Biology 1107: Mendel and Inheritance (Chapter 11 & 12)

(Chapter 12)

(Chapter 12)

Sex-linked gene

- A gene located on either sex chromosome. - Most sex-linked genes are on the X chromosome and show distinctive patterns of inheritance; there are very few genes on the Y chromosome. NOTE: - On test -

Sickle-Cell Disease

- Affects one out of 400 African-Americans • Caused by the substitution of a single amino acid in hemoglobin protein in red blood cells • In homozygous individuals ➝ all hemoglobin is abnormal (sickle-cell) • Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms - Symptoms include physical weakness, pain, organ damage, and even stroke and paralysis - About 1:10 African-Americans has sickle-cell trait à unusually high frequency of an allele with detrimental effects in homozygotes NOTE: - Two sickle-cell alleles are necessary for an individual to have sickle-cell disease and thus the condition is considered recessive - Fun Fact: The sickle-cell allele reduces the frequency and severity of malaria attacks.

1st: Alternative versions of genes account for variations in inherited characters

- Alternative versions of genes account for variations in inherited characters - Example: The gene for flower color in pea plants exists in two versions, one for purple flowers, and the other for white flowers. - Alleles - Alternative versions of a gene NOTE: - There was a difference in the way it was expressed

Tay-Sachs disease

- An inherited disorder in humans - Brain cells cannot metabolize certain lipids - As these lipids accumulate in brain cells, the child begins to suffer seizures, blindness, and degeneration of motor and mental performance and dies within a few years - Recessive: Requires two copies of the Tay-Sachs allele (homozygotes) - At the molecular level, it is codominant NOTE: - Very fatal - Triggered by dysfunctional enzymes - Enzymes that help break down lipids in the brain disfunctions, leading to a build up of lipids

Degrees of Dominance

- Complete dominance occurs when phenotypes of heterozygote and dominant homozygote are indistinguishable - Incomplete dominance ➝ phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties - Codominance ➝ two dominant alleles affect the phenotype in separate & distinguishable ways NOTE: - Alleles can show different degrees of dominance and recessiveness in relation to each other

Dominance and Disease

- For any character- the dominant/recessive relationship of alleles depends on level at which we examine the phenotype • 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 à the phenotype (i.e., the enzyme activity level) is incompletely dominant • At the molecular level ➝ the alleles are codominant NOTE: - This depends on what we are looking at when looking at traits and phenotypes

Chromosomal Basis of Sex

- Humans and other mammals have two types of sex chromosomes: a larger X chromosome and a smaller Y chromosome - Individuals who inherit two X chromosomes develop anatomy we associate with the female sex - Properties considered "male" are associated with the inheritance of one X and one Y chromosome NOTE: - Sex is the classification into a group with a shared set of anatomical and physiological traits - Gender is used to refer to an individual's own cultural experience of identifying as male, female, or otherwise - Sex is determined largely by chromosomes.

Human Traits & Mendelian Patterns of Inheritance

- Humans are not good subjects for genetic research • Generation time is too long •Parents produce relatively few offspring •Breeding experiments would be unethical - Basic Mendelian genetics utilized as foundation of human genetics - Recessive inherited disorders • Many genetic disorders are inherited in a recessive manner • These range from relatively mild to life-threatening NOTE: - Humans take a long time to gestate - Humans have 10 month gestation period - Once the fetus is born it is a baby, which has to grow up in order to fully be tested upon -

Chromosomal Basis of Sex-Humans

- In humans, the anatomical signs of sex depend on which genes are active - A gene that is located on either sex chromosome is called a sex-linked gene - Genes on the X chromosome are called X-linked genes - X chromosomes have genes for many characters unrelated to sex - Genes on the Y chromosome are called Y-linked genes • there are few of these • Many Y-linked genes help determine sex - Because of the complexity of the process of sex determination ➝ many variations exist NOTE: - A gene on the Y chromosome—called SRY, for sex-determining region of Y—is required for the development of testes. - In the absence of SRY, the gonads develop into ovaries, even in an XY embryo. - The fact that males and females inherit a different number of X chromosomes leads to a pattern of inheritance different from that produced by genes located on autosomes.

