry out a test cross: breeding the mystery individual with a homozygous recessive individual (a white one in this case) If any offspring have the recessive phenotype (white flowers), then the mystery parent had to be heterozygous (Pp). Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 Figure 14.7 The testcross 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

The Law of Independent Assortment
Mendel derived the law of segregation by following a single character (like flower color) The F1 offspring in Mendel’s experiment were monohybrids – heterozygous for one gene. When 2 monohybrids mate, we call it a monohybrid cross. These would be monohybrids: Pp, Tt, Yy, Ss These would be dihybrids: BbCc, PpTt, TtYy These are neither: BBCC, BB, pp, TTYy, Pptt Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Mendel identified his second law of inheritance by following two characters at the same time
He crossed a plant that was true breeding for 2 things (smooth and yellow) with a plant that was true breeding for 2 different things (wrinkled and green). The offspring were dihybrids: hybrid for the pea color and the pea texture. Here, he looked at pea color (yellow vs. green) and skin texture (smooth vs. wrinkled) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are inherited together as a package, or whether they are inherited independently Are all yellow seeds smooth? Or can they be smooth or wrinkled? Does what trait you have for one gene affect what trait you have for another gene?

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 Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? 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

Using a dihybrid cross, Mendel developed the law of independent assortment
The law of independent assortment states that each pair of alleles randomly separates into different gametes independently of each other pair of alleles. One exception to this rule: genes located near each other on the same chromosome tend to be inherited together (more on that in chapter 15) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Inheritance patterns are often more complex than predicted by simple Mendelian genetics
Mendel studied simple traits. Lots of traits are more complicated. 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Genes are more complicated than Mendel thought!
Environment influences the expression of genes. = “Nature vs. Nurture” Genes provide the plan for development, but how the plan unfolds also depends on environmental conditions. Here, environment means everything in the world around you – what you eat, how you’re raised, interactions with other people, etc.

Hydrangea color varies depending on how acidic the soil is
Fig Figure The effect of environment on phenotype Hydrangea color varies depending on how acidic the soil is

Some traits have more than 2 allele choices. EX: blood type Allele choices A, B, O

Some traits are determined by more than 1 gene. = polygenic trait EX: human height intelligence, skin & eye color

Traits determined by more than one gene may have multiple “in between” phenotypes. These are usually polygenic traits – caused by more than one gene. You aren’t either a genius or a dunce. Most of us are in the middle.

Kinds of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

COMPLETE DOMINANCE Dominant allele masks the recessive one
PATTERN ? Recessive allele returns in a 3:1 ratio in the F2 generation

INCOMPLETE DOMINANCE Don’t see expected 3:1 ratio in F2 generation
Heterozygous organisms with one dominant and one recessive allele show a blended in-between trait Image modified from:

Fig P Generation Red White RR WW Gametes R W When a trait shows incomplete dominance or codominance, people tend to use 2 different letters for alleles, instead of a capital for dominant and a lowercase for recessive. Figure Incomplete dominance in snapdragon color

P Generation Red White RR WW Gametes R W Pink F1 Generation RW Gametes
Fig P Generation Red White RR WW Gametes R W Pink F1 Generation RW Figure Incomplete dominance in snapdragon color Gametes 1/2 R 1/2 W

P Generation Red White RR WW Gametes R W Pink F1 Generation RW Gametes
Fig P Generation Red White RR WW Gametes R W Pink F1 Generation RW Figure Incomplete dominance in snapdragon color Gametes 1/2 R 1/2 W Sperm 1/2 R 1/2 W F2 Generation 1/2 R RR RW Eggs 1/2 W RW WW

CO-DOMINANCE Both traits are expressed at the same time (no blending) in heterozygote A roan horse has patches of red hair and white hair side by side

CO-DOMINANCE Both traits are expressed together (no blending) in heterozygote People with an A allele AND a B allele have blood type AB

REMEMBER Membrane proteins with carbohydrates attached that help cells recognize self = Glycoproteins

Blood types have more than 2 allele choices
The pattern of carbohydrates that is attached is determined by genes Allele choices are: A, B, and O

BLOOD TYPES An A allele tells the cell to put “A” glycoproteins
on its surface

BLOOD TYPES A B allele tells the cell to put a
different “B” glycoprotein on its surface

BLOOD TYPES An O allele tells
the cell NOT to put anything on the surface

A and B are CO-DOMINANT A cell with BOTH an A and a B allele has BOTH
glycoproteins on its surface

