Mendel and the Gene Idea

Important words and concepts from Chapter 14, Campbell & Reece, 2002 (1/29/2005):

by Stephen T. Abedon (abedon.1@osu.edu) for Biology 113 at the Ohio State University

Course-external links are in brackets

Click [index] to access site index

Click here to access text’s website

(1) Chapter title: Mendel and the Gene Idea

(a) “The best way to gain an understanding of genetics is to work with it. The fundamental principles discussed (below) will become clear to you, and you will grasp them more surely, if you carefully think through . . . problems which illustrate the various patterns of inheritance…” (Keeton, 1980, Biological Science third edition, W.W. Norton & Company, p. 621)

(b) [Mendel and the gene idea (Google Search)] [Genetic Science Learning Center] [Genetics (a journal) (The Genetics Society of America)] [Three neglected advances in classical genetics] [index]

(2) Gene [genotype]

(a) A gene is a discrete heritable unit

(b) Genes are found at specific loci on chromosomes

(d) Most genes generate traits (phenotype) via their coding for the synthesis of proteins

(e) [gene (Google Search)] [index]

(3) Genotype

(a) The sequence of nucleotides on chromosomes represents genotype

(b) Genotype is subdivided into discrete, heritable units called genes

(d) [genotype (Google Search)] [index]

(4) Phenotype

(a) Phenotype is specified by genotype

(b) Phenotype is the stuff that you can see and measure about an organism

(d) Phenotype imperfectly maps onto genotype

(e) That means that there may exist numerous genotypes for any one phenotype

(f) An example of such imperfect mapping are the phenotypes associated with dominant and recessive alleles

(g) See Figure 14.5, Genotype versus phenotype

(h) [phenotype (Google Search)] [index]

(5) Allele [genotype]

(a) The genes found at a given locus may differ between homologous chromosomes

(b) Often the differences represent only one or a few nucleotides

(c) Two genes that are found at the same locus of a homologous pair of chromosomes, that differ in nucleotide sequence, are referred to as different alleles

(d) Different alleles may or may not elicit (code for) different phenotypes

(e) When a gene has a dominant form and a recessive form, these forms are considered to be separate (i.e., different) alleles which give rise to distinct phenotypes (although it is also possible for two distinct alleles to give rise to the same phenotype)

(f) A diploid organism has up to two alleles present in their genome per locus, or as few as one

(g) See Figure 14.3, Alleles, alternative versions of a gene

(h) [allele (Google Search)] [index]

(6) Character [phenotype]

(a) A character is a heritable feature (phenotype) of an individual

(b) A character can vary

(d) See Table 14.1, The results of Mendel’s F₁ crosses for seven characteristics in pea plants

(e) [genetic character (Google Search)] [index]

(7) Trait [phenotype]

(a) A variant of a character is a trait

(b) Thus, we have genes that are responsible for a given character

(d) For example, a purple flower may have an allele which codes for a purple dye at the flower-color loci in the plant's genome ; flower color is the character and purple is the trait

(e) See Table 14.1, The results of Mendel’s F₁ crosses for seven characteristics in pea plants

(f) [genetic trait (Google Search)] [index]

(8) Diploidy [genotype]

(a) In diploid organisms, phenotype typically imperfectly maps onto genotype

(b) This is in part because diploid organisms (but not haploids) can possess up to two alleles at a given locus (one on each homologue)

(c) See Figure 14.3, Alleles, alternative versions of a gene

(d) Each of these alleles, whether identical or different, will interact to produce a trait

(e) The interaction between non-identical alleles results in interesting non-correspondences between genotype and phenotype

(f) [diploidy, diploid (Google Search)] [index]

(9) Segregation of alleles (Mendel’s law of segregation) [genotype]

(a) Note that during meiosis the allele found on one homologue will segregate from the homologous allele found on a the other homologue

(b) This is cytogenetical basis for Mendel’s law of segregation

(c) See Figure 14.4, Mendel’s law of segregation

(d) [segregation of alleles, Mendel's law of segregation (Google Search)] [index]

(10) True breeding [genotype]