Law of Independent Assortment

- Law of segregation ➝ a single character • F1 offspring produced in this cross were monohybrids ➝ individuals that are heterozygous for one character • A cross between such heterozygotes is called a monohybrid cross - Law of inheritance ➝ two characters at same time • Crossing two true-breeding parents differing in two characters produces dihybrids in F1 generation ➝ heterozygous for both characters • A cross between F1 dihybrids can determine whether two characters are transmitted to offspring as a package or independently

Multifactorial Disorders

- Many diseases have a multifactorial basis—a genetic component plus a significant environmental influence. - Example: Heart disease, diabetes, cancer, alcoholism, and mental illnesses have both genetic and environmental components - Lifestyle has a tremendous effect on phenotype for cardiovascular health and other multifactorial characters - Example: a tremendous effect on phenotype for cardiovascular health and other multifactorial characters. Exercise, a healthful diet, abstinence from smoking, and an ability to handle stressful situations all reduce our risk of heart disease and some types of cancer - Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease - Each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings - Genetic counseling relies on the Mendelian model of inheritance

Mendel's Experiment

- Mendel - scientific approach to identify 2 laws of inheritance: - Discovered basic principles of heredity by breeding garden peas in planned experiments - Peas have a specific variety of traits or characters ➝ selected two specific traits - He could strictly control mating between plants - Used varieties that were true-breeding ➝ mated these in process called hybridization • True-breeding parents = P generation • Hybrid offspring of P generation = F1 generation • F1 individuals self-pollinate or cross-pollinate with F1 hybrids = F2 generation NOTE: - Mendel chose to work with peas because there are available in many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits - He also chose to track only those characters that occurred in two distinct, alternative forms

Law of Segregation

- 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 - When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple • 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 - Felt the heritable factor for white flowers was hidden or masked in presence of purple-flower factor • Purple flower color a dominant trait and white flower color a recessive trait - Developed a model to explain 3:1 inheritance pattern observed in F2 offspring NOTE: - Example: All the F1 plants were purple. When they were self or crossed breed they showed three purple flowers and one white flower. - Purple flowers are the dominate trait. - White flowers are the recessive trait -

Locating Genes On Chromosomes

- Mendel's "hereditary factors" were genes ➝ segments of DNA located along chromosomes - Mitosis and meiosis were first described in late 1800s - Chromosome theory of inheritance states = • Mendelian genes have specific loci (positions) on chromosomes • Chromosomes undergo segregation and independent assortment • Behavior of chromosomes during meiosis can account for Mendel's laws of segregation and independent assortment NOTE: - Mendelian genes have specific loci on the chromones which can cause a lot of the differences - Location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

Overview - Law of Independent Assortment

- Mendel's dihybrid experiment results basis for Law of Independent Assortment • States = each pair of alleles segregates independently of any other pair during gamete formation • Law applies to genes on chromosomes that are not homologous or those far apart on same chromosome - Genes located near each other on same chromosome tend to be inherited together NOTE: - Each pair of alleles with segregate independently of any other pair during gamete formation

Mendelian Inheritance & Probability

- Mendel's law of segregation and intendent assortment reflect the same rules that ally to tossing coins, rolling dice, and drawing cards from a deck (rule of probability) • Outcome of one coin toss has no impact on the outcome of the next toss - In the same way, alleles of one gene segregate into gametes independently of another gene's alleles - There are Multiplication and Addition Rules in monohybrid crosses • Segregation in a heterozygous plant ➝ each gamete has ½ carrying dominant allele and a ½ chance of carrying recessive allele NOTE: - After saying tails its recessive - 3:1 ratio - Each garter has half of the dominant allele and inheritance of carrying the recessive allele

Gene's Alleles - Chromosome Pair Relation

- Morgan determined that the white-eyed mutant allele must be located on the X chromosome - Morgan's finding supported the chromosome theory of inheritance - The behavior of the members of the pair of sex chromosomes can be correlated with the behavior of the two alleles of the eye-color gene white - One experiment: Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) • F1 generation all had red eyes • F2 generation showed the classical 3:1 red:white ratio • But only males had white eyes - Morgan concluded that the eye color was related to the sex of the fly • the white-eyed mutant allele must be located on the X chromosome NOTE: - All the F1 offspring had red eyes, suggesting that the wild-type allele is dominant. - When Morgan bred the F1 flies to each other, he observed the classical 3:1 phenotypic ratio among the F2 offspring. - However, the white-eye trait showed up only in males. - All the F2 females had red eyes, while half the males had red eyes and half had white eyes. - The correlation between the trait of white eye color and the male sex of the affected F2 flies - This suggested that the gene involved in his white-eyed mutant was located exclusively on the X chromosome and that sex somehow affects eye color - Example: For a male, a single copy of the mutant allele would confer white eyes; since a male has only one X chromosome, there can be no wild-type allele (w+) present to mask the recessive allele. However, a female could have white eyes only if both her X chromosomes carried the recessive mutant allele