PHENOTYPE (BLOOD TYPE)
BLOOD TYPES & ALLELES GENOTYPE PHENOTYPE (BLOOD TYPE) AA A AO BB B BO OO O AB

A and AB see A as “like me” as Different!
DONOR BLOOD A and AB see A as “like me” B and O see A as Different! IMMUNE SYSTEM ATTACKS! Body images modified from:

B and AB see B as “like me” as Different!
DONOR BLOOD B and AB see B as “like me” A and O see B as Different! IMMUNE SYSTEM ATTACKS! Body images modified from:

O can donate to EVERY BLOOD TYPE
DONOR BLOOD O can donate to EVERY BLOOD TYPE = Universal donor Nothing on surface to recognize as “NOT SELF” YOU DON’T HAVE ANYTHING I DON’T HAVE! Body images modified from:

Only AB sees AB as “like me”
DONOR BLOOD Only AB sees AB as “like me” A, B, and O see AB as Different! IMMUNE SYSTEM ATTACKS! Body images modified from:

AB can only GIVE to AB BUT . . .
AB can RECEIVE FROM EVERY BLOOD TYPE = Universal Recipient Body image modified from:

BLOOD TYPE FREQUENCY IN USA
40% B 10% AB 4% O 46%

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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 population’s dominant allele Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 color and c for no color) determines whether the pigment will be deposited in the hair Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

Integrating a Mendelian View of Heredity and Variation
An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior (e.g. Type O blood is a phenotype, even though you can’t tell someone’s blood type by looking at them) An organism’s phenotype reflects its overall genotype and unique environmental history Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Many human traits follow Mendelian patterns of inheritance
Humans are not good subjects for genetic research – Generation time is too long – Parents produce relatively few offspring – Breeding experiments are unacceptable However, basic Mendelian genetics endures as the foundation of human genetics Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Key Male Affected male Mating Offspring, in birth order
Fig a Key Male Affected male Mating Figure Pedigree analysis Offspring, in birth order (first-born on left) Female Affected female

(a) Is a widow’s peak a dominant or recessive trait?
Fig b 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) Figure 14.15a Pedigree analysis 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 c 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) Figure 14.15b Pedigree analysis ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?

Pedigrees can also be used to make predictions about future offspring
We can use punnett squares to predict the probability of specific phenotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Behavior of Recessive Alleles
Many genetic disorders are inherited in a recessive manner You need two recessive alleles to get the disorder (aa, pp, etc.) Carriers are heterozygous individuals who can pass on the recessive allele but don’t have the disorder (Aa, Pp, etc.) Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Dominantly Inherited Disorders
Some human disorders are caused by dominant alleles Dominant alleles that cause a lethal disease are rare and arise by mutation Achondroplasia is a form of dwarfism caused by a rare dominant allele Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Parents Dwarf Normal Sperm Eggs Dwarf Normal Dwarf Normal Dd dd D d Dd
Fig Parents Dwarf Normal Dd  dd Sperm Figure Achondroplasia: a dominant trait D d Eggs Dd dd d Dwarf Normal Dd dd d Dwarf Normal

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 Many of these genes do not cause disease unless they are “activated” by certain environmental factors If you’re never exposed to those environmental factors, the gene won’t activate and you’ll stay healthy. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately Some of these tests are available during pregnancy Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fetal Testing In amniocentesis, the liquid that bathes the fetus is removed and tested Takes a few weeks to grow and test fetal cells In chorionic villus sampling (CVS), a sample of the placenta is removed and tested Much faster results, can be tested as early as 8-10th week of pregnancy Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

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

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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Heterozygous phenotype same as that of homozygous dominant PP Pp
Fig. 14-UN2 Degree of dominance Description Example Complete dominance of one allele Heterozygous phenotype same as that of homozygous dominant PP Pp Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes CRCR CRCW CWCW Codominance Heterozygotes: Both phenotypes expressed IAIB Multiple alleles In the whole population, some genes have more than two alleles ABO blood group alleles IA , IB , i Pleiotropy One gene is able to affect multiple phenotypic characters Sickle-cell disease

Relationship among genes Description Example Epistasis
Fig. 14-UN3 Relationship among genes Description Example Epistasis One gene affects the expression of another BbCc BbCc BC bC Bc bc BC bC Bc bc 9 : 3 : 4 Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc AaBbCc

Fig. 14-UN4

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