(a) Diploidy also results in interesting patterns of inheritance

(b) The simplest pattern results in all offspring (and their offspring, etc.) always resembling the parents

(d) This would be a total of four identical alleles between the two original parents

(e) Such characters are said to be true breeding because they fail to vary through the generations

(f) ["true breeding" (Google Search)] [index]

(11) Homozygosity (homozygote) [genotype]

(a) True breeding stems from homozygosity

(b) Homozygosity simply refers to a lack of non-identity (they’re the same) of the two alleles found at a given locus within a diploid individual

(d) [homozygosity, homozygote (Google Search)] [index]

(12) Heterozygosity (heterozygote) [genotype]

(a) An individual who possesses two different alleles at a given locus is said to be a heterozygote

(b) Note that the two alleles defining a heterozygote will segregate into different gametes such that 50% of gametes will posses one allele and the rest (50%) of the gametes will posses the other allele

(13) Homologous allele [genotype]

(a) A homologous allele is an allele that is found at the same locus on a different, homologous chromosome

(b) That is, a gene can consist of various alleles (up to 2 in a diploid individual), each of which within the same individual is considered homologous to each of these other alleles

(c) Thus, at a given locus a flower may contain an allele that codes for the trait purple flowers; the homologous allele, found on a homologous chromosome, might code for white flowers (or purple flowers or whatever), but not plant height, etc. (unless the gene has pleiotropic effects)

(d) [homologous alleles (Google Search)] [index]

(14) Dominance relationships (dominant alleles) [phenotype]

(a) Some alleles are capable of expressing a trait at the expense of a second, homologous allele within a heterozygote

(b) Such alleles (the former) are said to display dominance

(d) [dominance relationships genetics, dominant alleles (Google Search)] [index]

(15) Recessive alleles [phenotype]

(a) An homologous allele that fails to have an impact on phenotype when paired with a dominant allele is said to be recessive

(b) When we abbreviate alleles, typically the recessive allele is written in lower case (e.g., a)

(16) Homozygous recessive [phenotype]

(a) An individual who possesses only a recessive allele at a given locus is said to be homozygous recessive

(b) That is, at that locus the individual’s genotype would be aa where a is the abbreviation for the recessive allele

(17) Homozygous dominant [phenotype]

(a) An individual who possesses only the dominant allele at a given locus is said to be homozygous dominant

(b) That is, at that locus the individual’s genotype would be AA where A is the abbreviation for the dominant allele

CROSSES

(18) Cross [genotype]

(a) A cross is a mating between two individuals

(b) We abbreviate the occurrence of a cross as an x found between two genotypes, e.g., AaBBcc x aaBBcc is a cross between two individuals where we are keeping track of alleles found at three different loci (locus A, locus B, and locus C)

(19) Monohybrid cross [genotype]

(a) A monohybrid cross is a mating between two individuals who we are scoring for variation in a single character controlled by a single locus

(b) For example, flower color in peas

(d) Note that each individual participating in a monohybrid cross is heterozygotic at the locus in question

(e) See Figure 14.4, Mendel’s law of segregation

(f) [monohybrid, monohybrid cross, monohybrid cross problems (Google Search)] [index]

(20) P generation

(a) P stands for parental

(b) The parents are the first generation to be crossed (i.e., mated)

(d) See Figure 14.4, Mendel’s law of segregation

(e) ["P generation", parental generation (Google Search)] [index]

(21) F₁ generation (first filial generation)

(a) The F₁ generation is the product (the offspring) of the parental cross (P generation)

(b) F stands for filial

(c) See Figure 14.4, Mendel’s law of segregation

(d) [F1 generation, first filial (Google Search)] [index]

(22) F₂ generation (second filial generation)

(a) The F₂ generation is the product (the offspring) of the interbreeding of the F1 generation

(b) See Figure 14.4, Mendel’s law of segregation

(23) Cross between homozygous dominant and homozygous recessive [genotype & phenotype]

(a) See Figure 14.4, Mendel’s law of segregation

(b) AA x aa = P generation

(d) AA + Aa + Aa + aa = F₂ generation

(e) Say A is a dominant allele (e.g., codes for a purple flower color)