Multiple Alleles

- Most genes exist in populations in more than two allelic forms - Example: The four phenotypes of the A B O blood group in humans are determined by three alleles of the gene: 1. Iᴬ 2. Iᴮ 3. i NOTE: - Antigens determine the blood - If you have O then you have no antigens - If you have A then you have A antigens - If you have B then you have specific B antigens - Likewise, if you have AB then you have A and B antigens on your blood cell - The most common recipient is AB+ - If they don't have the correlation antigens it is recognized as a foreign invaders and the immune system will attach - The enzyme (I) adds specific carbohydrates to the surface of blood cells - Iᴬ adds the A carbohydrate and Iᴮ adds the B carbohydrate

Dominant Inherited Disorders

- Number of human disorders caused by dominant alleles - Dominant alleles causing lethal disease rare • Often cause death of affected individuals before can mature/reproduce - Lethal dominant allele may be passed to next generation if lethal disease symptoms first appear after reproductive age • Achondroplasia is form of dwarfism caused by rare dominant allele - A lethal recessive allele is only lethal when homozygous; it can be passed from one generation to the next by heterozygous carriers - A lethal dominant allele often causes the death of afflicted individuals before they can mature and reproduce, and in this case the allele is not passed on to future generations. - Huntington's disease is degenerative disease of nervous system NOTE: - Dominant is the most common - These also tend to be less lethal - If it was lethal the population would die out quicker - Recessive alleles tend to be more lethal - This is because those who have it may never make it to breeding age - As you increase in age you tend to increase in your possibility of passing on a disease

Chromosomal Basis of Sex

- Only ends of Y chromosome have regions homologous with corresponding regions of X chromosome • Regions allow X and Y chromosomes to pair and behave like homologs during meiosis in males - Each ovum contains an X chromosome - A sperm may contain either an X or a Y chromosome - Other animals have different methods of sex determination NOTE: - In mammals, the sex of an offspring depends on whether the sperm cell contains an X chromosome or a Y. (All eggs have an X.) - In mammalian testes and ovaries, the two sex chromosomes segregate during meiosis. - Each egg receives one X chromosome. - In contrast, sperm fall into two categories: Half the sperm cells a male produces receive an X chromosome, and half receive a Y chromosome. - If a sperm cell bearing an X chromosome fertilizes an egg, the zygote is XX, a female - If a sperm cell containing a Y chromosome fertilizes an egg, the zygote is XY, a male - Thus, in general, sex determination is a matter of chance—a fifty-fifty chance. - There are genes for things that extend outside sex

Recessive Alleles

- 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 - Most people who have recessive disorders are born to parents who are carriers of the disorder - If a recessive allele causes a disease is rare ➝ chance of two carriers meeting and mating is low - Consanguineous (between close relatives) matings increase the chance of mating between two carriers of same rare allele - Most societies and cultures have laws or taboos against marriages between close relatives NOTE: - They only show in the homozygous individuals - An allele that causes a genetic disorder (let's call it allele a) codes for either a malfunctioning protein or no protein at all. In the case of disorders classified as recessive, heterozygotes (Aa) typically have the normal phenotype because one copy of the normal allele (A) produces a sufficient amount of the specific protein. Thus, a recessively inherited disorder shows up only in the homozygous individuals (aa) who inherit a recessive allele from each parent. - Most people who have recessive disorders are born to parents who are carriers of the disorder but have a normal phenotype - Example: A mating between two carriers corresponds to a Mendelian F1 monohybrid cross, so the predicted genotypic ratio for offspring is 1 A : 2 Aa : 1 aa. Thus, each child has a ¼ chance of inheriting a double dose of the recessive allele; in the case of albinism, such a child will have albinism. From the genotypic ratio, we also can see that out of three offspring with the normal phenotype (one AA plus two Aa), two are predicted to be heterozygous carriers, a 23 chance. Recessive homozygotes could also result from Aa×aa and aa×aa mattings, but if the disorder is lethal before reproductive age or results in sterility (neither of which is true for albinism), no aa individuals will reproduce.