(f) Say a is a recessive allele (e.g., cods for a white flower color)

(g) Then the F₁ generation will have only purple flowers

(h) The F₂ generation will have three purple flowers for every white flower

(i) [cross between homozygous dominant and homozygous recessive (Google Search)] [index]

(24) Punnett square [genotype]

(a) The Punnett square is a means by which one attempts to keep track of expected genotypes following crosses, or by which one attempts to infer genotype information from phenotype information

(b)

(c) See Figure 14.4, Mendel’s law of segregation (see in particular the diamond shaped box associated with the F₂ generation)

(d) Note that each progeny organism inherits one allele (and only one allele) from each parent

(e) The Punnett square is limited in its power particularly because introducing multiple alleles (i.e., more than two) and, in particular, multiple loci into a cross results in an enormous mess; to transcend messes such as this we will instead employ probability theory

(f) [Punnett square (Google Search)] [index]

(25) Testcross [genotype & phenotype]

(a) Because phenotype maps imperfectly onto genotype, often we may not know the genotype of an organism, even if we know the phenotype and have a firm understanding of the underlying genetics

(b) For example, purple flowers may indicate either the heterozygous situation or the dominant homozygous situation

(d) The traditional way is to do a testcross

(e) In a testcross the individual with the unknown genotype is crossed with an individual with a known genotype

(f) What genotype is known?

(g) For the pea flower color example, it is the individuals with the white flowers

(h) That is, the homozygous recessive can produce only the white flower trait (in this example)

(i) In other words, the homozygous recessive condition typically displays a one-to-one mapping of phenotype onto genotype

(j) Examples of a testcross: Aa x aa or AA x aa (the former will give rise to two phenotypes whereas the latter will give rise to only one phenotype, thus the genotype of the first parent in each cross may be determined via this testcross)

(k) See Figure 14.6, A testcross

(l) [testcross (Google Search)] [index]

(26) Dihybrid cross (9:3:3:1 ratio) [genotype & phenotype]

(a) Upping the ante of complexity is the dihybrid cross

(b) A dihybrid is, for example, a genotype having an abbreviation of AaBb

(d) A dihybrid cross is AaBb x AaBb

(e) Follow this cross through the F₂ generation

(f) See Figure 14.7, Testing two hypotheses for segregation in a dihybrid cross

(g) Note the final phenotypic ratio of 9:3:3:1

(h) Make sure you understand this ratio as well as how to generate it

(i) [dihybrid, dihybrid cross, dihybrid cross problems, "9:3:3:1" genetics (Google Search)] [index]

(27) Independent assortment [genotype]

(a) The 9:3:3:1 ratio requires independent assortment of loci

(b) That is, for this ratio to appear, the two loci in question must be both randomly and independently shuffled (segregated) between gametes during meiosis

(c) You will understand what I mean by this when we consider the opposite situation, i.e., when loci are not independently shuffled but instead are linked together

(d) [independent assortment (Google Search)] [index]

(28) Trihybrid (and higher) crosses [genotype]

(a) The dihybrid cross can be understood employing the Punnett square

(b) However, Punnett squares become unwieldy as allele and locus number is increased

(d) A dihybrid cross looks like this: AaBb x AaBb

(e) A trihybrid cross looks like this: AaBbCc x AaBbCc

(f) A tetrahybrid cross looks like this: AaBbCcDd x AaBbCcDd, etc.

(g) To follow the products of these higher-order hybrid crosses it is far easier to employ the tools of probability theory

(h) [trihybrid, trihybrid cross, trihybrid cross problems (Google Search)] [index]

PROBABILITY THEORY (know to solve genetics problems)

(29) Probability theory

(a) In considering genetic variation, we have begun to touch upon the theory of probabilities

(b) As we delve further into genetic variation and heredity (i.e., genetics) we will also delve further into probability theory (e.g., Hardy-Weinberg equilibrium)

(c) It has been my experience that students who fail to grasp probability theory to some extent go on to fail to grasp genetics

(d) Probability theory, at its simplest, considers

(i) Statistical independence

(ii) A range of probabilities from 0 to 1

(iii) The law of multiplication

(iv) Calculating for events not happening

(v) The law of addition

(e) Probability theory will be found on exams only in the guise of genetics problems; that is, don’t worry about the terms (unless you find them helpful) so much as how to employ probability theory when doing your genetics problems

(f) [probability theory (Google Search)] [index]

(30) Statistical independence

(a) As a simplifying assumption, we will assume that all events occur independently

(b) This means that the occurrence of one event has no impact on the occurrence of another event

(c) Stated as an example, it means that when you toss two coins, which side one coin lands on does not influence which side the other coin lands on

(d) Note that statistical independence need not always apply but, for now, assume that it does always apply

(e) [statistical independence (Google Search)] [index]

(31) Range of probabilities

(a) Probabilities are assigned numerical values

(b) A probability of 0.0 means that an event will never happen

(d) An event with a probability between 0.0 and 1.0 will occur with some proportional likelihood such that the closer the probability is to 1.0, the more likely it will occur

(e) In other words, an event with a probability of 0.5 will occur half the times in which circumstances are such that such an event could occur (e.g., when a coin is tossed, half the time it will come up heads and half the time it will come up tails)

(f) An event with a probability of 0.17 will occur with much less likelihood than an event with a probability of 0.5 (e.g., when tossing dice, the likelihood of a 3 being tossed is 1/6 = 0.17; assuming the dice aren’t loaded, a probability of 1/6 should be true for any given number per toss)

(g) [range of probabilities (Google Search)] [index]

(32) Law of multiplication

(a) Assume that two events are statistically independent

(b) What is the probability that two such events will occur during two circumstances during which the event could occur (e.g., two subsequent tosses of a coin, what is the likelihood that both will be heads?)

(c) The answer is that the likelihood that two events will occur in two chances is the product of each event occurring during each chance individually (note that this ability to use the law of multiplication fails to work if the two events are not statistically independent)

(d) For example, the likelihood of obtaining two heads in two tosses of a coin are 0.5 x 0.5 = 0.25 (three tosses? 0.5 x 0.5 x 0.5 = 0.125; four? 0.5 * 0.5 * 0.5 * 0.5 = 0.0625 = 0.5⁴)

(e) Note that doing stuff like this can often be much more difficult should one choose to avoid exponential notation; this point can come back to haunt you all through biology

(f) ["law of multiplication" probability (Google Search)] [index]

(33) Calculating for events not happening

(a) Note that the law of multiplication can be used to calculate the probability of events not happening

(b) To do this, simply calculate the likelihood that an event will happen, then subtract that likelihood from 1.0

(c) For example, what is the likelihood of not rolling a total of exactly three 4’s in three rolls of dice? 1.0 – (0.17 x 0.17 x 0.17) = 1 - (1/6)³ = 0.995

(d) [events not happening probability (Google Search)] [index]

(34) Law of addition

(a) When a complex event (i.e., one requiring more than one step) can occur via more than one series of steps, what is the likelihood that the event will occur, independent of route?

(b) The answer is the sum of the probabilities that it will occur by all of the individual routes

(d) To answer this, first break down the problem into the many possible ways you can roll these numbers, distinguishing them by order: 3-4-5, 4-3-5, 3-5-4, 5-3-4, 5-4-3, 4-5-3

(e) Now, calculate the likelihood for each set: here this works out to (0.17)³ for each

(f) Now add together all of the probabilities of having the event by each possible path: (0.17)³ + (0.17)³ + (0.17)³ + (0.17)³ + (0.17)³ + (0.17)³ = 6 x (0.17)³ = 0.028

(g) The likelihood of not ending up with any of these combinations, by the way, is 1 – 0.028 = 0.972

Note that the sum of all possibilities must always add up to 1.0; you should always use this fact as a check on your calculations à if your probabilities do not all add of to 1.0 (and you have included all possibilities in your sum) then you have made some kind of mistake (and/or don’t understand the calculation)

(35) Probability theory/genetics practice problems:

(a) In squash an allele for white color (W) is dominant over the allele for yellow color (w). Give the genotypic and phenotypic ratios for the results of each of the following crosses:

(i) WW x ww

(ii) Ww x ww

(iii) Ww x Ww

(b) In peas an allele for tall plants (T) is dominant over the allele for short plants (t). An allele of another independent gene produces smooth peas (S) and is dominant over the allele for wrinkled peas (s). Calculate both phenotypic ratios and genotypic ratios for the results of each of the following crosses:

(i) TtSs x TtSs

(ii) Ttss x ttss

(iii) ttSs x Ttss

(iv) TTss x ttSS

(i) kkllmm

(ii) kkLLmm

(iii) kkLlmm

(d) Note that for this latter problem it is far easier to employ probability theory than it is to employ a Punnett’s square

(e) ["law of addition" probability (Google Search)] [index]

OK, BACK TO MENDELIAN GENETICS

(36) Genotype (properties of or impacting on as considered to varying degrees above and below)

(a) DNA nucleotide sequence

(b) Chromosomes

(d) Gene

(e) Allele

(f) Inheritance

(g) Variation

(h) Diploidy

(i) Haploidy

(j) Homozygosity

(k) Heterozygosity

(l) Multiple alelles

(m) Segregation of alleles

(n) Independent assortment

(o) Linkage

(p) Polygenic inheritance

(q) [genotype (Google Search)] [index]

(37) Phenotype (properties of or impacting on as considered to varying degrees above and below)

(a) What the organism “looks” like

(b) Character

(d) Dominant

(e) Recessive

(f) 9:3:3:1 ratios

(g) Incomplete dominance

(h) Complete dominance

(i) Codominance

(j) Pleiotropy

(k) Epistasis

(l) Quantitative characters

(m) Norm of reaction

(n) Nature versus nurture

(o) [phenotype (Google Search)] [index]

(38) Incomplete dominance [phenotype]

(a) Some alleles fail to completely dominate recessive alleles

(b) In this case we call the dominance relationship inco mplete dominance

(c) Molecularly, what is happening is that the protein produced by the incompletely dominant allele is not able to make up (completely) for the lack of activity exhibited by the protein made by the recessive allele

(d) See Figure 14.9, Incomplete dominance in snapdragon color

(e) White, in the example in the figure, is the product of a non-functional protein, i.e., no color was produced

(f) Note that with incomplete dominance the phenotypic ratios mirror the genotypic ratios (i.e., each genotype possesses a distinct phenotype)

(g) [incomplete dominance (Google Search)] [index]

(39) Complete dominance [phenotype]

(a) Complete dominance up to now we have referred to simply as dominance (or dominant)

(b) Molecularly, with complete dominance the protein produced can make up for a lack of activity exhibited by the protein produced by the recessive allele

(c) Alternatively, the effect of the recessive allele may be sufficiently slight that the phenotype associated with the completely dominant allele completely masks that associated with the recessive allele

(d) Example: brown and blue eye color

(e) Example: Black fur versus brown fur in mice

(f) Example: AO and BO in ABO blood group

(g) [complete dominance genetics (Google Search)] [index]

(40) Codominance [phenotype]

(a) Codominance occurs when two alleles both express functional proteins

(b) Very often the heterozygote possessing two codominant alleles displays the phenotypic product of both codominant alleles

(c) Note how this differs from the concept on incomplete dominance (i.e., in incomplete dominance only one allele produces a functional protein)

(d) Nevertheless, with codominance phenotype again mirrors genotype

(e) For example, the I^A and I^B alleles in the ABO blood group display codominance to each other

(f) In this case, the meaning of codominance is that both the A and the B phenotypes (oligosaccharides) are displayed by red blood cells

(g) Note that, at the molecular level, all dominance relationships are ones of codominance since the very act of describing, for example, one functional and one non-functional protein is describing two very real phenotypes associated with each allele

(h) [codominance (Google Search)] [index]

(41) Multiple alleles [genotype]

(a) The simplest case is one allele per locus

(b) We have been considering the two allele, one locus situations (as well as two alleles per loci, more than one loci situations, e.g., dihybrid cross)

(c) In addition there exist numerous examples of more than two allele, one locus genetics (and various dominance relationships can exist between different alleles)