Disorders of X-Linked Genes

- Some disorders caused by recessive alleles on X chromosome in humans: • Color blindness (mostly X-linked) • Duchenne muscular dystrophy • Hemophilia NOTE: - Males are color blind (NOT FEMALES) - (Be able to name at least three of these on the chart)

Addition rule states that

- States that to determine the probability of one event and the other occurring, we multiply the probability of one event (one coin coming up heads) by the probability of the other event (the other coin coming up heads) - By the multiplication rule, then, the probability that both coins will land heads up is ½ x ½ = ¼ NOTE: - It can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous - A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously - In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied - Example: With seed shape in pea plants as the heritable character, the genotype of F1 plants is Rr. Segregation in a heterozygous plant is like flipping a coin in terms of calculating the probability of each outcome: Each egg produced has a ½ chance of carrying the dominant allele (R) and a ½ chance of carrying the recessive allele (r). The same odds apply to each sperm cell produced. For a particular F2 plant to have wrinkled seeds, the recessive trait, both the egg and the sperm that come together must carry the r allele. The probability that an r allele will be present in the egg and in the sperm at fertilization is found by multiplying ½ (the probability that the egg will have an r) × ½ (the probability that the sperm will have an r). Thus, the multiplication rule tells us that the probability of an F2 plant having wrinkled seeds (rr) is ¼. Likewise, the probability of an F2 plant carrying both dominant alleles for seed shape (RR) is ¼

How are Genes passed along?

- The "blending" hypothesis is the idea that genetic material from the two parents blends together (the way blue and yellow paint blend to make green) - The "particulate" hypothesis is the idea that parents pass on discrete heritable units (genes) Mendel documented a particulate mechanism NOTE: - Parents are going to pass specific

Morgan's Fruit Fly Genetic Study

- The first solid evidence associating a specific gene with a specific chromosome came from the work of Thomas Hunt Morgan in the early 1900s - Several characteristics make fruit flies convenient organism for genetic studies • Produce many offspring • A generation can be bred every two weeks • Only four pairs of chromosomes - Morgan noted wild-type, or normal, phenotypes that were common in the fly populations - Traits alternative to the wild type are called mutant phenotypes - The first mutant phenotype he discovered was a fly with white eyes instead of the wild type red

Cystic Fibrosis

- This is the most common lethal genetic disease in US • 1:2500 people of European descent - Cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes • Leads to buildup of chloride ions outside the cell - Symptoms include: • Mucus buildup in some internal organs • Abnormal absorption of nutrients in the small intestine

Testcross

- We cannot tell the genotype of an individual with a dominant phenotype - Example: such as purple flowers - Example: Can you tell by looking at your classmate next to you their hair color? Such an individual could be either homozygous dominant or heterozygous - Breeding the individual with a recessive homozygote is called a testcross because it can reveal the genotype of that organism • If offspring display recessive phenotype ➝ mystery parent must be heterozygous NOTE: - There is a lot of genetic material - Just by looking at something you can't se the genotype only the phenotype

Duchenne muscular dystrophy

- X-linked disorder - Affects about one out of 3,500 males born in the United States - The disease is characterized by a progressive weakening of the muscles and loss of coordination. - Affected individuals rarely live past their early 20s. - Caused by an absence of a key muscle protein called dystrophin NOTE: - X-linked recessive disorder (only males)

Inheritance of X-Linked Genes

- X-linked genes follow specific patterns of inheritance - For a recessive X-linked trait to be expressed • A female needs two copies of the allele (homozygous) • A male needs only one copy of the allele (hemizygous) - X-linked recessive disorders are much more common in males than in females NOTE: - The male only has to have one copy - They only have one locus (hemizygous) - Females have to have two copies - As such, female will only express the phenotype if she is homozygous for that allele - Females are usually carries - Males usually have the condition - Any male receiving the recessive allele from his mother will express the trait - Far more males than females have X-linked recessive disorders. - The chance of a female inheriting a double dose of the mutant allele is much less than the probability of a male inheriting a single dose - Example: Color blindness is almost always inherited as an X-linked trait. A color-blind daughter may be born to a color-blind father whose mate is a carrier. Because the X-linked allele for color blindness is relatively rare, however, the probability that such a man and woman will mate is low.

Four types of blood groups:

1. A 2. B 3. AB 4. O NOTE: - These letters refer to two carbohydrates—A and B—that are found on the surface of red blood cells. - An individual's blood cells may have carbohydrate A (type A blood), carbohydrate B (type B), both (type AB), or neither (type O),

Dominant Inherited Disorders

1. Achondroplasia 2. Huntington's disease

Recessive Intertied Disorders

1. Albinism 2. Cystic Fibrosis 3. Sickle-cell disease

Three alleles of the gene:

1. Iᴬ 2. Iᴮ 3. i NOTE; - These determine the blood group gene

Two Laws of Inheritance

1. Law of Segregation 2. Law of Independent Assortment

Four Concepts of Mendel

1st: Alternative versions of genes account for variations in inherited characters 2nd: for each character an organism inherits two alleles -> one from each parent 3rd: if two alleles at locus differ -> dominant allele 4th: Law of segregation NOTE: - The dominant allele will always dominate (it will be expressed, you will see it) - The Law of segregation: Different heritable characteristics (alleles) can go to different gametes

4 Concepts of Mendel - #1 & #2

1st: alternative versions of genes account for variations in inherited characters - Example: Gene for flower color in pea plants exists in two versions • Purple flowers • White flowers - Alternative versions of gene called alleles - Each gene resides at specific locus on specific chromosome 2nd: for each character an organism inherits two alleles ➝ one from each parent - Two alleles at a particular locus may be identical - Alternatively two alleles at a locus may differ as in F1 hybrids

The F2 generation always produced a

3:1 ratio NOTE: - The dominant trait is present three times as often as the recessive trait. - Mendel coined two terms to describe the relationship of the two phenotypes based on the F1 and F2 phenotypes. - The hereditary determinants are of a particulate nature. - Example: When Mendel crossed a true-breeding variety that produced smooth, round pea seeds with one that produced wrinkled seeds, all the F1 hybrids produced round seeds; this is the dominant trait for seed shape. In the F2 generation, approximately 75% of the seeds were round and 25% were wrinkled—a 3:1 ratio

4 Concepts of Mendel - #3 & #4

3rd: if two alleles at locus differ ➝ dominant allele determines organism's appearance & recessive allele has no noticeable effect on appearance - Flower-color example: F1 plants had purple flowers because allele for trait is dominant 4th: Law of segregation => two alleles for heritable character separate during gamete formation & end up in different gametes - An egg or a sperm gets only one of two alleles present in organism - Segregation of alleles corresponds to distribution of homologous chromosomes to different gametes in meiosis

Dihybrid cross

A cross between F1 dihybrids can determine whether two characters are transmitted to offspring as a package or independently NOTE: - These can determine which of these two hypotheses is correct.

Monohybrid cross

A cross between two organisms that are heterozygous for the character being followed (or the self-pollination of a heterozygous plant).

Punnett Square

A diagrammatic device used for predicting the allele composition of all offspring resulting from a cross between individuals of known genetic makeup. NOTE: - Possible combinations of sperm and egg can be shown - It predicts results of a genetic cross between individuals of known genetic makeup • A capital letter represents a dominant allele • A lowercase letter represents a recessive allele • Example: P is the purple-flower allele and p is the white-flower allele In short: - The dominant letter is always dominant - The small letter is always recessive

Achondroplasia

A form of dwarfism that occurs in one of every 25,000 people. NOTE: - Heterozygous individuals have the dwarf phenotype - It is relatively harmless, but some dominant alleles cause lethal diseases

X-linked genes

A gene located on the X chromosome; such genes show a distinctive pattern of inheritance. NOTE: - The human X chromosome contains approximately 1,100 genes - Contains more genes than Y because it is bigger - X chromosomes have numerous genes for characters unrelated to sex. - X-linked genes in humans follow the same pattern of inheritance - Fathers can pass X-linked alleles to all of their daughters but to none of their sons. - Mothers can pass X-linked alleles to both sons and daughters.

Hemizygous

A gene present on the X chromosome that is expressed in males in both the recessive and dominant condition NOTE: - Male needs only one copy of allele (hemizygous) for an X-linked trait to express

Dominant trait

A genetic trait is considered dominant if it is expressed in a person who has only one copy of the gene associated with the trait. NOTE: - A genetic factor that blocks another genetic factor

Character

A heritable feature that varies among individuals NOTE: - Example: Flower color

Huntington's disease

A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms. NOTE: - Disease of the nervous system - Caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old - It is irreversible and inevitably fatal - A child born to a parent with the Huntington's disease allele has a 50% chance of inheriting the allele and the disorder

Recessive trait

A trait that reappears in the second generation after disappearing in the first generation when parents with different traits are bred.

Dominant allele

An allele that is fully expressed in the phenotype of a heterozygote. NOTE: - Determines the organism's appearance

Recessive allele

An allele whose phenotypic effect is not observed in a heterozygote.