(d) This latter case may be referred to as examples of multiple alleles

(e) Note that no matter how many alleles may be found at a given locus within a population of individuals, in a single, diploid individual a maximum of only two alleles may present

(f) [multiple alleles (Google Search)] [index]

(42) ABO blood group [genotype & phenotype]

(a) An example of a locus that displays multiple alleles is the locus that controls the ABO blood groups

(b) The ABO blood groups are controlled by three alleles, I^A, I^B, and i

(i) I^A is codominant to I^B

(ii) Both I^A and I^B are dominant to I

(iii) Note the different notation from that we have employed so far; variation in the notation used to specify genotype or phenotype is very common in genetics

(i) AB = I^AI^B (a codominant, heterozygous phenotype)

(ii) A = I^AI^A or I^Ai (a dominant phenotype)

(iii) B = I^BI^B or I^Bi (a dominant phenotype)

(iv) O = ii (the homozygous recessive phenotype)

(d) See Figure 14.10, Multiple alleles for the ABO blood groups

(e) [ABO blood group (Google Search)] [index]

(43) Pleiotropy [phenotype]

(a) The simplest case is where one locus controls only a single character (e.g., eye color)

(b) Often, however, a single locus will control more than one character

(d) Examples: albinism; white, cross-eyed tigers, etc.

(e) Note that typically, at some level of phenotype (e.g., molecular), the effect of an allele is more or less constant, but can have different effects on different systems (cells, tissues, organs) or with regard to different means of measuring the defect

(f) [pleitropy (Google Search)] [index]

(44) Epistasis [phenotype]

(a) The simplest case is where the phenotype controlled by one locus is neither affected by alleles found at other loci, nor affects the display of phenotype by alleles found at other loci

(b) Nevertheless, often one locus will influence the expression of a second locus (though not necessarily more than one character)

(d) Example (from text): Mouse coat color

(i) B = black coat color (dominant)

(ii) b = brown coat color (recessive)

(iii) C = coat is colored (dominant)

(iv) c = coat displays no color (recessive)

(e) Phenotypes (with underlying genotypes) include

(i) Black fur = BBCC, BbCC, BBCc, or BbCc

(ii) Brown fur = bbCC or bbCc

(iii) White fur (albino) = BBcc, Bbcc, or bbcc

(f) See Figure 14.11, An example of epistasis

(g) Note that the albino syndrome is actually a pleiotropy since albinism results in many defects in addition to a lack of fur (or hair) color, e.g., lack of skin color, red irises, poor vision, high susceptibility to skin cancer, poor eyesight, etc.

(h) [epistasis (Google Search)] [index]

(45) Polygenic inheritance (quantitative characters) [genotype & phenotype]

(a) Often one character is controlled by more than one locus (as with epistasis) but where loci display an additive effect (rather than qualitatively different effects as with the mouse example above)

(b) Such characters are termed quantitative and the associated genetics polygenic inheritance

(d) For example, height and skin color (in humans) are quantitative characters

(e) Animal and plant breeding typically involves quantitative characters (bigger melons, smaller dogs, higher yields, etc.)

(f) Figure 14.12, A simplified model of polygenic inheritance of skin color

(g) [polygenic inheritance, quantitative characters (Google Search)] [index]

(46) Norm of reaction (reaction norm) [phenotype]

(a) We’ve learned that any one phenotype may have a variety of genotypes as its genetic basis

(b) In addition, any one genotype may give rise to more than one variation on a character

(c) This is because genotypes interact among themselves in complex ways (e.g., epistasis and polygenic inheritance ) so that any one locus may not be able to solely control the resulting phenotype

(d) Additionally, the environment in which the organism develops and lives typically impacts on phenotype

(e) The norm of reaction refers to the degree to which a phenotype associated with a given genotype may vary, particularly as a function of variation in single environmental parameter (e.g., temperature)

(f) Some norms of reaction display no breadth (e.g., ABO blood groups , flower color)

(g) Other norms of reaction display significant breadth (e.g., polygenic characters and behavioral traits)

(h) See Figure 14.13, The effect of environment on phenotype

(i) [norm of reaction, reaction norm (Google Search)] [index]

(47) Nature versus nurture [phenotype]

(a) It is from the concept of reaction norm that we derive the concept of nature versus nurture, i.e., which is more important in determining a given phenotype , the underlying genotype or the environment?