Homozygous

An organism that has two identical alleles for a trait NOTE: - Female needs two copies of allele (homozygous) for an X-linked trait to express

Dihybrids

An organism that is heterozygous with respect to two genes of interest. All the offspring from a cross between parents doubly homozygous for different alleles are dihybrids. For example, parents of genotypes AABB and aabb produce a dihybrid of genotype AaBb. NOTE: - An organism that is heterozygous with respect to two genes of interest. - All the offspring from a cross between parents doubly homozygous for different alleles are dihybrids. - Example: Parents of genotypes AABB and aabb produce a dihybrid of genotype AaBb. - Example: Crossing two true-breeding pea varieties that differ two different characteristics—a cross between a plant with yellow round seeds (YYRR) and a plant with green wrinkled seeds (yyrr). This results in the two characters being followed in the cross (YyRR). • Yellow seeds (Y) - Dominant • Green seeds (y) - Recessive • Round seeds (R) - Dominant • Wrinkle seeds (r) - Recessive

Testcross

Breeding individuals with recessive homozygotes to reveal the genotype of that organism NOTE: - Is an experiment to determine what is going on - Example: Given a purple-flowered pea plant, we cannot tell if it is homozygous (PP) or heterozygous (Pp) because both genotypes result in the same purple phenotype. To determine the genotype, we can cross this plant with a white-flowered plant (pp), which will make only gametes with the recessive allele (p). The allele in the gamete contributed by the purple-flowered plant of unknown genotype will therefore determine the appearance of the offspring. If all the offspring of the cross have purple flowers, then the purple-flowered mystery plant must be homozygous for the dominant allele, because a PP×pp cross produces all Pp offspring. But if both the purple and the white phenotypes appear among the offspring, then the purple-flowered parent must be heterozygous. The offspring of a Pp×pp cross will be expected to have a 1:1 phenotypic ratio.

Trait

Each variant for a character NOTE: - Example: Purple or white color for flower

2nd: For each character, an organism inherits two alleles ➝ one from each parent

For each character, an organism inherits two versions (that is, two alleles) of a gene, one from each parent. NOTE: - Each somatic cell in a diploid organism has two sets of chromosomes • One set inherited from each parent - A genetic locus is represented twice in a diploid cell • Once on each homolog of a specific pair of chromosomes - The two alleles at a particular locus may be identical, • True-breeding plants of Mendel's P generation - The alleles may differ • The F1 hybrids

Y-linked genes

Genes located on the Y chromosome NOTE: - There are 78 genes on the human Y chromosome that code for about 25 proteins - Half of these genes are expressed only in the testis - The Y chromosome is passed along virtually intact from a father to all his sons. - Because there are so few Y-linked genes, very few disorders are transferred from father to son on the Y chromosome.

Genotype

Genetic makeup NOTE: - Because of the effects of dominant and recessive alleles, an organism's traits do not always reveal its genetic composition - Therefore, we distinguish between an organism's phenotype and its genotype, - In the example of flower color in pea plants, P P and P p plants have the same phenotype (purple) but different genotypes

3rd: if two alleles at locus differ ➝ dominant allele determines organism's appearance & recessive allele has no noticeable effect on appearance

If the two alleles at a locus differ, then one, the dominant allele, determines the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance. NOTE: - Accordingly, Mendel's F1 plants had purple flowers because the allele for that trait is dominant and the allele for white flowers is recessive.

Hybridization

In genetics, the mating, or crossing, of two true-breeding varieties.

Monohybrids

Individuals that are heterozygous for the one particular character being followed in the cross NOTE: - An organism that is heterozygous with respect to a single gene of interest. - All the offspring from a cross between parents homozygous for different alleles are monohybrids. - For example, parents of genotypes AA and aa produce a monohybrid of genotype Aa.

What about 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 single gene produces multiple phenotypes NOTE: - You can have different characters that can be inherited - Science always has acceptations <3 - It an allele is not completely dominant or allel - If a gene has more than two alleles

Law of Segregation

Mendel's first law, stating that the two alleles in a pair segregate (separate from each other) into different gametes during gamete formation.

Law of segregation

Mendel's first law, stating that the two alleles in a pair segregate (separate from each other) into different gametes during gamete formation. Example: - Mendel followed only a single character, such as flower color. - All the F1 progeny produced in his crosses of true-breeding parents were monohybrids, meaning that they were heterozygous for the one particular character being followed in the cross. - We refer to a cross between such heterozygotes as a monohybrid cross.

Law of Independent Assortment

Mendel's second law, stating that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes.

Chromosome theory of inheritance

Mendelian genes have specific loci (positions) along chromosomes, and it is the chromosome that undergo segregation and independent assortment.

Results of Mendel's F1 Crosses for Seven Characters in Pea Plants

NOTE: - 3:1 ratio - Understand what Mendel did and his 4 concepts

Genotype

NOTE: - The genetic trait that is being transferred along the chromosome - These can have three ratios - Ratio: 1:2:1

Phenotype

NOTE: - This is what is being prominently expressed - This is what you can see - Ratio: 3:1

Complete dominance

Occurs when phenotypes of heterozygote and dominant homozygote are indistinguishable NOTE: - Example: The F1 offspring always looked like one of the two parental varieties because one allele in a pair shows complete dominance over the other. - In such situations, the phenotypes of the heterozygote and the dominant homozygote are indistinguishable - They are balanced out

"Particulate" Hypothesis

Parents pass on discrete heritable units—genes—that retain their separate identities in offspring. NOTE: - An organism's collection of genes is like a deck of cards. - Like cards, genes can be shuffled and passed along, generation after generation, in undiluted form. - Mendel documented a particulate mechanism through his experiments with garden peas - It passes along the discrete heritable gene

Phenotype

Physical appearance NOTE: - Observable trait - Because of the effects of dominant and recessive alleles, an organism's traits do not always reveal its genetic composition - Therefore, we distinguish between an organism's phenotype and its genotype, - In the example of flower color in pea plants, P P and P p plants have the same phenotype (purple) but different genotypes

True breeding

Referring to organisms that produce offspring of the same variety over many generations of self-pollination. NOTE: - Example: Over many generations of self-pollination, these plants had reproduced only the same variety as the parent plant - Example: A plant with purple flowers is true-breeding if the seeds produced by self-pollination in successive generations all give rise to plants that also have purple flowers. - Example: The pea plant is the true parents. You get a hybrid offspring - P generation - grandparents - F1 generation - Parents - F2 generation - You!

F1 generation (first filial "son" generation)

The first filial, hybrid (heterozygous) offspring arising from a parental (P generation) cross.

"Blending" Hypothesis

The idea that genetic material contributed by the two parents mixes, just as blue and yellow paints blend to make green. NOTE: - It predicted that over many generations a freely mating population will give rise to a uniform population of individuals—something we don't see. - The blending hypothesis also fails to explain how traits can reappear after skipping a generation. - Both the male and female genes blended together - Think: Think of blending two colors of paint together

F2 generation (second filial generation)

The offspring resulting from interbreeding (or self-pollination) of the hybrid F1 generation. NOTE: -

Wild-type

The phenotype for a character most commonly observed in natural populations

Addition Rule

The probability that any one of two or more mutually exclusive events (one event or the other) will occur is calculated by adding their individual probabilities. NOTE: - Used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous - F1 gametes can combine to produce Rr offspring in two mutually exclusive ways: For any particular heterozygous F2 plant, the dominant allele can come from the egg or the sperm, but not from both. - The probability for one possible way of obtaining an F2 heterozygote—the dominant allele from the egg and the recessive allele from the sperm—is ¼. The probability for the other possible way—the recessive allele from the egg and the dominant allele from the sperm—is also ¼. Using the rule of addition, then, we can calculate the probability of an F2 heterozygote as ¼ + ¼ = ½ - Example: For a monohybrid cross of Yy plants, we can use a simple Punnett square to determine that the probabilities of the offspring genotypes are ¼ for YY, ½ for Yy, and ¼ for yy. We can draw a second Punnett square to determine that the same probabilities apply to the offspring genotypes for seed shape: ¼ RR, ½ Rr, and ¼ rr.

Incomplete dominance

The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele. NOTE: - The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties - It is between the two parents - Example: Red snapdragons are crossed with white snapdragons: All the F1 hybrids have pink flowers. This third, intermediate phenotype results from flowers of the heterozygotes having less red pigment than the red homozygotes. - Example: Interbreeding F1 hybrids produces F2 offspring with a phenotypic ratio of one red to two pink to one white. (Because heterozygotes have a separate phenotype, the genotypic and phenotypic ratios for the F2 generation are the same, 1:2:1.) The segregation of the red-flower and white-flower alleles in the gametes produced by the pink-flowered plants confirms that the alleles for flower color are heritable factors that maintain their identity in the hybrids; that is, inheritance is particulate.