(b) The answer to this question is sufficiently complex and ongoingly controversial that we won’t even attempt to answer this question here

PEDIGREE ANALYSIS

(48) Pedigree analysis

(a) Many of the advantages of doing genetics using experimental organisms—possessing in particular short generation times—are lost when it comes to human genetics

(b) Humans are long lived, with long generation times, and often prefer to avoid genetic manipulation (“But dear, it’s for the sake of science, and Johnny has such interesting traits. All we’re asking is that you have a very large family with him.”)

(c) On the other hand, some humans with large families have a predilection toward keeping good records of matings and births over decades and even centuries

(d) Large, well-kept pedigrees coupled with genetic diagnoses can be used to infer the genetics (e.g., dominance relationships ) of various human traits and diseases

(e) [pedigree analysis, pedigree analysis problems (Google Search)] [index]

(49) Pedigree graphic representation

(a) Pedigrees are represented graphically using certain conventions to symbolize individuals, matings, offspring-to-parent relationships, affected individuals, etc.

(i) Squares represent males

(ii) Circles represent females

(iii) Horizontal lines between individuals represent matings (often, of course, these are marriages)

(iv) Time flows from top to bottom

(v) Vertical lines represent offspring-to-parent relationships

(vi) Various branchings of vertical lines lead to siblings

(vii) Siblings typically are listed left-to-right in order of their births

(viii) Affected individuals (those displaying the trait) are represented by filled symbols

(b) The following is in more detail than you need to be concerned with in this course but still can serve as a handy reference:

(50) Autosomal dominant traits

(a) Dominant, autosomal traits have the following characteristics in a pedigree

(i) No skipping of generations

(ii) Typically only about half of offspring are also affected

(iii) No silent (i.e., not affected) carriers (at least in principle, that is)

(iv) Mutational genesis of early-onset lethal traits

(b) Examples of human autosomal dominant traits include (no need to memorize unless you are interested):

(i) Brown eyes

(ii) Widows peak

(iii) Free (not attached) earlobes

(iv) Huntington’s disease

(v) Etc.

(c) Note that lethal autosomally dominant diseases tend to be very rare because all affected individuals (all who carry the allele) die before passing that allele on to their offspring (evolution in action… literally); this is why such alleles tend to be the products of same-generation mutational genesis

(d) Huntington’s disease is excepted because the time of typical onset of symptoms is after child rearing (e.g., 35 to 45 years old)

(e) Consequent to the self-culling effect of early onset, lethal autosomally dominant alleles tend to be very rare

(f) See Figure 14.14a, Pedigree analysis

(g)

(h) [autosomal dominant, autosomal dominant traits, autosomal dominant traits pedigree analysis, autosomal dominant traits pedigree analysis problems (Google Search)] [index]

(51) Autosomal recessive traits

(a) Recessive, autosomal traits have the following characteristics in a pedigree

(i) Skipping generations (essentially same thing as “silent carriers” for early onset diseases)

(ii) Silent carriers (typically both parents)

(iii) One-fourth progeny affected (when both parents are not affected)

(iv) More likely early onset lethal than autosomal dominants

(v) For less serious conditions, parents can be homozygous recessives

(b) Examples of human autosomal recessive traits include (no need to memorize unless you are interested):

(i) Blue eyes

(ii) Lack of widow’s peak

(iii) Attached earlobes

(iv) Cystic fibrosis

(v) Tay-Sachs disease

(vi) Sickle-cell disease

(vii) Etc.

(c) For a serious (i.e., life threatening) autosomal recessive trait, the underlying alleles tend to be rare in populations; this is because the homozygous recessives tend to be “culled”; consequently, the majority of such alleles are found, within populations, in individuals who are heterozygous at the loci in question

(d) See Figure 14.14b, Pedigree analysis

(e) [autosomal recessive, autosomal recessive traits, autosomal recessive traits pedigree analysis, autosomal recessive traits pedigree analysis problems (Google Search)] [index]