4th: Law of segregation

The two alleles for a heritable character segregate (in other words, separate from each other) during gamete formation and end up in different gametes. NOTE: - Example: An egg or a sperm gets only one of the two alleles that are present in the somatic cells of the organism making the gamete. - In terms of chromosomes, this segregation corresponds to the distribution of copies of the two members of a pair of homologous chromosomes to different gametes in meiosis - Note that if an organism has identical alleles for a particular character then that allele is present in all gametes. - Because it is the only allele that can be passed on to offspring, that character in the offspring always looks the same as that of their parents in regard to that characteristic; this explains why these plants are true-breeding. - But if different alleles are present, as in the F1 hybrids, then 50% of the gametes receive the dominant allele and 50% receive the recessive allele.

Mutant phenotypes

Traits alternative to the wild type NOTE: - They are due to alleles assumed to have originated as changes, or mutations, in the wild-type allele.

P generation (parental generation)

True-breeding parents NOTE: - The true-breeding (homozygous) parent individuals from which F1 hybrid offspring are derived in studies of inheritance. - (P stands for parental.)

Law of inheritance

Two characters at same time NOTE: - Mendel derived the law of segregation by following a single character • F1 offspring produced in this cross were monohybrids ➝ individuals that are heterozygous for one character • A cross between such heterozygotes is called a monohybrid cross - Identified his second law of inheritance by following two characters at the same time • Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation ➝ heterozygous for both characters • A cross between F1 dihybrids can determine whether two characters are transmitted to offspring as a package or independently

Codominance

Two dominant alleles affect the phenotype in separate & distinguishable ways NOTE: - Variation on dominance relationships - The two alleles each affect the phenotype in separate, distinguishable ways. - Example: The human Mᴺ blood group is determined by codominant alleles for two specific molecules located on the surface of red blood cells, the M and N molecules. A single gene (L), for which two allelic variations are possible (Lᴹ or Lᴺ), determines the phenotype of this blood group. Individuals homozygous for the Lᴹ allele (Lᴹ Lᴹ) have red blood cells with only M molecules; individuals homozygous for the Lᴺ allele (Lᴺ Lᴺ) have red blood cells with only N molecules. But both M and N molecules are present on the red blood cells of individuals heterozygous for the M and N alleles (Lᴹ Lᴺ). Note that the Mᴺ phenotype is not intermediate between the M and N phenotypes, which distinguishes codominance from incomplete dominance. Rather, both M and N phenotypes are exhibited by heterozygotes, since both molecules are present.

Alleles are

alternative versions of a gene NOTE: - They reside at the same locus on homologous chromosomes - Each gene is a sequence of nucleotides at a specific place, or locus, along a particular chromosome. - The DNA at that locus can vary in its nucleotide sequence - This variation in information content can affect the function of the encoded protein and thus an inherited character of the organism - Example: The purple-flower allele and the white-flower allele are two DNA sequence variations possible at the flower-color locus on one of a pea plant's chromosomes. The purple-flower allele sequence allows synthesis of purple pigment, and the white-flower allele sequence does not.

An organism that has two different alleles for a gene is said to be

heterozygous for the gene controlling that character

Carriers are

heterozygous individuals who carry the recessive allele but are phenotypically normal NOTE: - Heterozygotes may transmit the recessive allele to their offspring

An organism with two identical alleles for a character is said to be

homozygous for the gene controlling that character

Mendel's "factors" are

segments of DNA located along chromosomes

At the organismal level ➝

the allele is recessive NOTE: - Alleles can show different degrees of dominance and recessiveness in relation to each other. - Alleles are simply variations in a gene's nucleotide sequence. - When a dominant allele coexists with a recessive allele in a heterozygote, they do not interact at all. - For any character, the observed dominant/recessive relationship of alleles depends on the level at which we examine the phenotype.

At the molecular level ➝

the alleles are codominant NOTE: - Whether alleles appear to be completely dominant, incompletely dominant, or codominant depends on the level at which the phenotype is analyzed.

At the biochemical level ➝

the phenotype (i.e., the enzyme activity level) is incompletely dominant NOTE: - For any character, the observed dominant/recessive relationship of alleles depends on the level at which we examine the phenotype. - Whether alleles appear to be completely dominant, incompletely dominant, or codominant depends on the level at which the phenotype is analyzed.

Heterozygotes are not

true-breeding NOTE: - Unlike homozygotes, which is

Law of independent assortment

two or more genes assort independently—that is, each pair of alleles segregates independently of any other pair during gamete formation. NOTE: - Each pair of alleles segregates independently of any other pair during gamete formation - This law applies only to genes (allele pairs) located on different chromosomes—that is, on chromosomes that are not homologous - Applies to genes that are very far apart on the same chromosome.